A microscopic image shows variability of grain size within a parallel-stratified portion of the "Slickrock" area in the martian rock outcrop examined by NASA's Opportunity. The dip from left to right is an artifact of the camera pointing angle. The grains in the rock define the fine layers, or laminae, and the variability in grain size along each lamina is small compared to the variability between laminae. Some of the fine layers have mostly smaller grains; others have mostly larger grains. Red arrows and labels indicate the sizes of a representative large grain (0.8 millimeter or 0.03 inch) and a representative small grain (0.3 millimeters or 0.01 inch).
Opportunity took this picture with its microscopic imager during the rover's 42nd sol on Mars (March 6, 2004), the sol before it took a similar picture (PIA05520) at the same location. Most of the grains visible resemble medium-sized sand grains and are well-rounded and well-sorted.
Image credit: NASA/JPL/Cornell/USGS
