These two images taken by the Compact Reconnaissance Imaging Spectrometer
for Mars (CRISM) show Mars' two small moons, Phobos and Deimos, as seen
from the Mars Reconnaissance Orbiter's low orbit around Mars. Both images
were taken while the spacecraft was over Mars' night side, with the
spacecraft turned off its normal nadir-viewing geometry to glimpse the
moons. The image of Phobos, shown at the top, was taken at 0119 UTC on
October 23 (9:19 p.m. EDT on Oct. 22), and shows features as small as 400
meters (1,320 feet) across. The image of Deimos, shown at the bottom, was
taken at 2016 UTC (12:16 p.m. EDT) on June 7, 2007, and shows features as
small as 1.3 kilometers (0.8 miles) across. Both CRISM images were taken
in 544 colors covering 0.36-3.92 micrometers, and are displayed at twice
the size in the original data for viewing purposes.
Phobos and Deimos are about 21 and 12 kilometers (13.0 and 7.5 miles) in
diameter and orbit Mars with periods of 7 hours, 39.2 minutes and 1 day,
6 hours, 17.9 minutes respectively. Because Phobos orbits Mars in a
shorter time than Mars' 24 hour, 37.4-minute rotational period, to an
observer on Mars' surface it would appear to rise in the west and set in
the east. From Mars' surface, Phobos appears about one-third the diameter
of the Moon from Earth, whereas Deimos appears as a bright star. The moons
were discovered in 1877 by the astronomer Asaph Hall, and as satellites of
a planet named for the Roman god of war, they were named for Greek
mythological figures that personify fear and terror.
The first spacecraft measurements of Phobos and Deimos, from the Mariner 9
and Viking Orbiter spacecraft, showed that both moons have dark surfaces
reflecting only 5 to 7% of the sunlight that falls on them. The first
reconstruction of the moons' spectrum of reflected sunlight was a
difficult compilation from three different instruments, and appeared to
show a flat, grayish spectrum resembling carbonaceous chondrite
meteorites. Carbonaceous chondrites are primitive carbon-containing
materials thought to originate in the outer part of the asteroid belt.
This led to a commonly held view among planetary scientists that Mars'
moons are primitive asteroids captured into Martian orbit early in the
planet's history. More recent measurements have shown that the moons are
in fact relatively red in their color, and resemble even more primitive
D-type asteroids in the outer solar system. Those ultra-primitive bodies
are also thought to contain carbon as well as water ice, but to have
experienced even less geochemical processing than many carbonaceous
chondrites.
The version of the CRISM images shown here were constructed by displaying
0.90, 0.70, and 0.50 micrometer wavelengths in the red, green, and blue
image planes. This is a broader range of colors than is visible to the
human eye, but it accentuates color differences. Both moons are shown with
colors scaled in the same way. Deimos is red-colored like most of Phobos.
However, Phobos' surface contains a second material, grayer-colored ejecta
from a 9-kilometer (5.6-mile) diameter crater. This crater, called
Stickney, is located at the upper left limb of Phobos and the
grayer-colored ejecta extends toward the lower right.
These CRISM measurements are the first spectral measurements to resolve
the disk of Deimos, and the first of this part of Phobos to cover the full
wavelength range needed to assess the presence of iron-, water-, and
carbon-containing minerals.
CRISM is one of six science instruments on NASA's Mars Reconnaissance
Orbiter. Led by The Johns Hopkins University Applied Physics Laboratory,
Laurel, Md., the CRISM team includes expertise from universities,
government agencies and small businesses in the United States and abroad.
NASA's Jet Propulsion Laboratory, a division of the California Institute
of Technology in Pasadena, manages the Mars Reconnaissance Orbiter and the
Mars Science Laboratory for NASA's Science Mission Directorate,
Washington. Lockheed Martin Space Systems, Denver, built the orbiter.