On August 6, 2022, the Curiosity rover reached a major milestone on Mars: 10 years of exploring Gale crater. Although getting on in years by rover standards (the Opportunity rover set the record of more than 14 years), Curiosity shows no signs of slowing down, and the discoveries related to the past potential habitability of Mars keep on coming.
Gale crater was formed by the impact of a large meteorite sometime between 3.5 and 3.8 billion years ago. The resultant crater is about 150 km (95 miles) across and is located just south of the Martian equator. It was selected as the landing site for Curiosity because of the different minerals detected from orbit that occur in the layered rocks forming the 5.5 km (3.4 miles) tall Mt. Sharp located near the crater center. Rocks on the lower slopes of Mt. Sharp display signatures that indicated clay-bearing rocks are present that are overlain by more sulfate-enriched rocks. Clay-bearing rocks often form in the presence of water, whereas sulfate-bearing rocks are commonly found in areas where considerable water has evaporated. By examining the clay-bearing rocks all the way up into the sulfate-bearing rocks, the goal for the rover and the team is to understand how the habitability of ancient Mars may have changed as conditions became drier over time.
On August 6, 2012, Curiosity became the first rover to be landed by being lowered to the surface via a Skycrane system. The rover landed between the north side of Mt. Sharp and the northern wall of the crater on a deposit of rocks and sand, called an alluvial fan, that were washed down the northern wall of the crater billions of years ago. From there, Curiosity literally took off for the hills. One of the first stops was a series of outcrops in an area near the landing site called Yellowknife Bay. There, Curiosity drilled into the rocks using its seven-foot arm and examined the resultant sample using its science payload of 10 instruments. The results were stunning and showed that an ancient lake had once filled the floor of the crater.
Subsequent exploration, drilling, and analyses during the 28.6 km+ (nearly 18 miles) traverse the rover has made up the northern flank of Mt Sharp are no less spectacular, as seen below. The individually thin lake beds total more than 300 m (close to 1,000 feet) in thickness, are clay-bearing, some contain organic materials, and point to a water chemistry that was likely habitable. Most agree that these rocks, known as the Murray Formation (named after Bruce Murray, a planetary geologist that made significant contributions to early Mars missions), record a long period of wetter, more clement, potentially habitable conditions on early Mars.
More recently, Curiosity has climbed higher and into the transition between the clay-bearing rocks and overlying sulfate-bearing rocks noted from orbit. Changes observed in the landscape and analyzed samples relate to a decrease in the decreasing amounts of clay with increasing elevation and the onset of rock units and forms (including mud cracks that form when wet mud dries and contracts, seen below) likely emplaced in a river setting rather than in a lake. In other words, the long-lived lake in Gale crater was drying out and being filled in as the environment transitioned to one where there was water only some of the time (rather than all of the time) in a large, deep lake.
Curiosity is currently just at the top of the transition between the clay-bearing and sulfate-bearing rocks and is documenting additional changes in the rock textures and composition that point to a time when the climate experienced further drying and became much more like it is today. Nevertheless, there is good evidence that the climate did not remain exclusively dry and that there was some late water activity that resulted in deposition of some features (e.g., small fans and possible deltas within the crater) and perhaps sourced a groundwater system that further modified the chemistry and cementation of the rocks.
The story of the Curiosity mission is still being written as the rover continues to explore, sample, and document the rocks ever higher on Mt. Sharp. Ultimately, the goal is to understand how the sediment, chemistry, and distribution of the various rocks not only record the drying of Gale crater and Mars over time, but also decipher clues as to the cause of why the Mars of today is so cold and dry relative to wetter, more habitable times in the past. Stay tuned!