Thematic Articles

Evidence of water on Mars dates back to the first observations of channeled landscapes made by Mariner 9 and Viking. More recent images from Mars Global Surveyor and Mars Express strikingly confirm that fluids have sculpted the Martian surface at least episodically through its history. The Mars Exploration Rovers Opportunity and Spirit have added evidence for extensive rock–water chemical interactions in the regions where these remote geologists landed, while OMEGA and THEMIS have shown that similar processes took place in many parts of the planet. Because of the close relationship between water and biological activity on Earth, such observations have been taken as hopeful signs that Mars, as well, might once have supported life and, indeed, might still do so in subterranean oases. There is, however, much more to consider. Water appears to be neces- sary for life, at least as it exists on Earth and can be contemplated on Mars, but it does not, by itself, insure habitability. In this paper, we review the broader requirements for biological activity as they relate to water and use these to constrain astrobiological inferences about Mars.

The Mars Exploration Rover Opportunity has examined sedimentary structures in the Burns formation at Meridiani Planum. The materials in this formation reflect, in part, subaqueous deposition of reworked, sulfate-rich, clastic sediments that likely formed in a playa–interdune setting. The chemistry and mineralogy of the sedimentary rocks record an origin by evaporation of sulfate- and chloride-rich brines mixed with a fine, altered, basaltic mud or dust component, prior to reworking. Cementation and postde- positional reactions to form hematite-rich concretions and crystal-mold porosity reflect diagenesis in a groundwater-saturated subsurface. More recent dehy- dration events are evidenced by polygonal textures in rocks within craters and exposed on the plains. The timing of formation of fracture fillings that cut across bedding is not well constrained and may be early postdiagenetic or later. The fracture fillings may have formed by solutions remobilized along zones of weakness. Alteration rinds may reflect more recent interactions between rock and atmospheric water vapor.

Martian meteorites have delivered to Earth water molecules and minerals produced by aqueous processes on Mars. The study of these samples, using powerful analytical instrumentation, provides a basis for understanding aqueous activity on Mars. Although most analyses are at the scale of microns, the conclusions reached have important implications for large-scale aqueous processes. Secondary minerals, such as clays, salts, and carbonates, are present at some level in all Martian meteorite subtypes and are especially important in the nakhlites and ALH84001. Light element stable isotope analyses show evidence for mixing between atmosphere and magmatic fluids into a crustal reservoir, and that this crustal water was not in equilib- rium with the host rocks. The mineralogical and isotopic data present a fairly consistent picture of the aqueous history of Mars: low levels of aqueous alteration are generally present but extensive aqueous processes are probably limited in space and time.

The poles and mid-latitudes of Mars contain abundant water in ice caps, thick sequences of ice-rich layers, and mantles of snow. The volume of the known reservoir is ≥≥5 x 106 km3, corresponding to a layer ~35 m thick over the planet. Hydrogen in subsurface H2O ice has been detected at latitudes poleward of 50°. Morphological features show downslope flow of ice-rich sediment, and recent gullies have been produced from subsurface aquifers or melting snowpacks. Variations in Mars’ orbit on timescales of 50,000 to 2,000,000 years produce significant changes in climate, which result in the transport of water from the poles, where it currently resides, to the lower latitudes, where it may play a critical role in surface geology, mineralogy, and geochemistry.

The Martian surface is dominated by primary igneous minerals common in basaltic rocks. Limited chemical alteration exists in fine-grained dust, and is likely in sands and rocks at high latitudes and in the northern lowland plains where materials have interacted with ice and snow. Evidence for extensive production of secondary phases is revealed at higher spatial resolutions, where alteration effects of unique, and perhaps time-limited, aqueous environments are observed. The distribution of ice on Mars thus appears to have a global influence on the production of alteration materials, whereas the effects of water are discovered in unique and locally diverse geological settings.

Martian landscapes and landforms indicate episodic activity by water and ice, extending from the planet’s earliest history up to the present day. Most of the relevant fluvial, glacial, volcano–ice, periglacial, lacustrine (even “marine”), and related landforms have direct counterparts on Earth. Moreover, they exist in causally related, holistic associations of space and time that confirm their relationship to a long-term history of water-related activity. Although strong geomorphological evidence for many of these relation- ships has been apparent for 30 years, its scientific importance has only been recently appreciated because of direct geochemical measurements of water and ice features by surface robotic and orbital instruments.