Scientific Benefit of a Hypervelocity Mars Atmospheric Sample Capture and Earth Return with the Scim Mission

Introduction: Atmospheric and volatile measurements are crucial to understanding both Mars' climate history and its potential for harboring life. Our present knowledge of the Mars atmosphere comes primarily from the Viking mass spectrometers, gases trapped in the martian meteorites, and from telescopic observations. The Viking landers measured the chemical composition of all major atmospheric species [1,2], but gave only high-uncertainty results on a few isotopic compositions. Thus, most of what we believe about the composition of the martian atmosphere is inferred from the martian meteorites. Gases contained in several of these meteorites are thought to be unfractionated martian atmosphere trapped by shock implantation during ejection from Mars. However, the derivation of the Mars atmospheric composition is not straightforward, as the meteorites also appear to have retained magmatic gases, and contain cosmic-ray spallation products from their journey through space. Additionally, the martian atmosphere found in these meteorites was trapped some time in the past. An important MEPAG goal is the high precision measurement of the present-day atmosphere. The Sample Collection for Investigation of Mars (SCIM) mission, presently in a competed Phase A study, proposes to return 1 liter STP of the Mars atmosphere to Earth for isotopic study. Here we discuss the great importance of obtaining such a sample for analysis in terrestrial laboratories. A companion paper discusses the SCIM dust sample return science [3]. and hydrogen isotopic compositions from gaseous and lithic sources can constrain the long-sought total primordial volume of water on Mars, and is key to understanding the surface-atmosphere interactions, and their relation to atmospheric escape. For example, most atmospheric isotope ratios strongly support the importance of atmospheric escape through time by recording heavy isotope enrichments due to preferential loss of the lighter isotopes from the atmosphere [e.g., 4]. However, the oxygen isotope ratio was found by Viking to be terrestrial (a result supported by martian meteorite analyses of lithic and hydrous phases), though with a Viking analysis precision of only ±10%. The O isotope ratio can be determined in terrestrial laboratories to better than ±0.02%, a factor of 500 improvement over Viking. A terrestrial-like O isotopic composition for the martian atmosphere would imply that oxygen in the atmosphere is buffered by a large non-atmospheric reservoir such as water or CO 2 in the polar caps or crust, or by crustal silicates, perhaps through the action of hydrothermal systems [5]. Unraveling the