25 January 2015

Cerious Predictions

"NASA Dawn Mission encounter with Ceres will be the first time the surface (or atmosphere) of a planet will be imaged for the first time by a spacecraft since Voyager 2 flew past Neptune in 1989. Dwarf planets like Ceres and Pluto/Charon (which will be encountered this July by New Horizons) are the most common type of planet in the solar system - and may be the most common type of planet in the universe. Dawn is the first mission to orbit and study such a body in detail to see how it works and compare it to other planets such as Earth. Dawn will be captured by Ceres' gravity on March 6. We are conducting a series of navigation and rotational characterization observations, each of which will be more exciting than the last, until we commence Survey mapping orbits on June 7 at an altitude of 4900 km (and a resolution of ~0.5 km/pixel), then move down to High Altitude Mapping Orbit on August 8 from 850 km (80 m/pixel), and finally Low Altitude Mapping Orbit on December 13 from 476 km (45 m/pixel). We'll be obtaining Framing Camera imagery in different filters, spectra from a visible-near-IR mapping spectrometer, and elemental compositional information from the Gamma-Ray Neutron Spectrometer. It is going to be a fun year (or two!!)."

The blog note above from Dawn nicely sums up the Ceres encounter, which is now quite literally almost on us.  We are now within the orbit of our Moon if it were orbiting Ceres, and this is very close indeed.  As Dawn's cameras are designed to map at much closer distances we won't be seeing a lot of detail until we get into mapping orbit but will instead slowly peel away Ceres' secrets as we move in on ion thrusters.  

To give a sense of what types of things we might see on Ceres at different stage of the mission, I put together some slides featuring Saturn's moon Dione, the closest 'twin' we have of Ceres.  Dione is rather similar in size to Ceres (Ceres 950~km; Dione 1120~km) [see also last weeks post for comparison shot].  The similar low densities (Dione is ~1.6 times as dense as wear ice, Ceres ~2 times) indicate that both have lots of water ice.  Ceres is a little bit rockier but both are believed to have an outer layer of water ice a 100 kilometers or more thick.  
Reposted from last weeks blog:  Dione as viewed by Cassini at resolutions comparable to Dawn at Ceres.  The center image approximates our view of Ceres in the middle of February, the last image our view during the last week of February.
But Ceres is not going to be Dione.  We may be making too much of the Dione analog, but it is a starting point.  First, Ceres is indeed a planetary object, and as noted above the first time we have explored such a body unresolved before since Voyager in 1989 (and Pluto is next).  Instead, Dione orbits Saturn and has been subject to its influence and that of it's neighboring moons.  I refer to the gravitational tides that power the jets of Enceladus and likely resurfaced much of Dione's leading hemisphere with smooth plains and flattened some  of its impact craters (more on that later, and I discussed some of these aspects in the previous post).  Second, Ceres is much closer to the Sun, which both warms the surface and interior, and makes the ice unstable over long periods.  This later effect, sublimation, can be seen in the northern states during winter as snow both melts and evaporates.  This is an erosive process that can seriously (or Ceriously?) degrade a landscape, as we saw on Callisto in Galileo images.  Callisto is of note because it too is icy on the inside but like Ceres is very dark reflecting only 10% of the Sun's light, contaminated by hydrated silicates and carbon-rich gunk.  (Callisto's global albedo is higher but I refer here to the dark stuff only.)  This similarity might be important.

So, here are some slides showing Dione and Callisto at different resolutions comparable to those expected during the Ceres approach phase.  Parts of Ceres may look like this but probably not.  The images should give us a sense of what types of features will be detectable as we move in, however. 
Views of Dione from Cassini at increasing resolution.  Views show the types of features that are likely to be resolvable as we map Ceres in 2015, including impact craters and fracture networks.  
Views of Callisto from Galileo at increasing resolution.  Views show the types of features that are likely to be resolvable as we map Ceres in 2015, except for a dark icy object undergoing sublimation erosion.  The bottoms view is of one of the ring scarps surrounding the giant impact Valhalla.
Now that the encounter phases of both the Dawn and New Horizons missions to Ceres and Pluto, respectively, have officially begun, and before we actually resolve any geologic features (which for Ceres may be a matter of just days away), I thought I'd venture into more hazardous territory and look ahead to what we might find, focusing this week on Ceres.  

I have been asked several times what we expect (predict?) to see on these small icy worlds orbiting the Sun, but each time I found my self stumped for a credible answer.  We have global maps for most of the icy moons (namely the 17 or so large enough one considered true worlds and not just battered limps; and I've helped produce a few of those as shown in earlier posts . . . )  We have catalogs of surface features and have evidence for almost every major geologic process occurring on at least one of these objects.  So we know what craters, volcanoes, faults, and landslides look like on icy bodies.    What we do't know is in what combination they will occur on either Ceres or Pluto.  Will Pluto and Ceres surprise us with new features and rewrite what we know about ice worlds?  It's very possible, of course.  Both are unique even among ice worlds, Ceres being unusually close to the Sun, and Pluto a large icy world orbiting the Sun at great distance and with 5 known moons of its own.  

Several "space" artists are also trying their hand at the predictive arts.  'streincorp' has a rendering of Ceres on Deviant Art that looks rather like Mars in some ways, without the canyons and river valleys.  Michael Carroll has another interesting view that tries to incorporate the HST and Keck observations, showing arcuate structures and the impact craters likely extending over most of the surface.

Speculation has also focused on whether or not Ceres and Pluto have been warm and active or cold and moribund.  Probably somewhere in between.  It would probably be a surprise if Pluto's geology was as extremely young (features formed in less than 100 million years) as Triton's.  Pluto may have had a violent beginning, having given birth to its system of moons in a violent Charon-forming collision long ago.  Triton on the other hand was completely remade in the violent events associated with being captured by Neptune.  Orbital tides and perhaps collisions with Neptune's original moons essentially melted Triton from the inside, resulting int he extremely contorted young surface we see today.

So, rather than speculate on what Ceres and Pluto look like, perhaps it would be more fruitful to consider what it would mean if we see different sorts of things on those bodies.  It is generally assumed that geologic activity on a planetary body implies there are higher levels of internal heat.  Probably the most compelling discovery would be volcanism (as in melting and eruption onto the surface of ice phases, including water, methane, nitrogen, and various similar compounds).  This would be direct evidence for very warm temperatures on the insides of these bodies.  Heat on either Ceres or Pluto is not likely to come from tidal focus as it does on Europa or Miranda or Triton to name a few.  

We will look for a variety of features, among them volcanoes, which would indicate temperatures hot enough omelet and mobilize water and other ices.  The extent and duration of any such volcanic terrains will tell us much about Ceres thermal history.   Diapirism, which is another name for convection in the solid-state, without large-scale melting, would also probably require considerable heat.   Diapirism is the rising of one layer upward into another, usually in the form of large blobs (quite a technical term, I know!).  Salt domes are perhaps the most common examples here on Earth.  The cantaloupe terrain on Triton is probably a vast diapir field (and one my early findings back in 1993).  The oval domes on Europa may be another example, and the coronae of Miranda may be diapers of upwelling ice on a planetary scale.  Several colleagues have suggested that convection could have occurred within Ceres and might be visible on the surface.  A simple density contrast within the crust, of a dense layer formed over a less dense layer could also trigger overturn.  It turns out hat the scale of such features tells us something about the heat levels and the thickness of the layers, so that if we see this on Ceres it will generate a lot of interest.
Diapiric convection occurs at different scales.  Examples include Triton's cantaloupe terrain, shown above at 350 meters (or Survey orbit) resolution.  The oval cells are 30 to 40 kilometers across, each representing an upwelling dome of ice.
We will also be looking closely at impact craters.  That is my specialty, but we will look at that more closely in a later post.  The key thing is that impact craters form predictable features.  Any alteration totem tells us about how hot the planet got or how eroded it became.  Measurements of crater shapes will be key to unraveling these questions, depending on what we find.

Ceres will be revealed in stages, with major structures coming into focus first.  Large rifts and fractures, the major basins and deep craters will be resolved, in part because of the shadows they cast, giving us an early indication of what type of planetary body we are going to map.  Circles and lines stand out.  Small features like crater chains, narrow fractures, (large) boulders, cliffs, landslides and other erosional processes, will become increasingly apparent as Dawn descends to tighter orbits.  Vesta was revealed in the same way.  Vesta is a fascinating object, but shares many familiar qualities with rocky objects like the Moon.  Ceres will be no less fun, in large part because it is so different from anything we have looked at before: a lone icy object orbiting the Sun.
An attempt to show how Ceres might look in our night sky if it were at the Moon's orbital distance.  Its a simplistic rendering using a digital photo at dusk over New Mexico two years ago.  Ceres is the small disk just left of the Moon.

13 January 2015

Year of the 'Dwarves': Ceres and Pluto Get Their Due

Is it irony or just poetic incongruity that at a gangly 6'4" I should devote much of 2015 working on the two missions that will be mapping the first icy 'dwarf planets' in our Solar System to be explored, Ceres and Pluto?   Having spent the last 30 years using Voyager, Galileo, and Cassini images to map and understand the icy worlds orbiting the giant planets that we have visited so far, you can imagine I'm quite pleased to be involved with the first missions to explore the two largest icy worlds orbiting the Sun!  In 2013 I was nominated and added as a new Science Team member to New Horizons on its way to Pluto, and then this winter asked to continue working on the Dawn Project as we approach the icy asteroid/planet Ceres.

This year we achieve the first exploration of these curious but fascinating objects, but I am still struck by the fact that we are 57 years into the Space Age and we are only now getting to these two bodies, diminutive in size compared to giant Jupiter but large in stature.  These are chief among a group of objects that are smaller than our Moon, orbiting the Sun, and large enough to be planetary in nature but sharing their 'orbital zone' with other similar objects.

iPhone captures of my observations logs of Ceres (bottom left) and Vesta (bottom right), through a 2.4" refractor in 
Buffalo, Summer 1978, a year out of High School and 9 months before Voyager at Jupiter.  
Did I contemplate exploration of either body those cool summer nights?  I'm sure I pointed the thing at Pluto, too, 
knowing it was well out of its range, hoping to record a 'Pluto-photon' on my retina.

The architecture of our Solar System seems to be more complex with each passing year.  In 1992 we discovered that Pluto was not alone and in fact part of a vast belt of smaller icy objects. The Solar System can be said to be constructed of 5 major zones: the rocky Inner Planets, the transitional Asteroid Belt, the Middle Zone of ice and gas-rich giant planets, the Outer Zone of the Kuiper Belt objects, including Pluto, and an Outer-Outer Zone of the Oort Cloud of comets. (We probably need some better names but this gets the point across.)

Ceres and Pluto are the dominant bodies of their two respective regions of the Solar System. Teeming with thousands of small objects, both the Asteroid Belt and the Edgeworth-Kuiper (or just plain Kuiper) Belt are key regions of our celestial neighborhood.  The Asteroid Belt, home of icy Ceres which holds fully 1/3rd its total mass, is the key transition zone between the (relatively) water-poor inner planets and the water-rich outer planets.  Ceres may also be carbon-rich.  Kuiper Belt objects have lots of carbon-rich material and exotic ices, like the methane, nitrogen and carbon monoxide that cover Pluto's surface.  This outer zone will tell us a lot about how the Solar System formed.
Ceres, second from the bottom, just above recently visited Vesta, is traditionally
placed among the Inner Planets (many of which are shown above),
but is ice-rich and is a transitional object.
Orbit of Ceres (in blue) within the Asteroid Belt population.

Asteroids visited to date, including Vesta, Dawn's mapping target in 2011.
Ceres is a lot darker than Vesta, as shown here. We will be able to use Dawn imaging rather soon!
The Voyager/Galileo/Cassini missions to the giant Outer (or Middle Zone?) Planets looked at their icy moons in close-up, revealing towering fault scarps, smooth ice volcanoes, impact craters flattened and 10-km-deep, disrupted ice rafts, and jets of water vapor and ice crystals venting into space (all the subject of numerous previous blogs).   When Dawn and New Horizons reach their targets this year, we will see up close for the first time ice worlds not orbiting large planets but orbiting the Sun.  This is a rather important distinction.  We now know that gravitational tides, like the ones that affect Earth's oceans but much more powerful, can radically change an orbiting body's geologic history.  This was a key discovery of the Space Age.  Examples of how tidal forces change worlds include the famous volcanoes wracking Io, but also the faulted and disrupted icy shell of Europa, the complex geology and interior of Ganymede, the icy fractures and vents of Enceladus, and the volcanoes, diapirs, and geysers of Triton.  All three of these icy bodies (and maybe a few more) are believed to be harboring liquid water oceans beneath their frigid surfaces.  Maybe we will find evidence of the same at Ceres, Pluto or both.  A key difference is that tidal forces are weak or negligible at both Ceres and Pluto.

Ceres is most similar in size to several of Saturn's icy moons and may be similar internally as well, being composed of 25% water ice by mass.  Dione is a pretty good match to Ceres, at least in basic properties of size and bulk density.  Dione has signs of past geologic activity in faults and volcanic resurfacing but is not active now.  One thing we have learned exploring the Solar System is to be prepared for anything.  Ceres is unlikely to be another Dione, but Dione will be useful as a benchmark of comparison when we do map Ceres.

Comparisons of Ceres with other prominent icy objects.  Dione is Ceres' closest twin in size and mass.
Pluto and the Moon are shown above for comparison. 
Global map of Dione
How the limited/negligible tidal heating will affect Ceres is unknown, but Ceres is closer to the Sun and warmer than the other ice worlds. This heat combined with radioactive heating and other sources may have resulted in internal convection, surface erosion or other internal activity.  Ceres also looks to be occasionally venting water vapor into space, as detected by the Herschel Space Observatory.  It is theoretically plausible that Ceres may have a liquid water ocean as well.  Whether the venting of water vapor is related to this ocean is unknown, and that would be rather exciting.  What might be hidden within such an ocean would be even more uncertain.  The first priority to to simply assess the geologic history of Ceres and to determine the origin of the venting.  Is it related to volcanoes, fissures, impact craters, or just warm ice vaporizing under the heat of the Sun?  It might be similar to the venting we see on Triton or Enceladus.  Maybe it's something we haven't thought of or seen yet.  We can only find out by mapping the geology, topography, and composition of the surface at high resolution.  We are about to do just that.


Dawn and New Horizons are two very different missions.  I will talk more about New Horizons next month, but in short, the Pluto encounter will be more than 6 months long but will be very fast, much like the Voyager encounter with Neptune in 1989.  Dawn will arrive at Ceres first, beginning in January, and this approach will be quite leisurely by contrast.   Orbit 'capture,' when Dawn is firmly under the gravitational influence of Ceres, will occur around March 6.  Dedicated mapping operations will start some time in late April at resolutions of ~1.5 kilometer per pixel.

Don’t expect lots of images during approach to Ceres, however.  To reach Ceres, the spacecraft must continue ion thrusting all the way in, and we can only point the cameras when the engines are turned off for brief intervals to peak at our target.  That's why we won't be seeing pictures every day!  We can't actually take pictures every day and still get to our target.  We should be getting a set of images once every 1-2 weeks during approach, though, and the science team and those watching with us will be most eager to see what those images tell us as we near our target.  

Once in orbit, we will map Ceres in stages, going down to progressively lower altitudes.   Resolutions will increase from ~400 meters to 140 meters to partial mapping at 35 meters!  Our best map of an icy body is that for Enceladus, which has recently been completed in color at 100 meter resolution, by the author.  For Dione we have a similar map at 250 meter resolution.

[See the series of seriously excellent Ceres blogs by Marc Rayman that describe in detail Dawn's approach and mapping plans.  Kudos to the Mission Team for the fabulous job of getting us to Ceres (and Vesta)!]
Simulated views showing the sorts of things we might see on approach to Ceres this winter.
I will be assisting in the geologic investigations of Ceres in a supporting role, as will many others on the Team.  I was first brought on board Dawn for the Vesta mapping phase in 2011-2012 and led the effort to understand the giant Rheasilvia impact basin discovered by HST at the South Pole, which I discovered was actually two overlapping basins.  (I will also be working to map Pluto and construct topographic maps from stereo images, though for Ceres I will just be using stereo for geology.)   

From a science perspective, I will be most interested in (and my main role) the nature of impact craters on Ceres.  Impact craters record many things about a planets history and its structure.  Impact craters can excavate material buried below the surface and eject it onto the surface to see.  Craters also record the thermal evolution of the interiors of icy bodies.  Ice can creep (that is, deform) if it is too warm, in much the way ice sheets move slowly downhill on Earth.  The warmer the ice layer, the more it will flatten or 'relax.'  Measuring the topography of craters will thus tell us how warm the interior has been.  But first we must go there and see if any craters have indeed relaxed.  Impact craters might also reveal if there is/was an ocean deep inside.  For all of these questions my experience mapping impact craters on the icy satellites of Jupiter, Saturn, and Uranus will come to bear.  Comparison with craters on Ganymede, Dione and other moons will be telling.  Crater shape and morphology statistics for the Jovian and Saturnian moons are complete and in hand, ready for use when we map Ceres' craters!

Some of the craters of Dione - the largest is 85 km wide.
Craters also tell us about the population of bodies that formed them.  In both cases, but Pluto and Charon especially, the sizes of the craters we see may tell us the numbers of small bodies that populate the zones both the Asteroid and Kuiper Belts.  I will be assisting on this effort, too, though not leading it.  Much of the crater work comes later when we have mapping orbit data in hand, but first order of business will be to survey the situation on dwarf planet 4 Ceres as we move in and assess what type of craters we see and how they have altered or been altered by the surface.

Color HST image showing dusky marking and a faint bluing near the poles. (STScI)
A new mapped version of the HST color images, compiled by Phil Stooke.
There are some artifacts, such as the curved streaks, but there might be
some 'bluing' at the poles, perhaps due to frost???
We don't know much about Cerean geology at this point.  The HST and Keck images show dusky markings of various shapes, including some dark spots and a bright ring which could be impact basins.  None of these features can be classified yet, however.  The few bright spots might be recent impact craters, but perhaps not (they tend to be craters on icy moons).  Ceres has quite an orbital inclination, almost 11°, but its axis is inclined only a few degrees, and as a result has almost no seasons.  How will the affect the evolution of the surface?  We don't see any obvious polar caps, but we might see some frost deposits in shadowed polar craters.  Speculations among the geologists run towards craters being relaxed, and the possibility of convection in the interior, which might lead to fracturing of the surface.  And of course, what is the source of true venting of water vapor?  It's not nearly as vigorous as Enceladus, but the source should still be mappable on the surface.  Lots to look for!
HST image showing possible bright ring (between 9 and 12 o'clock).
Is this an impact basin or maybe a tectonic feature like the coronae on Miranda?
Circular dark features in other areas may also be impact features, or . . . ?  (STScI)
Movie of HST images of Ceres shows a variety of faint markings. (STScI)
A future post will look at New Horizons and its encounter with Pluto this summer and its family of 5+ moons, including the relatively large Charon.  

08 January 2015

True Colors on Saturn's Icy Moons

One of the most frequently asked questions regarding planetary images is: "Are those the 'natural' colors?"  That's not so easy to answer.  Most imaging cameras, whether old style vidicon TV tubes or the more common CCD instruments have rather different light sensitivities than the human eye.  Thus its not so easy to replicate the apparent brightness we would see.  These cameras also tend to use color filters that don't line up very well with our R-G-B color sensitivity.  Nonetheless we can make an attempt to see what some of these bodies might look like to space travelers.  

Case in point, Cassini at Saturn.  In my previous post I showed some of the 'super' color maps that I recently released of Saturn's icy moons.  These are maps compiled from images acquired in IR, green, and UV filters.  They really bring out the color contrast between geologic materials, especially recently exposed materials like crater and fracture walls, which tend to have stronger UV signatures reflecting larger grains sizes.

Natural Color    -   DIONE   -   Super Color 
Natural Color - ENCELADUS - Super Color 
In this post I show some of the 'natural' versus 'super' color images that Cassini acquired.  These are not at very high resolution, however, mostly in the 1-5 kilometer range.  This is because when Cassini was close to these objects it was moving too fast to acquire the large number of images required to run through all the color filters, so it chose the minimum to accomplish its scientific tasks in the short time available, mostly the IR-Gr-UV sequences (~930 to 570 to 340 nanometers).  This gives the best geologic information.  These sequences were further away and Cassini acquired the full filter sequences.  It turns out that the centers of the R, G, and B filters (~700 to 400 nanometers) on Cassini are fairly close to optimal when attempting to simulate the human view of these bodies.

'Natural Color'

'Super Color'

300-meter-resolution color images of Enceladus, centered on longitude 180°W.

Most of the Saturnian icy satellites can be described as having a grayish tone, with a slight reddish or greenish cast.  This is because the strongest reflections from these surfaces tend to peak in the green-to-red portion of the spectrum.  Color variations on the surface would tend to be rather bland to our eye, though we would likely pick out the stronger features as subtle hue contrasts.  We would likely see the cliffs of Enceladus as pale bluish in tone, not unlike some terrestrial glaciers.  I will try to compile some additional shots in the coming weeks.  Enjoy!