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This paper provides an overview of Mars Exploration Rover SpiritMicroscopic Imager (MI) operations and the calibration, processing, and analysis of MI data. The focus of this overview is on the last five Earth years (2005–2010) of Spirit's mission in Gusev crater, supplementing the previous overview of the first 450 sols of the SpiritMI investigation. Updates to radiometric calibration using in‐flight data and improvements in high‐level processing are summarized. Released data products are described, and a table of MI observations, including target/feature names and associated data sets, is appended. The MI observed natural and disturbed exposures of rocks and soils as well as magnets and other rover hardware. These hand‐lens‐scale observations have provided key constraints on interpretations of the formation and geologic history of features, rocks, and soils examined by Spirit. MI images complement observations by other Spiritinstruments, and together show that impact and volcanic processes have dominated the origin and evolution of the rocks in Gusev crater, with aqueous activity indicated by the presence of silica‐rich rocks and sulfate‐rich soils. The textures of some of the silica‐rich rocks are similar to terrestrial hot spring deposits, and observations of subsurface cemented layers indicate recent aqueous mobilization of sulfates in places. Wind action has recently modified soils and abraded many of the rocks imaged by the MI, as observed at other Mars landing sites. The Microscopic Imager (MI) on NASA's Spiritrover returned the highest‐resolution images of the Martian surface available at the time of the 2004–2010 mission. Designed to survive 90 Mars days (sols) and search for evidence of water in the past, Spiritreturned data for 2210 sols, far exceeding all expectations. This paper summarizes the scientific insights gleaned from the thousands of MI images acquired during the last 5 years of the mission, supplementing the summary of the first 450 sols of the SpiritMI investigation published previously (Herkenhoff et al., [Herkenhoff, K. E., 2006]). Along with data from the other instruments on Spirit, MI images guided the scientific interpretation of the geologic history of the rocks and soils observed in Gusev crater on Mars. We conclude that the geologic history of the area explored by Spirithas been dominated by impacts and volcanism, and that water, perhaps very hot water, was involved in the evolution of some of the rocks and soils. More recently, winds have moved soil particles and abraded rocks, as observed elsewhere on Mars. These results have improved our understanding of Mars' history and informed planning of future missions to Mars. During the Mars Exploration Rover Spiritmission, the Microscopic Imager returned 5,923 full‐frame imagesThese images have guided interpretations of the geologic history of Gusev crater on MarsA table summarizing the MI images chronologically is appended, including target/feature names and sequence identifiers for Pancam images that provide context

We compare the thermophysical properties and particle sizes derived from the Mars Science Laboratory rover's Ground Temperature Sensor of the Bagnold dunes, specifically Namib dune, to those derived orbitally from Thermal Emission Imaging System, ultimately linking these measurements to ground truth particle sizes determined from Mars Hand Lens Imager images. In general, we find that all three datasets report consistent particle sizes for the Bagnold dunes (~110–350 μm and are within measurement and model uncertainties), indicating that particle sizes of homogeneous materials inferred from temperature measurements and thermophysical models are reliable. Furthermore, we examine the effects of two physical characteristics that could influence the modeled thermal inertia and particle sizes, including (1) fine‐scale (centimeter to meter scale) ripples and (2) thin layering of indurated/armored materials. To first order, we find that small‐scale ripples and thin (approximately centimeter scale) layers do not significantly affect the determination of bulk thermal inertia from orbital thermal data using a single nighttime temperature. Modeling of a layer of coarse or indurated material reveals that a thin layer (< ~5 mm; similar to what was observed by the Curiosity rover) would not significantly change the observed thermal properties of the surface and would be dominated by the properties of the underlying material. Thermal inertia and particle sizes of relatively homogeneous materials derived from nighttime orbital data should be considered as reliable, as long as there are no significant subpixel anisothermality effects (e.g., lateral mixing of multiple thermophysically distinct materials). The Mars Science Laboratory Curiosity rover spent approximately 20 Martian days interrogating an active sand dune field informally named the “Bagnold” dunes. The suite of measurements made by Curiosity provide a unique opportunity to link orbital data to ground truth. In this specific instance, we investigate if particle sizes derived from orbital data are reliable. Using a numerical model that describes how surface temperature varies as a function of a variety of input parameters, including observation geometry (e.g., season and time of day) and surface properties (e.g., reflectance, slope, azimuth, and elevation), we can model the response of the surface to various particle sizes. By using Curiosity's Ground Temperature Sensor as the link between fine‐scale Mars Hand Lens Imager ground truth data and the Thermal Emission Imaging System's orbital perspective, we conclude that indeed particle sizes determined from orbital observations are reliable. Furthermore, we find that relatively thin armoring lags of coarser particles and dune ripples do not dramatically affect these orbitally determined particle sizes. Thermally derived particle sizes of the Bagnold dunes from orbit and landed assets are consistent with direct particle size measurementsThermally derived particle sizes are not dramatically affected by surface ripples or thin layers of induration/armoringSubpixel mixing of sand with nearby materials likely resulted in overestimated particle sizes in previous orbital measurements

We present two photoclinometric profiles across a trough in the martian northern polar layered terrain. Complications caused by albedo variations were avoided by using an early springtime Viking image with a thin cover of seasonal CO2 frost. The topographic profiles were constrained with stereogrammetric elevations derived from summertime Viking images of the same region.We find that the photoclinometric profiles are consistent with a nearby MOLA (Mars Orbiter Laser Altimeter) track crossing the same polar trough. The trough is asymmetric, with higher relief and a steeper slope on the equatorward-facing wall. Individual layers are subdued and difficult to observe in the profiles. A decrease in both relief and elevation toward the eastern end of the trough suggests that layers become thinner to the east. Declining equatorward slopes in the eastern portion of the trough imply that erosion rates have varied along the trough. The variation in erosion rate may be linked to the change in layer thickness along the trough.Layers have an average thickness of 19±8 m in the center of the trough and 59±32 m on the northern wall. The northern wall is most likely composed of thinner layers that are obscured. To first order, we find that a 19-m layer requires 16,000 years of deposition to form. Although this timescale does not coincide with orbital variation periods of 105 and 106 years, deposition rates may not be constant and thus the 16,000-year layer formation time does not preclude layer formation during part of each orbital oscillation.

As the planet's principal cold traps, the martian polar regions have accumulated extensive mantles of ice and dust that cover individual areas of ∼106 km2 and total as much as 3–4 km thick. From the scarcity of superposed craters on their surface, these layered deposits are thought to be comparatively young—preserving a record of the seasonal and climatic cycling of atmospheric CO2, H2O, and dust over the past ∼105–108 years. For this reason, the martian polar deposits may serve as a Rosetta Stone for understanding the geologic and climatic history of the planet—documenting variations in insolation (due to quasiperiodic oscillations in the planet's obliquity and orbital elements), volatile mass balance, atmospheric composition, dust storm activity, volcanic eruptions, large impacts, catastrophic floods, solar luminosity, supernovae, and perhaps even a record of microbial life. Beyond their scientific value, the polar regions may soon prove important for another reason—providing a valuable and accessible reservoir of water to support the long-term human exploration of Mars. In this paper we assess the current state of Mars polar research, identify the key questions that motivate the exploration of the polar regions, discuss the extent to which current missions will address these questions, and speculate about what additional capabilities and investigations may be required to address the issues that remain outstanding.

Interpretation of the polar stratigraphy of Mars in terms of global climate changes is complicated by the significant difference in surface ages between the north and south polar layered terrains inferred from crater statistics. We have reassessed the cratering record in both polar regions using Viking Orbiter and Mariner 9 images. No craters have been found in the north polar layered terrain, but the surface of most of the south polar layered deposits appears to have been stable for many of the orbital/axial cycles that are thought to have induced global climate changes on Mars. The inferred surface age of the south polar layered deposits (about 10 Ma) is two orders of magnitude greater than the surface age of the north polar layered deposits and residual cap (at most 100 ka). Similarly, modeled resurfacing rates are at least 20 times greater in the north than in the south. These results are consistent with the hypotheses that polar layered deposit resurfacing rates are highest in areas covered by perennial ice and that the differences in polar resurfacing rates result from the 6.4 km difference in elevation between the polar regions. Deposition on the portion of the south polar layered deposits that is not covered by the perennial ice cap may have ceased about 5 million years ago when the obliquity of Mars no longer exceeded 40°.