The Martian atmosphere contains a significant amount of suspended dust. Dust is known to settle out of the atmosphere onto exposed surfaces, causing surfaces to slowly turn a dusty yellow-orange. More disturbingly, as the dust settles, it coats the surface of solar arrays and reduces the amount of power produced on Mars.
Up until now, the obscuration of the solar arrays by dust has been seen by the change in color of the arrays, and the effect has been measured from the current output of the arrays, but the dust and sand deposited on the arrays have not been examined directly. In this work, we used the Microscopic Imager (MI) to examine the surface of the solar array on the Mars Exploration Rovers (MERs) Spirit and Opportunity.
The solar cells on the MERs are mounted on panels approximately 0.8 m above the ground. The MI is an instrument on the extendable Instrument Deployment Device on the rover, mounted below the deck and the solar arrays. This was not designed to inspect the solar arrays, but ground testing on the engineering model showed that it had sufficient freedom of motion to reach portions of three solar cells on the front edge of the rover deck. During the extended mission, images of these cells at MI resolution were made on both the Spirit and Opportunity solar arrays.
Enlarged portion of a frame from the microscopic imager view of the dust-covered solar array on Spiriton sol 350, with an arrow showing one of the dark marks on the panel. Light-colored diagonal lines are the metallization pattern on the surface of the solar cell.
The preceding photograph shows an enlargement of a portion of one of the frames, taken after 350 Martian days (sols) on the surface of Mars. Dust-related features are evident. Before launch, the solar arrays were specular glass surfaces, covering a solar cell featuring parallel lines of metallization spaced at 0.625 mm on a dark cell surface. (These metallization lines are useful to show scale on the images.) After 11 months of exposure, the surface reflection is markedly diffuse in both the Panoramic camera (Pancam) and MI views. No clearly evident structure can be seen at MI resolution, with no visible aggregation or “fairy castle” structure. The dust coverage appears to be roughly uniform across the cells.
The size of particles suspended in the atmosphere at the MER sites has been estimated from optical scattering measurements to be approximately 1.5 μm. Since the MI has a resolution of 31 μm/pixel, it was not expected that any features would be visible at the resolution of the MI. Surprisingly, however, a distinct granularity is visible in the MI images. The following photograph shows an enlargement of one frame, with the contrast greatly enhanced to show detail. The fact that such details are visible is strong evidence that the deposits on the surface are significantly larger than the particles measured in the atmosphere by optical scattering. This could be either distinct larger particles, or conglomerate particles formed by agglomeration of micrometer-scale particles.
Enlarged MI view of the Spirit solar array surface, with the contrast greatly enhanced to show features.
An additional feature of interest is the presence of dark streak marks on the surface, such as the one indicated with a circle in the first photograph. These marks vary in width from a few pixels for the smallest to roughly 0.25 mm for the larger marks, with lengths from 0.2 to 0.9 mm. We interpret these as streak marks in the dust coating, made by the impact of saltating sand grains as they bounce off the dust-coated surface, removing dust to reveal the dark surface of the solar cell beneath the glass.
In addition to the surfaces of the solar cells, one frame was taken at the edge of the solar cell, with the exposed solar-cell wiring. A portion of one of these frames is shown in the following photograph after 350 sols on Mars. The pockets formed by the wiring have accumulated a load of sand-sized particles, evidently brought to the rover deck by wind in a process known as saltation. The largest of these particles hasa radius of ~150 μm. The number of particles seen trapped in this region increased during the mission. On Mars,the particle size most easily lifted by wind has been calculated to be that of fine sand, 80 to 100 μm in radius, consistent with the sizes of the particles seen on the deck.
MI image on sol 505 of the edge of one solar cell on Spirit with electrical wiring visible, showing sand grains accumulating at the edge of one panel on Spirit.
The solar-scattering properties of dust that settles on solar arrays differ from those for atmospheric dust. We see evidence for a three-component particle distribution:
Landis, G., et al.: Dust and Sand Deposition on the MER Solar Arrays as Viewed by the Microscopic Imager. Lunar and Planetary Science Conference XXXVII, Houston, TX, 2006.
Dr. Geoffrey A. Landis, 216-433-2238, Geoffrey.A.Landis@nasa.gov
Author: Dr. Geoffrey A. Landis
Headquarters program office: Office of Space Science
Programs/projects: Mars Exploration
Last updated: December 14, 2007
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