A lot more Martian
rocks were altered by water than scientists originally thought, suggesting that
early Mars was a very wet place.
New
observations made by NASA's Mars Reconnaissance Orbiter (MRO), currently
circling the planet, have revealed evidence that vast regions of the southern
highlands of Mars were altered by water in a variety of environments billions
of years ago.
Water is a key condition for life as we know it. Though there is no firm evidence that Mars has ever harbored life, knowing that the planet was once wet suggests that it was at least habitable in the past.
The key to
the finding is the discovery that rocks called phyllosilicates are widespread
on at least the planet's southern hemisphere. The water
present on Mars from about 4.6 billion to 3.8 billion years ago transformed
some rocks into these phyllosilicates, which include clays rich in iron,
magnesium or aluminum, mica, and kaolinite (an ingredient in Kaopectate).
"In a
phyllosilicate, the atoms are stacked up into layers, and all of the
phyllosilicates have some sort of water or hydroxyl [oxygen and hydrogen group]
incorporated into the crystal structure," said study team member Scott
Murchie of Johns Hopkins University.
Previous data
from an instrument called OMEGA - Observatoire pour la Mineralogie, l'Eau, les
Glaces et l'Activite on the Mars Express spacecraft had revealed only a few large
outcrops of phyllosilicates, suggesting they were a relative rarity on
Mars.
"It
sort of gave the false impression that rocks that were altered like this were
more restricted than they really are," Murchie said.
But the new
observations, made with MRO's Compact Reconnaissance Imaging Spectrometer for
Mars (CRISM) and detailed in the July 17 issue of the journal Nature,
reveal "thousands and thousands of outcrops that we can now resolve with
the higher resolution of the instrument, and they're scattered all over the
planet wherever the older rocks occur," Murchie told SPACE.com.
"What
that's suggesting to us is that we're seeing a pervasive subsurface layer that
goes back in time — it's been altered by water to clays and related minerals,
and it's outcropping all over the place," Murchie added.
The layer
of water-altered rocks sits below younger, volcanic rocks and the ubiquitous
windblown Martian dust and sand in many places. But in craters and scarps,
including Valles Marineris, across the terrain of the southern hemisphere, the
ancient clays and other minerals have been exposed.
"It's
like going to the bottom rock layer in the Grand Canyon," Murchie said,
where ancient layers underlie the whole area, but are only exposed in a few
places.
This
layering gives scientists a dividing line of about 3.7 to 3.5 billion years ago
for a transition in Martian geology: "Before that the rocks were altered
into clays, since then they're not," Murchie said.
The variety
of clays and other minerals formed also tells scientists that rock was altered
by water under a variety of conditions.
"There's
a variety of environments that are formed where the rock was lightly altered
where you see things like chlorite, to where it was altered with water at
really high temperature, where you see mica, to where a lot of water must have
flowed through the rock in order to dissolve out the iron and magnesium and
you're left with kaolinite," Murchie said.
The
alteration of later rocks, such as the sulfates found by the Mars Exploration Rovers (MER),
Spirit and Opportunity in the northern hemisphere, on the other hand, formed
under much more restricted conditions.
One
implication of these findings is that some of the environments that formed the
phyllosilicates would not have been antagonistic to any potential life, unlike
the conditions that formed the sulfates, which formed in a highly acidic
environment similar to battery acid, as Murchie put it.
Whether the
MER rovers can get a close-up peek at these phyllosilicates while the robots
still roam the Martian surface is uncertain, Murchie said, because so far the
rocks haven't been detected near the crafts. But they could be there and simply
be obscured in the north from the MRO instruments by dust.
"It
doesn't take much to hide something from our optical instrument in orbit,"
he said, just a few micrometers of dust. "So just brushing away the rock
surface could be enough," he added.
Whether or
not Spirit and Opportunity get a chance to investigate these intriguing rocks
up close, future rover missions, such as the Mars
Science Laboratory set to launch 2009, could certainly be aimed at known
phyllosilicate-rich sites, Murchie said, shedding more light on the mysteries
of early Mars.
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