March 2, 2004 — This image, taken by Opportunity's Microscopic Imager, shows the shapes of the vugs, or small cavities, located on the region called El Capitan. This image gives a glimpse into the nature of the rock material surrounding the vugs. Several vugs have disk-like shapes with wide midpoints and tapered ends. This feature is consistent with sulfate minerals that crystallize within a rock matrix, either pushing the matrix grains aside or replacing them. These crystals are then either dissolved in water or eroded by wind to produce vugs. Here, the rock matrix appears granular, delicately enhanced through wind abrasion. Some of these grains are well rounded, which could result from transport of rock fragments in air or water, or precipitation of mineral grains in water.