Several
lines of evidence point to the possibility of a past ocean on Mars, from
apparent ancient shorelines to chemicals in the soil.
Add
to the list a
new map of the red planet's valleys. The map shows extensive valley networks
around the equator and in the southern hemisphere, suggesting a warmer Mars
long ago, with extensive rainfall that would have fed an ocean in the northern
hemisphere.
"All
the evidence gathered by analyzing the valley network on the new map points to
a particular climate scenario on early Mars," Northern Illinois University
geography professor Wei Luo said. "It would have included rainfall and the
existence of an ocean covering most of the northern hemisphere, or about
one-third of the planet's surface."
The
valley networks are more than twice as extensive (2.3 times longer in total
length) than had been previously mapped out.
"The
presence of more valleys indicates that it most likely rained on
ancient Mars, while the global pattern showing this belt of valleys could
be explained if there was a big northern ocean," said Tomasz Stepinski of
the Lunar and Planetary Institute.
Luo
and Stepinski published their findings in the current issue of the Journal of
Geophysical Research — Planets.
Longstanding debate
For
nearly four decades, scientists have debated whether the valley
networks on Mars were created by surface water erosion or some other
process, such as groundwater sapping, which can occur in cold, dry conditions.
One
argument against rainfall had been that the valley networks on Mars were not as
dense as on Earth. The new map shows the densities on Mars are greater than
previously thought.
"It
is now difficult to argue against runoff erosion as the major mechanism of
Martian valley network formation," Luo said.
"When
you look at the entire planet, the density of valley dissection on Mars is
significantly lower than on Earth," he said. "However, the most
densely dissected regions of Mars have densities comparable to terrestrial
values."
The
mapping project, funded by NASA, used topographic data from Mars satellite
missions, fed into a computer algorithm.
"The
only other global map of the valley networks was produced in the 1990s by
looking at images and drawing on top of them, so it was fairly incomplete and
it was not correctly registered with current datum," Stepinski said.
"Our map was created semi-automatically, with the computer algorithm
working from topographical data to extract the valley networks. It is more
complete, and shows many more valley networks."
Explaining Mars
The
Martian surface is characterized by lowlands located mostly in the northern
hemisphere and highlands located mostly in the southern hemisphere. Given this
topography, water would accumulate in the northern hemisphere, where surface
elevations are lower than the rest of the planet, thus forming an ocean, the
researchers said in a statement today.
"Such
a single-ocean planet would have an arid continental-type climate
over most of its land surfaces," Luo said.
The
northern-ocean scenario meshes with a number of other characteristics of the
valley networks.
"A
single ocean in the northern hemisphere would explain why there is a southern
limit to the presence of valley networks," Luo added. "The
southernmost regions of Mars, located farthest from the water reservoir, would
get little rainfall and would develop no valleys. This would also explain why
the valleys become shallower as you go from north to south, which is the case.
"Rain
would be mostly restricted to the area over the ocean and to the land surfaces
in the immediate vicinity, which correlates with the belt-like pattern of
valley dissection seen in our new map," Luo said.
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