Touchdown on Titan

Yesterday I attended all the talks on Cassini and Huygens. There has been masses of interesting science gained from both missions so in the next couple of posts I shall list some of the things I noted down.

I'll assume that you know the history of the Cassini spacecraft. If not, hopefully Tom or Ian can fill in the details. It finally arrived at Saturn, after a seven year journey, late last year and delivered the Huygens lander to Saturn's largest moon Titan. I'll come back to Cassini in another post, but for now let me describe what happened to Huygens....

Huygens entered the atmosphere of Titan at a height of about 1200 km, although the MASI instrument had been taking measurements of the acceleration from 2800 km up. If you know the mass of the lander and its drag coefficient (how much it gets slowed down by different amounts of air), which we do, you can use the acceleration to work out the density of the air you are moving through. So MASI was able to basically work out how much atmosphere there was. The precise nature of the deceleration even gave a hint that there may be some convection in the atmosphere.

There were also measurements of the temperature (the upper atmosphere was warmer than expected), pressure (there is more as you get nearer the surface!) and chemical structure. A lot has been learnt about Titan's atmosphere. For instance, it turns out that there were six inversion layers at 1020,980,800,680,600 and 510 km, and the atmosphere mainly consists of nitrogen (like the Earth), methane, hydrogen and a little argon. Some theories had suggested there could be other noble gases such as krypton or xenon but none were seen.

As with most space missions something did go wrong. It turned out that one of the two transmission channels, onboard Huygens, malfunctioned and there was a worry that data from the doppler wind experiment was lost. Thankfully, there was a backup plan, so radio telescopes from around the world (including Green Bank, Parkes, Mauna Kea and Pie Town) came to the rescue. They were able to effectively recover some of the lost information which tells us about the wind speeds during the rather bumpy descent. It was bumpy; it swung about by up to about 50 degrees at some points and this can be confirmed by matching up images taken with the cameras. The measurements show that although the upper atmosphere moves in the same direction as the planet rotates, the bottom kilometre had a slight wind blowing in the opposite direction.

There was a speed of sound experiment onboard involving a speaker and microphone. As the temperature or pressure changes the speed of sound also changes, so measuring it gives you a handle on those values. As Huygens approached the surface echos of the speaker could be heard reflecting from the ground! This means that it was possible to measure the speed of Huygens very accurately during the final stages of descent.

The first part of Huygens to hit the ground - at a leisurely 5 metres per second - was the penetrometer (part of the Surface Science Package). This was basically a metal pole that stuck out the base of the lander and measured the impact. The plots of the force (in Newtons), against time from impact, show some interesting features. It looks as though it passed through a very soft top layer, possibly hit a thin crust or ice pebble and then pushed into some wet 'sand' below the surface.

Just after landing the Gas Chromatograph Mass Spectrometer (GCMS) detected a sudden increase in methane. It is thought that the 'sand' underneath the lander was probably damp with liquid methane and so the relatively hot lander made it evaporate; this would give the GCMS, with its opening underneath the lander, a whiff of methane and so explain the 30% increase.

So what chemicals are present on the surface? The data show methane, ethane, carbon dioxide, C2H6 (if you know the proper name of that molecule post your answer below?) and argon. Now I think it is frankly amazing what a few chemicals can actually tell you about a planet. The argon possibly indicates some geoplogical activity although that isn't certain; The ratio of different isotopes of carbon suggests that there is no life - certainly not as we know it anyway. The ratio of isotopes of nitrogen even suggests that Titan has lost several times its present atmosphere over its lifetime.

Even though data were only collected for seventy two minutes after touchdown (hey, that is 26 times longer than originally planned for) further measurements were made. Around 100 images were taken on the surface but as the camera couldn't move they all look the same! The tilt detector in the surface science package noted that the lander was at an angle of 10.5 degrees and even tilted a further half a degree during the hour after landing.

(Note: I shamelessly stole the title for this entry from the title of the public lecture last night).

Posted in astro blog by Stuart on Tuesday 06th Sep 2005 (12:30 UTC) | Permalink
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