Friday, July 15, 2005

Highest Resolution images of Enceladus

The JPL Cassini site has this image on their front page. This is a wide-angle image from the UVIS occultation ride-along. I felt this deserves its own topic if only because this is the wide-angle image, imagine what the narrow angle image will look like.

This wide-angle image has a resolution of approximately 20 meters/pixel.

Update: the NAC view is now available. this image has a resolution of 2 m/pixel. You can clearly see several large boulders in a region of high tectonic deformation. The WAC image that was above can still be seen on the JPL Cassini Front Page.

Enceladus-2 Raw Images

I'll post the best raw Enceladus images when they become available.

The Cassini website has the following posted as of 2:12pm PDT:
The Cassini spacecraft conducted its closest flyby yet, coming within 175 kilometers (109 miles) from the surface of Enceladus. Data collected is currently being downloaded. Raw images are expected to appear on this web site at around 10 p.m. PDT.
UPDATE: 4:30 pm: The last of the Rhea images are showing up on the JPL raw images page now. Next up, Enceladus...
UPDATE: 4:50 pm: It has begun...
UPDATE: 5:00 pm: CICLOPS has begun post some select raw images from yesterday's flyby. So check out 10 of these at the CICLOPS website. I have put my favorite above. Trust me, it gets better....
UPDATE: 8:40 pm: Most of the NACs should be up soon. here is a list of interesting images thus far:
Obviously, feel free to post your favorites in the comments for this post!
UPDATE: 09:30 pm: Here are a few more (and Epimetheus):

Enceladus-2 Coverage Map

CICLOPS has released a coverage map for yesterday's encounter with Enceladus. The colored outlines represent regions on the surface of Enceladus that will be imaged by Cassini's narrow angle camera. This map does not include images taken during a "ride-along" sequence with UVIS' Gamma Orionis occultation at close approach. As you can see from this map, most of the images planned are of the southern trailing hemisphere of Enceladus, with some distant imaging of the leading hemisphere ridged plains. The highest resolution prime observations (prime observations mean that the imaging team will be controlling the orientation of the spacecraft during a block of time) will be of a region of heavily tectonically deformed terrain near 60 South, 190 West.

Note, due to unplanned changes in the orientations of the camera, the region that will be imaged could shift slightly from the outlines shown above. This mainly effects regions where the frames are much smaller than the disk, not so much the region outlined in red.

On a related subject, the JPL Cassini Website has a flyby page for this encounter. Check back here or on that page for the latest from other Cassini instruments, if and when they release products.

Rhea images starting to show up

The first of the Rhea images taken before yesterday's Enceladus flyby are starting to show up. Above is an example of one of the early images showing the full disk of Rhea. At its peak, images with resolutions up to 1 km/pixel should show up. The region seen here is the southern leading and anti-Saturnian hemisphere. South is to the right. More images to come.

UPDATE: most of the Rhea images downlinked this morning are now down, suggesting a 8 hour lag between when the images are downlinked and when they are posted on the JPL raw images page. The available images show a largely cratered landscape with several large impacts, one with a nice central peak.

Thursday, July 14, 2005

Enceladus Flyby Today

The Enceladus flyby took place 3 hours ago so most of the data from the flyby should now be onboard the spacecraft :D We will have to wait till tomorrow for the data so make sure to come back here tomorrow afternoon and Saturday for the latest on the Enceladus-2 flyby.

New Titan Image: Clues in the Bright and Dark

CICLOPS has released this view of Titan taken by Cassini on June 4, 2005 from a distance of 1.2 million km (for a resolution of 7 km/pixel). This view was taken shortly before the "lake" image, and was discussed here last month. This image shows surface features in and around the large bright region known as Xanadu. Xanadu here is fairly mottled with small dark spots along with regions of brighter terrain, like the "smile"-shaped surface feature. The "Smile" was also seen by VIMS during T4 and T5 and was found to be the bright feature on the surface of Titan at 5 microns. The bright area at 4 o'clock is actually a patch of clouds, perhaps the same area of clouds seen a day later in the lake image released earlier.

Wednesday, July 13, 2005

Raw Images of the Day

Only a few more days till the Enceladus encounter and orbit 11 periapsis so the moons are starting to get larger in the window...

Concession Speech

The votes have been tallied and the results are in: the colorized surface photo of Titan taken by Huygens is the winner of the Photo Contest held on the Cassini-Huygens website. While I never expected my two enterants, Encountering Iapetus and Titan Mosaic to win, I was shocked at the low poll results, I mean, I lost to a moon smaller than the city I live in :o No worries, I will be working on an image product over the weekend, that, if successful, will surely win a year 2 contest...

Tuesday, July 12, 2005

Enceladus-2 and other Rev 11 Encounters

View of Enceladus an hour and a half before close approach (sub-spacecraft point: 47.75 South, 194.5 West). The map used is by Steve Albers (best online maps that include updated Cassini images). Phase angle at this time is 44 degrees so much of area near the bottom and to the right will be in darkness (terminator not shown here due to limitations in my available software). This plot was generated in ISIS. I have also created a plot using the online Saturn Viewer tool to show the terminator.

Enceladus-2 facts
  • Occurs on July 14 at 1:19pm PDT
  • Close Approach Distance = 169.4 km (lowered from 1000 km)
  • Relative speed WRT Enceladus = 8.2 km/sec
  • Closest Approach Lat and lon = 23.2 South, 325.4 West
  • Low-phase inbound, high-phase outbound
  • Phase Angle at T-2 hours: 46.7 degrees
It has been nearly 3 months since T5, the last Cassini targeted encounter of one of Saturn's many satellites. On Thursday, July 14, Cassini will fly within 175 km of the surface of the icy satellite Enceladus, the closest encounter of any target by Cassini thus far. This is the third close encounter with Enceladus: the first, a 1500-km non-targeted flyby on February 17, the second, a 500-km targeted encounter on March 9. Images from those two encounter taught Cassini scientists quite a bit about Enceladean geology and grain size distribution (course grain ice was found within outcrops along the walls of fresh fractures and ridges). Other instruments, like the magnetometer, the Cosmic Dust Analyser, and RPWS, provided tantalizing hints of an atmosphere and maybe even a magnetic field.

This week's encounter is designed to persue many of the unanswered questions left over from those two flybys as well as further explore question brought up by the data from earlier this year. This encounter was lowered in April from 1000 km to 170 km to explore the discoveries made by the magnetometer and other field-and-particles instruments. Data from these instruments improves as the altitude is lowered. These instruments will be looking at the particle environment of Enceladus by measuring the composition of the particles coming off Enceladus (CDA), by examining how that environment is modified through plasma and radiation bombardment (MIMI), or by examining the atmosphere (MAG). UVIS will also be looking at the density of the atmosphere by monitoring an occultation, during close approach, of gamma Orionis (Bellatrix) by the atmosphere and by Enceladus proper. UVIS failed to detect the atmosphere during an occultation of lambda Sco in February, though this was likely the result of using a poor occultation star. Bellatrix is much brighter in the UV range UVIS observes in and should provide better quality data compared to lambda Sco.

A few additional highlights of this flyby:
  • ISS will image Enceladus throughout the encounter. ISS has major prime observations prior to close approach, observing the region above at 50 m/pixel to 1 km/pixel. This is the southern trailing hemisphere region of Enceladus. The trailing hemisphere was observed during the February and March encounters but the more southerly trajectory allows for improved viewing of the south polar region. As you can see in the plot above and in an image taken in May, there are four dark stripes (some with resolvable bright lanes in the middle) cutting through the south polar region (unofficially called "tiger stripes"). The nature of these stripes is still unknown. Higher resolution images taken in March suggest they might be ridges surrounded by blue material, but the ridge interpretation is suspect given the oblique viewing angle. Images from Thursday's encounter should help to resolve the issue. The rest of the region to be imaged was also seen earlier this year, but the differing viewing angles could provide an excellent stereo opportunity.
  • INMS, normally associated with Titan observations, will be measuring the positive ion and neutral environments of Enceladus.
  • VIMS will be preforming compositional mapping of the surface. Prior observations earlier this year found only water ice of varying grain size with none of the expected contaminants, like CO2 and Ammonia found. However, given the possible unusual nature of the south polar region, it is possible that VIMS might find non-ice materials there.
  • UVIS, in addition to their gamma Orionis occultation, will be mapping the surface composition, focusing on the distribution of grain sizes on the surface. Water ice is dark at wavelengths less than 160 nm and turns bright over a range from 160 to 190 nm. Where the change in brightness takes place in the spectrum is diagnostic of the grain size.
  • RADAR, another usual suspect for Titan, will be working in scatterometry mode to determine the roughness at centimeter scales. RADAR will also be in radiometry mode determining the energy balance of Enceladus.
  • Early in the mission, Cassini Radio Science (RSS) determined that Enceladus was much denser than previously expected. At 1.6 g/cc, Enceladus must contain significant amounts of non-ice material in its interior. RSS will also use this flyby to further refine the mass estimate for Enceladus.
  • CIRS will be preforming several scans of Enceladus' surface to determine the thermal and compositional characteristics which can be used to sample the regolith structure.
Given the wealth of data to be taken on this flyby, it is hoped that we will understand quite a bit more on how Enceladus' evolved, how it got to look so young in regions, and how Enceladus interacts with its environment. The observations also leave open the possibility of a serendipitous discovery, like a plume. The next close pass of Enceladus will take place in March 2008, when Cassini will pass within 100 km of the surface, even closer than this pass.

Enceladus isn't the only body that will be observed on this encounter. There are planned observations of Rhea and Epimetheus as well. Rhea will be observed by both RADAR and ISS during this Voyager-class distance encounter (~180,000 km). This encounter will allow imaging of the south polar region as well as the southern anti-Saturnian hemisphere, a region poorly observed by both Voyager and Cassini so far. This would also allow imaging of the circular moat and possible degraded impact basin seen in today's daily release.

Epimetheus will also be observed on this orbit from a distance of 84,000 km, a little farther and at higher phase than the images taken in late March of this moon. This observation will also be a stereo opportunity, allowing for improved determination of Epimetheus' shape. These images will be taken after the Enceladus encounter.

Images and other data taken during the Enceladus, Rhea, and Epimetheus encounters will be returned on Friday, July 15. There will be two playback periods. The first runs from 12:14am till 3:04 am PDT, when the images from the Rhea encounter and perhaps the first of the Enceladus images will be returned. The second runs from 7:54am till 4:54pm, when the rest of the Rhea and Enceladus data will be returned as well as the Epimetheus images. Generally, raw images show up on the JPL raw image page several to 12 hours after the images are returned from the spacecraft though you can expect some select raw images could be posted on the CICLOPS website Friday afternoon.

Thanks to Steve Albers for the map used in the plot above and to the PDS Rings Node for the Saturn Viewer.

New Promtheus Image: Solar Eclipses… Daily

CICLOPS has released this low resolution view of Saturn's small inner moon Prometheus. This image was taken on June 3 from a distance of 2.1 million km, leading to a resolution of 13 km/pixel. Prometheus is seen here shortly after emerging from the shadow of Saturn, which it enters daily. Satellites farther from Saturn, like Enceladus or Titan, can also undergo these eclipses, but only during the few years surrounding equinox, which next takes place in Aug. 2009. However, satellites like Prometheus undergo eclipses even near solstice, meaning an observer near the sub-Saturnian point on the satellite would never see the sun at high noon.

Monday, July 11, 2005

Hyperion Rev09 Releases

CICLOPS has released an anaglyph and a movie of images taken of Saturn's spongy moon Hyperion last month. These images show newly resolved features on these, the highest resolution images ever taken of Hyperion. Unfortunately, many of these images were over-exposed, but they still provide plenty of insights into the processes that have taken place on this small moon. Among the notable features is Hyperion's irregular shape. Hyperion is rather large for an irregular satellite, 328 by 260 by 214 km (dimensions given in the caption are the radii values). This has led some to suggest that Hyperion may be the remnant of a larger satellite that was torn to pieces by an impact and failed to coalesce back due to tidal interactions with Titan. While these images don't go either way on this issue, it does show signs that Hyperion has trouble re-accreting material from impacts on it surface. Many of Hyperion's craters are strangely formed, often with dark material within their floors and bright material along the walls and rim. Titan's gravity, in addition to causing impact ejecta to not fall back on Hyperion, may also cause Hyperion to tumble as it orbits Saturn.

The sponge-like appearance of the surface is followed to the interior, where signficiant porosity has led to a very low density body, with a density of only 0.6 g/cm3, based on pre-flyby, but Cassini NAV mass values. This also suggests that Hyperion is likely made up of nearly pure water ice, with components that make up the dark material seen on crater floors constituting very little of the interior, otherwise, pore space within Hyperion could make up 50% or more of the interior.

New Rhea Image: Diversity of Impacts

CICLOPS has released this processed view of Rhea's leading hemisphere taken on June 2 from a distance of 1.8 million km (for a resolution of 11 km/pixel, though this view has been magnified 2x to improve feature visibility). This image shows a range of impact phenomena on the surface of Rhea. On the right side of the disk, we can see the now famous ray crater as well as many as two large impact basins near the terminator. The northern impact basin is Tirawa which was measured using this image as being 360 km across (+/- 20 km). There is a second impact basin south of Tirawa more in the shadows, which is at least as large as Tirawa

Phil mentioned a third impact basin 1 basin width south of the southern basin. Maybe Phil can correct me here but I think he is talking about the circular ring seen here near the terminator 1 basin width south of the "southern" basin.