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The Planetary Society Blog
By Emily Lakdawalla
Mars Express catches Phobos' shadow
Feb. 18, 2006 | 09:29 PST | 17:29 UTC
The Mars Exploration Rovers have recently been observing transits of Phobos and Deimos across the Sun from the surface of Mars, and those pictures are pretty cool. | Transit of the Sun by Deimos
This animation consists of 15 frames captured 10 seconds apart by Opportunity on sol 706. Credit: NASA / JPL / Cornell / Doug Ellison | | Transit of the Sun by Phobos
This animation consists of 24 frames captured 10 or 20 seconds apart by Opportunity on sol 707. Credit: NASA / JPL / Cornell / Van Driel |
Transits of the Sun by Mars' moon Phobos and Deimos occur during "eclipse seasons" lasting a couple of weeks. They happen pretty fast; here are some Quicktime movies put together by Doug Ellison to show what the eclipses would look like in real time.
Realtime Phobos transit simulation by Doug Ellison from NASA / JPL / Cornell data »
Realtime Deimos transit simulation by Doug Ellison from NASA / JPL / Cornell data »
Now, ESA has released an image showing the same event, only seen from above the surface instead of on it: | Phobos' shadow on the surface of MarsOn November 10, 2005, Mars Express spotted the shadow of Phobos crossing the surface of Mars. The shadow is smeared because both Phobos and Mars Express were moving as the image was taken over the course of several seconds. Mars Express' High Resolution Stereo Camera (HRSC) is a "pushbroom" style imager, which captures long image strips along Mars' surface one line at a time as the orbiter moves from south to north at 12,600 kilometers per hour (7,800 miles per hour). With each line advanced by HRSC from bottom to top, Phobos' shadow had shifted slightly from west to east at 7,200 kilometers per hour (4,500 miles per hour). The shadow is also darker at the center than the middle because, as seen from the surface of Mars, Phobos' diameter is much smaller than the disk of the Sun. Source Credit: ESA / DLR / FU Berlin (G. Neukum) |
As a matter of fact, HRSC caught several images of the moving shadow because of the way that the camera works. It's actually 9 separate "pushbroom" channels, each pointed at different angles forward, nadir, or backward along the north-south ground track of the orbiter. Here is a Quicktime movie of all of the HRSC channels observing Phobos' shadow. All the channels work simultaneously, but because they are pointed at different angles along the ground track, they image a single position on the ground at slightly different times, spanning a period of a few minutes.
In the movie, five channels are shown. The most-forward and most-backward-looking channels are "panchromatic" -- they cover all wavelengths, and the differences in their points of view are how the HRSC team generates 3D topography from their images. The three channels that are closest to downward-pointing are a forward-pointing green-filtered channel, a panchromatic, nadir-pointing channel, and then a backward-pointing blue channel. In the movie, the first, most forward-looking channel didn't see Phobos' shadow at all. Then Phobos' shadow appears to move from left to right as you look at the green, then nadir, then blue channels, and finally, in the most backward-pointing channel, which was captured last, Phobos' shadow has passed outside the field of view again. Normally things don't happen on Mars fast enough for the slightly different times at which HRSC takes each color image to make any difference, but in this case, it mattered. Cool, huh?
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