Liquid water on Mars may have once drizzled from the sky
or collected as dew on the ground.
The falling water left signs in the Martian soil measured
by NASA's Viking, Pathfinder, and rover missions, a new study suggests.
"By analyzing the chemistry of the planet's soil, we
can derive important information about Mars' climate history," said Ronald
Amundson, UC Berkeley ecologist and the study's lead author.
It was already known that it snows
around the north pole of Mars. And there is ample evidence that liquid
water used to well up from beneath the surface, as hot springs or groundwater.
But water falling from the Martian sky is something
that's never been proven.
The frigid climate on Mars has locked the planet's water
in ice, but scientists agree that a warmer climate 4.6 billion to 3.5 billion
years ago allowed for liquid water in lakes and rivers. Now the new study
suggests that liquid water still existed in the Martian atmosphere even more recently,
from 3.5 billion to 1.8 billion years ago.
The study will be detailed online in Geochimica et
Cosmochimica Acta this month and in a print issue in August.
Amundson's team saw evidence that the Martian soil had
lost much of the elements in the original rock fragments which formed the soil.
That pattern suggests water once moved downward through the dirt and washed
those elements away.
The Mars record also shows a long period of drying out,
as seen in surface patterns in the sulfate-rich soil. The researchers compared
that to very similar patterns in Chile's Atacama Desert, the driest region on
Earth with just 1 millimeter of annual rainfall on average.
"The Atacama Desert and the dry valleys of Antarctica
are where Earth meets Mars," Amundson said. "I would argue that Mars
has more in common geochemically with these climate extremes on Earth than
these sites have in common with the rest of our planet."
Sulfates appear to have accumulated on the surface with layers of chloride salt
beneath — a pattern suggesting water fell onto the soil from the Martian
atmosphere. Because sulfates are less soluble in water than chloride, they tend
to separate out from water first. That means water moving downward would carry
chlorides further beyond the sulfates.
Scientists still debate how much Earth's climate can
compare to that of Mars, but Amundson argues that the Martian soil stands as a
"museum" containing the history of water on the planet. Looking to Earth may
help unlock that record.
"It seems very logical that a dry, arid planet like
Mars with the same bedrock geology as many places on Earth would have some of
the same hydrological and geological processes operating that occur in our
deserts here on Earth," Amundson said.
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