Space Topics: Mars
Phobos
Color view of Phobos from Mars Reconnaissance Orbiter
Credit: NASA / JPL / U. Arizona |
Diameter: 27 x 21.6 x 18.8 kilometers, smaller than many asteroids
Orbital distance: 9,378 kilometers from the center of Mars (about 5,980 kilometers
above the surface)
Orbital period: 0.31891 days (7.65 hours)
Discovery: 1877 by Asaph Hall
Phobos is the larger and inner of Mars’ two tiny moons. Its orbit
is so close to Mars that it moves faster than Mars spins. As a result,
from Mars’ surface, Phobos appears to rise in the west and set in the
east, usually twice a day. Another consequence of its close orbit is
that it is not even visible (it travels below the horizon) from the surface
at high latitudes.
When Mars is near its vernal and autumnal equinoxes, the
equatorial orbit of Phobos and Deimos carry them across the Sun in a series
of daily eclipses. These
events have been imaged many times by the Mars Exploration Rovers, and
are also visible from space as the shadows of the moons move across Mars' surface.
It is likely that both Phobos and Deimos are captured C-type
asteroids. Their
densities are very low, so they must be porous. Phobos is riddled with
craters, including the giant Stickney crater on one side. It has long
been thought that the Stickney impact may be responsible for the noticeable
grooves crossing Phobos’ surface, but recent images from Mars Express
have ruled out that possibility; it is now thought that the
grooves represent chains of secondary impacts from craters that formed on
Mars,
6,000 kilometers below.
Many spacecraft have imaged Phobos. One of the most picturesque data sets
was from the Phobos 2 mission, which ultimately failed but did return 13 sets of images
of Phobos from its "VSK" television camera. The Planetary Society has a
page presenting the entire Phobos 2 VSK
data set.
Phobos over Mars
The Soviet Union's Phobos mission to Mars returned many images of Mars' inner
moon, including this one of Phobos poised across the limb. Credit: Phobos
mission VSK / © Ted
Stryk
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Mars Global Surveyor imaged Phobos on five occasions, four of them in August
and September of 1998 and a final one in June 2003. The raw image data
can be downloaded here, and Malin Space Science Systems presented captioned
versions of images from all but the third encounter.
As the only current Mars orbiter with an elliptical orbit, Mars
Express is the only modern spacecraft that is capable of imaging anything other
than Phobos' sub-Mars hemisphere (the hemisphere that always faces Mars
as Phobos circles it). The first view shows the same, sub-Mars hemisphere,
but the rest show other perspectives. |
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Phobos -- up close and personal
This image, taken by the High Resolution Stereo Camera (HRSC), is one of the highest-resolution pictures so far of the Martian moon Phobos. It shows the Mars-facing side of the moon, taken from a distance of less than 200 kilometers with a resolution of about seven meters per pixel during orbit 756, on August 22, 2004. Due to geometric reasons the scale bar is only valid for the centre of the image.
Credit: Courtesy ESA / DLR / FU Berlin (G.Neukum)
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Phobos above the Martian limb
In this image, taken on January 10, 2007 on Mars Express' 3,868th orbit, Phobos floats just above the Martian limb. Phobos is the larger and inner of Mars' two moons. The image has been enhanced slightly to bring out details on the moon.
Credit: ESA / DLR / FU Berlin (G. Neukum)
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Phobos below the Martian limb
In this image, taken on January 22, 2007 on Mars Express' 3,909th orbit,
Phobos floats just below the Martian limb. Likely a captured asteroid, it is
significantly darker in color than Mars. The image has been enhanced slightly
to bring out details on the moon. Credit: ESA / DLR / FU Berlin (G. Neukum)
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Maps of Phobos
Currently available maps of Phobos, and names of features, are predominantly
based on Viking image data, from which the United States Geological Survey
prepared airbrushed base maps.
Map of Phobos, with features labeled
The irregularly-shaped moon Phobos is squashed into a cylindrical map. Because of Phobos' flattened shape the cylindrical map distorts features near the poles even more than it does for a spherical body. However, the map makes it easy to identify latitude and longitude coordinates of features on Phobos' surface.
The left half is the leading hemisphere. The right half is the trailing hemisphere. The center of the map represents the sub-Mars hemisphere, and the two edges mark the middle of the anti-Mars hemisphere.
Credit: USGS / Peter Thomas / Phil Stooke
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The leading hemisphere of Phobos
This map of Phobos' leading hemisphere is in a "morphographic" projection, based on a shape model of the irregular body, so it distorts areas and distances less than traditional azimuthal projections.
Credit: USGS / Peter Thomas / Phil Stooke
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The trailing hemisphere of Phobos
Credit: USGS / Peter Thomas / Phil Stooke
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Make Your Own Phobos!
The image below can be printed on card stock, cut out, and folded into
a reasonable facsimilie of the shape of Phobos. Visit The
Planetary Society Weblog for more detailed instructions.
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Constant-scale natural boundary map of Phobos
This map of Phobos can be cut out and assembled into a good three-dimensional representation of the odd-shaped moon of Mars. The map was produced through a technique called constant-scale natural boundary mapping, developed by Georgia architect Chuck Clark.
Credit: Chuck Clark / Phobos base image by USGS 
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