Cassini studies atmospheric chemistry

Spacecraft has already provided detailed information from Jupiter.

The Cassini spacecraft may have only recently arrived at Saturn and just begun its main four-year mission, but it has already provided planetary scientists with valuable new information.

Between October 2000 and March 2001, as part of its seven-year journey to reach Saturn, Cassini flew past Jupiter, and used its composite infrared spectrometer (CIRS) to analyse Jupiter’s atmosphere. Experts from the US, UK, France, Germany and Italy have now analysed the data sent back by Cassini, providing greater insight into the chemical composition of Jupiter’s atmosphere but also generating a few surprises.

The CIRS comprises two combined interferometers that operate in the far-infrared and the mid-infrared. Their prime objective is to analyse Saturn’s atmosphere, as well as its rings and the surfaces of its moons, but the Jupiter fly-by gave scientists a prime opportunity to enhance their knowledge of the atmospheric composition of the largest gas giant in the Solar System. The information will not only aid in the study of Jupiter, but will also help scientists understand the chemical composition of extrasolar planets, many of which are thought to be similar to Jupiter.

Cassini identified two new chemical species in Jupiter’s atmosphere: the methyl radical (CH3) and diacetylene (C4H2). Researchers think that both these compounds are important to Jupiter’s stratospheric photochemistry, with CH3 involved in the synthesis of methane, and C4H2 potentially involved in the formation of stratospheric haze. The data from Cassini also enabled development of a much more detailed picture of the distribution and transport of the most abundant hydrocarbon species in Jupiter’s upper atmosphere, such as C2H6 and C2H2.

Scientists on Earth had already detected hydrogen cyanide, carbon dioxide and carbon monosulfide in Jupiter’s atmosphere. These chemical compounds were all produced as a result of Comet Shoemaker-Levy’s collision with Jupiter in July 1994. However, the data from Cassini revealed that the diffusion of CO2 and HCN away from the impact site had been very different, with HCN showing a much broader latitudinal distribution than CO2. The scientists admit that this difference in distribution is surprising and hard to explain, and suggest that a range of different atmospheric and chemical processes are probably involved.

Jon Evans