Noble Gases on Mars

While there are probe data for the atmosphere of Mars, high precision information has also been obtained from Martian meteorites. These have also provided some information about the interior source regions of Martian volcanics. The most recent review is by Swindle (2002, this volume). 

The radiogenic Xe budget Martian atmospheric Xe clearly contains a 

The fissiogenic budget The amount of Xe produced in Mars or accreting materials, assuming that the silicate portion of Mars has 16 ppb of at present (Wanke 

The isotopic composition of the Martian interior is only available for Xe. The nonradiogenic isotope ratios found in Chassigny (a dunite that is possibly mantle-derived) appear to be indistinguishable from solar values (Ott 1988 Mathew and Marti 

and so do not exhibit the strong fractionation seen in the atmosphere. The relative abundances of Xe and Xe are also close to solar, and so there is little scope for radiogenic additions, indicating that this reservoir had a high Xe/Pu ratio, at least during the lifetime of In contrast, this solar Xe appears to be accompanied by a Kr/Xe ratio that is fractionated with respect to the solar value. Data from other meteorites indicate that there are other interior Martian reservoirs that contain solar Xe but with 

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In summary, as a result of the Martian meteorites, we know more about the noble gases on Mars than about those on any planet besides Earth. As our knowledge of Martian noble gases grows, we continue to find results that are unexpected, and, at the moment, inexplicable, providing a valuable check for models that we devise based on our more detailed knowledge of the Earth-Moon system. 

Noble gases are most abundant in planetary atmospheres, although even there they are only minor components. They have been measured in the gas envelopes of Venus, Earth (of course), Mars, and Jupiter. We will consider their utility in understanding planetary differentiation and atmospheric evolution shortly, but first we will focus on their rather miniscule abundances in meteorites and other extraterrestrial materials. 

Figure 7.1 Elemental abundance of noble gases relative to cosmic abundance (Anders Grevesse, 1989). Data for Earth (atmosphere), SW (solar wind implanted on A1 foils on the moon), Lunar (solar wind implanted on lunar soils), Q (chondrites), and Mars are from Table 3.2.

While considerations of the origin of planetary noble gases have been predominantly focused on those presently found in the atmosphere, noble gases still within the Earth provide further constraints about volatile trapping during planet formation. A wide range of noble-gas information for the Earth s mantle has been obtained from mantle-derived materials, and indicates that there are separate reservoirs within the Earth that have distinctive characteristics that were established early in Earth history. These must be included in comprehensive models of Earth volatile history. Also, data are now available for the atmospheres of both Venus and Mars, as well as from the interior of Mars, so that the evolution of Earth volatiles can be considered within the context of terrestrial-planet formation across the solar system. 

The principal difficulty encountered by these mixing models is their inability to account for differences in nonradiogenic noble-gas isotopic distributions between Earth and Mars, and between both of these and solar compositions. Experiments specifically designed to investigate isotopic fractionation in the gas-trapping process showed maximum heavy-isotope enrichments which are too small to explain the observed offsets of martian Ar/ Ar and of xenon on both Mars and Earth from solar ratios (Notesco et al., 1999). Also, models that rely on supply of noble gases from material trapped in the outer parts of the solar system cannot explain the abundances of... 

In the first evolutionary epoch, note that the extent of xenon fractionation from primordial to present composition is similar on both Earth and Mars despite the much smaller mass of Mars, the apparent differences (U-Xe versus SW-Xe) in their precursor xenon, the much greater overall depletion of martian noble gases, and the possibility that escape episodes were powered by... 

Table 1 summarizes the techniques used to measure noble gases. By far the most important is mass spectrometry. Mass spectrometers in space are used, e.g., for solar wind and solar energetic particle measurements or atmospheric analyses on Venus, Moon, Mars and Jupiter, while mass spectrometers in the laboratory allow us to analyze extraterrestrial samples available on Earth, i.e., lunar samples, meteorites, interplanetary dust or solar corpuscular radiation trapped by foils exposed in space. Of course. 

Ozima M, Wider R, Marly B, Podosek FA (1998) Comparative studies of solar, Q-gases and terrestrial noble gases, and implications on the evolntion of the solar nebula. Geochim Cosmochim Acta 62 301-314... 

However, there are several other meteorites, the SNC meteorites, which appear to be close relatives of BETA 79001, so these other meteorites probably come from Mars as well (McSween 1994). Hence, the noble gases within them can be used to study volatile reservoirs and transport processes on Mars. A listing of the Martian meteorites described in the scientific literature as of the time of publication is given in Table 1. McSween has written three valuable reviews of Martian meteorites as the topic has evolved (McSween 1985, 1994, 2002), and the Johnson Space Center has produced a useful compendium of information about them (Meyer 1998). More recently, Treiman et al. (2000) have reviewed the arguments that these meteorites are indeed from Mars, and Volume 96 of Space Science Reviews, stemming from a 2000 workshop on the Chronology and Evolution of Mars, contains several reviews of specific aspects of Martian meteorites, several of which will be referenced below. 

Swindle TD (2001) Could in situ dating work on Mars Lunar Planet Sci XXXII, Abstr 1492 (CD-ROM) Swindle TD (2002) Noble gases in the Moon and meteorites Radiogeiuc components and early volatile chronologies. Rev Mineral Geochem 47 101-124... 

The discussions here are necessarily based on conclusions of more detailed evaluations of available data. More information can be found in this volume in the reviews of terrestrial noble gases (Graham 2002), of models for terrestrial degassing (Porcelli and Ballentine 2002), of solar and planetary characteristics (Wider 2002), of Mars (Swindle 2002), and of meteorites (Ott 2002). For earlier reviews or alternative viewpoints on the origin of planetary volatiles, see, e.g., Lewis and Prinn (1984), Ahrens et al. (1989), Pepin (1991, 1992, 1994, 1997), Jakosky et al. (1994), Hutchins and Jakosky (1996), and Ozima and Podosek (2001). 

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