Water on Mars, so challenging to find today, may once have covered the planet with rivers and oceans. The planet is smaller than Earth, with less gravity and a thinner atmosphere. Over time, as liquid water evaporated, more and more of it escaped into space, allowing less to fall back to the surface of the planet. Vast deposits of water appear to be trapped within the ice caps at the north and south poles of the planet. Each summer, as temperatures increase, the caps shrink slightly as their contents skip straight from solid to gas form, but in the winter, cooler temperatures cause them to grow to latitudes as low as 45 degrees, or halfway to the equator. The caps are an average of 2 miles (3 kilometers) thick and, if completely melted, could cover the Martian surface with about 18 feet (5.6 meters) of water.
Liquid water appears to flow from some steep, relatively warm slopes on the Martian surface. Researchers studying images taken by the High Resolution Imaging Science Experiment (HiRISE) camera aboard the Mars Reconnaissance Orbiter (MRO) noticed dark streaks that appear during warm weather but fade away when temperatures drop. Spectral analysis of these streaks, called recurring slope linae (RSL), lead scientists to conclude they are caused by salty liquid water. More water may be frozen just beneath the surface, covered by the dry red dust that blankets the planet. Some high-latitude regions seem to boast patterned ground-shapes that may have formed as permafrost in the soil freezes and thaws over time.
NASA’s Curiosity rover is the fifth robot to land on the surface of the Red Planet in the last 15 years. Pathfinder, Phoenix, Spirit and Opportunity all took detailed measurements of the planet; all but Phoenix traveled across the surface collecting a treasure trove of information. In 2008, Phoenix turned up small chunks of bright material that disappeared after four days, leading scientists to surmise that they were pieces of water ice. The lander went on to detect water vapor in a sample it collected and analyzed, confirming the presence of frozen water on the red planet. Spirit and Opportunity, the twin rovers, found traces of water enclosed in rock. In a shining example of a problem becoming a solution, a broken wheel on Spirit scraped into the top of the Martian surface, revealing a layer beneath rich in silica that had most likely formed in the presence of water. Curiosity has found yet more evidence of water flowing on ancient Mars.
In addition to examining the relatively recent (geologically speaking) presence of water, the various missions have also studied the surface of the planet in a historical context. In addition to understanding how Mars may have changed and developed over time, scientists hope that finding water will help them to find something even more valuable — life, either past or present. Recent orbiters, landers and rovers sent to Mars were designed to search for water, rather than life, in the hopes of finding environments where life could have thrived. The next NASA rover — a car-size robot based heavily on Curiosity’s basic design — will blast off in 2020 to look for evidence of past Red Planet life.