the atmospheres of giant planets are mostly made-up of hydrogen, helium and methane. Now, studies at the Infrared Space Observatory ( iso ) Science Operation, Villafranca, Spain, show that the icy satellites and the rings which surround these planets, could be an important source of oxygen to the atmospheres of Saturn, Uranus and Neptune ( Nature , Vol 389, No 6647).
The short wavelength spectrometer at the iso has detected water in gaseous form in the upper atmospheres of these planets. The findings may have an important bearing on the photochemistry and ionospheric properties of these atmospheres.
A team of researchers led by H Feuchtgruber of the Max Planck Institue fur Extraterrestrische Physik, Garching, Germany, looked into the emission lines that are characteristic of water vapour in the far infrared wavelengths. The spectrum showed the presence of several emission lines at wavelengths between 28 and 45 micrometres, which coincide with the rotational lines of water vapour in this range.
Using two different models of atmospheric composition of these planets, the researchers obtained an estimate of the amount of water. The amount of water thus deduced in the atmospheres can be accounted for the bombardment of the atmospheres by water ice and silicate dust. The source of the ice and the dust would be the rings, the icy satellites, comet nuclei and even interplanetary dust surrounding the planets. The water is deposited at high altitudes in the atmospheres and then mixes downwards because of atmospheric motion.
The researchers also confirm the presence of carbon dioxide ( co 2 ) on Saturn and Neptune, but not on Uranus. However, the source of co 2 is yet not certain as it is thought that any external source should be able to provide comparable amount to all three planets.