A new study took a closer look at the clays in the Mawrth Vallis region of Mars and found the clays lie in a uniform sequence of layers, according to the journal Science Friday.
The study indicates the chemistry of water on Mars changed over time and sheds light on the planet's history of water and life.
Previous observations made by an instrument onboard NASA's Mars Reconnaissance Orbiter (MRO) already have shown substantial clay deposits that formed about 4 billion years ago in two regions of Mars, Mawrth Vallis and Nili Fossae, that indicate that water was more widespread in those areas than was initially thought.
"We see different clays, but the way we see them there, it's kind of like ... a layer cake, where we, every time we, every place we get a glimpse of what's there, it's the same order," said study leader Janice Bishop of the SETI Institute in Mountain View, Calif.
"There was a varied chemistry, and it was pervasive, because everywhere we look we see this same trend," she added.
MRO's Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) detected the sequence, which features iron and magnesium smectites in the lowest layer, overlain by a layer enriched in reduced iron. Next is a layer of silica opal with a layer of aluminum-rich clays on top.
But the really interesting middle layer, the one with the reduced iron, was formed after the iron and magnesium-rich layer, Bishop said.
Forming deposits of reduced, or ferrous, iron "usually ... takes microorganisms," she said. For instance, microbes on Earth can transform iron from its ferric to its ferrous state.
But the finding doesn't prove that microbes once existed on Mars, as other processes could account for the iron transformation, Bishop cautioned. Organic carbon, perhaps from an impacting comet, could have reduced the iron or some change in water chemistry could also have done the job. Alternatively, the iron could have been deposited and dried too quickly to oxidize. But which of those processes is correct is anybody's guess at this point.