The Planetary Society Blog
By Emily Lakdawalla
New scientific publications on Titan and Huygens
Jul. 27, 2006 | 14:44 PDT | 21:44 UTC
There were two sets of Titan-related scientific papers published this week. A pair of papers in Nature describes Titanian weather, including clouds and methane drizzle, and a passel of papers in the Journal of Geophysical Research is devoted to the Earth-based campaigns to observe Titan before, during, and after the Huygens mission.
One of the Nature papers was by Tetsuya Tokano and six coauthors and talks about evidence from the Huygens Gas Chromatograph Mass Spectrometer (GCMS) and Atmospheric Structure Instrument (HASI) for "Methane drizzle on Titan." Cool start! They calculated a vertical profile of the relative humidity of methane for the lower 40 kilometers of Titan's atmosphere. When the relative humidity of water in Earth's atmosphere exceeds 100%, you get clouds, fog, or rain. They also accounted for the presence of nitrogen (lots of it) in Titan's atmosphere, which (when dissolved in methane) lowers the freezing point of methane by 10 to 15 Kelvin, an effect that allows methane to exist as a liquid up to 15 kilometers above Titan's surface.
Their vertical profile shows that, at least along Huygens' trajectory, the relative humidity of methane starts at 45% near the surface, is 90% at 6 kilometers, and approaches 100% above that. So, they infer, there should be a cloud or something at least up to an altitude of roughly 8 kilometers up to the level where methane freezes; at altitudes above that, there is an ice cloud, with a small gap in between the two. They note that Huygens' camera did detect a methane haze at around 21 kilometers, with a gap below 20 kilometers. The camera didn't see the methane-nitrogen cloud at around 8-16 kilometers, but they explain how such a cloud could be "subvisible." From these cloud densities they can estimate a methane rainfall rate for the Huygens landing site, which they determined to be 50 millimeters per year. This isn't a lot -- it would be considered a desert rainfall rate on Earth -- but it would be enough to wet the surface. They conclude: "Large-scale stratiform precipitation -- drizzle -- may constitute a more persistent component of Titan's whole methane cycle than the optically thick but sporadic clouds."
The other Nature paper had a slightly more dramatic title: "Methane storms on Saturn's moon Titan," by Hueso and Sánchez-Lavega. They produced a computer model that indicates that "severe methane convective storms accompanied by intense precipitation may occur in Titan under the right environmental conditions....Raindrops of 1-5 millimeters in radius produce precipitation rainfalls on the surface as high as 110 kilograms per square meter and are comparable to flash flood events on Earth." They describe some issues with their models failing to match some of the observations of cloud locations, but it's an interesting start. I wonder what the terminal velocity of a 1-centimeter methane raindrop on Titan is? The air's thicker and the gravity's lower than on Earth, so it's probably slower than Earth rain.
Here's a few highlights of the papers from the Journal of Geophysical Research.
Ralph Lorenz and 10 coauthors published an article titled "Huygens entry emission: Observation campaign, results, and lessons learned." The gist of the article is that they were trying actually to detect from Earth the fireball that was created as Huygens plunged into Titan's uppermost atmosphere. The paper describes the predictions that suggested that observing this tiny flash might be possible, as well as the frustrating circumstances that prevented Hubble, Palomar, and the complex of telescopes at Mauna Kea from observing the event at all. (Hubble was stymied by the failure of the power supply of the Space Telescope Imaging Spectrograph in August 2004, and Palomar and Mauna Kea by exceptionally bad weather.)
They did get observations with both Keck II and IRTF. Neither one of them conclusively detected a fireball, but the lack of detection did at least set an upper limit on how big the fireball could have been. The Keck II result was a little less useful, because of the geometry of the entry: the probe itself probably blocked much of the fireball from view. So the meat of the paper is the "lessons learned" section at the end -- what does the lack of detection mean for future, similar attempts? The paper notes that if things had gone badly and the probe had broken up during entry, the fireball would have been much easier to detect, making the observations very useful for a forensic investigation. The paper also notes that the best possible platform for viewing the fireball wasn't able to because of budget constraints. Cassini was much closer to Huygens than any of the Earth-based assets, but a mission redesign in 1992 to cut costs eliminated both a scan platform for the optical remote sensing instruments and an independently steerable probe relay antenna. Those two cuts meant that Cassini's high-gain antenna had to be pointed at Titan during the descent, making observations impossible.
Hartung, Herbst, Dumas, and Coustenis contributed an article titled "Limits to the abundance of surface CO2 ice on Titan." They used the Very Large Telescope to look for spectral evidence for carbon dioxide ice on the surface of Titan just before the Huygens descent, in December 2004. Again, the paper presents a negative result -- no detection. Again, though, even a non-detection tells you something; they give an upper limit to the possible abundance of carbon dioxide ice on the surface of 7% (for bright areas) to 14% (for dark areas).
Bill Folkner and 16 coauthors contributed "Winds on Titan from ground-based tracking of the Huygens probe," that is, the results of the Doppler Wind Experiment. This is an update to a preliminary analysis published by DWE principal investigator Michael Bird in Nature last year. The most interesting piece of this paper is a graph showing very high resolution measurements of the wind speed and direction performed by the Green Bank Telescope during a couple of different segments of the descent. The graph makes it clear just how choppy and turbulent Huygens' passage through some of the "atmospheric boundary layers" must have been.
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