Swept Away: Evidence of Erosion on Mars

Global Topography of Mars
Global topography of Mars acquired by the Mars Global Surveyor spacecraft. Old, heavily cratered highlands occur in the south and smooth plains blanket the northern part of the planet. The eroded terrain of western Arabia Terra is indicated and may have been a source for much of the sediment in the northern plains. The Tharsis volcanoes likely led to the development of an early atmosphere that would allow water to erode and transport large portions of western Arabia.
Credit: NASA

Massive erosion shaped the surface of Mars, according to planetary scientists at Washington University in St. Louis. While hunting for safe but geologically-rich landing sites for NASA’s 2003 Mars Exploration Rover (MER) missions, researchers have combed images of the sandy plains in a region of Mars called Arabia Terra.

Brian M. Hynek, doctoral candidate in Earth and Planetary Sciences, and Roger J. Phillips, Ph.D., professor of earth and planetary sciences and director of Washington University’s McDonnell Center for the Space Sciences, suggest that early in Mars’ history, western Arabia Terra, an area the size of the European continent, experienced an extensive erosion event caused by flowing water.

"This entire region has been massively eroded," says Hynek. "The region used to look like the rest of the [southern] highlands, but a vertical kilometer of material – enough to fill the Gulf of Mexico – has been relocated downslope and spread out into the northern plains."

The researchers used high-resolution topographic data from the Mars Orbiter Laser Altimeter (MOLA) instrument on the Mars Global Surveyor mission to construct detailed maps of the planet’s surface.

"This research would not be possible without the superb data acquired by the MOLA team," says Hynek. "Not only does it allow us to look at local, regional, and global flow patterns, we are for the first time able to separate out the older, high-standing cratered inliers from the younger depositional materials. This information was crucial in determining when the denudation took place."

MOLA’s accuracy – to within half a meter – and the more than half a billion data points it has collected, reveals many previously unknown features of Mars’ surface. The research was published in the May issue of the journal Geology.

"We have tried to provide clear evidence that water was, at least in part, responsible for shaping the landscape in this region of Mars", says Hynek.

Oblique view of western Arabia Terra
An oblique view of western Arabia Terra. The eroded highlands are evident throughout the center of the image. Much of the material was likely carried into the topographically lower smooth plains near the top of the image. View is looking northwest and the vertical exaggeration is 12 x.
Credit: NASA

Mars is divided into two main areas: the older Southern Highlands, with lots of craters and small-valley networks; and the younger Northern Lowlands, with few craters and no valley networks. Many scientists believe that billions of years ago massive amounts of eroded material from the highlands were washed northward and deposited in the low-lying northern plains.

"MOLA and Mars Orbital Camera (MOC) data continue to back the notion that the northern plains are sedimentary in origin, as previously hypothesized," says Hynek. The new research by Hynek and his colleagues reveals a likely source for this sedimentary material.

When the researchers began studying maps from the new data, they noticed that one region, western Arabia Terra, is a kilometer lower than the rest of the highlands and borders the lowlands to the north. Before the Mars Global Surveyor mission (the orbiting spacecraft that contains the MOC and MOLA instruments), this region was lumped in with the rest of the highlands. But the new topography reveals that there is more going on here than previously thought.

Looking carefully at western Arabia, they noticed that it differs from the rest of the highlands: It contains very few large craters, few traces of valley networks, and numerous erosional remnants.

"This combination makes it very likely that the entire region was swept away," said Hynek.

But how can you push around all that Martian sand and spill deposits into the northern plains?

"Lots of things can erode planets. Wind is very effective on long timescales; volcanoes, ice and glaciers can all erode features, but on this large of a scale these are unlikely explanations," Hynek says. The massive size of the eroded area and the remnants of valley networks, he concludes, suggest running water was responsible.

Erosion on Mars
Erosion has exposed hundreds of layers of the same thickness and pattern on the floors of craters throughout western Arabia Terra. This example comes from a crater at 8°N, 7°W. In this view, dark windblown sand helps make the layers "stand out" in sharp detail. Layers of repeated thickness and physical expression such as these indicate repeated (cyclic or episodic) patterns of change in the deposition of the sediments that formed these rocks billions of years ago.
Credit: NASA/JPL/Malin Space Science Systems

The researchers believe the erosion event took place very early in Mars’ history, during the Late Noachian period, and ceased around 3.8 to 3.5 million years ago. This timing coincides with other water features found on the planet and heavy outflow of lava from volcanoes early in Mars’ history, as the researchers noted in the journal Science earlier this year. Volcanic eruptions emit great amounts of water, carbon dioxide, sulfur and other greenhouse gases as well as lava and ash. This could have led to the development of an atmosphere on Mars that persisted for a few hundred million years, long enough to raise surface temperature above freezing and maintain liquid water on the planet’s surface.

"Mars has not always been cold and dry with little happening on the surface. At one time it had a heyday," says Hynek.

What’s Next

The researchers are now focusing attention on a large outcrop of hematite occurring within western Arabia Terra, the Terra Meridiani region. The discovery of hematite on Mars is exciting because on Earth hematite, an iron oxide, typically forms in the presence of liquid water.

The hematite deposits on Mars were recently discovered and the community is still struggling to understand their origin.

"It is evident from the last landing site selection workshop that one of the two rovers will go to the Terra Meridiani hematite site, situated in the middle of our study area. We want to go where the water was."