A basic tenet of sedimentology, the field of geology that studies
sedimentary processes, is that the farther a piece of rock travels from
its source, the smaller and rounder in shape it becomes as the materials
suffer impacts with other grains during transport by wind or water.
Grains that have not traveled as far are more angular and less rounded.
A comparison of photographs taken by the microscopic imager on NASA's Mars
Exploration Rover Spirit suggests that sand that has accumulated in drifts
on the plains of Gusev Crater traveled farther from the source rock than
similar sand grains in the hills inside Gusev. In a microscopic image
taken on the plains of Gusev Crater early in the mission (PIA05288), sand
deposits were made up of rounded grains. In contrast, this more recent
microscopic image of a sand drift near the top of the "Columbia Hills"
shows poorly sorted, more angular grains of sand, which suggests they
were transported a relatively short distance from a local source.
This image is of grains in a sand drift informally named "Cliffhanger"
because of its proximity to the edge of steep slopes that bound the summit
region of "Husband Hill," highest of the Columbia Hills. Spirit took the
image with its microscopic imager on the rover's 607th martian day, or sol
(Sept. 9, 2005). The photo covers an area 3 centimeters (1.2 inches)
across. The scale of the image (31 microns or one one-thousandth of an
inch per pixel) allows features as small as 0.1 millimeter (four
one-thousandths of an inch) to be resolved.
