Massive erosion shaped the surface of Mars, according to planetary scientists at Washington University in St. Louis.

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 western Arabia Terra, an area the size of the European
continent, experienced an extensive erosion event caused by flowing water.

“We argue that this entire region has been massively eroded,” said Hynek.
“The region used to look like the rest of the 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. “Before this mission,
topography was known only within a kilometer at best; now we are accurate to
within half a meter at any given point on the surface of Mars,” said Hynek.

MOLA’s accuracy, 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.

Mars is divided into two main areas: the older Southern Highlands with lots
of craters and valley networks, and the younger Northern Lowlands with few
craters and no valley networks.

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, this region was lumped in with the rest of the highlands.
But the new topography reveals that there is much more going on here than
previously thought. Looking carefully at western Arabia, they noticed that
it differs from the rest of the highlands in having very few large craters,
and only a 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 remove all this material and carry it away?

“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,” said Hynek. He said that
the massive size of the eroded area and the remnants of valley networks
suggest running water was responsible.

The researchers believe the erosion event took place very early in Mars’
history, during the Late Noachian, and ended by around 3.8 to 3.5 million
years ago. The 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,” said Hynek.

The researchers are now focusing attention on a large outcrop of hematite
occurring within western Arabia Terra, the Terra Meridiani region. Hematite,
an iron oxide, forms in the presence of water on Earth.

“This is very likely to be one of two Mars Rover landing sites in 2004,”
said Hynek. “We want to go where the water was.”