When our solar system formed, some 4.5 billion years ago, the giant planets were massive enough to retain most of the gas from which they formed. They are therefore witnesses to the early stages of the solar-system history. For a long time, it was believed that all giant planets had similar atmospheres, composed mostly of hydrogen and helium with roughly cosmic abundances. Data from the Voyager space probe, from ground-based spectroscopic measurements, and recently from the Galileo spacecraft, have shown that there are in fact significant differences in the chemical abundances and in the elemental and isotopic ratios, which are important tracers of the processes by which the planets formed and evolved.
Measurement of the deuterium-to-hydrogen ratio is of particular interest. Deuterium (heavy hydrogen) was formed in the first few seconds after the Big Bang and has been continuously destroyed in stars as the universe has evolved. The two largest planets, Jupiter and Saturn, grew mainly by the accretion of gas from the solar nebula, with only a small core composed of the icy material. The molecular hydrogen (H2) and deuterated hydrogen (HD) in their atmospheres will have remained unchanged to the present day. Measurement of the deuterium-to-hydrogen (D/H) ratio in these planets should therefore give us the deuterium abundance in the gaseous component of the solar nebula.
In contrast, the atmospheres of Uranus and Neptune are thought to contain material which originated from a mixture of gas and icy planetesimals. It is known that this icy material tends to be enriched in deuterium compared with the gas so that deuterium is expected to be more abundant in the atmospheres of Uranus and Neptune than in Jupiter and Saturn. An accurate comparative measurement of D/H in all the giant planets would allow these ideas to be tested quantitatively, and would have important cosmological and cosmogonical implications.
Current estimates of the D/H ratios in Saturn are derived from ground-based spectroscopic observations of deuterated methane (CH3D) at optical wavelengths. These measurements are difficult to make and to interpret, so the uncertainties are large. One of the biggest problems is in translating the deuterium abundance in the form of CH3D to an equivalent in HD; this relies on a somewhat uncertain understanding of the chemical processes involving CH3D, CH4, H2 and HD. The Galileo probe which entered the at mosphere of Jupiter in December 1995 made an in situ measurement of the D/H ratio. The result was a big surprise to astronomers because it was higher than expected and higher than the value measured from CH3D observations.
ISO gives direct access to far-infrared spectral lines of HD at 56 and 112 micrometres, allowing astronomers to measure the quantity of HD directly and to compare the results from all four planets. The LWS team has made preliminary observations of the 56 -micrometres HD transition in the atmosphere of Saturn and has clearly seen the characteristic absorption feature of HD. Its provisional estimate of the D/H ratio is in good agreement with the estimates from CH3D but barely compatible with the lower limit of the Galileo measurement on Jupiter. More detailed observations of all four giant planets will be made soon to measure the HD abundance with greater accuracy. But according to Dr Matt Griffin of Queen Mary and Westfield College, "If the abundances in Jupiter and Saturn turn out to be very different or if they are outside the expected range, it will be a major puzzle for solar system astronomers."
The Long-Wavelength Spectrometer is one of four instruments in the European Space Agency's Infrared Astronomy Satellite ISO. It covers the wavelength range 43 to 198 micrometers at either moderate (around 200) or high (about 10,000) spectral resolution. It was designed by a consortium of scientists and engineers from Canada, France, Italy, Sweden, the UK and the USA. Laboratories in France, Italy and the UK provided hardware and software, funded by national agencies including CNES, CNR and PPARC. Additional funding of individual scientists is provided by these agencies as well as NSERC and NASA.
For more information, contact:
| Dr Matt Griffin | Telephone: +44-(0)171-975-5068 |
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| E-Mail: m.j.griffin@qmw.ac.uk |