August 8, 2008

Clays Shed Light on Water, Suggest Past Mars Microbes

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.

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.

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.

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).

pretty common, and we see those in a lot of areas on Mars," Bishop told
"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."

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.

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.

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.