Sunday, September 16, 2007

THEMIS and Atmospheric Dust Levels

Credit: NASA/JPL-Caltech/Arizona State University

THEMIS's infrared camera is able to detect infrared energy at ten different wavelengths. Nine of these have wavelengths between 6 and 13 micrometers, an ideal region of the infrared spectrum to determine thermal energy patterns characteristic of silicate minerals. At a tenth wavelength, 14.88 micrometers, the atmosphere of Mars becomes opaque, so that THEMIS cannot see the surface of the planet.

Using the 9 micrometer wavelength, THEMIS is able to show the atmosphere's opacity due to the level of dust in the atmosphere. This was very beneficial in the past few months as a major worldwide dust storm blew around Mars. The dust storm erupted during the last week of June 2007. Beginning in the equatorial region west of Meridiani Planum, it moved into the heavily cratered southern highlands. The storm took roughly a week to grow large enough to spread around the planet south of the equator. Dust also drifted into the northern hemisphere as well.

The dust storm affected operations for all five spacecraft operating at Mars. For the orbiters, the storms obviously interfered with visible light observations of the surface. However, the dust storms were mission-threatening for the twin rovers, Spirit and Opportunity, both of which went into hibernation mode to wait out the dust storms. The problem was that the dust storms caused a steep decrease in the amount of electricity generated by the rovers' solar panels, threatening the rovers' survival. The solar panels on the rovers can normally generate up to 700 watt hours per day, of which any output below 150 watt hours forces the rovers to rely upon batteries to operate their heaters, which keeps the rovers operational. At the worst part of the dust storms, Spirit's electrical output fell as low as 261 watt hours, and 128 watt hours for Opportunity. However, by early August, the Martian skies began to clear enough so that Opportunity could fully recharge its batteries and Spirit could bring its batteries' energy levels to a nearly full charge. (Opportunity has since resumed its mission, beginning its exploration of the interior of Victoria crater; meanwhile, on September 5th, Spirit climbed onto its long-term destination called Home Plate, a plateau of layered bedrock bearing clues to an explosive mixture of lava and water.)

In the above GIF animation, THEMIS's infrared maps of the atmosphere show the atmosphere's opacity between June 15th through July 19th of this year, covering a significant portion of the dust storms' creation and expansion. The scale bar's values run from nearly clear (0.05) to roughly a one-third reduction in sunlight (0.40). (Note: For some reason, only the first slide of the nine in the animation is showing up; please let me know in the comments if you are or are not able to see the entire animation.)

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