Return to Dione

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

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!

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

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)

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!)