New research tracking Mars extensive network of valleys and adds to the growing body of evidence suggesting the Red Planet once had an ocean covering a large portion of the northern hemisphere. In a new study, scientists from Northern Illinois University and the Lunar and Planetary Institute in Houston used an innovative computer program to produce a new and more detailed global map of the valley networks. The findings indicate the networks are more than twice as extensive as had been previously shown in the only other planet-wide map of the valleys forming a belt around the planet between the equator and mid-southern latitudes.
Scientists have previously hypothesized that a single ocean existed on ancient Mars, but the issue has been hotly debated.
"All the evidence gathered by analyzing the valley network on the new map points to a particular climate scenario on early Mars," NIU 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.".
Since the networks were discovered in 1971 by the Mariner 9 spacecraft, scientists have hotly debated whether they were created by erosion from surface water, which would point to a climate with rainfall, or through a process of erosion known as groundwater sapping, which can occur in cold, dry conditions.
The large disparity between river-network densities on Mars and Earth had provided a major argument against the idea that runoff erosion formed the valley networks. But the new mapping study reduces the disparity, indicating some regions of Mars had valley network densities more comparable to those found on Earth.
"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 relatively high values over extended regions indicate the valleys originated by means of precipitation-fed runoff erosion—the same process that is responsible for formation of the bulk of valleys on our planet," he added.
The researchers created an updated planet-wide map of the valley networks by using a computer algorithm that parses topographic data from NASA satellites and recognizes valleys by their U-shaped topographic signature.
"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.".
"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.
Source: Northern Illinois University