Clay deposits found in one of the oldest riverbed-like channels on Mars shows some unusual signatures that may shed light on the history of water ? and possibly life ? on the red planet.

Observations made by an instrument onboard NASA's Mars Reconnaissance Orbiter (MRO), currently circling the planet, 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. Those findings were detailed in the July 17 issue of the journal Nature.

Now, a new study, detailed in the Aug. 8 issue of the journal Science, took a closer look at the clays in the Mawrth Vallis region and found that they lie in a uniform sequence of layers that indicates that the chemistry of water there changed over time.

""We see different clays, but the way we see them there, it's kind of like ? a layer cake, where 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 (clays rich in those particular minerals) in the lowest layer, overlain by a layer enriched in reduced iron (making it distinct from the iron in the first layer. Next is a layer of silica opal with a layer of aluminum-rich clays on top.

Bishop says that the iron and magnesium smectites were likely formed as the water in a huge lake transformed underlying basaltic ash or rock (formed by volcanism).

"They're pretty common, and we see those in a lot of areas on Mars," Bishop told SPACE.com. "That's what happened first and that was probably pervasive; there was probably a lot of water for a long time and that happened in the whole area."

The aluminum-rich top layer probably formed during a subsequent watery period where some type of acid-leaching removed the iron and magnesium, and aluminum was all that was left, Bishop explained.

But the really interesting middle layer, the one with the reduced iron, formed after the iron and magnesium-rich layer when "something kind of weird happened," 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.

"Right now we have more questions than answers," Bishop said.

But as more CRISM images are analyzed and future robotic missions are sent to Mars, more information might be gleaned on this unique geology which could help scientists "build a better story," as Bishop put it.