Signs of old hydrothermal vents greet the rover after a seven year drive.

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.



While Cape York is named for a place on Earth, Shoemaker Ridge and Greeley Haven are named in memory of planetary scientists Eugene Shoemaker and Ronald Greeley. Greeley helped design some of the instruments used on the Opportunity rover, known as the Athena payload.



Opportunity approached Endeavour along a relatively low rise known as Shoemaker Ridge at the lip of the crater, which itself is part of a region known as Cape York. For the current study, the rover analyzed rocks at two locations, known as Chester Lake and Greeley Haven, respectively at the southern and northern ends of Cape York. Since these rocks are separated by about 700 meters, Squyres et al. assume they are representative of the entire Shoemaker formation.

In both locations, Opportunity examined rock within outcrops (the matrix) and broken off fragments (clasts), some of which were ejected from the impact that formed Endeavour. Grinding small amounts off rocks allowed Opportunity to test their chemical composition and the hardness of the minerals, both of which reveal information about formation and history.

One type of rock known as breccias (which are well known from Earth) consists of solid fragments embedded in fine-grained stone. Analysis of breccias in near Chester Lake revealed the presence of a lot of zinc in high concentrations. On Earth, such features are produced in hydrothermal vents, when volcanic heating sends water through fissures, allowing it to pick up minerals.

In addition, when Opportunity studied rocks in the Greeley Haven region, it found veins of material threading the matrix. Based on the chemical analysis and modeling, Squyres et al. concluded this material is gypsum, a soft white calcium sulfide compound containing water (CaSO 4 •2H 2 O). On Earth, gypsum is most commonly found when water has evaporated, leaving previously dissolved minerals behind. Based on the temperature at which gypsum forms, the researchers postulated a warm shallow body of water, perhaps transient—and possibly habitable.

Both the presence of zinc deposits and veins of gypsum are very suggestive—water was likely once present in the Endeavour Crater region. Comparison with sandstone found elsewhere on Mars hints that Cape York's water was transitional. The hydrothermal deposits mark the early period, when volcanic activity was more common, while the evaporative deposits show a later period, when seas covered much of the Martian surface. The warm water required to form and precipitate gypsum hints that perhaps transient pools may have been habitable.

Science, 2012. DOI: 10.1126/science.1220476 (About DOIs).