Posted on Wednesday 22 December 2004

Okay, I did kind of imply that I would still be posting from time to time.

The Planetary Society has a great article up on its site about the current state of knowledge about Titan and what it means for the Huygens space probe descent into its atmosphere. I thought this method of ascertaining the structure of Titan's atmospher by looking at star light shining though it was rather neat.

But the most important result to come out of the second Titan encounter was the result of two occultation experiments performed by the UVIS team. An occultation experiment involves staring at a bright light source -- in this case, the bright stars Spica and Shaula -- and watching how the intensity of their light varies as they appear to cross behind a semitransparent target. Occultation experiments will be performed throughout the mission on the atmospheres of Titan and Saturn, as well as on Saturn's rings. UVIS is sensitive to ultraviolet wavelengths, so it probes the uppermost atmosphere, the region in which Huygens will be relying upon the friction between her heat shield and the atmosphere to brake.

Once the data came down from the spacecraft on Monday afternoon, Pacific time, the UVIS team worked around the clock in order to analyze what the flashes of light from Spica and Shaula meant for the vertical structure of the atmosphere. Early Thursday morning was a critical event for the Huygens mission, a "GO / NO-GO" meeting for the Probe Targeting Maneuver, a burst of Cassini's engines that will set the spacecraft on a collision course for Titan. If the atmospheric models proved wrong, the mission would be forced to scuttle the plans for a January descent for Huygens.

Fortunately, the calculated values for the density of Titan's atmosphere -- the most critical number -- came "within three percent of predictions," reported UVIS Principal Investigator Larry Esposito. Because of the near-perfect match between predictions and observations, "We got the green light to proceed for the next step," said Jean-Pierre Lebreton, Project Scientist for Huygens. "The UVIS team did a great job in analyzing the data within 24 hours. In a sense it's almost disappointing -- we did not have to change anything."

It seems as though the computer models have got Titan's atmosphere just about right so the big mystery reamins: Where are those big sloshing hydrocarbon oceans that should have been raining down on Titan's surface? Something definitely has been raining down because Titan appears to be geologically new but currently there is no sign of anything actually wet down there.