Dr. Schenk's 3D House of Satellites

The Snows of Enceladus

2011-05-24T17:18:00.000-05:00

I'm attending an Enceladus Workshop in Mountain View CA today and tomorrow and just finished my official report. Here is a summary. It needs a lot of editing but should still gets the basic idea across.

I know snow. Snow was a constant winter companion growing up in the south Buffalo snow belts, where the average seasonal accumulation exceeded 100 inches and 3 feet could accumulate in a single storm. So, i think i know snow. The Solar System offers little opportunity to study snow (the snows on Mars and even Triton today amount to little more than flurries; Pluto awaits). There is now one major exception to that conclusion: Enceladus. The large plumes of icy gas and dust from the south pole of that small world discovered by John Spencer and colleagues on the Cassini project give rise to a host of consequences, not least of which is the "painting" of neighboring satellites with E-ring dust (see my earlier postings here on the Saturn moons color topic). Now it looks like a thick blanket of "snowpack" may be accumulating on its fractured surface.

Buffalo NY (1964)

First a definition. By snow I do not mean large heavy snowflakes familiar to those who live in the Great Lakes snow belts or familiar with Nor'easters or Sierra snowstorms. The snows of Enceladus are small crystals smaller than a millimeter that slowly fall to the surface from the giant plumes of icy gas and dust jetting from the south pole. This "snowfall" is hardly noticeable but is persistent and steady.

One of the stories revealed by the global color mapping I did in 2009-2010 of all of Saturn's non-Titan icy satellites (see my postings from last year and my Icarus article in Volume 211, January 2011, Pages 740-757) was that the global color patterns on Enceladus did not match those of the other satellites. There was a symmetric global pattern with two opposite zones each of bluish and reddish areas, but the center of that symmetry was offset 40°W compared to that seen on the other moons. working independently was the team of Sasha Kempf and Jurgen Schmidt and colleagues in Germany who were mapping the fallback of Enceladus' plume particles back onto the surface. To our mutual delight the predicted deposition and observed color patterns matched surprising well (figure to come). This appears to be a direct confirmation that significant amounts of plume dust were falling back onto the surface in discrete patterns. It turns out that by contrast to Mars, Enceladus is caught in a decades-, if not millenia-long blizzard.

So how much plume dust (lets call it "snow") has accumulated on the surface. The answer has implications for how long the plumes have been active and what is the nature of this moon's cold surface. The model predicts that the density of accumulated snow is highly variable across the surface, reaching it thickest along the 40° and 220° W longitudes. Few direct observations are available, except in the form of a single high-resolution image a bit north of the active south polar region. Topography data are now available over this site, thanks to my particular genius in extracting such data from stereo and photocllinometry. The image was acquired at 12 meter resolution, good enough to distinguish objects as small as 40 feet across.

High resolution image (12 meters/pixel) of Enceladus showing a heavily fractured 10-kilometer wide impact crater (bottom center). In between the numerous fractures are smooth lightly cratered plains. Fizzy areas are from lower resolution context images. The color is not natural but rather color-coding of topography (blues are low, reds are high).

The scene has many similarities to terrains covered by a heavy snowfall or indeed ashfall from volcanoes. Except for the numerous deep fractures crossing the scene, the terrains are surprising flat, with shallow ghost-like remains of older fractures, several pit chains, and rounded scarp edges. Thick deposits of fine-grained debris (deposited ballistically or from above) such as snow obscures sharp narrow features like street curbs, fire hydrants or small dogs, as any car driver from up north will testify. This is what we seem to be seeing here. Our best estimate right now is that the snow deposit here is on the order of 100 to 150 meters thick (less thick elsewhere). Hopefully, additional images of the surface will be acquired at better than 10 m resolution. These measurements also has implications for how long the plumes have been active, but these and other details I will have to reveal later after we verify our calculations!

This perspective view was rendered using digital topography of the area generated by the author and is roughly 7 kilometers wide. Note the rounded scarps of the large fractures and the probable drainage pits on top the smooth plateau at center left. A fresh sharp edged fracture is visible at upper right.

If correct, then this finding gives us a new insight into what the surface of Enceladus might be like. First, it is not certain this snowfall would be visible from inside your space helmet standing on the surface, except perhaps as a faint glow in the sky towards the south or the occasional sparkling sun-glint off random descending ice crystals. There would certainly be no swirling winds (or any wind for that matter), towering snow drifts or the like. The intensity of the snow fall would be exceedingly light. The intensity would also differ depending on location. Near the south polar jets, the falling particles might be more visible, but still not as intense as light flurries. It is its persistence over many years that gives rise to the tens of if not hundred meters of accumulation. It is not quite snow as we know it in the temperate climates. The particles on the surface are expected to be sub-millimeters in size and will likely accumulate in a dense but unconsolidated blanket. Unlike our terrestrial snow packs, however, the extreme cold temperatures mean the icy crystals won't stick together very easily. You might be able to kick this snow around a bit. Traction might be a problem for astronaut and rover alike.

This perspective view of Enceladus was rendered using digital topography of the area generated by the author and is roughly 5 kilometers wide. It shows the heavily fractured rim of an old 10-kilometer-wide impact crater.

To view the FLYOVER MOVIE these stills are extracted from,

go to The Snows of Enceladus movie.

Mimas Video Show

2011-03-13T21:53:00.001-05:00

Yes it has been a while since my last post. Here is a video I made from the final new Mimas topography map, which I will talk about in a later posting. The best portion of this new map is centered on the Herschel basin and this video circles the 140-kilometer-wide 12.5 kilometer deep bowl-shaped impact basin. The conical central peak rises 5 kilometers above the floor. The video also flies over part of a wide chasm just beyond the crater rim, the origin of which remains a total mystery! Actually, it is likely either due to global spin changes or to impact induced fracturing. There is more on this feature in my previous post!

Link to Video: Herschel Impacts Mimas

Mimas Picture Show

2011-01-12T21:09:00.001-06:00

At last I have my hands on the February 2010 Mimas encounter data (I am not on the project so I have to wait 12 months to use it . . . ). The encounter produced several mosaics which allow us to produce detailed topographic and color maps of the surface centered on the great 130-km-wide Herschel impact basin. I will be writing a detailed report on this relatively young giant crater later, but in the meantime here are some cool views of the interior of the crater and its surroundings. It is shaped like a big 12-kilometer-deep cereal bowl with a large 5-kilometer-high mound in the middle. One can also see a little bit of icy debris slumped along the base of the inside rim scarp, which rises 2 kilometers above the surrounding plains (1 kilometer = 0.6 miles). (A preliminary profile based on earlier data was presented in my post in February last year.) A note bout the views, my current 3d renderer assumes the terrain is flat and so we don't yet see the curvature of Mimas, which would be very pronounced in a normal view. I'm working on implementing a new renderer soon.

Happy Holidays from your Icy Satellites!

2010-12-29T14:29:00.000-06:00

From a Texas Christmas snowfall, December 24th, 2004

A New View of Tethys

2010-12-10T11:13:00.002-06:00

A prime objective of the Cassini orbital mission at Saturn is to characterize the nature and evolution of Saturn's extended family of icy satellites. Cassini observations since the beginning of the prime mission in mid-2004 have made possible the first global maps of these diverse bodies. A team of scientists lead by Dr. Paul Schenk at the Lunar and Planetary Institute in Houston have produced the first global color and topographic maps of these satellites. These two views of Tethys show the high-resolution color (at left) and the topography (at right) of the leading, or forward-facing, hemisphere of this 1060-km-diameter ice-rich satellite. The color map shows the prominent dusky bluish band along the equator, first seen by Voyager in 1980, and shown by the team of scientists lead by Dr. Schenk to be due to the bombardment and alteration of the surface by high energy electrons traveling slower than the satellite's revolution period. These findings were published in the journal Icarus (see previous posts). The general reddish tones may be due to the coating of the Tethyean surface by dust-sized particles ejected by Enceladus' south polar plumes. The view at right is a color-coded topographic map of the same region (blues are low, reds are high). The total range of topography shown is 10 kilometers from highest to lowest point. The dominant feature is the 8-kilometer-deep and 440-kilometer-wide Odysseus impact basin at upper left. Straddling the view like a belt is a previously unknown topographic ridge between 2 and 3 kilometers high. To the east of the ridge lies ordinary rolling cratered plains, but between the ridge and Odysseus the surface is scoured and pockmarked. The ridge may be a tectonic feature related to the impact event or may be a deposit formed when the ejecta blasted out of Odysseus slammed back onto the surface at high velocity. At bottom right can be seen the globe-circling trench Ithaca Chasma, formed as part of Tethys stretched apart. This map is part of a set of new global topographic maps produced by Dr. Schenk's team for each of Saturn's icy satellites and previewed on-line on this very blog (see previous post)!

Leading Hemisphere of Tethys (base mosaic resolution is 400 meters).

Gloabl color base mosaic (left) and global color-coded topography (right).

New Moons

2010-11-30T15:10:00.018-06:00

New Moons - First Global Topographic Maps of (Saturn's) Icy Moons

At the 2010 Div. of Planetary Sciences meeting in Pasadena in October I presented some unique maps of Saturn's icy moons. Now these are different from the ones showed in 2009 and which are now published in Icarus (Schenk et al., [2010], Icarus, doi:10.1016/j.icarus.2010.08.016, now on-line). Those were global color maps as part of my persistent efforts to map these moons globally. These moons are "midsize," not as big as Ganymede or Titan but large enough to be roundish in shape and have some internal geologic history. They are Mimas, Enceladus, Tethys, Dione, Rhea and Iapetus (Figure 1a).

Figure 1a. Global color mosaics of the 6 midsize icy satellites of Saturn. Maps are shown to scale. Rhea, the largest of these moons, has a diameter of ~1525 km.

So what are these new maps? These are nearly global topographic maps of these ice-rich moons (except for the north poles), the first true global topographic maps of icy satellites we have ever had. Until now we haven't been used to seeing icy satellites in this way because the Galileo mission was not able to return global scale mapping mosaics that would have allowed topographic maps of this type (it did return a number of higher resolution local stereo maps which have been used to make the views shown in earlier weblogs!). Why are these maps important? Topography directly reflects the geologic history of a world, and reveals the tectonic, volcanic or thermal processes that have modified or altered the interior. This weblog summarizes those finding, which will be reported on in more detail in a publication Spring 2011.

These maps are derived from medium-resolution (150-2000 m/pixel) stereo images obtained by Cassini (additional high resolution stereo and shape-from-shading (photoclinometry) mapping components have been partially completed and will be added later). They reveal a wealth of geologic information. The maps (Figure 1) are first presented to scale, both horizontally and vertically. Each are shown at 1 km resolution (horizontal) and have been jpeg compressed to show the topography between -5 and +5 km relative to the approximate mean elevation (this is the range which contains roughly 95% of the topography on most of these worlds.

Figure 1a. Global topographic mosaics of the 6 midsize icy satellites of Saturn. Maps are shown to similar horizontal and vertical scales. Topographic range shown is +/-5 km. North polar areas will be filled in during the ongoing extended Cassini mission.

Global Comparisons

The presentation of satellite topography at -5 to +5 km in Figure 1b reveals some interesting features. The most obvious feature is that Iapetus and Enceladus stand out from the others. Enceladus is very active and has high heat flows, resulting in lo topography generally. The exceptions are the 100-km-wide dimple-like depressions that Bill McKinnon and myself talked about back in 2009 (Schenk and McKinnon [2009], Gephys. Res. Lett., vol. 36, CiteID L16202). Iapetus topography, on the other hand, is saturated in Figure 1b because its topographic range is ~ -12 to +12 km, roughly twice that of the other satellites (it is shown at the proper jpeg compression in Figure 2.) The ancient age of Iapetus and its large deep basins are evident here and have been noted before by others. But Mimas, Tethys, Dione, and Rhea all have much lower topographic ranges, indicating that if the deep topography seen on Iapetus ever formed on these worlds, it was erased or reset very early on by global thermal heating event(s), followed by the tectonic and cratering record we now see on those surface. This means that these satellites experienced significant global heat production early on.

Figure 2. Global topographic mosaic of Iapetus, icy satellite of Saturn. Full range of topography is shown. This map is not as complete as only 2 high-resolution encounters were planned for this satellite by Cassini orbiter.

Satellite Stories

The story is not the same for each satellite, however. On Tethys, there are several large craters with relaxed topography (Telemus, for example in Figure 1b), but also a similar number of large deep craters (Odysseus is at least 8 km deep, Figure 3). On Dione, however, all the large impact basins have experienced significant relaxation. Evander is similar in relative age to Odysseus and almost the same size yet has been essentially relaxed away (Figure 4), leaving only the rim and central structures. The implication is that Dione’s thermal heating episode lasted much longer than on Tethys. Both satellites have smooth plains that may be volcanic reminders of these thermal episodes. Rhea is different still. Although there are several large basins on Rhea, they are not as frequent as on Iapetus (which is similar in size) and are half as deep. Clearly Rhea experienced a lot more thermal heating than Iapetus. It is not as geologically complex as Dione but also experienced a period of global expansion, forming the network of extensional graben seen in last years Rhea encounter (Figure 5).

Figure 3. A radially averaged profile across Odysseus impact basin, Tethys. Data from global stereo topographic map.

Figure 4. A radially averaged profile across Evander impact basin, Dione. Data from global stereo topographic map. The horizontal bar shows where the crater floor is relative to the prominent central peak and outer rim structures.

Impact Impact

Internal heating is not the only signature we see on these small icy moons. 2:10 PMhe large impacts so evident in the topography (Figure 1b) also appear to have produced large-scale geologic disturbances. In the Tethys map, a large 2 to 3 km high ridge extends in an arc due east of the Odysseus impact basin. East of the ridge (to the right in the maps) we see fairly normal looking cratered terrains, but between the ridge and Odysseus, the texture is dominated by small irregular pits. This ridge could be a mega-ejecta ridge formed by this enormous basin (a massive computer simulation is currently underway to test this hypothesis).

A second large and very ancient basin can be seen in the center of the trailing hemisphere (left half) of the Dione topography map (Figure 1b). Radial to this basin are several prominent trenches or gouges that may have been carved when this basin formed long ago. Radial scour is also evident around Evander basin on Dione. Numerous radial troughs are also present on Iapetus although the resolution of the topographic maps in those areas is rather poor. Evidence for seismic shaking on the surface of Mimas may also be present in the form of flattened craters opposite to the location of the large Herschel impact there. Evidently, large impacts can have a big impact on the geology of these icy moons. (As an aside, I will be looking closely for similar effects when we arrive at Vesta next summer.) More details about these maps will be shown in future reports.

Figure 5. Perspective views of Rhea’s graben network. These are located near the center of the moon's trailing hemisphere (the left half of the maps in Fig. 1). Produced from high-resolution stereo/photoclinometric topographic map of the region.

The Iapetus Story

As shown in Figure 2, the topographic map of Iapetus is incomplete but it does tell us that large impact basisn 8-12 km deep dominate much of the surface. The other main feature is the equatorial ridge. The surprise here is that the ridge is definitely not continuous. It has a maximum height of approximately 18 km (final numbers will be posted later), but in other areas is only a few kilometers high and is divided into a series of widely spaced knobs. These knobs bear a striking resemblance to the blue patches on Rhea, which were also widely spaced along the equator. These were attributed in our Icarus article late this year to impact on the surface of ring debris (now apparently gone) in orbit around Rhea. The difference here is that the ring system around Iapetus proposed by Wing Ip was much more massive and accumulated much more debris onto the surface of Iapetus than on Rhea. The key is that on Rhea we see the bluish deposits prefentially only on one side of the highest standing topography, indicating the accumulation of low-flying debris onto obstacles. This could easily build the promontories seen on Iapetus. Sounds like a good target for some enterprising computer particle modelers.

To cite these results, the required citations are:

Schenk, P. (2010) Global Topographic Mapping Of Saturn's Midsize Icy Satellites: System-wide Thermal And Impact Effects, Amer. Astron. Soc., D.P.S. meeting 42, abstr. 9.16.

Schenk, P. (2010) New Moons – First Global Topographic Maps of (Saturn's) Icy Moons,

http://stereomoons.blogspot.com/2010/11/new-moons.html.

Colors Published

2010-10-15T22:05:00.005-05:00

Our major paper on the colors of Saturn's moons was published last week online at the journal Icarus. I have indeed posted on this before (see last October and February), but as always I have a few new graphics to share.

One of the more interesting features are the broad equatorial blue bands on Mimas (discovered here first) and on Tethys (discovered by Voyager first), which are in fact due to the impact of lots of high energy MeV or greater electrons that travel more slowly in Saturn's magnetic field and appear to be spiraling in "retrograde" into the front side of these satellites. (This is opposite of what happens on Europa due to the different energies involved). What is so amazing is that tiny electrons have the power to alter the surfaces of these satellites. Why the blue (really ultraviolet) brightening is unclear but the subsequent observation that these same areas are colder in the daytime than they should be (observed by my friends on the Cassini CIRS team) adds a key tot he puzzle and suggests that the surface structure is being altered on the microscopic level enough to change the thermal inertia (ability to conduct heat) of the upper centimeter of the surface! Who would have thought. Scientists are looking at this now with new data expected over the next few months.

Enhanced color view of the leading hemisphere of Mimas, showing the large crater Herschel and the broad ultraviolet band across the equator (shown in blue in this RGB rendering)

The second excitement is from Rhea. First the plasma teams observed very odd signatures around Rhea in 2007 which looked rather like the telltale signs of a thin ring around this otherwise heavily cratered satellite about the size of Alaska. Then, looking at stereo images of the craters I noticed an odd blue (really ultraviolet) patch that seemed to be right on the equator. "What a minute," I says to myself "Thats odd." So I made a global map and sure enough the spots went almost all the way around! Only a ring could do that! But then, when the imaging camera was trained to look specifically for a Rhea ring and the next close pass for the plasma instruments happened in 2009, neither time was a ring observed. Hmmm . . . Here we had direct evidence for surface impact onto the surface of small bits from a ring around Rhea (disturbing the dusty coating on the icy surface) and yet the ring turns out to be some sort of as yet unexplained phantom. Well, we don't need the ring to be present today to explain the ultraviolet splotches on the surface. They could have formed a few thousand or million years ago and still exist on the surface today. Probably not much longer than that but thats very young for the Solar System. Its times like these when I really enjoy my job.

Enhanced color medium resolution (2009) view of equatorial region of Rhea from Cassini orbiter. The ring deposits are the dark splotches running east-west along center frame. Turns out these patches don't have the same color shift as those on the leading hemisphere, perhaps due to the presence of E-ring dust on that hemisphere.

http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WGF-50X2NKM-1&_user=10&_coverDate=08%2F30%2F2010&_rdoc=40&_fmt=high&_orig=browse&_origin=browse&_zone=rslt_list_item&_srch=doc-info(%23toc%236821%239999%23999999999%2399999%23FLA%23display%23Articles)&_cdi=6821&_sort=d&_docanchor=&_ct=87&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=8c64b08a5f9d269449c99dedb4544294&searchtype=a

Mountain climbing on Iapetus

2010-09-10T22:55:00.000-05:00

A short note here to publicize the latest Iapetus video. This one features a site 300 km to the east of the views I showed a few months ago. In the region the ridge rises 18 km high and is more continuous in length, although there are obvious gaps. Also there are none of the bright ice-rich patches seen in the western site. Here the surface is completely dominated by the dark carbonaceous material that coats the leading hemisphere.
The video itself is posted on Youtube. I have also posted an updated version of the first Iapetus video on a site called ExposureRoom. I believe the ExpRoom posting is higher quality. Both Links are pasted here, so please comment if you like the ExpRoom format.

http://www.youtube.com/galsat400#p/u/0/o6J69uqPVGY
http://exposureroom.com/members/DrSchenk/ab1461b0ab884401b1665b5570089518/

Vestal Directions

2010-08-10T14:25:00.001-05:00

Two weeks ago I received the news that, after a rigorous competition, I was selected to be one of a several new members of the DAWN mission to Vesta and Ceres! Woo Hoo! It's really a pleasure and honor to be part of this flight, the first detailed exploration of what are now termed "dwarf planets," which is just another name for the larger members of the two belts of small planetoids that orbit between Mars and Jupiter and beyond Neptune. (I am also part of the New Horizons mission to "dwarf planet" Pluto in 2015, so maybe this is fitting.) I will be working on cratering studies of Vesta, which is of some importance because several small asteroids and the eucrite meteorites all appear to have been knocked off of Vesta in impact collisions in the past. We won't get to Vesta until next summer, so I won't have much to report on until then, when we start getting high-resolution images. But there is always something exciting about seeing a world for the first time, and while the new images of the asteroid Lutetia we saw on-line last month are excellent, this will be the first time we will see the larger asteroids, worlds that may have been capable of generating molten rock and volcanic flows on the surface. Should be exciting indeed.

NASA/ESA views of Ceres and Vesta from the Hubble Space telescope. Not much surface detail except for the patches of bright and colored materials, some of which are likely impact craters, and the distorted shape of vesta, due to the large south polar crater that has been identified there.

Dawn mission website: http://dawn.jpl.nasa.gov/

Saturn Triple Play - Tethys, Rhea & Iapetus

2010-08-07T11:44:00.002-05:00

With the official release (in the UK anyway) of the "Atlas of the Solar System" by myself and Cambridge Univ. Press this past week, I thought it high time to release some new goodies. Okay it is more views of the icy Saturnian satellites, but I do have several new Galilean satellite views ready to be posted when the book begins to appear on buyers desks. The views shown here are clips from three new movies of Rhea, Tethys, and Iapetus. Those of Rhea and Tethys feature distinctive impact craters. On Rhea we see what may be the youngest large crater in the Saturn system, Inktomi, while on Tethys, we fly over one of the largest impact basins in the Saturn system, Odysseus. Both are rather deep, but Inktomi is distinct for its extensive bright ray system radiating from the crater rim. These rays are diagnostic of very young impact craters. For Iapetus we fly over a broken section of the infamous and equally puzzling linear ridge that straddles the Iapetan equator. In this region, we see several 15-20 kilometer high peaks along the trace of the ridge in a region where large portions are missing.

Link to videos: http://www.youtube.com/galsat400

INKTOMI - RHEA'S BRIGHT RAY CRATER

This perspective view features Inktomi, a prominent 48-kilometer-wide bright ray crater on the icy surface of Rhea, Saturn's second largest satellite. Inktomi is the youngest and most spectacular of Rhea's many large impact craters, and most likely formed within the last few million years. The Cassini spacecraft, in orbit around Saturn, observed this crater in September 2007 at high resolution, in color, and in stereo, providing one of the best color topographic maps of Saturn's icy satellites. These perspective views highlight the steep 6 to 7 kilometer high rim wall cliffs of Inktomi. Rolling hills on the floor of the crater are debris mounds formed when portions of the rim collapsed onto the crater floor. The blue color of the rim scarp (which is actually nearly white: color is exaggerated here for clarity) reveals the crustal composition of Rhea and indicates that larger ice grain sizes or a more pure ice composition may dominate below the surface. The surface beyond the rim has been covered and scarred by icy material ejected from within Inktomi, material that also forms the extensive system of bright rays radiating away from crater center. These perspective views are based on stereo topography derived from Cassini orbiter imaging data and are excerpted from a digital movie showing a hypothetical flight over Inktomi. Mosaic base is at a resolution of 40 meters per pixel. Image processing, stereo topography, and visualization were performed by Dr. Paul Schenk, Lunar and Planetary Institute, Houston.

NISHANU - ANCIENT CRATER OF RHEA

This perspective view of Nishanu, a large 120-kilometer-wide impact crater on Saturn's icy moon Rhea, illustrates the complexity of these ancient cratered surfaces. The steep 6 kilometer deep rimwall of this impact basin is easily identified by the wide bluish ring scarp (a portion of the bluish rimwall of a smaller impact crater is also visible at lower left). The blue color is actually nearly white and the color of the scene is exaggerated here for clarity. It may be due to the exposure of crustal ice with larger grain sizes or less rocky impurities beneath the darker redder surface. The bright patches at the top of the scene are icy material ejected from the nearby very young bright ray crater Inktomi. These perspective views are based on stereo topography derived from Cassini orbiter imaging data and are excerpted from a digital movie showing a hypothetical flight over Inktomi. Mosaic base is at a resolution of 40 meters per pixel. Image processing, stereo topography, and visualization were performed by Dr. Paul Schenk, Lunar and Planetary Institute, Houston.

TETHYS - ODYSSEUS IMPACT BASIN

The 420 kilometer diameter Odysseus impact basin dominates Saturn's icy moon Tethys. This view shows the eastern two-thirds of Odysseus. Odysseus is relatively young, perhaps only 1 billion years old. It is therefore well preserved. The 2 to 3 kilometers high rim ridge defines the outer edge of the basin. The total depth from rim to floor is 8 kilometers, making Odysseus one of the deepest impact features in the Solar System. A rugged central mountain complex (at center right) forms a crude topographic ring rising 3 to 5 kilometers above the basin floor, roughly half way up to the level of the surrounding crater rim. These perspective views are based on stereo topography derived from Cassini orbiter imaging data and are excerpted from a digital movie showing a hypothetical flight over Odysseus. Mosaic base is at a resolution of 450 meters per pixel. Image processing, stereo topography, and visualization were by Dr. Paul Schenk, Lunar and Planetary Institute, Houston.

EQUATORIAL MOUNTAINS OF IAPETUS

One of the many surprises of Saturn's icy moon Iapetus is the prominent topographic ridge that straddles the eauator like a walnut. This perspective view looks east along the length of the equatorial ridge and is based on stereo topography derived from Cassini orbiter imaging data. The Cassini orbiter acquired a strip of color and stereo images along this ridge in September 2007, near the boundary between the dark and bright hemispheres. This stark albedo contrast has been observed since the late 1600's and was famously described in Arthur C. Clarke's novel adaptation of "2001: A Space Odyssey." The origin of the ridge is unknown but Cassini's stereo data indicate the ridge at this site is broken into several sharp peaks 15 to 20 kilometers above the surrounding dark cratered plains. These are among the highest peaks in the Solar System. Patches of bright pure water ice can be seen flanking these dark peaks, which have the brightness of soot. The scene is ~350 kilometers across from top to bottom and is excerpted from a new movie showing a hypothetical flight over Iapetus. Mosaic base is at a resolution of 55 meters per pixel. Image processing, stereo topography, and visualization were completed by Dr. Paul Schenk, Lunar and Planetary Institute, Houston.

PEAKS OF IAPETUS

One of the many surprises of Saturn's icy moon Iapetus is the prominent topographic ridge that straddles the eauator like a walnut. The Cassini orbiter acquired a strip of color and stereo images along this ridge in September 2007, near the boundary between the dark and bright hemispheres. This stark albedo contrast has been observed since the late 1600's and was famously described in Arthur C. Clarke's novel adaptation of "2001: A Space Odyssey." The origin of the ridge is unknown but Cassini's stereo data indicate the ridge at this site is broken into several sharp peaks 15 to 20 kilometers above the surrounding dark cratered plains. These are among the highest peaks in the Solar System. This perspective view looks southeast toward one of these peaks and is based on stereo topography derived from Cassini orbiter imaging data. Patches of bright pure water ice can be seen flanking these dark peaks, which have the brightness of soot. The scene is ~70 kilometers across and is excerpted from a new movie showing a hypothetical flight over Iapetus. Mosaic base is at a resolution of 55 meters per pixel. Image processing, stereo topography, and visualization were performed by Dr. Paul Schenk, Lunar and Planetary Institute, Houston.

EQUATORIAL TOPOGRAPHY OF IAPETUS

The Cassini orbiter acquired a strip of color and stereo images along the equator of Saturn's icy moon Iapetus in September 2007, near the boundary between the dark and bright hemispheres. This view shows an area of the equator on the trailing hemisphere crossed by low ridges and fault scarps. The prominent equatorial ridge seen elsewhere is mostly missing in this scene and replaced by low ridges and fault scarps trending north-south. This perspective view looks south across the equator and is based on stereo topography derived from Cassini orbiter imaging data. Patches of bright pure water ice can be seen flanking some of the darker cratered slopes. The scene is ~100 kilometers across and is excerpted from a new movie showing a hypothetical flight over Iapetus. Mosaic base is at a resolution of 55 meters per pixel. Image processing, stereo topography, and visualization were performed by Dr. Paul Schenk, Lunar and Planetary Institute, Houston.

Return to Dione

2010-04-06T09:26:00.003-05:00

This week Cassini makes its second very close flyby of complicated Dione. It will search for signs of activity on this enigmatic icy satellite of Saturn. Although only 1100 km across, it is perhaps second to Enceladus in terms of complexity. Despite that, no obvious current activity has been detected. Way back in the 1980's, shortly after the Voyager flybys first mapped these moons, several investigators including Jeff Moore argued that the smooth plains on Dione's leading face were formed by some type of effusive volcanism involving water and water ice. At a Lunar and Planetary Science conference a few years ago, Jeff Moore and I further proposed that a set of oddly-shaped craters seen by Cassini near Dione's equator during its ongoing mapping efforts were in fact volcanic calderas, formed by more energetic styles of volcanism, including explosive and collapse volcanism. These pits are 30 to 40 km across and very shallow.

The views shown here include perspective views of these putative volcanic features, as well as relaxed craters, smooth plains and narrow sinuous canyons that could be volcanic flow channels. Hopefully Cassini will help answer some of these questions.

Dione: North Polar Region. The large relaxed impact crater is 150 km across. The narrow canyons originate near the north pole and could be lava channels.

Dione: Equatorial Smooth Plains. The irregular pits in the background are 30 to 40 km across and could be volcanic calderas.

Dione: Smooth Plains. The largest crater is 105 km across. Its large central peak towers 3 km above the plains and is a classic indicator of viscous relaxation and a sign that Dione's internal heat flow was once much higher than today.

THE VIDEO:
http://www.youtube.com/galsat400

Mimas Rejoinder: Those Pesky Icy Satellites Between the Rings and Titan

2010-03-02T07:50:00.003-06:00

In addition to the Rhea views I released last week (see preceding posts for Blue Streak on Rhea!) to mark the Rhea encounter scheduled for later today, here are some of the other neighboring satellites. Included is a perspective view of Herschel, the large 140-km-wide 11-12 km deep impact basin on Mimas that lends it the infamous Death Star appearance. It is based on lower resolution data from 2005-2007 (I would use the newest images but I am expressly forbidden to use Cassini data until it is 1 year old). Other views include some nbew ones of fractures on Tethys and Dione (Enceladus is extensively covered in older posts). Enjoy!

Herschel impact crater on Mimas (left). Herschel is 140 kilometers across and 11-12 kilometers deep, one of the deepest in the Solar System!

The rugged cratered highlands of Mimas (right). One of this small icy moon's linear grooves, formed when the moon fractured, crosses in the foreground. The largest craters here are about 35 kilometers across.

Cratered highlands and fractures of Tethys (left). The linear fracture valley is part of the giant Ithaca Chasma fracture system. The largest crater here is 45 kilometers across.

Cratered highlands and scarps of Tethys (right). The band of linear scarps is part of the global Ithaca Chasma fracture network and here is about 125 kilometers across.

Craters and fractures of Dione. The largest crater is 100 kilometers across. (Here color is topography, blues are low, reds are high)

Rhea's Blue Streaks - Rings and Other Things!

2010-02-25T12:15:00.003-06:00

This perspective view shows one of a series of relatively blue patches that form a very narrow band only 10 kilometers wide that straddles Rhea's equator. The bluish material is fresh ice reexposed when material from Rhea's ring struck the surface along the equator, and will be a target of investigation during the March 2 Cassini flyby of Rhea. The smaller craters with the bluish deposits in the center of the frame are typically 3 to 10 kilometers wide and less than one kilometer deep. This view is looking toward the west along the equator. This view was created using stereo topography generated by Dr. Paul Schenk of the Lunar and Planetary Institute in Houston Texas from Cassini imaging data returned in 2008. The colors have been enhanced to highlight the color differences between these patches and the cratered terrains of Rhea.

It's a busy time for satellites on Cassini. An extremely close pass of Rhea a scant 100 km above the surface March 2, a brush with tiny Helene the day after, and a 500 km pass of Dione in April are all coming up very quickly, close on the heels of our only close pass of Mimas, the subject of my most recent post. As posted here in October, I've been working extensively on color mapping of these moons and their neighbors and this work has now been submitted for publication at the journal Icarus. These color maps reveal a host of interesting phenomena and a few surprises, and the new data will help us understand these features better.

But first heavily cratered Rhea. Going into orbit 6 years ago, Rhea was regarded as the ugly stepchild, the "Callisto," of the Saturn system. Little was remarkable about this second largest of Saturn's icy satellites, but it should be axiomatic by now that the closer you look at a planetary object the more surprises you see. The biggest surprises so far are the belt of graben fractures running north-south on the trailing hemisphere and the purported circum-Rhea debris ring reported in 2007. The fractures betray a degree of thermal activity on Rhea. Perhaps they are stretch marks formed when the satellite cooled and the icy lithosphere contracted, stretching the surface (though why they would form only on one side is not clear).

The debris ring around Rhea is even more puzzling. In fact the discovery has been greeted with some scepticism in the ring community (ring particles are too small to map geologically so I haven't paid too much attention, until now). Unlike the main rings or Enceladus' plumes, the Cassini cameras see no glow from Rhea when it is backlit, indicating that the debris ring is not very dusty. So it came as a pleasant surprise when our group discovered the very narrow bluish streaks along Rhea's equator (this feature was also discovered independently by two other groups). We have completed the first mapping of these features and report on it in our new journal article (see my blog of October for detailed images). Although the streaks are not continuous, they can be traced over at least 270 degrees of Rhea's circumference and lie within ~2 degrees of the equator. Most likely, discrete portions of Rhea's surface have been impacted onto its surface. A further curiosity is that the blue streaks formed only on high-standing rises or ridges. In one area, for example, they formed only on the high-standing rim of a large crater straddling the equator. This indicates that the particles were in low orbit and preferred to strike the highest standing topography first. This leads us to the videos I am releasing today. They feature the heavily cratered terrains of Rhea. The second of these videos ends with a pass over one of these bluish features, which lies across a broad cratered ridge crossing the equator of Rhea. The nature of these patches suggests they were small and disrupted the surface, reexposing blue material.

In our paper we also discuss the equatorial blue bands of Mimas and Tethys (see my preceding post this February and the post in October). Without going into too many details, we find that the patterns on both moons are almost exactly matched by the impact pattern of a particular type of retrograde trapped electrons hitting the front side of these two satellites. Yes, some particles do drift backwards within Saturn's magnetic field. We also find that the color pattern on Enceladus differs from all the others. In this case, the pattern matches the predicted fallout and deposition particles from Enceladus' icy plume back onto the surface. It also seems that the plume material blocks other particles from hitting Enceladus and thats why we do not see the equatorial bands on this satellites. Two more mysteries solved! The annual Lunar and Planetary Science Conference is coming up next week so I will be busy but stay tuned to this iChannel for more news on these bodies in the next two weeks.

These perspective views of Saturn's second largest moon, Rhea, shows the western half of its second largest impact basin, Tirawa. This ancient impact basin is 370 kilometers across and roughly 6 kilometers deep. The broad arcuate scarp cutting across scene center is the battered rim of Tirawa. The floor of Tirawa, at right in top view, left in bottom view, is heavily cratered, indicating it formed in ancient times when a 30-40 kilometer wide comet struck Rhea. The two large craters just beyond the rim are 55 and 60 kilometers across. This view was created using stereo topography generated by Dr. Paul Schenk of the Lunar and Planetary Institute in Houston Texas from Cassini imaging data returned in 2008. Although enhanced, the color in this view is an approximation of what we might actually see.

This view of Rhea's blue streaks is looking toward the north across the equator shows one of a series of relatively blue patches that form a very narrow band only 10 kilometers wide that straddles Rhea's equator. The bluish material is fresh ice reexposed when material from Rhea's ring struck the surface along the equator. This view was created using stereo topography generated by Dr. Paul Schenk of the Lunar and Planetary Institute from Cassini imaging data returned in 2008. The colors have been enhanced to highlight the color differences between these patches and the cratered terrains of Rhea.

The videos:

http://www.youtube.com/galsat400

Mimas Rising

2010-02-10T12:09:00.004-06:00

In less than a weeks time (February 13 in fact), the Cassini spacecraft makes its first very (and only) close pass of Mimas, the innermost of Saturn’s major icy satellites. Like Galileo at Io, Jupiter’s volcanic moon, this inner moon was deferred to late in the missions but for slightly different reasons. Io is deep in Jupiter’s lethal radiation belts and the Io encounters were postponed to minimize damage to the vehicle, and getting down to Io’s orbit also required a lot of orbit reduction. But the mountains and volcanoes of Io did not disappoint and fantastic discoveries were the result.

View of Mimas similar to that expected from Cassini, February 2010. The prominent large crater is at left is Herschel, the largest on this small icy moon

Topographic profile across crater Herschel, one of the deepest in the Solar System

Who Cares for Mimas?

Mimas is close to the outer edge of Saturn’s huge ring system and Cassini has not ventured very close, until now. The radiation danger is much lower at Saturn, but Mimas is not Io. Despite being only just a little smaller than crazy Enceladus, with its massive erupting ice plume, Mimas is in fact rather duller. Voyager made Mimas famous when it took the infamous Death Star image showing the large crater Herschel looking much like that battlestar’s large laser dimple. It would be a mistake to disregard Mimas as a cold cratered lump of ice, however. All of Saturn’s other icy moons have betrayed some level of geologic activity in their past. The largest crater Herschel, about 130-140 kilometers across and 11-12 kilometers deep, is one of the deepest in the Solar System. It may also be relatively young, providing an opportunity to study how craters on these icy worlds form and what they might tell us about the interior. Voyager mappers, myself included, saw sets of linear grooves that are most likely extension fractures crosing the globe. Whether these fractures formed when Mimas cooled and expanded or when Herschel knocked Mimas for a loop remains to be determined. Herschel was potentially large enough to have fractured Mimas throughout.

A map of Mimas grooves I made in 1989 based on Voyager images

Mimas Looms Large

Diminutive Mimas, named after a giant (!) of greek mythology, is only 400 kilometers across. It is also distinctly non-spherical as its polar axis is 10% shorter than the longer equatorial axes. The degree of sphericity could be used to tell whether Mimas has a small rocky core or not (it is otherwise mostly made of ices). Plus there is something very new (to us at least). I reported on this in one of my October blogs on this site. It is a lens-shaped deposit of “bluish” “stuff” along the equator of Mimas’ leading (forward-facing) hemisphere. This material appears to coat the cratered surface. We now have an explanation for this feature, and a similar one on Tethys. Unfortunately I can’t tell you the exact answer right now, as this paper has been submitted for review and we should wait until it is approved. I will post all the details on this phenomenon in a few weeks!

The best current map of Mimas from Cassini. Map resolution is ~400 m and in color

Mimas Soon

What can we expect at Mimas? The Cassini flyby on the 13^th occurs near orbit perigee (peri-kronos?) and will be very fast indeed. Although it will pass only 9500 kilometers above the surface, it will have less than an hour or so near closest approach to get the highest resolution images at something like 50-75 meters resolution (our best so far are only 350 meters or so). This should be good enough to get an image or two in which Herschel fills the camera frame. This encounter should thus provide an excellent view of Herschel and fracture systems to the east of the crater. The whole of Mimas will almost fill the wide-angle camera at this time, as well. As Cassini recedes it will likely do global disc mapping of the Herschel side of Mimas, including some of the fractures to the west. It should be an exciting weekend, this being the first good look at a body we haven't yet seen at high resolution.

http://www.youtube.com/watch?v=CwWBZFIBkr0

Rotating Mimas, based on my current global map (shown above)

Padova and Galileo - 1610-2010 Celebration (New Videos)

2010-01-06T02:58:00.001-06:00

Buongiorno!
If you have noticed that I havent posted anything here in the past two months, it is mainly because I have been preparing a big set of new videos of the Galilean satellites to support the 400th anniversary celebration here in Padova, where the actual event occurred on a cold winter night not unlike the cold weather we are having now. I sit in the cozy comfort of our hotel room this morning overlooking the cold (1°C today) train station uploading video to facebook. It will probably take the rest of the day to complete. I will have more to report tonight after the official reception for the meeting being hosted here. I am supposed to lead a tour of some large posters I contributed to the celebration. In the meantime, I post some stills below as videos continue to upload.
ciao!
paolo

Youtube channel: www.youtube.com/galsat400

(watch for updates to this post over the next day or so!)

Galileo's Moons: 4 Moons at 400 Years

2009-10-19T15:14:00.028-05:00

After focusing on Europa (and other bodies) for the past few weeks, it is time to show off the rest of Galileo' s four Jovian moons. With the 400th anniversary of Galileo's Jupiter discoveries approaching next January, it is also timely to release 4 movies, one each of Io, Europa, Ganymede and Callisto, four large bodies worthy of the name planets, despite Jupiter's domination. Each data set is different in character. The mosaics used here are from the soon to be published Atlas of the Galilean Satellites (P. Schenk, Cambridge Univ. Press, 2009).

The Io video over Tohil Mons is based on a preliminary topographic map and is in lower resolution than the final data, but it gives a good overview of the entire mountain. The Europa video is based on a a combination of stereo and shape-from-shading and is also in color. Those of Ganymede and Callisto are in grey tone because high-resolution color does not exist for these sites, or indeed most of their surfaces. All are limited by the size of the base mosaics, the result of the failure of Galileo's main antenna, limiting the downlink and the number of images that could be returned. Hence, mosaics tended to be short and narrow, limiting the turning area in which to design such movies. But enough complaining, here are Europa and her siblings.
[NOTE ADDED: all videos posted here and on YouTube are reformatted for the Iphone/Ipod. Full resolution versions are 20-40 Mb and can be made available on special request.]

Tohil Rising: Tohil Mons, Io

Tohil Mons rises 9 kilometers above the volcanic plains of Io and was a frequent target of Galileo observations. Both stereo and low-Sun imaging were acquired, allowing us to complete both stereo and shape-from-shading maps, which were combined here to produce a detailed topographic map as companion to this 340-meter resolution mosaic. Tohil Mons is one of the most complex of Io's 150 or so large mountains, many of which apparently formed due to global compression and thrust faulting resulting from Io's constant volcanic resurfacing. The video begins south of Tohil Mons. At the center of Tohil Mons lies a deep circular depression, which may have formed by volcanic collapse. Formation of this caldera may be slowly destroying the steep walls of the mountain. The low plateau to the east of the main promontories appears to be slowly collapsing or eroding. To the north we see a circular plateau standing 3 to 4 kilometers high and heavily incised by a number of fault-bounded troughs. The vibrant colors overlapping the northern part of Tohil Mons are sulfur-rich plume deposits from nearby Culann Patera, a volcano with almost no vertical relief. The Galileo cameras were sensitive in the ultraviolet and infrared, providing these enhanced colors. A higher resolution topographic map and video is currently in production.

Tohil 1: View from the northeast, across Tohil and Radagast Paterae. Reddish deposits are plume fallout from Culann Patera.

Tohil 2: View from the northwest. The central amphitheatres are dramatically apparent in this view.

Tohil 3: View from the southeast. The 100 meter high basal scarp along the eastern plateau is visible in the foreground.

Tohil Mons Rising, Io - LINK TO VIDEO

Agenor Linea: Europa's Drag Strip

The bright band Agenor Linea was one of the more intriguing features seen during Voyager 2's low resolution peak at Europa. Stretching 1400 kilometers across the face of icy Europa, Agenor Linea still mystifies. Mapping of this 9-frame 55 meter resolution Galileo mosaic suggests a combination of strike-slip and compressional faulting, not unlike that on parts of the San Andreas fault system. In this region, Agenor Linea is 20 kilometers across with only a few hundred meters of relief. At several points in the track, we see regions of chaos disruption adjacent to Agenor, highlighted by the radical tilting of ice blocks several kilometers across. Towards the end of the track, we also see several walled depressions cut into the surface of Agenor Linea, formed when parts of the icy shell dropped down.

Agenor 1: A typical section of Agenor, including the elevated margin in the foreground. The color of the band also changes from dark reddish at bottom to brighter at top.

Agenor 2: This is one of the widest parts of Agenor and occurs along a bend in the trace of the lineament.

Agenor 3: Agenor Linea lies in the foreground, while a patch of chaos highlighted by tilted blocks and rugged matrix is in the upper right.

Agenor Linea, Europa - LINK TO VIDEO

Arbela Sulcus: Cutting Ganymede

This video, based on mosaics obtained during Galileo's 28th orbit of Jupiter, crosses Arbela Sulcus, a narrow lane of bright terrain deep within the ancient cratered terrain of Nicholson Regio on Ganymede. The 4-frame mosaic, at 130 meter resolution, features three major terrain types common to Ganymede. We start over a region of grooved bright terrain, that has been that has been intensely faulted, forming a series of parallel ridges and troughs. The deepest of these troughs or grooves, in the lefthand foreground, is about 600 meters deep. The smaller ridges are typically 100 to 150 meters high. The first large crater is 6 kilometers wide and almost 700 meters deep. As we move forward, the grooved terrain comes to an abrupt end, cut by Arbela Sulcus, which here is 24 kilometers wide. This band of smooth bright terrain is nearly free of faulting and is relatively, though not entirely flat. Subtle fault-lines run parallel to the edges of the band, which most likely formed by the emplacement of now-frozen water lavas across the surface of a downdropped block of older terrain. The video ends over a highly fragmented section of Nicholson Regio proper. This terrain is heavily cratered, very rugged and very ancient. Such terrains cover only approximately 33% of Ganymede today. The narrow troughs are on the order of ~500 m deep and show that parts of ancient dark terrain were heavily fractured without forming bright terrain. The topographic data used to make this video are derived from stereo image analysis, which are not as high in resolution as the final mosaic.

Arbela 1: Ridges and troughs of grooved terrain, west of Arbela Sulcus, Ganymede. Relief across the larger ridges is up to 650 meters.

Arbela 2: Arbela Sulcus. The small crater at left is only a few kilometers across, the smooth band of Arbela Sulcus is 24 kilometers across.

Arbela 3: Extensional troughs within ancient cratered dark terrain of Nicholson Regio.

Arbela Sulcus, Ganymede - LINK TO VIDEO

Callisto's Scars: Asgard Impact Basin

Galileo obtained a 90 meter resolution 12-frame mosaic of the Asgard Basin, extending south from basin center. The video begins at the southern end of this radial mosaic in heavily cratered terrain. Immediately visible in one of the outer concentric rings of this 3000-kilometer wide structure. The outer rings here consist of walled graben or down-dropped troughs, which follow a snake-like path around the center of the basin. Also apparent is the deep erosion that has removed large portions of the graben walls as well as the rims of many craters in this region, leaving a dark surface residue. Despite the erosion, these graben remain at least 1 kilometer deep. As we fly forward we see more craters and additional graben rings. If you look closely you can see small landslide deposits on the floors of some craters. As we fly further north we begin to transition to the inner ring zone, which have the form of ridges rather than graben fractures. Further north, the terrain brightens and is less cratered. This is in fact the outer zone of a large 115-kilometer wide crater Doh, which formed some time after Asgard. At the center of Doh is a 25-kilometer-wide central dome. The surface of the dome is densely fractured, suggesting that it expanded upward as it cooled, cracking the surface. Formation of Doh obscures any features that might have existed at the center of Asgard. No stereo imaging was obtained with this mosaic, all topographic data are based on shape-from-shading techniques.

Asgard 1: Outer Ring Graben. The rugged eroded trough snaking across the scene is deep graben fracture roughly 20 kilometers across and at least 1 kilometer deep.

Asgard 2: Eroded Cratered Highlands. A small landslide is seen on the floor of the 1 kilometer deep crater at right.

Asgard 3: Inner ring ridge of Asgard impact basin. Hummocky material overlying upper half of the scene is impact ejecta from Asgard and the 115 kilometer-wide Doh crater to the north.

Asgard 4: Central dome and inner ring massif of Doh impact crater. The dome is 25 kilometers across and 1 kilometer high.

Callisto's Scars: Asgard - LINK TO VIDEO

All images, data and video credit to: Paul Schenk, Lunar and Planetary Institute, Houston (2009).

Video Ooops

2009-10-16T09:37:00.004-05:00

Not the post I wanted to make today. But via a blunder trying to set up a second account I deleted my current YouTube account and the 3d videos with it. I am currently restoring those on a new Channel. That new Channel is:
http://www.youtube.com/user/galsat400

Collision at Callanish

2009-10-15T20:03:00.002-05:00

Perhaps the most violent events in a planets history are the impact of a large asteroid or comet on its surface, which leave a record in the form of large depressions. Europa is no exception, but its dynamic geologic record has erased all but the most recent of these events. Only two craters have been identified that are larger than 30 kilometers across and both of these are very unusual. One is Tyre, the other, shown here, is Callanish.

Callanish was observed twice at high resolution by Galileo as mosaics across the center of the crater, but at different times of day. Normally this difference in lighting (late evening and early morning in this case) would preclude stereo, but it turns out that if you reverse the contrast in one of the mosaics, you get an eerie but very effective stereo pair! Its a little difficult for the eye to view but the computer can actually make sense of the information (except in the deep shadows cast by the taller ridges). Why is getting stereo important? The unusual ringed morphology (there is no crater rim, central peak or other classical feature we expect in a crater this size) indicates that the ice shell of Europa, which is believed to float on a global water ocean, is relatively thin. Getting a handle on the topography and determining what happened to this crater may help us constrain how thin that shell really is, or rather how deep the ocean lies. The new topographic map, which combines the stereo map with high-resolution shape-from-shading mapping, shows that there is very little depression at the center of Callanish. The concentric ridges that surround it can be as high as 200 meters, however. Total relief across the map area is less than 500 meters.

Callanish is Valhalla-type multiring basin, so-called because the 3000-kilometer wide Valhalla basin (which will someday appear on this blog) was the first discovered. These basins have up to several dozen concentric rings in the form of ridges or graben fractures, formed in a large impact event if the planetary lithosphere is thin and easily fractured (credit my thesis advisor Bill McKinnon for deducing this!). The fact that such a small impact crater (nominally only 33 kilometers across, which we deduce in this case fromt he range at which we can identify the numerous small secondary craters that surround it).

The comet that likely formed Callanish was only 3 to 5 kilometers across. Was it enough to punch through the icy shell and splash out ocean water on the surface? Probably not, but it came close. Numerical modeling of cratering in an icy shell like Europa's suggests that the crater was large enough that the ice shell could not support a true crater morphology and collapsed in on itself during excavation, but the ocean was not likely breached directly. Some water might have gotten mixed in during the event, however, which might explain some of the darker orange coloring of Callanish and its ejecta deposit.

Callanish Morphology

The center of Callanish is relatively flat but also very rugged with a coarsely textured surface. This texture extends across much of Callanish and is likely refrozen impact melt and debris (composed of water ice in this case). Numerous short but concentric ridges surround the center, and extend out some distance before changing to depressed fractures, called graben by geologists. Altogether, this system of ridges and graben have a diameter of 90 kilometers, although the true crater is only 33 km across. Within and beyond the fracture zone lies a coarsely textured deposit that cover or obscure older terrains. This is the ejecta deposit, which in turn is surrounded by normal Europa terrains peppered by small secondary craters from Callanish.
The natural color video starts in this outer zone of secondaries, sweeps across the ring system and past the center back to the graben zone, and then back once more across the basin center before coming to an end over the ejecta deposit, which can be seen partly burying older bits of Europa. The colors shown are based on very low resolution global images and do not unfortunately show much in detail. The synthetic color video uses the topographic map as an overlay of color, with blues being low and reds being high. The perspective views shown here a based on these videos.

Basin Center

The Outer Ring Graben

Outer Ejecta Deposit and Secondaries

Callanish

Collision at Callanish: The Movie

Two videos of Callanish are presented on YouTube and FB.
The first video is in natural color - http://www.youtube.com/watch?v=5asr3q4pj8Y
http://www.facebook.com/paul.schenk?ref=profile#/video/video.php?v=1127515597030
The second video features colorized topography - http://www.youtube.com/watch?v=ueEmhQWkQXY
http://www.facebook.com/paul.schenk?ref=profile#/video/video.php?v=1127584678757&ref=mf

Imaging is from Galileo Orbiter. Topography and rendering by and credited to: Dr. Paul Schenk, Lunar and Planetary Institute, Houston.

The Colors of Saturn's Mid-Sized Icy Satellites

2009-10-05T11:32:00.007-05:00

October 5 - Fajardo, Puerto Rico

Okay, this isn't stereo or topography but it is some cool stuff. Research can take some interesting twists, and these maps are a case in point. In the course of examining some of the high resolution images from Cassini's Rhea pass in 2007, searching for stereo images, I found some were in color. In the process of registering these images I produced a color mosaic and noticed some odd bluish spots (it turns out the Cassini team had already seen a few of these odd spots, but I didn't know that at the time). Well, my natural curiosity was aroused by their location very close to the equator, so, with the report by Jones and colleagues of ring around Rhea, it was obvious that a global color map was necessary. That proved to be an effort but this "feature" was very obvious in this new global map, and so were some other interesting patterns too. This lead to a third bout of curiosity: why not map all the icy moons? Do they show any similar patterns? These color maps (of Mimas, Enceladus, Tethys, Dione, and Rhea) are what I show here. The maps contain more than a few surprises: these moons are not so bland after all! To understand these patterns I have called on the aid of several of my colleagues, and the discussion below represents some of our early conclusions. We will have much more to say over the next few months.

The Data
Voyager first mapped the icy saturnian satellites from a distance (something my friend Bonnie Buratti did much of the work on), but Cassini has two advantages: it can map each moon at full resolution (400 to 750 meters) and over a broader spectral range: from UV (0.38 microns) to IR (0.93 microns). This allows us to map out global and local patterns with confidence. The most dramatic way to map out these patterns is to ratio the infrared reflectance by the ultraviolet reflectance (what we call the IR/UV ratio). I show both versions.

The Maps
Each map is in cylindrical projection. They extend from pole-to-pole and 0° to 360° (from right to left). Thus the leading hemisphere is on the right half and the trailing hemisphere on the left half.

The Patterns

Global Asymmetries
Several curious features are apparent in the new color maps. First is the basic color asymmetry apparent on Tethys, Dione and Rhea. This is an enhancement in the redness of the surface on the trailing hemisphere of each of these satellites (visible as a brightness in the IR/UV ratio maps). There is also a subtle enhancement on the leading hemisphere. This pattern suggests that each satellite is getting bombarded by particles and charged plasma on both sides. One candidate for the front-side color pattern is E-ring particles, which are suspected to coat (or blast) the surfaces of these moons. It turns out that the Enceladus pattern may be related to the fallout of plume material back onto the surface.

The Mimas Band
The second unusual feature are the narrow lens-shaped IR-dark, UV-bright features across the equator of both Mimas and Tethys. The Tethys feature was known from Voyager days, but the Mimas feature has not been recognized previously. It isn't new, but Voyager did not have the spectral range to detect it and it does not contrast as strongly on Mimas as on Tethys. High-energy charged particles in saturn's magnetosphere can make a very similar pattern on the surfaces of these two inner moons, and we are currently testing this model with numerical predictions tuned to each moon. We should have an answer soon.

The equatorial Line on Rhea
(I forgot to note initially that the above image is taken from the IR/UV ratio map. It shows the leading hemisphere only. The features thenselves are not dark, but rather more blue than regular terrains. In fact they are not distinctive in ordinary images of the surface.)
A third unusual feature is a very narrow set of small UV-bright spots on Rhea. Normally this is not a cause for excitement, as fresh crater rims have this signature, but these are lined up along a great circle trace very close to Rhea's equator. This alignment is not a random coincidence. No other satellite has comparable features. (The Cassini team is planning higher resolution observations during next year's Rhea flyby). This feature is only a few kilometers across, but its linear pattern across nearly 2/3rds of Rhea's circumference and alignment within 2 degrees of the equator indicate it is quite plausibly material from Rhea's proposed ring system that has struck the surface of Rhea. A higher resolution color observation (the one that started this entire project) suggests that this material would be composed of discrete but incoherent packets of ring material that this the surface at scattered intervals along the equator, and I should be posting that image here later. These and the other observations make an intriguing story but one that requires a lot more work to fully understand.

email: schenk@lpi.usra.edu

Broken Land: Touring Conamara Chaos, Europa

2009-09-18T21:52:00.005-05:00

At last we move on (or back) to Europa. I will be posting a series of Europa views as I work through my Galileo image and data archives. These data are on the order of 10 years old, and I have started to combine some of my new color mosaics (generated for the forthcoming Atlas of the Galilean Satellites, which I will describe later this month), with the stereo and photoclinometry based Digital Elevation Models (DEM) or topographic maps I have been generating over that same time. I start with one of the best of Galileo's data sites, Conamara Chaos, a type example of this broken and jumbled terrain.

Over the next days and weeks I will post more of these, interspersed with an odd Ganymede or Callisto set. These satellites did not get as good coverage from Galileo. Io will not be forgotten, but requires additional processing. The key difference for Galileo was the antenna failure, which crippled communications and limited mapping and stereo coverage to small mosaics. These limited areas require tight turns and makes video production more challenging. I might get an Academy Award yet!

Paul

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Broken Land: Conamara Chaos

These perspective views show parts of the interior of Conamara Chaos, a region where the ridged crust of Europa has been disrupted into smaller plates set amidst a crazed and rugged terrain, termed matrix, that is in reality crushed ice. Although some areas look like frozen over liquid water, it is as likely, if not more so, that these are the result of diapirs, rising blobs of warm ice from below that have broken through to the surface. The total relief across Conamara Chaos is only 500 meters, so don't expect towering mountains. Individual ridges and blocks can be 100 to 200 meters high. The color shown here is actual surface color, enhanced to bring out contrasts. Galileo's camera was sensitive to infrared and ultraviolet radiation and so these colors are a little stronger than what we would see. The original images have a basic resolution of about 55 meters. The large blocks are typically 5 to 10 kilometers across.

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Broken Land: Conamara Flight
The video can be seen on Facebook (at least thats what I'm trying to link to here).
http://www.facebook.com/photo.php?pid=30381827&op=1&o=global&view=global&subj=1501400606&id=1223295839#/video/video.php?v=1112355938048

Credit: Paul Schenk/Lunar and Planetary Institute, Houston

Miranda's Warning

2009-09-10T08:26:00.007-05:00

Miranda, always taking the spotlight. Nobody cared much about this small world, about the size of Louisiana, until January 24, 1986, when a (relatively) primitive but stout robot Voyager 2 sailed past - just 4 days before Challenger. (I had the opportunity to return to JPL to witness the encounter but as my doctoral exams were not going well, it was "suggested" I stay at Wash U and study.) Expectations were not especially high, and Miranda was expected to look a bit like Saturn's lumpy moon Mimas, although given Voyager's history, anything was possible. Voyager's path to Neptune took it closest to Miranda of Uranus' five larger classical moons. Resolutions as high as 250 meters were possible, among the best of the entire Voyager mission. Happily, most of these images came back unsmeared and sharp. They revealed a complex and evolved landscape. Half of Miranda looked indeed a bit like cratered Mimas, but the other half was paved over if you will by lanes of ridges and grooves vaguely (and misleadingly) reminiscent of grooved terrain on Ganymede, all concentrated in three ovoid or retangular regions called coronae. This tiny moon with the multiple personalities has fascinated researchers ever since. Most now agree that Miranda attempted to turn itself inside out due to residual formation or forced tidal heating. Soft warm ice oozed up in three (or more) convective cells, forming the ovoid coronae on he surface where some of this ice breached the surface. Alas, only 45-50% of the surface was visible in sunlight at the time due to the fact Uranus and its moons being at southern solstice in 1986. Efforts to return to Uranus and Neptune have so far met with frustration. Perhaps the images shown here of Miranda, Ariel and Triton will relight the fires of interest in these planets and their active moons.

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Flyover of Miranda, Uranian moon.
Imaging data acquired January 1986 by Voyager 2, topography derived by P. Schenk. The total topographic relief shown here is roughly 10 kilometers. Note that we are seeing roughly half of the entire moon here (which is roughly 500 km across), but the data are shown in flat map projection (the renderer cannot as yet handle a true sphere).
The movie begins with an approach toward Elsinore Corona, one of the ovoid regions of ridges and grooves. It then turns toward and over Inverness Corona, a smaller region of resurfacing very close to the south pole. We end with a "landing" along the edge of the 10 km deep Verona Rupes fault canyon system.

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Elsinore Corona
The first two views of Elsinore Corona, an ovoid shaped region resurfaced by ridges composed of water and possibly other ices. These ridges stand up to 2 kilometers high in some locations. The rugged terrain nearby is ancient cratered highlands, which has relief of 5 kilometers or more.

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Inverness Corona

The next two views show the border between Inverness Corona and the more rugged cratered highlands. Inverness, like Elsinore, has been resurfaced by viscous ices that flowed onto the surface, forming ridges several hundred meters to a kilometer or so high. The cratered highlands have relief of several kilometers.

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Verona Rupes and Inverness Corona

This area is the most rugged terrain known on Miranda. Up to 10 kilometers of relief has been mapped here, a complex area formed by the intersection of multiple tectonic features.

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New Global Image Mosaic and Topographic Map of Miranda
These maps (image mosaic on top, topography on bottom) are in lambertian equal area projection, centered on the south pole. They have been reduced in size by a factor of 2 for the web.

All images may be used with permission.
Credit: NASA/JPL and Paul Schenk/Lunar and Planetary Institute, Houston

The Turn of Enceladus

2009-09-03T21:39:00.028-05:00

When we think of Saturn, its tiny icy moon Enceladus is right there on the list of fun stuff. The south polar region is where the current action is, so naturally I am curious about its topography. The August 2008 Cassini pass produced a set of high resolution images with resolutions from 12 to 30 meters, and synoptic coverage at 50 to 200 meters resolution. Using my magic topo maker TOPO, I have produced the first high resolution map of this terrain based on this image set. A more complete map is in progress, but it will improve this one only by degrees. Like Europa and Triton, relief is not very high, rarely exceeding a few hundred meters locally (the story on Europa is much more complex, but that is a future blog). For now I present these fun images and movies of three of the "tiger stripe" tectonic ridges that scar this terrain.

I will be editing this post with more pix and details over the weekend! I have added the Bagdhdad Sulcus movie today (Sunday). Higher resolution versions of the movies can be made available for special needs. Uncompressed versions of the videos are available on Facebook under my name http://www.facebook.com/paul.schenk?ref=profile

Please advise me when any of these are used! I like to keep track of where this stuff ends up.

All images and videos credit: NASA/JPL and Paul Schenk/Lunar and Planetary Institute, Houston

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Looking Down the Throat of Damascus Sulcus

This double ridge is a site for one of the source jets for the south polar plumes. The ridges are 100 to 150 meters high, and the medial trough is 200 to 250 meters deep. The small elongate blocks at the base of the trough formed when blocks slid off the wall scarps or were thrust up from the interior along the central fault. The numerous parallel ridges within the plains formed by crumpling of the icy surface. These smaller ridges stand tens of meters high.

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Baghdad Sulcus

This view shows a deformed area between two branches of this tiger strip. These ridges stand is 80 to 100 meters high. Like Damascus Sulcu, numerous small elongate blocks have formed at the base of the medial troughs, which are 200 to 250 meters deep. The second view below shows an area extending into the ridged plains.

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Cairo Sulcus

The ridges of Cairo stand 150 to 200 meters high. This view shows the steep wall scarp of the medial trough that splits the feature into two ridges. Vertical striations and large boulders 10s of meters can be seen. The rolling fractured ridges in the foreground most likely formed due to buckling of the icy surface. The largest of these ridges are nearly 50 meters high.

Flyover on Triton - Voyager +20 Years

2009-08-25T08:35:00.002-05:00

Twenty years is a long time, but that is how long its been since Voyager 2 completed its primary mission during a close pass of the planet Neptune. The planet's largest Moon would steal much of the thunder. I myself was a relatively new post-doctoral researcher working at Jet Propulsion Lab, where I had been an intern 10 years earlier during the Voyager 2 Jupiter encounter. I was not part of the Project at that time, but a friend escorted me and another friend into the imaging control center to watch as the Triton images came in. We weren't really supposed to be there I guess and no doubt we ruffled a few feathers but I doubt many would truly begrudge us the experience.

In those days lab workers could watch the mission unfold as each new image of Neptune and its storm clouds flashed on our monitors. Each day it grew larger and more detailed. Triton was the real mystery as even a week before the big August 25th encounter, the actual size of this cold moon was still unknown! The first high resolution images of Triton flashed on the screens and everyone could see that the surface was geologically complex and very young. There were very few craters on the surface (its age is still uncertain but is likely even younger than that of ocean-covered Europa and most of Enceladus). Long ridges, volcanic craters and an odd terrain that looked remarkably like the skin of a cantaloupe marked the surface.

To this day, Triton's surface looks alien and unlike any we have seen elsewhere. Much of this is related to the ice-rich composition, with water, methane, nitrogen, CO, CO2, and other ices in abundance. Triton is likely a captured Kuiper Belt object and residual heat from that event has keep it very warm inside. There may evn be an ocean beneath its surface. Triton bears some remarkable similarities to Pluto but we will have to wait until summer 2015 to find out iif the two bodies look even remotely similar. Nonetheless, there are still things to be learned about Triton from 20 year old data. One of those is the topography of the surface. I have put together a new topographic map of the surface and used it to make a flyover movie simulating the navigations of a vehicle a few thousand kilometers above the surface. A word of warning, the surface of Triton does not have mountains higher than a mile so you will not see towering volcanoes like on Mars or deep basins like on the Moon. What you can see is a complex landscape scarred by small ridges, mounds and pits, many of which are volcanic in origin. There are even a few small impact craters, as well as walled volcanic plains. [By the way, Wikipedia is not entirely correct. The diapir hypothesis for cantaloupe terrain originated with Schenk and Jackson, in Geology, 21, 299, 1993!]

The video begins near the western edge of Neptune-facing hemisphere with an approach over cantaloupe terrain and two large smooth walled plains. The video tracks due east for roughly 1500 kilometers over a large province of volcanic pits, calderas and smooth plains. The video was produced from using a new topographic map of Triton, combined with a 1.65-kilometer resolution image mosaic. Topographic mapping was based on shape-from-shading analysis of the original Voyager images. Data such as these are being used to help plan New Horizons encounter with Triton's estranged twin Pluto in 2015.

Vertical relief has been exaggerated by a factor of ~25 to aid interpretation. It has been formatted to be iPod and iPhone compatible, and can also be downloaded at www.unmannedspaceflight.com and the NASA Photojournal. Additional still images from the movie can be found in the second post to this blog and on the NASA Photojournal.

All Images Credit: NASA/JPL and Paul Schenk, Lunar and Planetary Institute.
Use is not restricted, but a request for use (or for a quality digital copy) should be forwarded to the author. Proper credit is always appreciated!

More Ariel

2009-08-23T13:42:00.000-05:00

While I prepare the new Triton Movie for release on August 25 (the 20th anniversary of the Voyager flyby of Triton and Neptune), I will show some more of the images and data from Ariel (original data acquired January 1986). Included are a movie and the digital topography. Perspective views can be found in the original post a few days ago.

Ariel: The Movie

The Image data have a resolution of 1 kilometer. The topographic base map has a variable resolution but can resolve features taller than about 250 meters (I think). The topography is based on both stereo image analysis (stereogrammetry) and shape-from-shading (photoclinometry, PC). Stereo data were produced across most of the mapping area while PC is only available for the portion near the terminator (shadow) line. The two maps have been merged here to produce the topographic map we now see (the original data have been JPEG compressed for display.)

Image mosaic of southern hemisphere of Ariel (top). Topographic map of same region (bottom).

There are a variety of interesting features to see on Ariel. Most obvious are the 50 to 140 kilometer wide troughs along the terminator. The floors of these downdropped blocks are 3 to 5 kilometers deep and have been resurfaced by water or ammonia lavas long since frozen over. Note also the narrow ridges and troughs to the top, and the two large craters near center, which are 65 to 85 kilometers across. One of these craters is deep, the other shallow (due to relaxation or volcanic filling).

Ariel is interesting because it is even more geologically deformed and resurfaced than Miranda, which gets all the good press. Almost no ancient surface remains on Ariel, although Voyager only saw ~40% of the surface in 1986. The heat source responsible for making Ariel so volcanically active (it is likely quiet today) is unknown but is probably related to gravitational tides.

A New Global map of Ariel

This is my best current map of Ariel showing the global mapping coverage acquired by Voyager. Note that all the good stuff is south of the equator (the horizontal line in this cylindrical projection). The fuzzy area north of the equator was actually captured in Uranus-shine, and although poor in quality allows us to see another bright craters and the continuation of severa troughs. (I thank Ted Stryk for discovering that these dark-Ariel data exist.)

All Images Credit: Paul Schenk, Lunar and Planetary Institute.
Use is not restricted, but a request for use (or for a quality digital copy) should be forwarded to the author. Proper credit is always appreciated!

Surface of Triton

2009-08-21T20:30:00.000-05:00

Perspective Views of Triton

The views shown here are derived from topographic mapping of Voyager images obtained August 1989, 20 years ago this week! Triton was the last solid object visited by the Voyager spacecraft on their epic 10 year tour of the Outer Solar System. The surface of Triton is very rugged, scarred by diapirs, faults and volcanic eruptions and flows composed of melted ices. The surface is also extremely young and sparsely cratered. It may even be younger than the surface of Europa, one of the first objects visited by Voyager 30 years ago this summer. These views show volcanic pit chains and terrains. The middle view shows two large walled plains 150 to 200 kilometers across. Each view is on the order of 500 km across. A new version of the flyover movie posted on my Facebook site will be released later this week to commemorate the Voyager anniversary.

All Images Credit: Paul Schenk, Lunar and Planetary Institute.
Use is not restricted, but a request for use (or for a quality digital copy) should be forwarded to the author. Proper credit is always appreciated!

Stereo Moons

2009-08-21T14:41:00.001-05:00

Stereo images, and especially those of planetary surfaces, are a big part of what I do. I've been searching for a suitable place to host these images and some of the products I've been making from them. So if you like, please leave some feedback so that I post more! I will explain more about these in the coming days and weeks, right now I just want to begin uploading some of the dozens of images and Videos I have been generating the past 20 years or so . . .

Paul

Perspective Views of Ariel

Data derived from stereo topographic mapping of Voyager images. (Thanks to Ted Stryk for 2 desmeared images)

Credit: Paul Schenk, Lunar and Planetary Institute

All images credit: NASA/JPL and Paul Schenk/Lunar and Planetary Institute, Houston
All data files credit: Paul Schenk/Lunar and Planetary Institute, Houston