A UFO enthusiast might think that strange geometric shapes on the surface of Mars are proof of an extraterrestrial civilization, but scientists say they could be good evidence for the existence of an ancient Martian sea.

Various NASA space probes eyeing the Martian surface have been able to pick out polygonal formations, which stand out because of their relatively hard edges. Most of these are located on the northern plains of Mars. Some are relatively small, about 10 or 20 meters across; others are much larger, with sides 100 or more meters long.

In a new paper in the journal GSA Today, University of Texas at Austin researcher Lorena Moscardelli and her colleagues took a close look at imaging data of the Earth's sea floor from the oil and gas industry, looking for features similar to the Martian polygons.

"If there was a deep-water ocean on Mars, then chances are that deep-water polygonal features, as well as other deep-water features that we can see on Earth, could have formed in a very similar fashion," Moscardelli said in an email.

What they found seems to support that notion. The seismic data from Earth's sea floor revealed deep-water polygons more than a kilometer in diameter. The researchers think that topographical features of the sea floor can directly affect how these polygons form and persist - on both Earth and Mars, gentler slopes and flatter surfaces mean more regular shapes and sizes, while steep slopes and the presence of other formations can break up the polygons.

Smaller polygon shapes are thought to be caused by the contraction and expansion of ground ice, of which Mars has plenty. But scientists still debate how the larger formations were made.

Some scientists have thought that the huge formations are forged by the tectonic movements of plates, or possibly from stresses caused by volcanic activity, according to Joe Levy, an Oregan State University researcher unaffiliated with the paper.

While Moscardelli and her colleagues can't prove conclusively that the large polygonal formations on Mars were formed underneath a deep ancient ocean, they've certainly rekindled the debate, according to Levy.

"I think previously people had rejected this marine mechanism explanation because they said there weren't any marine polygons [on Earth], but now they've shown evidence of them," Levy said in a phone interview.

It's still not clear exactly how these shapes form, but we do know that they're made soon after sediments are deposited on the sea-floor. Some sort of fluid expulsion, possibly caused by small physical or chemical interactions within the ocean sediments, generates the outlines of the polygonal shape. When the fault lines appear, if that changes the slope of the sea floor, sediments can then spill into the formation to keep it from closing up again.

"It's like having broken glass on a table and then slowly turning the table over," Moscardelli explains.

There's other evidence for old oceans on Mars. Scientists have mapped formations that look like ancient shorelines, and Moscardelli has previousy argued that various teardrop-shaped islands on the planet's surface were made deep beneath an ancient sea, where sediment flows were warped by craters into elongated shapes.

"The presence of these big-scale polygons is just an additional observational element that reinforces the whole story," Moscardelli says.

Mars' northern hemisphere is also, as a whole, lower than the planet's southern half, so it makes sense that water would have pooled in that region, Levy says.

Moscardelli is quick to emphasize that she and her colleagues can only hypothesize about the Martian formations -- for now.

"The only way to know for sure will be to put a geologist with a hammer on Mars," Moscardelli says.

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