Benzene in Saturn and the origin of water in giant planets
20 October 1998The European Space Agency's ISO infrared telescope has detected for the first time the presence of the molecule benzene in Saturn's atmosphere, an unexpected result that poses now the new problem of having to explain how this molecule has been produced. This is one of the new findings presented today at the international meeting "The Universe as seen by ISO" that is being held this week in Paris and is attended by 400 astronomers.
During today's session - entirely dedicated to the Solar System - ISO scientists debated also a controversial topic: a solution for the mystery of the origin of the water in the atmospheres of all four giant planets and Saturn's moon, Titan.
Benzene is a common molecule on Earth (it's a component of car fuel) but apparently not so much in the other planets. Only in certain areas of Jupiter's atmosphere had it been detected before. ISO's. spectrometers see it now also in Saturn, along with other three hydrocarbons also detected for the first time in this planet (methyl, propyne and diacetylene). "ISO results are showing us the chemistry of the Solar System with an accuracy well beyond our expectations, and with a great diversity," said Emmanuel Lellouch, from the Observatoire de Paris Meudon. "We are getting now a more detailed picture of the processes taking place in the planets' atmospheres, and also a better knowledge of how the whole Solar System formed five billion years ago."
"Actually, ISO has also obtained the first detection of a new molecule containing deuterium (a form of hydrogen) in the atmospheres of Neptune and Uranus, a result that will contribute to understand the birth of the Solar System out of a collapsed proto-solar nebula. The models explaining this process will now have to take into account the new ISO data," Lellouch added.
The mystery of water
One of ISO's first discoveries was the presence of water vapour in the higher atmospheres of all giant planets. The amount of water detected indicated an inflow of 10 litres of water ice per second in a planet like Jupiter. Initial interpretations involved two possible explanations: either an interplanetary source or a local source, the latest meaning that the water was being brought by material falling from planetary rings, or even by a continuous shower of micrometeorites on the planets - including Earth.
Further analysis of the ISO data and the recent detection of water on Titan suggest that the first explanation was valid. According to Helmut Feuchtgruber, from the Max Planck Institute in Garching (Germany), the water comes simply from the interplanetary dust, which in turn is full of scattered grains from comets. This solution is supported by measurements of the amount of water ice in the Earth's higher atmosphere, showing a similar inflow of water on Earth and on the other planets.
New spectra were also taken from Mars. The results regarding vertical distribution of water in Martian atmosphere are in agreement with those obtained by NASA's Pathfinder mission.