19 October 2009

Galileo's Moons: 4 Moons at 400 Years

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.

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.

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.

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.

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

16 October 2009

Video Ooops

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:

15 October 2009

Collision at Callanish

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



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
The second video features colorized topography - http://www.youtube.com/watch?v=ueEmhQWkQXY

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

05 October 2009

The Colors of Saturn's Mid-Sized Icy Satellites

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