Mooning over Titan's atmosphere

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  • Published: Oct 15, 2006
  • Channels: Base Peak
thumbnail image: Mooning over Titan's atmosphere

Of all the places to visit in our solar system, Titan, the largest of Saturn's moons, may seem a strange choice to the man in the street. After all, some of the planets are closer and, being planets, will be more interesting to earthlings than one of the many moons orbiting around a distant planet. Well, Titan has a secret that has drawn knowing scientists into its spell. It has a thick atmosphere which some believe may have been similar to that on primordial Earth.

It was for this reason that a joint mission by NASA, the European Space Agency and the Italian Space Agency sent the Cassini-Huygens probe to Titan, launching in October 1997. Seven years later, it reached Saturn's system where the Cassini spacecraft is now orbiting the planet, making 75 orbits over 4 years, gathering data on Saturn and several of its moons.

The Huygens probe was jettisoned on Christmas Day 2004 (no holiday for the mission crew on Earth), taking 20 days to reach Titan, where it was given a controlled descent by parachute. It travelled through the rich atmosphere which extends almost 400 miles into space, taking measurements as it went, and landed on Titan's surface where it continued operating.

One of the key instruments on board was a GC/MS system fitted with an aerosol collector and pyrolyser ports. It was used primarily to study the components of the moon's atmosphere and identify the substances that give it that orange-brown hue. The finger of suspicion had been pointed squarely at tholin, a complex organic material that does not occur naturally on Earth. Named after the Greek for muddy, it forms an effective UV screen for the surface and may even form amino acids, the stuff of life, on the surface.

The main gaseous atmospheric components are nitrogen and methane (roughly 95:5%), compared with the Earth where nitrogen is accompanied by oxygen and smaller amounts of other gases like methane and carbon dioxide. It is believed that tholin was produced by lightning discharges in the early atmosphere but scientists find it hard to explain why there is still so much present.

In order to interpret the Huygens GC/MS data, experiments have been carried out in Earthly labs on what is believed to be synthetic tholin. Scientists in the US led by senior reporter Megan McGuigan from the University of Michigan treated a nitrogen-methane mixture (9:1, by volume) with a cold plasma discharge and the orange tholin film was scraped off the glass container.

This material was subjected to pyrolysis-GC/MS using a flash pyrolysis unit that heated the sample up to 1000 °C in less than 1 second. The breakdown products were separated by two-dimensional GC and analysed by time of flight mass spectrometry. These techniques were required to analyse the highly complex pyrolysis mixture, as more than 6000 peaks were detected.

The pyrolysis products included many low-molecular-weight nitriles, alkylpyrroles, alkylbenzenes, PAHs and hydrocarbons. Acetonitrile was present down to 250 °C, but at 600 °C the dominant compounds were the pyrroles and non-cyclic nitrogen-containing compounds. When standards of the individual compounds found in the pyrolysate were pyrolysed alone, they were little changed. This stability suggests that these structures -methylpyrroles and nitriles - are likely to be present in thiolin.

A second Titan-related study has also been published recently by scientists in Europe and the USA, led by planetologist Cyril Szopa from the Service d'Aeronomie in France. This team were trying to establish how the local conditions in the atmosphere above Titan would affect the operation of the GC/MS system, so that the data could be interpreted correctly. They carried out tests simulating the changes in pressure and temperature that might have been encountered before and after the probe launch, during its long journey through space and during its descent towards Titan's surface.

Pressure variations did not affect the separation power on the particular columns used, so that atmospheric pressure changes during the probe's descent were deemed unimportant. They did, however, affect the time of the analysis, retention times increasing nearer the surface, so that some compounds would not be chromatographed at lower altitudes compared with higher ones. Temperature also affected retention time but column aging did not influence either the column efficiencies or separation powers.

These influences were considered when the GC/MS data transmitted from Huygens was being analysed, establishing a reference for interpretation. Szopa tell us that data analysis is continuing and the data "could bring important information on Titan's atmosphere and its history".

It has since been confirmed on the mission website that GC/MS data from Huygens on the atmospheric particles found in the haze resembles that of tholins.

For those of you who are interested, the Huygens data archive, including the GC/MS measurements, is now in the public domain.

The atmospheric layers of titan
The upper atmosphere of Titan has many haze layers extending several hundred miles above the surface, shown in this UV image of Titan's night side processed to look like true colour.
Image: courtesy NASA/JPL/Space Science Institute 

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