Thursday, June 30, 2005
Anyways, I wouldn't want to skew the results, but I really encourage you to vote. You all know how to vote, right? ;-)
I am shocked and dismayed by the lack of Enceladus images in the contest...
These images are in their original format. So you will need a viewer. I use ISIS, which allows me to create those mosaics you see released of Titan. I will look for additional viewers.
Wednesday, June 29, 2005
Despite the general lack of targeted icy satellite observations (save the Rhea and Prometheus observations mentioned below), a few icy satellite "Opnavs" have shown up on the raw images page. These images were designed for optical navigation of the spacecraft, but they are still quite useful for scientific purposes.
The best of bunch is shown above. This image of Tethys shows Odysseus impact basin along the terminator. A few smaller craters are visible within the basin. Note the appearance of the rim of this basin. Also note the relative "smoothness" of the plains to the southeast of Odysseus. This view has a resolution of 3 km/pixel.
Additional images include:
Next up in the series of small, inner satellite images started Monday with Janus, CICLOPS has released this view of Epimetheus. This image was discussed here earlier when it was taken last month. The view shows the irregular, lumpy shape of Epimetheus, also seen in an higher resolution image from March. This image has a resolution of 2 km/pixel, though it has been magnified 2x to improve feature visibility.
Tuesday, June 28, 2005
CICLOPS has released this view of Titan's south pole along with a movie showing cloud movement, both data taken during Cassini's non-targeted encounter on June 6, 2005. This is Cassini second look at the south polar region, the first coming nearly a year ago, shortly after SOI. At the time, Casssini cameras saw the south pole of Titan shrouded in a field of clouds. Clouds continue to appear in the south polar region for several months following that distant encounter, until the disappeared almost entirely in December, both from Cassini images as well as ground-based observations. As you can see in the movie, clouds have returned to the south polar region, but luckily for mappers (...like myself), the clouds are not exactly over the south pole (noted by the red cross in the still image). The clouds move around the pole, though some small, isolated clouds, don't appear to move at all but do appear to change shape or dissipate (in some cases).
Perhaps the most exciting surface feature seen in this view is the dark, roughly peanut-shaped feature in the upper left in the movie and left of center in the still. This feature is much darker than many of the other features in the scene. In addition, the smooth morphology of the margin suggests that it has been modified by erosive processes, rather than tectonic forces like the western margin of Xanadu. It is thought that this feature might be a reservoir (a "lake" if you will) for rain run-off from the many clouds that have been seen in the region. Some clouds, like those seen south of the main cloud field in July 2004, have appeared very close to this feature. Cassini imaging scientists feel this is the strongest candidate for a body of liquid on the surface of Titan they have seen thus far. However, no confirmation of that interpretation has been obtained. One such confirmation is to look for the specular point, but it was too far south and west at the time of this month's observations for it to show specular reflection. This leaves open the possibility that this feature is not a lake. It is possible that this is just a depression filled with dark sediment, like nearly all the other dark regions seen on Titan thus far. It is also possible that this is some kind of sinkhole or even an Io-style cryovolcanic caldera. Cassini scientists will continue to search for opportunities to view this feature at higher resolution. As summer drags on, and the sun marches north in Titan's sky, the window for viewing this feature through the ISS and VIMS instruments is rapidly closing (this feature is near 75 deg. South latitude). RADAR might be another possibility and opportunities to view this feature through RADAR SAR or the south polar region in general will be examined very closely in the coming months and years.
UPDATE: 06/28 10:49am: Carolyn Porco, the Cassini Imaging Team leader, has updated her Captain's Log posting she has on the CICLOPS homepage, mostly with reflections on the last year in orbit and on this recent discovery.
UPDATE: 06/28 3:31pm: Here are a few links from media organizations reporting on the lake:
Possible lake on Saturn's moon Titan [CNN/Reuters]
Cassini Probe Spies Lake-Like Feature on Titan [Space.com]
Candidate Lake on Titan? [Planetary Society]
On result from Enceladus that I hadn't remember hearing (or I have but not from an official source) is that the CDA instrument, the Dust analyzer on Cassini, detected an up-tick in dust particle hits around the time of close-approach with Enceladus, suggesting it is a source of dust in the system. Intriguing...
Continuing the series of small, inner satellite images started yesterday with Janus, CICLOPS has released this view of Pandora. Pandora orbits just outside the F ring. This image was discussed here earlier when it was taken last month. Very few craters are seen in this view, though that could be a result of the relatively low phase angle, as numerous craters were noted in Voyager images of this satellite. This image has a resolution of 2 km/pixel, though it has been magnified 2x to improve feature visibility.
Monday, June 27, 2005
Interested in gaining a better grasp on icy satellite geology, and Europa in particular, I purchased Europa the Ocean Moon by Richard Greenberg, a planetary scientist here at the University of Arizona. I have avoided Europa like the plague for many years but I felt it was time that I know a little more about the subject. I know I said I would never post news regarding Europa, but for this, I will make an exception.
In the Europa community, there are thick ice people and there are thin ice people. Maybe that is a bit simplistic, but this is the picture you get from reading this book. Richard Greenberg and his research team are thin ice proponents. They believe that the ice shell surrounding Europa's ocean is fairly thin, less than 10 km thick, that the ice lithosphere is conducting, and that the ocean directly "communicates with the surface". In other words, the features seen on the surface, like chaos terrain and double ridges, are the result of oceanic material reaching the surface through tidal and local heating effects. Much of Greenberg's book lays out the evidence for his hypothesis, and frankly this is where the book shines. I bought this book to be informed about Europa and when Greenberg sticks to the evidence in hand and discusses Galileo results, I felt I got at least some of my money's worth. I even enjoyed the description of how these discoveries were made and the work involved.
Greenberg's discussion of the other major hypothesis, that the ice layer at least 20 km thick, consists of a brittle upper layer and a convecting layer, and does not allow for communication with the surface, is the book's major weakness. Oftentimes, Greenberg resorts to almost ad hominem attacks on the thick ice theory's major proponents, Bob Pappalardo in particular. In some cases he does go into the problems with the theory, his discussion of "pits, spots, and domes" is his best stated argument against the thick ice theory, but his other arguments against tend towards the ad hominem. At several points in the book, Greenberg describes how he felt marginalized by the establishment, by "The Man" (my words). This line of argument is often childish and was definitely the weakest part of the book.
Europa the Ocean Moon, when it sticks to the subject of Europa, is enlightening and a useful resource for the thin ice theory for the evolution of Europa's surface features. When it diverts to the discussion of the people studying Europa, particularly those outside its research group, the book borders on the childish. I would recommend this book once it hits paperback or comes down in price from its current 90 dollars.
Though Rev 10 is largely a rings orbit (don't miss the zero phase Ring images), and very few satellites observations are planned, there are a few gems to be found. First, there is a sequence of zero phase angle Rhea images (one is shown above). These allow Cassini imaging scientists to look at the opposition surge effect on Rhea as well as look albedo without topographic shading getting in the way, though you will notice that many craters are marked by albedo features along their rims, usually bright frost on the poleward-facing slopes. In addition, Rhea's large ray crater system is quite prominent.
The other raw images now available show the small, inner satellite Prometheus. These images show the irregular shape of this small moon.
CICLOPS has released a processed (and not jaggy) version of the Janus image taken last month showing dark spots on the surface. As mentioned at the time, Janus' surface is covered in darkish spots. Consider our experience with Phoebe. On that moon, the surface was quite dark, with bright spots seen on the surface at low resolution. At higher resolution, those spots were revealed to be small impact craters which punched through the dark surface layer to bright icy interior. The impact had brought up material from that bright interior and had deposited it on the dark surface. A similar effect maybe happening on Janus. The dark spots could indicate that the interior of Janus is darker than the surface and small impacts bring that dark material to the surface. The individual craters are not resolved in these spots, except for a comparatively large impact at upper right. That crater has dark material surrounding it, perhaps conforming to this hypothesis. There are other craters in this view, similar in size to the one at upper right, that don't have dark material surrounding them. Somehow, a space weathering process is brightening the surface, perhaps from ring material.
The original image has a resolution of 2 km/pixel though this view has been magnified 2x to enhance feature visibility. This image was taken on May 20, 2005 from a distance of 357,000 km. The phase angle on this image is very small, only 6 degrees so most of what you are seeing here are albedo variations, not topographic shading.