Opportunity took this image while stuck in the sand ripple dubbed Purgatory for over a month. The panorama (partly shown here) was named Rub Al Khali after the Arabian Desert's “Empty Quarter.” NASA/JPL-Caltech/Cornell University

Tiny spherules pepper a sandy surface in this 3-centimeter (1.2-inch) square view of the Martian surface. The largest one is broken in half and shows little internal texture—typical of these “blueberries” on the Meridiani Plains. Opportunity took this image while the target was shadowed by the rover’s instrument arm. NASA/JPL-Caltech/Cornell/USGS/Cathy Weitz

Ralph Bagnold, an early pioneer of dune studies, remarked that—compared to the nearly static chaos that seems to characterize slowly crumbling, weathering landscapes—sand dunes can “move inexorably, in regular formation, over the surface of the country, growing, retaining their shape, even breeding, in a manner which by its grotesque imitation of life, is vaguely disturbing to an imaginative mind.” Perhaps more so, if we’re on Mars? NASA/JPL-Caltech/Cornell University

This was one of the first images of the sulfate-rich outcrops on the Meridiani Plains of Mars to show a broken spherule, or “blueberry” (below center). The spherules do not deflect the crosswise layers of finer sediments, indicating that the spherules and sediments were not deposited at the same time. The image shows a 3-centimeter (1.2-inch) square section of the rock Robert E in Eagle Crater. NASA/JPL-Caltech/Cornell/USGS

These loose, BB-sized, hematite-rich spherules are embedded in this Martian rock like blueberries in a muffin and released over time by erosion. The Mars Rover Opportunity found this cluster of them at its Eagle Crater landing site and analyzed their composition with its spectrometers. Hypotheses about their formation have contributed to the story of water on Mars. NASA/JPL-Caltech/Cornell University

The El Dorado Dune Field dominates this 160-degree view. The Mars Rover Spirit spent several days exploring this area and acquiring images and data on the physical characteristics of this large basaltic sand sheet, before moving downhill toward Home Plate. NASA/JPL-Caltech/Cornell University

The piece of metal with the American flag on it is made of aluminum recovered from the site of the World Trade Center towers in New York City. It serves as a cable guard for Spirit’s rock abrasion tool as well as a memorial to the victims of the September 11, 2001, terrorist attacks. Opportunity has an identical piece. NASA/JPL-Caltech/Cornell University

Spirit obtained this view of the area called Home Plate while parked atop Husband Hill. The colors emphasize differences in rock weathering. A large dust devil appears as the V-shaped discoloration of sky at the top right. NASA/JPL-Caltech/Cornell University

The Mars Rover Spirit took this sublime view of a sunset over the rim of Gusev Crater, about 80 kilometers (50 miles) away. Taken from Husband Hill, it looks much like a sunset on Earth—a reminder that other worlds can seem eerily familiar. Sunset and twilight images help scientists to determine how high into the atmosphere the Martian dust extends and to look for dust or ice clouds. NASA/JPL-Caltech/Texas A&M/Cornell

Full-scale model of the Mars Exploration Rover. The twin Mars Exploration Rovers Spirit and Opportunity were launched toward Mars in the summer of 2003. They arrived months later in spectacular fashion, bouncing down safely on the surface after a harrowing six-minute descent through the thin atmosphere. Spirit arrived on January 3, 2004, and Opportunity on January 24, 2004. Photo by Eric Long, Smithsonian Institution