Over seven years into its (originally) 90-day mission, the Mars rover Opportunity arrived at the rim of Endeavour Crater. While the crater itself was formed from an ancient meteorite impact, the rocks at its rim show signs of a watery past. Chemical analysis found deposits typical of hydrothermal vents on Earth, along with features usually associated with evaporation. Together, these pieces of evidence suggest warm, shallow water formerly existed in the region of Endeavour.

In a new Science paper, S. W. Squyres and colleagues describe the process Opportunity used to obtain and analyze the rock samples. The landscape around Endeavour is very old, dating back to the era when Mars was under constant bombardment by meteorites, which is why it was chosen as a site for exploration by rover. If Mars’ history parallels Earth’s in any way, the early cataclysmic period gave way to calmer times, and water—possibly life—may have been present. Based both on the sedimentary and evaporative characteristics of the rocks around the crater, the researchers conclude the region may have been habitable for at least a short period of time.

As on Earth, Mars has regions where the rocks date back to early times, shortly after the planets cooled sufficiently to allow the surface to solidify. On Mars, this era is known as the Noachian period (in reference to the biblical figure Noah), as the now-dry planet probably had a lot of surface water. This period was also characterized by heavy meteorite bombardment: Noachian landscape is heavily cratered, and bears some resemblance to the highland regions on Earth’s Moon.

Endeavour Crater is the site of one such meteorite impact. The crater is about 22 kilometers across, and the layers of rock exposed by the impact resemble similar structures on Earth. Thus, Endeavour provides a good laboratory for understanding early Martian history, so scientists selected it for exploration by the Opportunity rover during its extended mission.

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