Pancam and Microscopic Imager observations of dust on the Spirit Rover: Cleaning events, spectral properties, and aggregates

Abstract

Approach: The Mars Exploration Rover Spirit has experienced over 2000 martian days (sols) of interaction w ith the martian dust c ycle. Much d ust accumulated o n the vehicle, but there have also been t hree major "cleaning e vents" when s ignificant a mounts o f d ust w ere removed. Pa ncam d ust monitoring observations acquired every ~10 sols throughout the mission allowed visual inspection of the amount of dust collecting on a portion of the solar array and on the Pancam and Mini-TES calibration targets. Pancam multispectral d ata reveal three classes of airborne d ust adhering to the capture and filter magnets: 1) "bright" capture magnet dust 2) "dark" capture magnet dust, and 3) filter magnet dust. Microscopic Imager (MI) images of the capture and filter magnets and a portion of the solar array aid in interpretation of behavior and morphology of the dust deposits. Results: Three s ignificant periods of "cleaning e vents" removed s ubstantial a mounts o f d ust from the rover. Each event occurred during southern hemisphere early spring and summer, and occurred later i n the s eason of each s uccessive M artian y ear. Pancam m ultispectral d ata o f "bright" capture magnet d ust exhibit the h ighest r eflectance l evels of t he t hree cl asses. T hese sp ectra a re a lso characterized by steep visible spectral slopes (434 to 673 nm), strong 535 nm absorptions, and are relatively featureless and c onvex in shape i n the NIR region. "Dark" capture magnet dust has the lowest reflectance levels of the three classes, much shallower visible slopes and are much less "red" than the bright capture dust. They also have shallower 535 nm absorptions and are featureless in the NIR. They exhibit positive slopes from 900 to 1000 nm. Filter magnet dust shares spectral characteristics with the "dark" capture magnet dust, and together they represent a darker, more strongly magnetic component. Both Pancam and MI images reveal that the dust particles on the rover b ody cohere an d f orm larger ag gregates, measured at 100 µm to several m m. MI i mages of the rover magnets and body show that grains collected range from assumed (unresolved) dust size (< 4 µm) to fine sand (250 µm), and show that sand can saltate to a rover deck height of 70 cm in strong w inds. T hrough c omparisons with o ther d ata s ets and previous work, w e think w ind speeds exceeding 20 m/s are responsible for the removal of dust during the cleaning events. Implications: The spectral characteristics of the "bright" capture dust described here are consistent with multispectral observations of dust and bright soil elsewhere on the martian surface and support the theory that martian dust is globally mixed and deposited. This expanded data set also provides further e vidence that the d ust has a d arker, more s trongly magnetic component than typical b right dust. The large size of observed dust aggregates (≥ 100 µm) should make them easier to entrain by wind than individual dust grains, and therefore represent another process by which dust is lifted into the at...

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