Scientists studying the Martian landscape said yes, a river
ran through it — and not just one. The ancient red planet also seems to have
experienced rain, they say.
The rivers may have cut the deep valleys in the Martian
highlands near the equator, and also left calling cards elsewhere. Three Mars
spacecraft spotted signs of fan-shaped river deltas inside
ancient craters which some valleys clearly flow into.
"We can see layered sediments where these valleys open
into impact craters," said Ernst Hauber, a geologist at the DLR (German
space agency) Institute of Planetary Research in Berlin-Adlershof. "The
shape of certain sediments is typical for deltas formed in standing
water."
Rivers carry sediment downstream until the currents become
too weak and let the material fall to the river bottom. The flow almost drops
to zero at places where rivers empty into a larger body of water, such as a
lake-filled crater.
Hauber and other researchers focused on possible ancient
river valleys crisscrossing the Xanthe Terra highland region. They examined
crater images taken by the European Mars Express, NASA's Mars Global Surveyor,
and NASA's Mars Reconnaissance Orbiter.
One small 3-mile (5-km) crater contains possible proof in
the form of a fan-shaped delta, where the Nanedi river flows into the crater
from the south. Sediment has almost completely filled the crater up to 164 feet
(50 meters) deep in an area covering slightly less than 9 square miles (23 sq
km).
Researchers also counted the number of craters to roughly
determine the age of the planetary surface in the area. Their crater count
revealed that water flowed through the valleys sometime between 3.8 and 4
billion years ago. Additional work by Maarten Kleinhans, a geologist at the
University of Utrecht in the Netherlands, showed that the crater sediment
deposits formed in no less than a few hundred thousand years.
The valleys flowing into and out of the craters allow
researchers to be "fairly certain that there were lakes on Mars,"
Hauber noted.
An ongoing debate is whether rain and snowmelt or
groundwater may have played a bigger role in creating the valleys. The most
recent findings presented at the European Planetary Science Congress in
Muenster, Germany provide more evidence for the former.
"Our findings also point in this direction and we are
convinced that both processes have played an important role in Xanthe
Terra," Hauber said.
Other research has suggested that water flowed in the plains
around the Valles Marineris region until even more
recently, around 3.7 billion to 3 billion years ago. But no evidence so far
has shown flowing surface water still exists on Mars.
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