Tuesday, September 06, 2005

DPS News and Notes

The AAS' Division of Planetary Sciences Conferences is taking place as I type in Cambridge, England. No, I am not there. But the Planetary Society's Emily Lakdawalla is and she has a great blog post with some of the results from yesterday's Cassini talks. Among the highlights include Larry Soderblom's confession that the DISR mosaics may have north off by as much as 15 degrees counter-clockwise, which is a major relief for me given my work to try to find the landing site in ISS images. Given their current understanding of "north", I could not find any site that matched orientation of the islands seen in the DISR images. Now with this revelation, there is a solution (easternmost tip of Adiri). Larry also mentioned the planned coverage by RADAR SAR of the landing site in T8. This swath is an east-west pass across central Belet and Adiri. The RADAR team extended the Beam Three path (at the expense of altimetry) to try to cover the landing site, but the signal/noise ratio maybe a bit low, I'm afraid. Hopefully, something good will come out of it and we can start making sense of all our data. FYI, RADAR SAR swaths are composed of five beams (you may have noticed the seams in early RADAR products). Beam three is the center beam and has the best signal to noise ratio. The Principal Investigator of the HASI instrument on Huygens reported that his instrument found a number of inversion layers in the Titan thermosphere. This confirms the INMS result that showed a number of waves in the methane and temperature profiles in the upper atmosphere. Apparently this wave structure continues down to at least 510 km. Now why is this so important... The SSP and the Permittivity Sensor on HASI both detected slight changes in conditions at the surface after landing. The increase in permittivity and changes in the speed of sound and spacecraft tilt maybe related to the vaporization of methane by Huygens as it sat on the surface, but that is just one possibility.

There were a number of results from the instrument teams from onboard Cassini. Many of the major results announced today were related to the rings and Saturn, and rightly so since admittedly they have gotten very little press coverage compared to the satellites so it is good they have their day in the sun. But this is a blog about the satellites, so who cares about those results ;-) Most of the Enceladus results were announced earlier at a press conference but some instrument teams did clarify them. VIMS last week announced they had found crystalline ice (absorption band at 1.65 microns) and simple organics (absorption band at 3.44 microns). During his talk, Bob Brown mentioned that the aborption band is attributed to a C-H stretch. In other words, it is some kind of organic compound, but VIMS can't tell what it is with just this one absorption band. Brown also announced that VIMS had not found any ammonia on Enceladus, shutting the door on ammonia-water volcanism. I'm not sure I agree with him there, but we may want to look at models that don't include ammonia. This also fails to explain how INMS saw N2. The CIRS principal investigator, Mike Flasar reported on results from Enceladus. In addition to the hotspot in the south polar region, CIRS found a relatively low thermal inertia, indicating that Enceladus is covered in unconsolidated material. Perhaps due to infalling E-ring material.

Finally, the RADAR team presented their results from T3. The most interesting features in that swath were the numerous linear to sublinear dark streaks, nicknamed cat scratches. These features have been interpreted as linear or longitudinal dunes, similar to those found in Saudi Arabia and Australia (near "Lake" Eyre). However, the RADAR team has been comparing the scratches to icy dunes in Antartica, which are visible in RADAR images of the continent, but not so much from the ground. The dunes take the form of changes in grain size rather than topographic features that one could see from the ground. Such an interpretation would jive with ISS' inability to resolve the scratches if both the dunes and the material that fills in between them are dark at 938 nanometers.

RADAR also released an image yesterday showing these dunes and a couple of drainage channels west of the "Circus Maximus" impact basin.


Anonymous jerry said...

Another possibility is that the cat scratches are fins, formed when a sedimentary layer is uplifted by a dome forming or plate uplifting. Sand dunes, and to a lesser extent snow dunes tend to be more periodic. The relief also appears to be a little steep for dunes. (although I have never seen a snowdrift at 70k)

A good example of fins can be found in the geology of Arches National Monument.

9/06/2005 11:43:00 AM  
Blogger Jason said...

The cat scratches are really too widespread to be fins. Fins occur over a small geologic region, and even in Arches. So, I don't see how fins could explain what we see throughout Fensal. We also don't have slope info on these dunes.

9/06/2005 12:31:00 PM  
Anonymous michael chisnall said...

Linear features sometimes show up in marine SAR images here on earth and can be caused by a number of things such as wind and atmospheric gravity waves. Although the penetration depth of radar into the ocean is on the order of mm's it is still possible, sometimes, to indirectly make out features on the bottom of very shallow stretches of water (a few tens of meters) because of their influence on overflowing currents and hence on the surface. This can also generate linear features that sometimes loosely look like the "cat scratches". How this applies to Titan is less clear however since the oceans are predicted, depending on composition ratios and solutes, to have penetration depths on the order of tens to, maybe, hundreds of meters. In very shallow oceans on titan it may be possible to directly image the bottom with radar. Dendritic features can sometimes be seen on terrestrial seafloors.

9/23/2005 02:39:00 AM  
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10/05/2005 01:15:00 AM  

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