CRISM Views Phobos and Deimos
Date: 22 Oct 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. The spacecraft turned off its normal nadir-viewing geometry to glimpse the moons.
The image of Phobos, shown at left, was taken at 0119 UTC on 23 October 2007 (9:19 p.m. EDT on Oct. 22), and shows features as small as 400 m (1,320 feet) across. The image of Deimos, shown right, was taken at 2016 UTC (12:16 p.m. EDT) on 7 June 2007, and shows features as small as 1.3 km (0.8 miles) across. Both CRISM images were taken in 544 colors covering 0.36-3.92 mm, and are displayed at twice the size in the original data for viewing purposes.
Phobos and Deimos are about 21 and 12 km (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 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-km (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.
Last Update: 18 Aug 2011 (AMB)