NASA's Curiosity rover has found a wide diversity of minerals in rock samples from Mars, which suggests that conditions changed in the water environments on the red planet over time.
Layers of rocks at the base of Mount Sharp on Mars accumulated as sediment within ancient lakes around 3.5 billion years ago.
Previous research has shown that the mountain's lowermost layers have variations in minerals that suggest changes in the area have occurred.
In a study published in the journal Earth and Planetary Science Letters, scientists from NASA's Johnson Space Centre in the US described on the first four samples collected from the lower layers of Mount Sharp.
"We went to Gale Crater to investigate these lower layers of Mount Sharp that have these minerals that precipitated from water and suggest different environments," said Elizabeth Rampe from NASA.
"These layers were deposited about 3.5 billion years ago, coinciding with a time on Earth when life was beginning to take hold. We think early Mars may have been similar to early Earth, and so these environments might have been habitable," said Rampe.
The minerals found in the four samples drilled near the base of Mount Sharp suggest several different environments were present in ancient Gale Crater. There is evidence for waters with different pH and variably oxidising conditions.
Studying such rock layers can yield information about Mars' past habitability, and determining minerals found in the layers of sedimentary rock yields much data about the environment in which they formed.
At the base are minerals from a primitive magma source; they are rich in iron and magnesium. Moving higher in the section, scientists saw more silica-rich minerals.
In the "Telegraph Peak" sample, scientists found minerals similar to quartz. In the "Buckskin" sample, scientists found tridymite.
Tridymite is found on Earth in rocks that formed from partial melting of Earth's crust or in the continental crust - a strange finding because Mars never had plate tectonics.
In the "Confidence Hills" and "Mojave 2" samples, scientists found clay minerals, which generally form in the presence of liquid water with a near-neutral pH, and therefore could be good indicators of past environments that were conducive to life.
The other mineral discovered here was jarosite, a salt that forms in acidic solutions. The jarosite finding indicates that there were acidic fluids at some point.
There are different iron-oxide minerals in the samples as well. Hematite was found near the base; only magnetite was found at the top. Hematite contains oxidised iron, whereas magnetite contains both oxidised and reduced forms of iron. The type of iron-oxide mineral present may tell scientists about the oxidation potential of the ancient waters.
"We have all this evidence that Mars was once really wet but now is dry and cold," Rampe said. "Today, much of the water is locked up in the poles and in the ground at high latitudes as ice," he said.
"We think that the rocks Curiosity has studied reveal ancient environmental changes that occurred as Mars started to lose its atmosphere and water was lost to space," he added.