Could it be that Mars - the dry, red planet - once had an ocean so huge it covered around one-third of its surface … and that rain created it? New research by scientists from Northern Illinois University (NIU) and the Lunar and Planetary Institute in Houston would indicate that could be the case. Using an innovative computer program to produce a highly detailed global map of the valley networks on Mars, their findings indicate the networks are more than twice as extensive (2.3 times longer in total length) as had been previously shown in the only other planet-wide map of the valleys. These valley networks roughly form a belt around the planet between the equator and mid-southern latitudes, which would indicate the presence of a long-gone ocean.
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 Prof 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."
Luo, together with Tomasz Stepinski, a staff scientist at the Lunar and Planetary Institute, published their findings in the current issue of the Journal of Geophysical Research — Planets.
"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," Stepinski said.
Similar to river systems on Earth, the valley networks on Mars suggest the Red Planet was once warmer and wetter than present. Since the networks were discovered in 1971 by the Mariner 9 spacecraft, scientists have argued whether they were created by erosion from surface water, which suggests a climate with rainfall, or through groundwater sapping. Groundwater sapping can occur in cold, dry conditions.
"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.
Updated planet-wide map
The researchers created an updated planet-wide map of the valley networks by using a computer algorithm that analyzes 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," said Stepinski, who developed the algorithms used in the mapping. "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 basic idea behind our method is to flag landforms having a U-shaped structure that is characteristic of the valleys," Stepinski added. "The valleys are mapped only where they are seen by the algorithm."
The Martian surface is characterized by lowlands located mostly in the northern hemisphere and highlands located mostly in the south. 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.
"Such a single-ocean planet would have an arid continental-type climate over most of its land surfaces," Luo said.
"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.
The research was funded by NASA.