Jonathan Lunine talks Titan, Huygens

Jonathan Lunine, a Cassini Interdisciplinary scientist from the University of Arizona, recently gave a webcast lecture, giving an overview of Titan and putting the early Huygens results into context. Many of the comments he gave were his personal views of the data returned so far. I took some notes during the webcast so here are some of the comments I thought were significant.

• 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.

That's it for now from that talk. A lot of information given. The link above goes to the Real Player stream of the lecture so that you can watch it for yourself.

UPDATE 03/25/05: The talk now appears to be offline. You can find edited transcripts of the talk on the Astrobiology Magazine website.