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)