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[1] A full dust devil ''season'' was observed from Spirit from 10 March 2005 (sol 421, first active dust devil observed) to 12 December 2005 (sol 691, last dust devil seen); this corresponds to the period L s 173.2°to 339.5°, or the southern spring and summer on Mars. Thermal Emission Spectrometer data suggest a correlation between high surface temperatures and a positive thermal gradient with active dust devils in Gusev and that Spirit landed in the waning stages of a dust devil season as temperatures decreased. 533 active dust devils were observed, enabling new characterizations; they ranged in diameter from 2 to 276 m, with most in the range of 10-20 m in diameter, and occurred from about 0930 to 1630 hours local true solar time (with the maximum forming around 1300 hours) and a peak occurrence in southern late spring (L s $ 250°). Horizontal speeds of the dust devils ranged from <1 to 21 m/s, while vertical wind speeds within the dust devils ranged from 0.2 to 8.8 m/s. These data, when combined with estimates of the dust content within the dust devils, yield dust fluxes of 3.95 Â 10 À9 to 4.59 À4 kg/m 2 /s. Analysis of the dust devil frequency distribution over the inferred dust devil zone within Gusev crater yields $50 active dust devils/km 2 /sol, suggesting a dust loading into the atmosphere of $19 kg/km 2 /sol. This value is less than one tenth the estimates by Cantor et al. (2001) for regional dust storms on Mars.

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[1] The Microscopic Imager (MI) on the Mars Exploration Rover Spirit has returned images of Mars with higher resolution than any previous camera system, allowing detailed petrographic and sedimentological studies of the rocks and soils at the Gusev landing site. Designed to simulate a geologist's hand lens, the MI is mounted on Spirit's instrument arm and can resolve objects 0.1 mm in size or larger. This paper provides an overview of MI operations, data calibration, processing, and analysis of MI data returned during the first 450 sols (Mars days) of the Spirit landed mission. The primary goal of this paper is to facilitate further analyses of MI data by summarizing the methods used to acquire and process the data, the radiometric and geometric accuracy of MI data products, and the availability of archival products. In addition, scientific results of the MI investigation are summarized. MI observations show that poorly sorted soils are common in Gusev crater, although aeolian bedforms have well-sorted coarse sand grains on their surfaces. Abraded surfaces of plains rocks show igneous textures, light-toned veins or fracture-filling minerals, and discrete coatings. The rocks in the Columbia Hills have a wide variety of granular textures, consistent with volcaniclastic or impact origins. Case hardening and submillimeter veins observed in the rocks as well as soil crusts and cemented clods imply episodic subsurface aqueous fluid movement, which has altered multiple geologic units in the Columbia Hills. The MI also monitored Spirit's solar panels and the magnets on the rover's deck.