Status Report

Mars Picture of the Day: Phobos Over the Martian Limb

By SpaceRef Editor
June 23, 2003
Filed under , , ,

Mars Global Surveyor Mars Orbiter Camera

MGS MOC Release No. MOC2-400, 23 June 2003

MOC Wide Angle View

MOC Wide Angle View

MOC Narrow Angle Camera View

All Images Credit: NASA/JPL/Malin Space Science Systems

Mars has two natural satellites, or moons, Phobos and
Deimos. On 1 June 2003, the Mars Global Surveyor (MGS) spacecraft
was slewed eastward to capture these views of the inner
moon, Phobos, shortly before it set over the
afternoon limb. Phobos orbits Mars about 3 times
a day at an average distance of 9,378 km (5,828 mi).
About 0.006 times the size of Earth’s Moon, Phobos is
a potato-shaped object with dimensions approximately
27 by 22 by 18 kilometers (about 17 by 14 by 11 miles).

The first picture shown here is a color composite
of four MGS Mars Orbiter Camera (MOC) wide angle images;
the second is the same as the first, but indicates
the location of Phobos. The third view is a MOC narrow
angle image, taken at the same time as the wide angle
views, showing details on the surface of the tiny moon.

Phobos is one of the darkest objects in the Solar System.
Thus, four wide angle images were obtained to make the
picture of Phobos over the martian limb: a pair of red and
blue wide angle images was acquired for the limb, and a pair
of separate images were required to see Phobos. The wide
angle images illustrate the fact that Phobos is
mostly colorless (dark gray); the faint orange/red hue
in the wide angle picture is a combination of slight
differences in the focal lengths of the blue and red cameras
and the orange/red illumination provided by reflection of
sunlight off Mars. To a person standing on Phobos, the
red planet would fill most of the sky.

The high resolution image (bottom) was taken at the same time
as the wide angle views. MGS was about 9,670 kilometers (6,010 miles)
from Phobos when the picture was taken. At this distance, the
image resolution is about 36 meters (470 ft.) per pixel;
the maximum dimension of Phobos as seen in this image (the diagonal
from lower left to upper right) is just over 24 km (15 mi). This is the
“trailing” hemisphere, the part of Phobos that faces opposite the
direction that the moon orbits Mars. This is a part of Phobos that
was not seen by MOC in 1998, when MGS made several close flybys of
the tiny moon.

The rows of grooves and aligned pits on Phobos are related to,
and were probably caused by, a large meteor impact that occurred
on the side of Phobos that is not seen here. That large crater,
Stickney, was named for the maiden name of the wife of the astronomer
that discovered Phobos and the other martian satellite, Deimos,
in 1877, Asaph Hall.

Examples, with descriptive captions, of the views of Phobos
obtained by MOC in 1998 can be seen at:

All of the previous MOC images of Phobos are available in the
MOC Gallery at:

Malin Space Science Systems and the California Institute of Technology
built the MOC using spare hardware from the Mars Observer mission.
MSSS operates the camera from its facilities in San Diego, California.
The Jet Propulsion Laboratory’s Mars Surveyor Operations Project
operates the Mars Global Surveyor spacecraft with its industrial
partner, Lockheed Martin Astronautics, from facilities in Pasadena,
California and Denver, Colorado.

SpaceRef staff editor.