The Mars Exploration Rover mission was launched in 2003 with the goal of finding out more about the geological history of the planet and investigating the role liquid water may have played there. Life, at least all life as we know it, cannot survive without liquid water. While Mars has been declared a cold and lifeless place, scientists have had reason to believe that it may have once had a wetter and warmer environment able to support life. In an effort to find out if there was ever a chance for life on Mars, NASA's exploration strategy was to "follow the water."

On Earth, when water interacts with rock or sediment substrates, it may leave behind a variety of telltale signs. For example, flowing water carves out surface features or produces characteristic formations in rocks and sediments. Small holes in rocks, called vugs, may be found where crystals once existed but have since been either dissolved by a change in the water chemistry or eroded away. High concentrations of sulfate minerals are found in rocks that were formed in or have had long exposures to water. Certain minerals, such as gray hematite, are often formed by interaction with water. Other minerals, such as the uncommon jarosite, are only formed in acidic water. If these or similar features were found on Mars, they would be clues that water once existed there.

Twin robotic geologists, Spirit and Opportunity, were sent to Mars to look for such evidence. Both rovers were designed to have great mobility and were equipped with identical sets of five custom-made scientific instruments. Each rover body has a panoramic camera that creates high-resolution images and a spectrometer to determine mineralogy. Each rover also has a robotic arm that holds and maneuvers miniature versions of instruments used by geologists on Earth: a spectrometer to identify the elemental composition of rocks and soil, another spectrometer to specialize in iron-rich minerals, and a microscopic imager to enable close-up views of Martian features — the first ever seen. In addition, the rovers also have a rock-abrasion tool to scrape off the outer surfaces of rocks and expose fresh interiors for study. Using evidence collected by all these tools and their knowledge of how water interacts with materials on Earth, many scientists have become convinced that Mars once had a watery environment that may have been able to support life.

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