[meteorite-list] MRO CRISM Views of Phobos and Deimos

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed, 28 Nov 2007 09:14:19 -0800 (PST)
Message-ID: <200711281714.JAA10927_at_zagami.jpl.nasa.gov>


CRISM Views Phobos and Deimos
Release Date: Nov 27, 2007

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

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

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.

The Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) is one
of six science instruments on NASA's Mars Reconnaissance Orbiter. Led by
The Johns Hopkins University Applied Physics Laboratory, the CRISM team
includes expertise from universities, government agencies and small
businesses in the United States and abroad.

CRISM's mission: Find the spectral fingerprints of aqueous and
hydrothermal deposits and map the geology, composition and stratigraphy
of surface features. The instrument will also watch the seasonal
variations in Martian dust and ice aerosols, and water content in
surface materials - leading to new understanding of the climate.

Received on Wed 28 Nov 2007 12:14:19 PM PST

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