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TIMESTAMPS
It had long been know from Voyager data and remote sensing that the atmosphere of Jupiterwas primarily hydrogen and helium with trace species such as methane and ammonia. The GPMS and another instrument on the probe (the Helium Abundance Detector) were able to measure the ratio of helium to the major species hydrogen with considerably better precision and accuracy than had previously be achieved. Many other atomic species and molecules, both organic and inorganic, were also detected during the descent of the Probe. Some of these, such as hydrogen sulfide and the heavy noble gases, had never before been detected by remote sensing. The Galileo Probe Mass Spectrometer measured the abundance of many these species as a function of altitude, including regions below the clouds not accessible by remote sensing from the Galileo Orbiter.
This data serves to constrain mechanisms postulated for the formation of Jupiter. The ratio of many elements in our Sun to the major species H is well known. To facilitate comparison of Jupiter and the Sun, two significant reservoirs of solar system material, it is useful to normalize the mixing ratios found at Jupiter to the solar values as is done in the figure. Helium and neon were found to be depleted in the atmosphere of Jupiter. This is believed due to their precipitation as rain-like droplets in the metallic hydrogen that is the state of this major species at the high pressures of Jupiter's interior. Remarkably, the abundance of all the other elements (except for oxygen - discussed below) was found to be present between 2 and 4 times the solar abundance in the deep atmosphere sampled by the GPMS. For a further discussion of the interpretation of these results, see the annotated reference list.
An unexpected result from the GPMS experiment and other measurements from the probe, was that at the point of entry the abundance of these species were highly depleted at a depth where it had been assumed they would condense and at increasing depth form clouds of ammonia ice, ammonium hydrosulfide (NH 4 SH), and then water. Instead of thethick cloud layers expected, instruments on the probe detected only thin clouds - consistend with the depletion of these species as measured by the mass spectrometer. It turns out that the probe by chance had landed in a somewhat unusual region of the atmosphere - a "hot spot" where the relatively clear atmosphere allowed radiation from below to penetrate upward. This soon became evident from infrared and visible images of the entry site. In fact both ammonia and water apparently reached their average atmospheric composition well before the end of the Probe's mission, but water was still depleted even at the deepest point in the atmosphere where measurements were taken (>20 bar). While this was indeed an interesting result, this still leaves theaverage oxygen abundance in the atmosphere of Jupiter yet to be determined by a future mission, possibly one that could penetrate even deeper into the atmosphere using multiple probes to sample regions of different abundance of these condensible species.A detailed discussion of these results can be found in the papers listed in the references at the bottom of this page.
An isotope of an element has the same nuclear charge, but a different atomic mass due to a different number of neutrons in the nucleus. Isotope ratios were measured at Jupiter by the GPMS for the noble gases helium, neon, argon, and xenon and also for nitrogen, carbon, and hydrogen. The later isotopic compositions were obtained from measurements of ammonia, methane, and H 2 respectively. It is particularly interesting to measure isotope ratios at Jupiter because these may provide reference (protosolar) values for the average material from which the solar system bodies formed. Various physical processes acting on the atmospheres of the terrestrial planets will over time change (or fractionate) these isotopic compositions for various elements. Knowledge of the protosolar values provides a reference state for understanding the relative importance of these physical processes such as sputtering losses from the solar wind, thermal escape, and hydrodynamic escape (dragging of heavier species away by lighter species such as hydrogen). A detailed discussion of the GPMS isotope measurements can be found in the papers listed in the references at the bottom of this page.
