HiRISE acquired two dramatic views of the Martian moon, Phobos, on 23 March 2008. Observation PSP_007769_9010, acquired at a distance of 6,800 kilometers from Phobos, provides surface detail at 6.8 m/pixel scale and a object diameter of about 3,200 pixels. The second observation, PSP_007769_9015 providing a closer look at 5,800 km, views the surface at slightly more detail (5.8 m/pixel with an object diameter of about 4,000 pixels).

The two images were taken within 10 minutes of each other and show roughly the same features, but from a different angle so that they can be combined to yield a stereo view. (Watch a short clip of both observations: 204KB, QuickTime.)

The illuminated part of Phobos visible in the images is about 21 km across. Images from previous spacecraft have been of smaller pixel scale (for example, Mars Global Surveyor got data at 4 m/pixel, because this spacecraft came closer to Phobos), but the HiRISE images have greater signal-to-noise, making the new data some of the best ever for Phobos.

The most prominent feature in the images is the large impact crater Stickney, in the lower right. With a diameter of 9 km, it is the largest feature on Phobos. A series of grooves and crater chains is obvious on the other parts of the moon. Although many appear radial to Stickney in the images, previous studies show that the grooves radiate from a different point on Phobos. Hypotheses for their formation vary. Some scientists believe the grooves and crater chains are related to the formation of Stickney, whereas others think they may have formed from ejecta from impacts on Mars that later collided with Phobos. The lineated textures on the walls of Stickney and other large craters are landslides formed from materials falling into the crater interiors in the weak Phobos gravity (less than 1/1000th the gravity on Earth)

IRB COLOR
Both PSP_007769_9010 and PSP_007769_9015 were made by combining data from HiRISE's blue/green, red, and near-infrared channels. The color data accentuate details not apparent in the black and white images. For example, materials near the rim of Stickney appear bluer than the rest of Phobos. Based on analogy with materials on our own moon, this could mean this surface is fresher, and therefore younger, than other parts of Phobos.

“MARSSHINE”
This is a close-up and enhanced part of image PSP_007769_9015. In the original image, detail in the black, un-illuminated portion is not apparent. However, seen in enhanced detail here, craters are clearly visible. This faint illumination is from reflected light off of Mars ("Marsshine"). This is directly analogous to "Earthshine," where reflected sunlight from our planet illuminates the dark side of the Moon. The ability to see features on Phobos illuminated by Marsshine demonstrates the high sensitivity of the HiRISE camera.


BACKGROUND
Phobos, and the second Martian moon, Deimos, are interesting for several reasons. Both objects are small, with average diameters of just 22 and 12 km, respectively. At this size, their gravity is insufficient (less than 1/1000th of Earth) to pull them into spherical shapes, in contrast to the larger moons and planets in the Solar System.

Both moons are tidally locked to Mars, meaning, like our own moon relative to Earth, they present the same side to Mars all the time. The small size and composition (determined from spectroscopy) of Phobos and Deimos make them very similar to some asteroids. Most asteroids are located in a belt between Mars and Jupiter, with others having orbits that cross that of Mars. Therefore, it is possible that Phobos and Deimos are captured asteroids. Other hypotheses are that they formed with Mars in the early Solar System, or are composed of material blasted off of Mars by impacts.

HiRISE can provide new imaging, color, and topographic (from stereo) data on these objects, thereby helping to constrain their origin and subsequent evolution. While this caption focuses on observations of Phobos, images of Deimos may be acquired at a later date. The CRISM (Compact Reconnaissance Imaging Spectrometer for Mars) instrument on MRO imaged Phobos and Deimos in 2007.

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Written by: Nathan Bridges   (9 April 2008)