File:Phobos shadow.jpg

Credit: NASA/JPL/Malin Space Science Systems [1] archive copy at the Wayback Machine [2]

The penumbral shadow of Phobos is visible on the landscape of Mars, as photographed by the Mars Global Surveyor. The center of the shadow was at approximately 10.9°N 49.2°W (Map of area archive copy at the Wayback Machine Map zoom archive copy at the Wayback Machine) at 04:00:33.3 UTC Earth time.

The image shows western Xanthe Terra on August 26 1999 at about 2:41 p.m. local solar time on Mars. The image covers an area about 250 kilometers across and is illuminated from the left. The dark spots on the crater floors are probably dark sand dunes. None of the craters pictured currently have names. Nanedi Valles is the meandering valley at the bottom right. The picture covers about 7°–15°N vertically and 52°–48°W horizontally. The vertical orientation of the image is 3.01° to the west of north; a north-pointing arrow superimposed on the image would point slightly to the right.

To determine the time of the shadow, we can look up the original image files at M04-03241 archive copy at the Wayback Machine (red) and M04-03242 archive copy at the Wayback Machine (blue). The "image start time" was 03:26:13.01 UTC, the "line integration time" is 80.4800 milliseconds, and the "downtrack summing" factor is 4. Since the shadow is centered at 6400 pixels from the bottom of the original 10800-pixel-high image (Mars Global Surveyor had a south-to-north sun-synchronous orbit), we add (6400 * 0.08048 * 4) = 2060.3 seconds = 34 minutes 20.3 seconds to get a time of 04:00:33.3 UTC for the center of the shadow.

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External links

• NASA press release (November 1 1999)
• MOC Wide-Angle Images, Region Lunae Palus, Subphase M04 archive copy at the Wayback Machine
• Mars Global Surveyor Mars Orbiter Camera Image Gallery