A simple, idealized model for the rapid escape of a hydrogen thermosphere provides some quantitative estimates for the energy-limited flux of escaping particles. The model assumes that the atmosphere is "tightly bound" by the gravitational field at lower altitudes, that diffusion through the lower atmosphere does not limit the flux, and that the main source of heating is solar euv. Rather low thermospheric temperatures are typical of such escape and a characteristic minimum develops in the temperature profile as the escape flux approaches its maximum possible value. The flux is limited by the amount of euv energy absorbed, which is in turn controlled by the radial extent of the thermosphere. Regardless of the amount of hydrogen in the thermosphere, the low temperatures accompanying rapid escape limit its extent, and thus constrain the flux. Applied to the Earth and Venus, the results suggest that the escape of hydrogen from these planets would have been energy-limited if their primordial atmospheres contained total hydrogen mixing ratios exceeding a few percent. Hydrogen and deuterium may have been lost in bulk, but heavier elements would have remained in the atmosphere. These results place constraints on hypotheses for the origin of the planets and their subsequent evolution.

The four giant planets in the Solar System have abundances of 'metals' (elements heavier than helium), relative to hydrogen, that are much higher than observed in the Sun. In order to explain this, all models for the formation of these planets rely on an influx of solid planetesimals. It is generally assumed that these planetesimals were similar, if not identical, to the comets from the Oort cloud that we see today. Comets that formed in the region of the giant planets should not have contained much neon, argon and nitrogen, because the temperatures were too high for these volatile gases to be trapped effectively in ice. This means that the abundances of those elements on the giant planets should be approximately solar. Here we show that argon, krypton and xenon in Jupiter's atmosphere are enriched to the same extent as the other heavy elements, which suggests that the planetesimals carrying these elements must have formed at temperatures lower than predicted by present models of giant-planet formation.

Abstract. The Galileo probe mass spectrometer determined the composition of the Jovian atmosphere for species with masses between 2 and 150 amu from 0.5 to 21.1 bars. This paper presents the results of analysis of some of the constituents detected: H2, He, Ne, Ar, Kr, Xe, CH4, NH3, H20 , H2S , C 2 and C3 nonmethane hydrocarbons, and possibly PH 3 and C1.4He/H2 in the Jovian atmosphere was measured to be 0.157 _+ 0.030. 13C/12C was found to be 0.0108 +_ 0.0005, and D/H and 3He/4He were measured. Ne was depleted, -<0.13 times solar, Ar -<1.7 solar, Kr -<5 solar, and Xe -<5 solar. CH 4 has a constant mixing ratio of (2.1 _+ 0.4) x 10 -3 (•2C, 2.9 solar), where the mixing ratio is relative to H 2. Upper limits to the H20 mixing ratio rose from 8 x 10 -7 at pressures <3.8 bars to (5.6 _+ 2.5) x 10 -5 (•60, 0.033 _+ 0.015 solar) at 11.7 bars and, provisionally, about an order of magnitude larger at 18.7 bars. The mixing ratio of H2S was <10 -6 at pressures less than 3.8 bars but rose from about 0.7 x 10 -5 at 8.7 bars to about 7.7 x 10 -5 (328, 2.5 solar) above 15 bars. Only very large upper limits to the NH 3 mixing ratio have been set at present. If PH 3 and CI were present, their mixing ratios also increased with pressure. Species were detected at mass peaks appropriate for C2 and C3 hydrocarbons. It is not yet clear which of these were atmospheric constituents and which were instrumentally generated. These measurements imply (1) fractionation of 4He, (2) a local, altitudedependent depletion of condensables, probably because the probe entered the descending arm of a circulation cell, (3) that icy planetesimals made significant contributions to the volatile inventory, and (4) a moderate decrease in D/H but no detectable change in (D + 3He)/H in this part of the galaxy during the past 4.6 Gyr. IntroductionThe Galileo probe mass spectrometer (GPMS) obtained useful data concerning the composition of the Jovian atmosphere along the probe trajectory between pressure levels of 0.51 and 21.1 bars. Among species detected were H 2 and HD, 3He and 4He; the isotopes of the noble gases Ne, Ar, Kr, and Xe; the volatiles CH4, NH 3, H20 , H2S; a chlorine compound which may have been HC1, and a large number of C2 and C3 nonmethane hydrocarbons (NMHCs). Mixing ratios, or data capable of being translated into mixing ratios, have been obtained from the mass spectra acquired by direct sampling of the atmosphere and information provided by two enrichment cells for numerous species. Striking aspects of the abundance profiles were (1) the very low mixing ratios of condensable volatiles such as H2S and H20 at pressures <8 bars and their gradual increase at higher pressures along the probe trajectory, InstrumentThe GPMS has been described in detail [Niemann et al., 1992]. A quadrupole mass filter scanning in integral mass steps from 2 to 150 atomic mass units (amu) provided mass analysis. A one half-second integration time was allotted to each mass step. A secondary electron multiplier detected the ions transmitted by the mass filter. The ...

The deuterium-hydrogen abundance ratio in the Venus atmosphere was measured while the inlets to the Pioneer Venus large probe mass spectrometer were coated with sulfuric acid from Venus' clouds. The ratio is (1.6 +/- 0.2) x 10(-2). The hundredfold enrichment of deuterium means that at least 0.3 percent of a terrestrial ocean was outgassed on Venus, but is consistent with a much greater production.

Results from the occultation of the sun by Neptune imply a temperature of 750 +/- 150 kelvins in the upper levels of the atmosphere (composed mostly of atomic and molecular hydrogen) and define the distributions of methane, acetylene, and ethane at lower levels. The ultraviolet spectrum of the sunlit atmosphere of Neptune resembles the spectra of the Jupiter, Saturn, and Uranus atmospheres in that it is dominated by the emissions of H Lyman alpha (340 +/- 20 rayleighs) and molecular hydrogen. The extreme ultraviolet emissions in the range from 800 to 1100 angstroms at the four planets visited by Voyager scale approximately as the inverse square of their heliocentric distances. Weak auroral emissions have been tentatively identified on the night side of Neptune. Airglow and occultation observations of Triton's atmosphere show that it is composed mainly of molecular nitrogen, with a trace of methane near the surface. The temperature of Triton's upper atmosphere is 95 +/- 5 kelvins, and the surface pressure is roughly 14 microbars.