This is a global map depicting the dissection density of valley networks on Mars, in relation to the hypothesized northern ocean. Two candidate sea levels are shown: contact 1 with mean elevation at -1,680 meters and contact 2 with mean elevation of -3,760 meters.
Credit: Wei Luo, Northern Illinois University
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.
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."
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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