[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.

[1] Wind-related features observed by the rover Spirit in Gusev crater, Mars, include patches of soil on the surface, some of which are organized into bed forms. Windblown grains include dust (inferred to be <3 mm in diameter), sands (up to a few hundred mm in diameter), and granules (>2 mm in diameter). Microscopic Imager data show the sands and granules to be rounded and relatively spherical, typical of grains transported long distances by the wind. The interior of bed forms exposed by rover operations suggests the infiltration of dust among the grains, indicating that these sands are not currently experiencing saltation. Orientations of 1520 features (such as bed forms and ventifacts) along Spirit's traverse from the landing site (the Columbia Memorial Station) to West Spur in the Columbia Hills suggest primary formative winds from the north-northwest, which correlate with measurements of features seen in orbiter images and is consistent with afternoon winds predicted by atmospheric models. A secondary wind from the southeast is also suggested, which correlates with predictions for nighttime/early morning winds. Wind abrasion is indicated by ventifacts in the form of facets and grooves cut into rocks, the orientations of which also indicate prevailing winds from the north-northwest. Orientations of many aeolian features in the West Spur area, however, have more scatter than elsewhere along the traverse, which is attributed to the influence of local topography on the patterns of wind. Active dust devils observed on the floor of Gusev from the Columbia Hills demonstrate that dust is currently mobile. Sequential images of some dust devils show movement as rapid as 3.8 m/s, consistent with wind velocities predicted by atmospheric models for the afternoon, when most of the dust devils were observed. Sands accumulated on the rover deck in the same period suggest that some sands in the Columbia Hills experience active saltation. ''Two-toned'' rocks having a light band coating at their bases are considered to represent partial burial by soils and subsequent exposure, while ''perched'' rocks could represent materials lowered onto other rocks by deflation of supporting soils. Measurements of the heights of the light bands and the perched rocks range from <1 cm to 27 cm, indicating local deflation by as much as 27 cm.

[1] Spirit began operations in Gusev Crater in January 2004 and has returned data on three seasons of dust devil (DD) activity. Total DDs observed were 533 in season one, 101 in season two, and 127 in season three. Their general characteristics are the same within factors of 2 among the seasons, with median diameters of 19 m in season one, 24 m in season two, and 39 m in season three, and dust flux values for individual vortices ranging from 4.0 × 10 −9 to 4.6 × 10 −4 kg m −2 s −1 in season one, 5.2 × 10 −7 to 6.2 × 10 −5 kg m −2 s −1 in season two, and 1.5 × 10 −7 to 1.6 × 10 −4 kg m −2 s −1 in season three. All three seasons were initiated with the onset of southern Martian spring within 14 sols of the same L s (181°) and their frequency increased to the period corresponding to late southern spring. The occurrences decreased monotonically in seasons one and three but apparently ended abruptly in season two when a large dust storm occurred; although the dusty atmosphere might have precluded the detection of active DDs, the abrupt cessation could result from conditions such as thermal stability of the atmosphere due to the presence of dust which could halt DD formation. Dust devils can contribute significant quantities of dust to the atmosphere, although it is unclear as to whether this dust stays locally or is injected into higher-altitude winds and is distributed elsewhere. In the three DD seasons observed through Spirit, DDs in Gusev Crater injected a minimum average of ∼18 × 10 6 kg of material into the atmosphere each season.

[1] Linear, low-albedo patterns (termed dark wind streaks) formed on the floor of Gusev crater between September 2003 and February 2004, as seen on High Resolution Stereo Camera images taken on board the Mars Express Orbiter. Pancam images from the Mars Exploration Rover Spirit show that the rover crossed a dark streak during its traverse to Bonneville crater. Spirit Microscopic Imager data reveal that sand grains within the dark wind streak are relatively free of dust, whereas grains outside the streak are mantled with dust. During the September 2004 solar conjunction, Spirit remained in one location from sol 240 to sol 260. Comparison of images taken before and after the conjunction shows that patches of soil beneath the rover darkened with respect to the adjacent soils, suggesting removal of relatively bright material. Two MI mosaics taken 18 sols apart of the surface within 0.5 m of the nearest dark patch show that some larger (1-2 mm) sand grains moved as far as 0.7 mm. These observations support the hypothesis that some dark surface patterns result from the removal and/or repositioning of fine-grained material by winds, exposing a relatively lower albedo substrate, such as coarse sand grains. Other variable features on the Gusev floor seen from orbit faded between September 2003 and February 2004 and are interpreted to represent settling of dust from the atmosphere, consistent with the accumulation of dust observed on Spirit. The observation of dark streaks fading with time, while some dark streaks were newly formed, is consistent with local wind gusts or the passage of dust devils that locally sweep dust from the surface or cause a redistribution of fine grains among larger particles.