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Boulders provide new clues to ancient ocean on Mars

Illustration showing how the ancient Oceanus Borealis may have looked on Mars.
Illustration showing how the ancient Oceanus Borealis may have looked on Mars.
ESA / C. Carreau

The possibility of an ancient Martian ocean is an enticing one, and there has been growing evidence that it did indeed exist (dubbed Oceanus Borealis), covering most of the northern hemisphere, and about a third of the planet, billions of years ago. Now, some new observations of boulders in what likely used to be the ocean bottom have given scientists additional clues as to what this ocean was like, it was announced this past Saturday (February 15, 2014).

In this relatively flat region in the northern hemisphere, larger boulders are scattered across the landscape. A new study by Lorena Moscardelli, a geologist at the University of Texas in Austin, shows that the distribution of these boulders is similar to those on Earth which were moved around by underwater landslides. Her paper was published this month in a journal of the Geological Society of America.

Some other scientists have assumed the boulders, seen in images taken by the Mars Reconnaissance Orbiter, were simply the result of meteorite impacts, but Moscardelli disagrees.

"That's possible for some of the boulders, especially those found close to craters," she said. "But how do you explain boulder fields that can cover thousands of square kilometers without any impact craters around? The submarine hypothesis provides a feasible alternative."

She adds:

"We know that 'submarine landslides' can transport big boulders - sometimes as big as a house - for hundreds of kilometres into the deep-water of the Earth oceans. Imagine a huge landslide affecting the entire state of Texas, but happening in the ocean."

She shows that on Earth, similar occurrences can be found in the Pennsylvanian Jackfork Group of south-central Arkansas, the outcrops of the Guandacol Formation in the Pangazo Basin, Argentina and in the Santos Basin, offshore Brazil as a few examples.

As well as the boulders, other geologic features seen here can also be found underwater on Earth, such as teardrop-shaped islands and polygonal-shaped cracks in the surface.

The Viking spacecraft had already spotted what looked like the remains of shorelines in this area back in the 1980s. These northern plains are also lower in elevation than the terrain in the southern hemisphere, similar to ocean basins on Earth.

We know that there is a lot of ice still buried just below the surface in this region. Could it be left over from the former ocean? The Phoenix lander even directly scooped up some of this ice several years ago. We also now know from the rovers and orbiters that there used to be Martian lakes and rivers; were there oceans too?

Further study will still be necessary to see if this explanation is accurate, but when combined with other evidence it appears to strengthen the case for a former Martian ocean. If so, the next question of course is whether anything was alive in those waters?

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