Archive August 2010
This mosaic of the Viking 1 Orbiter images f826a33 to 38, taken on September 21, 1978, shows the feature colloquially called “White Rock” which is located on the floor of Pollack Crater in the Sinus Sabaeus region near the Martian equator.
The mosaic was rotated to put north up.
Image Credit: NASA/JPL/Arizona State University/Mosaic by astroarts.org
White Rock got its nickname more than 30 years ago, when scientists first spotted the feature on the floor of Pollack Crater in images taken by the Mariner 9 spacecraft.
Pollack, which is about 90 kilometers wide, has a dark floor, especially over its southern half, where White Rock lies. At the time of Mariner 9, rather contrasty image processing gave White Rock, which measures about 15 by 18 km, a chalky-bright appearance.
This brightness led many scientists to propose White Rock was made of water-deposited sediments, like the salty residue of a dried-up desert lake.
In 2001, however, scientists working with the Thermal Emission Spectrometer (TES) on NASA’s Mars Global Surveyor (MGS) found that White Rock has a dry origin and is built of wind-blown sediments. The bright blocks and ridges have the same brightness as light-colored, dusty regions elsewhere on Mars, and White Rock’s spectra likewise matches these and shows no trace of water.
The wind-sculpted ridges that make up White Rock rise about 300 meters above Pollack Crater’s floor, which shows as dark lanes cutting into the light-colored formation.
While the light material stands up as buttes and ridges, lots of loose material nonetheless surrounds White Rock. At the feature’s northern end lies a field of dunes made of dark, basaltic sand grains. These sand grains probably eroded from the lava that covers the floor of Pollack. The dune shapes suggest that some winds blew from the east or southeast. It is possible these were funneled by the channel, some 500 meters wide, that cuts straight through White Rock.