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Titan: Sol's Biggest Lightweight

Huygens will help unveil a whole new world largely hidden by the worst "smog" in the solar system. Artwork by David Seal
by Bruce Moomaw
Cameron Park - March 20, 2001
In our last report on the Cassini mission we looked at Saturn's strange moon Titan -- or, rather, the little we know about it, since Titan is arguably one of the most mysterious worlds to be explored in our Solar System.

In this chapter, we will look at how Cassini and its Titan probe Huygens will seek answers to the many unanswered questions about Saturn and Titan -- and how the mission's controllers plan to solve a serious design problem that threatens to limit the amount of data Huygens can radio back to Cassini.

The original plan for the mission runs as follows: On Nov. 6, 2004 -- about three-quarters of the way through Cassini's first elongated orbit around Saturn, on its way back to the planet -- Cassini, having set itself on an impact course for Titan, will release the 319-kg Huygens probe, and will then itself veer slightly away from Titan to enable it to make a close flyby of Titan while delaying its arrival until 4 hours after Huygens gets there.

After that, Cassini and Huygens will travel separately for three weeks until Nov. 27, when Huygens will dive into Titan's atmosphere at 21,000 km/hour.

Indeed, Huygens will hit Titan pretty much head-on -- at an angle of only 26 degrees off vertical -- which sounds pretty rough, but is actually a less brutal entry than that carried out by the four Pioneer atmospheric probes which NASA smashed head-on into Venus' atmosphere and which survived without trouble back in 1978.

Its heat shield will allow it to survive that entry easily, and when it slows down to 1400 km/hour it will deploy an 8-meter parachute, dump the heat shield, and start making scientific measurements.

As I mentioned in my last report, Titan has a combination -- unique in the Solar System -- of weak gravity and a bulky atmosphere with a great deal of gas in it. In fact, its surface gravity is actually slightly weaker than our own Moon's -- but because of that thick layer of gas, its surface air pressure is nevertheless 1.5 times that of Earth's.

The result is that Titan's atmosphere towers to amazing heights above its surface, such that when Huygens deploys its parachute, it will be 160 km above the surface with an atmospheric pressure 1/300 that of Earth. Moreover the probe will already be entering the upper fringes of the orange organic smog that blankets Titan.

The purpose of the chute is to slow it down for the first 15 minutes of its descent so that it can study the upper atmosphere in more detail.

Then -- still at about 115 km above Titan's surface -- it will cut loose the chute and release a smaller 3-meter chute that will increase the rate of descent, to ensure that Huygens gets to Titan's surface before Cassini moves beyond its horizon and its brief communications period ends.

Because of that remarkable combination of low gravity and dense atmosphere, it will take fully another two hours - plus or minus 15 minutes - before it actually hits the surface. If Titan has any actual clouds at all - made of droplets of liquid methane and perhaps ethane - they will be probably be located only about 15-25 km up. But recent Earth observations suggest that while Titan does indeed have such methane clouds, they're very sparse and Huygens is likely to miss them.

During all this period, Huygens will be studying Titan with an impressive collection of sophisticated instruments. Italy has provided a package of sensors for air temperature and pressure, accelerometers to record Titan's upper air density during its initial fiery plunge, and sensors to detect radio bursts from possible distant lightning bolts and measure the electrical conductivity of Titan's air, as well as a microphone to sense any local thunder.

During its close 1980 flyby, Voyager 1 failed to detect any radio bursts from Titanian lightning -- proving that if it exists at all, it must be extremely rare -- but the possibility can't be completely ruled out.

A German radio beacon will allow the Cassini mother ship to do very precise Doppler tracking of Huygens, thus measuring the speed of Titan's winds as they blow the probe along sideways -- and theoretical studies suggest that Titan's winds probably blow as fast as 200-400 km/hour in its upper atmosphere, although they gradually drop to only about 3-4 km per hour at its surface.

More important, though, is Huygens' bulkiest experiment: a sensitive American Gas Chromatograph/Mass Spectrometer combination to analyze Titan's atmosphere, which can sense gases in the part-per-billion range.

It will precisely measure Titan's major atmospheric gases including argon, which could represent several percent of its air -- and whose quantity, as I noted earlier, can probably tell us whether Titan originally got its nitrogen in its current form or as ammonia that was then broken down by the Sun's UV light.

It will also measure faint trace isotopes which will tell us more about Titan's atmospheric history -- and it can analyze the complex organic compounds which we know are being slowly formed out of the methane and nitrogen in Titan's air by the Sun's UV light, cosmic rays, and Saturn's own radiation belts.

These organic compounds are sure to exist in a very wide variety, and Huygens' mass spectrometer can detect them all the way up to a molecular weight of 146 AMU -- but many such complex compounds are sure to have the same, or almost the same, total molecular weight, making it impossible for the mass spectrometer by itself to tell them apart.

So, while the spectrometer will constantly analyze direct samples of Titan's air, it will also constantly analyze other samples drawn through three gas chromatograph columns stuffed with different chemical powders which will "adsorb" different gases to varying degrees -- that is, different types of gas molecules will cling to the powder granules for varying lengths of time before coming loose again, thus sorting out the initial flow of Titan's gases so that the mass spectrometer can analyze the gas flow one piece at a time and sort out all the dazzling variety of trace organics which must exist there.

However, that leaves Titan's orange organic smog itself -- made out of other organic compounds, some of them very complex, which have actually formed smoke like microscopic solid particles - including some that are probably made out of natural polyethylene plastic!

To analyze these, Huygens will need another experiment -- a French "Aerosol Collector and Pyrolyzer" which is really an auxiliary attachment to the GCMS.

Between the altitudes of 150 and 45 km, it will suck Titan's air through a fine filter to capture such particles -- and then it will then gradually heat the filtered material all the way up to 650 deg C, finally vaporizing even the most temperature-resistant organics so that their vapors can be fed into the GCMS for analysis.

Between 30 and 15 km, it will collect a second filtered sample of Titan's air, and then gradually roast that sample to analyze the organics in Titan's lower atmosphere.

Click For Part Two

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