wind inputs will allow a determination of how these processes operated through time and how important loss to space has been over time. These measurements will require global-scale observations that can be made through a combination of in situ and remote-sensing approaches from orbit. They would address the current inadequate information in this area today.

If Mars has active geological processes that involve gas exchange between the surface and the subsurface (such as outgassing associated with volcanism or gas exchange between the atmosphere and a deep aquifer), there should be evidence of this in atmospheric trace gases. There also is likely to be trace-gas evidence if Mars has a large, active surface or subsurface biosphere. Methane has been discussed in this context recently, due to its reported detection by three different groups. However, atmospheric methane can result from either of these sets of processes. Distinguishing between the two would require measurements of other trace gases (e.g., formaldehyde) that would be produced by only one of the processes. Thus, global-scale measurements of atmospheric gases, with sufficiently high precision and accuracy to allow detection and characterization of trace gases, would be a valuable astrobiology objective. There also is the possibility that observations can be made in a nadir-pointing mode—the preferred approach when trying to identify a localized source—that would provide high-spatial-resolution mapping of trace gases, which could in turn allow identification of a localized source of trace gases and their potential temporal variability.

Other regional- and global-scale measurements would contribute to astrobiological science goals. These might include high-resolution spectroscopic mapping that would allow identification of mineralogy associated with water- or volcanic-related geological features, geophysical measurements that would determine the thermal history and subsurface structure, and mapping of polar deposits at all wavelengths that would provide information on their history and the potential occurrence of liquid water.

GEOLOGICAL AND ENVIRONMENTAL CONTEXT AT LOCAL SITES

The plausibility of life existing at a given landing site depends on the location having a habitable environment. The recognition and characterization of present environments are straightforward. If such an environment existed in the past, then the geological history of that site must have allowed preservation of an environmental record and of traces of organisms that populated it. Past environments and historical geology can be retrieved from rocks using in situ instruments, as demonstrated by rovers such as Mars Pathfinder’s Sojourner and the Mars Exploration Rovers, Spirit and Opportunity.1,2 These missions clearly demonstrate that the keys to unraveling geological and environmental context are mobility and a complementary suite of instruments for observations and measurements.

The geologically active surface of Earth constitutes a major hurdle for recognizing the remains of its ancient life. Surface rocks on Mars, as far as we know, have not experienced the thermal (metamorphic) and deformational (tectonic) events that so commonly obscure the record of life in terrestrial Precambrian rocks. However, several processes can potentially complicate astrobiological studies of Mars rocks:

  • Volcanism. Much of the planet’s surface is covered with lava flows that would destroy any surface or near-surface organisms. The limited number of impact craters on some terrains and the young ages of many martian meteorites indicate that volcanic activity has continued throughout Mars’s history. Understanding the timing of volcanism at a local site, relative to the age of any putative life forms, is critical to any hypothesis for martian life.

  • Shock metamorphism. Meteor impacts transmit large shock pressures to target rocks, transforming minerals, pulverizing rocks, and sometimes melting them. Shock metamorphism is pervasive in martian meteorites, and the high density of craters in ancient Noachian terrains (arguably the most intriguing sites for life) argues that most of these rocks have experienced shock. Cratering also excavates materials, thus disrupting the outcrop stratigraphy that is so useful in reconstructing geological history and environments.

  • Weathering. The spectral identification of readily weathered igneous minerals (olivine and pyroxene) at regional and local scales suggests that weathering processes on Mars might be dominated by physical rather than chemical weathering. However, alteration processes in soils and alteration rinds on rocks reveal chemical dissolution reactions that could potentially obscure evidence for life.3 In that case, mechanisms for accessing fresh rock



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