Tuesday, September 27, 2005

Rhea's Tirawa Basin

As Voyager 1 receded from its close flyby of Rhea, it snapped this view of Rhea's Tirawa basin, a large impact structure seen on the terminator in this crescent view. It is on of the worst-imaged parts of this world, and this Voyager view is still the best. Rhea is a moon that shows some similar features to more active Dione and Tethys, but seems to have been inactive for longer, as evidenced by the fact that all of its surface is extremely worn from impacts and the evidences of internal activity are much more subtle.

Voyager's Hyperion

Today Cassini is returning images from its flyby of Saturn's moon Hyperion. I have reprocessed some Voyager 1 images. This is a sequence from they flyby:

Here is a super-resolution color view. It is amazing to see the intricate patterns of bright
and dark areas. Cassini is now revealing this peculiar world.

Cassini Approaching Tethys (And Hyperion Too!)

This orbit by the Cassini orbiter will include flybys of both Hyperion and Tethys. The Tethys flyby was added a few months ago due to increasing interest in this moon as a contributor of ring material. Here are the best Voyager views of this moon.

This view, from Voyager 1, clearly shows a dark band across the center of the disk. The existance of this has been seen in Cassini's images too, but the orgin is not yet known.

This is a color view of Tethys from Voyager 2's approach to the moon. It shows on the terminator the degraded Ithaca Chasma, a huge canyon bisecting this moon.

This is the last Voyager 2 view, showing Ithaca Chasma in the center and to the left. Within the next view days, Cassini should supercede this view. This was the best view (5 km/pixel) that either Voyager obtained before pointing errors and scan platform problems ruined imaging sequences.

With the scan platform sticking and the spacecraft off target, this is the only frame from the highest resolution mosaic of Tethys, taken at 2.2 km/pixel, to catch a corner of the moon. For years, this stood as the mysterious best view of this world. Cassini has already doubled this resolution, and at last the weekend has arrived where Tethys receives true closeup treatment! Stay tooned!

A Craggy Rock On Venus

I have watched with great excitement how the Mars Exploration Rovers have explored the red planet. However, Venus, while it has had more successful landers on its surface, was the subject of much less capable spacecraft. There are four color image sets of the surface, all with blank or nearly blank blue channels. Venera 13 sent back one full pan in color, which is the best set we have, and another partial one (complete in black and white) that hs much poorer color data. Venera 14, which landed in a much rockier area, took a pretty good partial color pan (again, complete in black and white), but while the other pan was complete in both black and white and through color filters (althoug again the blue was almost useless), the color data in this set was horribly underexposed. Below is the Venera 14 partial pan, my favorite of the set because of the cool rock right near the lander. It seems to be sitting on the rocky plain...it makes one wonder how it got there. I don't see anything else like it in the Venera pans, although given their limited coverage, it doesnt mean there aren't perhaps a few more rocks like it around. But still , it is lucky it was so close to the lander and in a color zone.

Here is the other Venera 14 pan. The color data is OK on the sides, but in the center, all color data is nearly blank, making any color variations dubious in that part of the image.

Here is a composite of Venera 13 images, looking towards a ridge on the horizon.

New Color Views of Europa

Below is a mosaic of color versions I have processed of all the Galileo global color views of Europa. The large mosaic on the bottom is partially from color from the dataset used to make the greyscale data and partially from the G1 orbit. Despite Galileo's limitations, these are some real gems. (Please note that the large mosaic on the bottom is at greatly reduced resolution, and the image in the upper left hand corner is reduced somewhat from the super-resolution product size, although it is larger than the original image size.

Remnants of a Lost Approach

When Galileo approached Jupiter in the 1990s, the antenna problem, as well as a tape recorder problem in fall of 1995 that cost the images from the arrival orbit pretty much wiped out the return of images from this part of the mission, other than the Ida encounter, from which the images were piddled back over many months. In 1990, before even the Gaspra encounter, Galileo took some images for calibration that could be returned more easily while it was still in the inner solar system. Here is a color view of Jupiter and some of its moons generated from this data. It has the appearance of being taken through a small telescope.

Galileo would next image Jupiter during the SL9 imacts, a year and a half before arrival. It used a technique known as an "on chip mosaic," taking multiple images on the same frame so it could be returned as one picture. There were several techniques used that involved smearing the image to look for flareups from the impact. For the last impact, Galileo took multiple unsmeared images, providing a neat perspective, as its images are the only ones to see the impacts directly, and not coming over the limb. Perhaps another entry will be dedicated to these images. But here is one showing the impact and Jupiter, produced from several images on the on-chip mosaic.

The tape recorder problem caused the end of imaging until the G1 orbit in the summer of 1996. However, as Galileo approached in 1995, it took optical navigation frames showing Ganymede slowly looming larger to help plan for the G1 orbit. It is a little taste of the sights Galileo saw that fall but could not show us.

Galileo Mosaic of Ganymede

This is a galileo mosaic of Ganymede taken on the G8 orbit. It both reminds me of Cassini mosaics of the Saturnian moons and reminds me of what the Galileo mission might have been like without the antenna problem. It does suffer from compression artifacts and a few data dropouts, and is not mutispectral. Also, there are a few missing spots I filled in with a blank disk. But all in all, it is a nice mosaic. I am surprised it was never released on the planetary photojournal. But then, I am always surprised at the gems that never get released to the public.

Here is a link to a full resolution jpeg version.

Reconstructed Pioneer View of Io

I have updated my website with my newly reprocessed version of Io from Pioneer 11. I also have the raw images in lossless format there. Below is a version of the Pioneer image from a print copy I enhanced (I removed the terminator area because it was too problematic, which gives the appearance of a different phase), and to the right, a version made frmo the original data. My hope is that someone can use this to look for albedo changes over the years. These days HST and groundbased telescopes take images routinely that are better than this, if not from this angle, above the north pole. And of course there is Galileo and Voyager. But this is by far the best pre-Voyager view we have of this rapidly changing world. Here is a link to the page on my site.

Update On Mimas: A Record of Impacts

Cassini flew by Mimas on August 2, 2005, and showed what had been expected: A battered world, with few if any signs of internal activity.

This leaves us with a mystery. Mimas is about 400 km in diameter. But moons of its size vary greatly. In the image below, from left to right, there is Neptune's moon Proteus, which could not even pull itself into a somewhat round shape, battered Mimas, the Uranian moon Miranda, which shows signs of intense internal activity that has now ceased (note the large grooved features), and Enceladus, which is still active today. It is indeed a complex solar system!

Sedna...Once A Big Discovery

A few years ago, the discovery of Kuiper Belt object about 1000 km in diameter was heralded as a major find. This HST image was taken soon after it was found to pin down its size. Now, with several rivaling or possible a good deal larger than Pluto, it is just one of the zoo. So many worlds to explore!

The View Trapped In Low Earth Orbit

In 1972, astronauts walked on the moon for the last time. Since
then, the Space Shuttle Program and the Space Station program, in its
various incarnations that would eventually materialize into the
International Space Station, have drained manned spaceflight, keeping
it trapped in low earth orbit. Hopefully, with the Shuttle
nearing the end of its life and the new Moon-Mars initiative, manned
spaceflight will soon join our wonderfully successful unmanned
planetary exploration program. While scientifically not very
productive, the ISS astronauts have gotten some nice photographs,
including many showing the moon, a bittersweet reminder of the heights
our space program once reached. Here are some of the best images.

One can only hope that soon the manned spaceflight program is back at the moon and looking at earth in this fashion!

Triton...A Glimpse of a Changing World

Voyager flew by Triton in 1989, showing us an active world with an atmosphere, clouds, and erupting geysers. However, since then, we have only been able to study Triton from afar, and that situation will continue for the forseeable future. Only one data set, from the Hubble Space Telescope's Faint Object Camera in 1995, shows the resolved disk, barely. Below is the best I could get out of the HST views, which show one full rotation.

Bryan Flynn, the scientist behind this imagery, produced maps from the data, showing that the polar cap had enlarged since Voyager and hinting at variations between visible and ultraviolet images. Below are Flynn's maps:

Finally, here is a composite of Neptune and Triton I made from HST images from the mid 1990s. I hope that soon another telescope (or HST again) can image Triton at least at this resolution. Other data (light curves, spectra) have indicated increasing change, making continuing the baseline of coverage all the more important. With the discovery of planet size objects in the Kuiper belt, of which Triton is a captured member, understanding of this world is made all the more important. It is, after all, the only one we have seen closeup.

Iapetus from Cassini's Approach

I have been working with some Cassini approach images from Saturn. I can't wait until the releases of raw data from the time in Saturn orbit. Here is the best approach view of Iapetus I could make.

Despite being pre-orbit, it is comparable to Voyagers best views:

Exciting times are ahead!

The Galilean Satellites and Titan: A View From The Pioneers

Paving the way for the Voyager encounters, Pioneer 10 and 11 flew through the Jovian system in 1973 and 1974, respectively. They carried extensive particle and fields instruments, but due to the fact that they were spin stabilized, they did not carry cameras. However, one instrument, the Imaging Photopolarimeter, was capable of producing crude, 6-bit images in red and blue channels by scanning pixel by pixel, line by line as the spacecraft spun, slowly buiding an image. This worked pretty well for producing images of Jupiter's cloudtops, but the ability to image the moons of Jupiter was very limited. However, the Pioneers were able to produce the best images to date of these worlds, showing mottled albedo features that tantalized scientists. Pioneer 10 would have gotten a pretty good shot of Io, but unfortunately radiation effects prevented this. It did detect an ionosphere around Io, but this was detected through radio occultation. Pioneer 11 headed to Saturn, where it had a very successful encounter. However, when it reached Titan, a lot of data was lost due to the fact that Saturn was rapidly heading into solar conjunction. The images show the hemispheric brightness difference, the orange cloudtops, and little else (Titan alternately is brighter in its northern and southern hemispheres). It was a surprise that no other features in the clouds showed up, for we didn't know just how thick the clouds would prove.

Above are the best views of the Galileans and Titan. From left to right are Io, Europa, Ganymede, Callisto, and Titan. Some research was done using Pioneer images in the 1980s due to the fact that they had a red channel (the closest thing Voyager has is an orange filter). I would like to track down the original digital data for the Io image - if it were cleaned up, perhaps it could be used to look for change in albedo features compared to the Voyager 1979 images. This view shows it looking down on the north pole.
These views show these planet-sized moons on the edge of becoming worlds to us - these images made us wonder, but yielded few secrets.

The View From Earth

The image below appears to be a typical telescopic view of the moon from earth. It is not what this picture shows that makes it special, but what it was taken with. It was obtained my the context imager for NASA's Mars Reconnaisance Orbiter during a test last fall. It will launch on August 10th. This camera will provide context images for HIRISE, an extremely powerful camera capable of taking images of the martian surface at 25 cm/pixel. This should be an exciting mission. For now, here is the context imager's view of the moon.

I33 - Galileo's Last Snapshots

In January of 2002, Galileo swooped down on the Jovian system, the last time it would do this with cameras on. There was a hard fought campaign to fund this, led by Jason Perry, due to the fact that it would allow photography of portions of Jupiter's moon Io that had never before been seen at close range.

However, this was not to be. Galileo would suffer a radiation-related glitch, common during the later portion of the mission, causing it to miss the entire Io imaging sequence. Largely this orbit was a loss, and it drove the final nail in the coffin of imaging on Galileo's final orbit, A34, during which it would fly by tiny Amalthea. In addition to funding problems, it was decided that turns for imaging would make it to likely that the spacecraft would shut down before the flyby.

A few views did get returned from the I33 orbit. They stand as a last testament to Galileo's camera. It brought us some unforgettable glimpses of the Jovian system. However, we only saw a small fraction of what we would have, had Galileo's main antenna opened properly.

This view shows the top of the red spot on the limb of the planet. It is amazing to see all the intricate paterns in the clouds.

This view, taken as the red spot rotated further onto the disk, is a false-color view, generated from various infrared views. It certainlys shows great contrast between the spot (which appears blue here), the cloud belts, and the thunderheads that can be seen on the lower left.

This final view shows the spot rotating across the terminator, into the Jovian night.

On its way in and out, Galileo took these two snapshots of Amalthea. The purpose was to improve navigation for the upcoming flyby of this little moonlet, but such images also help in the study of the shape of irregular worlds.

Galileo also took a series of views of the Jovian cloudtops much further into the infrared than its camera could see, trying to study the temperature and composition of the clouds. This one shows Io, the bright spot to the left of the disk, and its shadow on the cloudtops, just left-of-center. It would be Galileo's last look at Io, and a bittersweet one, considering what might have come from this flyby.

This final image is of Europa. Taken to study the way Europa reflects light when it is viewed at a "full moon" illumination angle - called opposition surge - it is Galileo's last view of a world where it discovered an ocean beneath the icy surface, and a world that Galileo turned into a top priority for future exploration.

Metis and Adrastea...Sources of the Jovian Rings

Metis and Adrastea are the two inner innermost moons of Jupiter. Being deep within the radiation belts, they are difficult to study with a spacecraft. The problem is worsened by the fact that they are only 20 and 12 km in diameter, respectively. Metis was viewed several times during the Galileo mission. Here is a collection of all the angles the spacecraft got on this moonlet.

Using data from other orbits, I was able to colorize the lone image from Galileo's 26th orbit. It appears reddish, likely due to deposits from Io. It was taken from about 290,000 km at 3km/pixel.

Adrastea was the worst observed of the inner moons, the best galileo views simply show it as a tiny disk.

This image below, from NASA's planetary photojournal, shows how Metis and Adrastea fit into Galileo's ring scheme. They are different from Amalthea and Thebe, the sources of the Gossamer ring, in that they are more icy, less heavy in silicates. It is not know whether they are of different origins, or something caused these little moons in their inner orbits to lose their volatiles. It is possible that the rings and all four inner moons are pieces of the same parent body.

Perhaps NASA's upcomming Juno mission will be able to make some improvements. Its focus will be on studing Jupiter's internal structure, but it will orbit very close to Jupiter's cloudtops, likely allowing better study at least Amalthea but maybe, just maybe, some of these even less-known inner moons. Perhaps they are the leftovers of a collision - perhaps they were once as prominent as Saturn's rings. Here is a view of the rings from Galileo, with Europa in the background. This hard-to-explore area calls out for exploration.

Umbriel...A Shadowy World Gaurds Its Secrets

Voyager 2 flew by the Uranian moon Umbriel in 1986. Umbriel, 1,200 km in diameter, was only seen from half a million kilometers away. It reflects only 16 percent of the light hitting it, making it significantly fainter than the other major Uraniam moons. Distance and darkness conspired to limit the quality of Umbriel imaging. These images, taken during approach, show a dark world, battered with craters. A few bright markings appear. On the left-hand image, there seems to be a huge crater with a bright central peak near the bottom. Fainter, large scale albedo features can be seen a the top of this image and others, as well as in the image on the right. Paul Helfenstein, Peter C. Thomas, and Joseph Veverka proposed in a 1989 letter to the journal Nature that polygonal paterns can be seen in these features and the at they are the remnants of an ancient tectonic system similar to those seen on other moons of similar size. This remains an intriguing possibility.

The surface of Umbriel is ncient one. A collection of impact craters of various sizes was found to saturate the landscape. As Voyager drew closer, became clear that most or all of the bright features are related to craters. On the terminator, one can see two interesting craters. One, Skynd, has a bright central peak. The others, Wunda, at the top of the image, has a bright rim and no apparent central peak. This is the best color image obtained by Voyager.

This is a composite of the best Voyager images, slightly better than the original 8 km/pixel resolution. It appears to be a battered world - no features appear to be generated by anything other than impacts, unless the bright areas are produced by internal activity.

Here is a closeup systhesis from all the closeups of Wunda. It is hard to tell if the craters dug up the bright material, or if they had something to do with the impacts. Maybe it was derived from eruptions or melting caused by imacts. Or maybe they are some sort of frost left over from impacts. This issue may not be resolved until another spacecraft observes Umbriel. Here (below, left) is the best view of Wunda that can be generated with the available data.

Voyager left Umbriel behind, but the study of this world using earth-based instruments continues. The right hand image above shows a crescent view taken as Voyager receded. Perhaps compositional studies will yield clues that help us resolve some of the secrets Umbriel holds. We are left to wonder at this faint, fleeting, and fuzzy glimpse of this intriguing world.