Friday, January 28, 2005
Thursday, January 27, 2005
Discussion on the Dark Terrain: Part II
With ISS and Keck having already shown that widespread liquids in the dark terrain was unlikely and Arecibo and Cassini RADAR perhaps finding a few isolated areas of surface liquid, it was up to Huygens to perhaps resolve the mystery of the dark material. Luckily, Huygens landed near a boundary between dark and bright material so that both types of terrain can be seen up close for easy and unambiguous comparison. Even better, Huygens landed in a dry patch of dark terrain. Features seen in the dark material from altitude such as dark streams between small patches of bright terrain and streamlined shapes to the larger bright patches within the dark terrain suggest that although the dark material at the landing site was dry, fluid had at one time flowed in the area. This is supported by the observation of channels in the bright terrain terminating at the boundary between dark and bright regions and by the eroded shapes of ice rocks in the vicinity of the Huygens. Images from the surface using the medium-resolution imager were affected by the lamp but suggest that the close up texture of the surface is akin to gravel or sand.
In addition to imaging, the GCMS instrument, the penetrometer on the SSP instrument, and the spectrometer on the DISR instrument examined the composition and texture of the dark material. Spectra from altitude by DISR shows that the terrain is mostly made of water ice with some kind of darkening agent. The data from the penetrometer on the Surface Science Package has been interpreted as showing that that dark material at the landing site was covered in a thin crust of material with icy sand below. This is consistent with the DISR-MRI view of the surface. Finally, the GCMS instrument, with its heated inlet, found methane percolated into the instrument after being heated up to boiling.
Putting together these data, it appears that the area where Hugyens landed as modified by fluids, possibly from rainfall during a rainy season but that's still an early hypothesis, especially since we have yet to see clouds at that latitude, perhaps since clouds have only been observed for a few years (though a large cloud may have been see at northern mid-latitudes by HST in 1995). However, the views from above and from the surface both suggest that the terrain has been modified by fluid flow and erosion. The GCMS, SSP penetrometer, and MRI images seem to indicate a dry initial surface layer of ice sand with liquid methane 15-30 cm below the surface mixed in with the ice sand in a kind of methane slushee.
ISS - Imaging Sub-System (Cassini's camera)
VIMS - Visual and Infrared Mapping Spectrometer (Cassini's remote spectrometer)
INMS - Ion and Neutral Mass Spectrometer (Cassini's insitu spectrometer)
DISR - Descent Imager and Spectral Radiometer (the camera on Huygens)
GCMS - Gas Chromatograph and Mass Spectrometer (the insitu spectrometer on Huygens)
ACP - Aerosol Collector and Pyrolyzer (instrument on Huygens)
SSP - Surface Science Package (on Huygens)
DWE - Doppler Wind Experiment
OPNAV - OPtical NAVigation images used by Cassini to determine position by looking at known targets
There is a new opnav image of Titan taken yesterday up on the JPL Raw images page. The image was taken in the two clear filters on the Cassini Narrow Angle Camera and really doesn't show much thanks to the ubiquitous haze in the shorter wavelength filters. Occassionally these images can be stretched to tease out some bright/dark boundaries, but it is still pretty useless. Still, it's nice to know it's still there.
This image is a great new view of Mimas with the rings in the same field of view has just be published in the Ciclops website and on the Cassini JPL website. The caption says that this image was taken on December 18, but I think it is much closer to January 18 since I remember seeing this image come down just after the Huygens landing. I will check and make sure.
Wednesday, January 26, 2005
Discussion on the dark terrain: Part I
On January 14, the Huygens probe landed on the surface of Titan within the large equatorial belt of dark material in anti-Saturnian hemisphere. The nature of the dark material on Titan has been important question for geologists ever since the first disk-resolved images of surface taken by HST were released in the mid-1990s. One of the more popular explainations prior to the Cassini mission was that the dark material consisted of seas of liquid methane and ethane. This hypothesis was supported by speckle imaging in the late 1990s by a group out of the Lawrence Livermore Lab using the Keck I telescope showing that the dark material had an albedo at 2 microns of around 2%.
A more conclusive remote test for liquids is to search for specular
reflections. A specular reflection can be seen when looking at the sun's reflection on a body of water while on a day at the beach. All that is required is that the surface be smooth at the wavelength of the light source. In this case, five seperate data sets can be brought to bear on this problem, four of which are known to this blogger. First, Campbell et al. used the Arecibo Radio Telecope in Puerto Rico to look for a specular reflection at a wavelength of 13 cm. They found specular reflections in some places, but not everywhere. Finding a specular reflection at 13 cm is not necessarily diagnostic of liquids, but it does mean that the surface in the areas where a specular reflection was seen had to be smooth at the scales of 13 cm. The RADAR experiment on Cassini also observed Titan at a different wavelength in late October. While this is not aware of any specular reflection seen by that experiment, it did see a region of very low radar reflectivity (nicknamed Cici's Halloween Cat) that might be a lake of liquid methane but the jury is still out on that. More diagnostic is the Keck and Cassini ISS data sets. ISS does have higher resolution, but Keck has better longitudinal coverage at the present time. In neither data set has a specular reflection been found, including in the dark terrain.
- DISR images showed that the haze layer extended down to at least 20 kilometers, well within the troposphere. Before Huygens, the haze was expected to extend down to around 50-60 kilometers and that the atmosphere should be clear below the tropopause at an altitude of 40 kilometers, assuming a well-mixed troposphere.
- The Gas Chromatograph and Mass Spectrometer (GCMS) instrument on Huygens failed to find ethane in the atmosphere or on the surface to within the sensitivity of the instruments. For the surface, this can be explained two ways. First, the ethane could simply not be there, either by draining away from the area where Huygens landed or the photochemical models for Titan's atmosphere overestimated the amount of ethane that would be produced in Titan's lower haze layers, and instead other simple products like acetylene were produced. The other possibility is that liquid ethane was there along with the liquid methane. Ethane's vapor pressure was 3 orders of magnitude less than methane given the instrument setup (GCMS had a inlet heated to 90 degrees Celsius while on the surface). So the inlet may not have been hot enough to vaporize enough ethane for GCMS to sense it. In the atmosphere, the thicker haze layer may mean that ethane is locked up in the lower haze layers rather than precipitating out of the haze.
- Given the the thicker haze, the optical depth of the haze layer may be more than previously thought. The DISR team estimates that the haze may have an optical depth of 3-4 at 1 micron, compared to a value of 1 predicted before the mission. However, with such an optically thick haze, how can we see the surface in ISS images? I would certainly like to see how they arrived at that number. My first guess would be using the solar aureole imager.
- Lunine offered an alternative hypothesis to explain the sharp spike in the SSP penetrometer data. Instead of a thin crust, as posited by the SSP team, Lunine thinks Huygens first came down on a grouping of ice pebbles seen in the surface images, then slid down and settled on the surface. However, the question I have is whether the accelerometer data, which shows a 15 G deceleration in 40 ms, supports that hypothesis.
- In the last post, I mentioned that doppler data recieved on Earth directly from Huygens indicated that Huygens had a fairly rough ride above the tropopause. Lunine mentioned in his talk that DISR saw the solar aureole with the side looking imager, which in a normal orientation looks about 10 degrees above the horizon. This means that at higher altitudes, Huygens was swinging by as much as 60 degrees from normal, nearly at the collapse point for the parachute.
- Finally, Jonathan Lunine discussed the developing ammonia story on Titan. A water-ammonia mixture has a lower melting point than pure water, so it takes less energy to maintain a interior liquid layer than if the layer was composed of pure H2O. This may help explain the non-zero orbital eccentricity of Titan, which would dissipate if the liquid layer, between the Ice-I surface layer and the higher pressure ice below, froze. A conformation of an interior liquid layer could come on four flybys of Titan (two at periapsis and two at apoapsis) by Cassini designed to look at Titan's interior through the Doppler shift in Cassini's signal as it flys by. The non-detection of non-radiogenic Argon (mass=38) suggests that Titan did not get its nitrogen from molecular nitrogen locked into Titan's ices since Argon-38, which has about the same volatility as molecular nitrogen, would condense at a rate proportional to the relative abundance of Nitrogen and Argon in the solar nebular (~10x). Since, the abundance is much less than the 1-10% this condensation should give, nitrogen could not have come to Titan as N2. Instead, ammonia likely condensed with the water ice, and was later brought to the surface. Once ammonia was in an atmosphere, sunlight broke up the nitrogen and the hydrogen, with the nitrogen remaining and the hydrogen escaping into space. RADAR SAR data from October showed evidence for cryovolcanic flows and Venusian-style pancake domes, indicating recent volcanic activity. This would be supported by the possibility of a present liquid layer and would likely be composed of Water and Ammonia, since the ammonia would give the water the bouyancy it needs to reach the surface and would give the right mobility to match the RADAR observations, so that it resembles basalt. Cryovolcanism (or venting at least) is supported by the INMS (an instrument on Cassini) and GCMS measurements of Argon-40, a by-product of the radioisotope Potassium-40.
UPDATE 03/25/05: The talk now appears to be offline. You can find edited transcripts of the talk on the Astrobiology Magazine website.
Doppler data also showed the ride higher up in Titan's atmosphere was quite bumpy (more on this in a post later today).
This new view of Tethys was taken last month and has a resolution of 10 km/pixel (as opposed to imaging from October at 2.5 km/pixel). This view clearly shows the albedo asymmetry on Tethys.
Tuesday, January 25, 2005
Tonight Show Titan Joke
A probe recently landed on the surface of Saturn's moon Titan, finding rivers and clouds made of liquid methane...or it landed in New Jersey.
The Titanese Times
On the use of Titanese: it has been rather difficult to come up with a suitable adjectival form for Titan. Titanian would be the obvious first choice, but that could become easily confused with the adjectival form for the Uranian moon Titania. Titanic has become a favored form for some ISS team members, but it is unsuitable for me since it just brings to mind a very BAD movie. Titanese is also a possibility, though the -ese suffix is usually limited to nationalities from east Asia (Vietnamese, Chinese, Japanese, etc.). However, given the unsuitability of the first two, I am forced to concede to Titanese. If anyone has any other suggestions, don't hesitate to send me an email.
As stated in the intro, I will strive to continue to post on this blog for news primarily for Titan and Io, though I can't ignore the other satellites, except Europa, that moon will get no press here. Why? Because I feel like it. This blog will also not cover Mars, because it is not an outer planetary satellite and it gets enough press elsewhere. I will also post on the other Saturnian satellites as new results come in and are published. I may occasionally sway to other topics, like politics, but given the current political climate, I may try to avoid that. We'll see.
In terms of contributions, I will obviously look at them and post them as I see fit. Since this blog has a fairly small audience, I don't think an additional author is needed, but if you want to offer your services, I'll look into a dual author or team authorship arrangement, however I'd rather not have a site run by committee.
I'm going to sign off for now though later today, I hope to publish my thoughts on the recent Huygens images. Titan certainly looks like a very interesting place that continues to astound, amaze, and confuse.
More Spacedaily articles
Monday, January 24, 2005
Keck image of Titan from last Thursday, January 20 showing a relatively cloud-free day over the trailing hemisphere of Titan. The filter used for this image cover the methane window at 2.05 microns.
Unbeknownst to me until last week, the Keck telescope in Hawaii has been making regular obervations of Titan this observing season like they did last year. New this year is the emergence of mid-summer southern mid-latitude clouds, also seen by Cassini and Roe et al. starting late 2003. Polar clouds are still seen this observing season, though seamingly larger and more sporatic than last year. Only time will tell whether clouds will be seen at lower latitudes, like near the Huygens landing site as Titan approaches Equinox in 2009.