boundary constraint on the current climate. The NS and HEND are both sensitive to near-surface H and are being used to map ground ice at high latitudes and bound water or ice at lower latitudes.

THEMIS is mapping surface composition using infrared multispectral imaging. This is able to identify surface mineralogy, which provides strong constraints on geological processes and, in particular, on places where aqueous processes have been relevant. In the visible mode, it is imaging morphological features that constrain the geological history, which is relevant to volcanism and tectonism, as well as water-related processes. In both modes, it is mapping the polar caps, which again provides constraints on the present climate and on how to extrapolate it to other epochs. THEMIS also is mapping surface physical properties using temperature measurements and, with both infrared and visible imaging, is being used to help understand potential landing sites for the Phoenix and Mars Science Laboratory missions.

Mars Express

Mars Express is the European Space Agency’s first Mars orbiter. Its instrument complement includes a high-resolution stereo camera, a visible/infrared mineralogical mapping spectrometer, a subsurface sounding radar altimeter, Fourier and ultraviolet/infrared spectrometers for atmospheric studies, and an energetic neutral-atoms analyzer for studying the properties of the upper atmosphere.

The mineralogical mapping spectrometer—known as its French acronym, OMEGA—is able to map surface composition using reflectance spectroscopy by identifying characteristic absorption features. In its early mission phases, it was able to identify sulfate minerals on the surface that are strong indicators of aqueous geochemical processes, especially as constrained by the in situ measurements of the rover Opportunity.

The instruments designed to study the upper atmosphere are able to measure ion abundance. It has thus been possible to demonstrate that the martian upper atmosphere is being lost at present, although the net loss rate and the relationship to upper-atmospheric and solar-input processes have not been measured.

The radar sounding experiment uses long-wavelength radar that can penetrate as much as kilometers below the surface. It is able to identify subsurface structure that is related to the layering in the polar caps (Figure 6.2) and in sediments associated with impact basins.

The high-resolution stereo camera can be used to map morphology that tells researchers about ongoing geological processes. Of special interest has been the recent history of aqueous processes and of volcanism.

Mars Reconnaissance Orbiter

The Mars Reconnaissance Orbiter (MRO) is just beginning its primary science mission at this writing, and only a few preliminary results have been obtained to date. The instruments on board are a high-resolution camera (HiRISE), a high-resolution imaging spectrometer (CRISM), and a shorter-wavelength radar that can provide higher-vertical-resolution information on subsurface structure and on possible subsurface liquid water. Each of these measurements will be making significant contributions to landing-site selection for Phoenix and MSL, and each provides information that is of high value to astrobiology science objectives.

Mars Exploration Rovers

The rovers, Spirit and Opportunity, have been operating for more than 3 years and have returned a tremendous wealth of science data. Both rovers carry a panoramic imaging system (Pancam), a miniature thermal-emission spectrometer (Mini-TES), a Mossbauer spectrometer, an alpha particle x-ray spectrometer (APXS), a microscopic imager, and a rock abrasion tool (RAT). This instrument package has proved capable of characterizing astrobiologically relevant materials at landing sites selected for their high potential for having had liquid water—i.e., evaporates at Meridiani Planum and aqueously altered basaltic rocks at the Gusev crater. Not only have the rovers’ instruments returned a wealth of pertinent data confirming aqueous processes at both landing sites, but both spacecraft also continue to explore the surrounding terrain and are expected to remain operational for the foreseeable future.

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