Teardrop-shaped islands on Mars could have formed deep underwater.

A new study highlights surprising similarities between these Martian landforms and streaky seafloor mounds off the coast of Trinidad, bolstering evidence for an ancient ocean in Mars' northern plains.

"Based on this analogy, I am humbly suggesting that teardrop-shaped islands on Mars formed underwater in a relatively deep ocean," said geologist Lorena Moscardelli of the University of Texas, Austin, lead author of a paper in the July Geology.

Planetary scientists started suspecting Mars might have had a standing ocean in the late 1980s and 1990s, when images from the Viking Orbiter showed what looked like shorelines and river channels flowing into Mars' Chryse Planitia region.

Images from the Mars Odyssey spacecraft, which arrived at Mars in 2001, revealed even more evidence that a lot of liquid had once surged through Chryse Planitia, including the raised, streamlined mounds Moscardelli calls teardrop-shaped islands. The islands look like they were formed during a catastrophic flood – but whether they were carved out of dry land or underwater was unclear.

Moscardelli and geologist Lesli Wood of the University of Texas, Austin examined the shapes and sizes of streamlined features 3,280 feet underwater off the coast of Trinidad.

These features, called erosional shadow remnants, are comet-shaped streaks up to several miles long anchored by a mud volcano on one end. They form when mixtures of sediment and water flows around the mud volcano and gathers on the opposite side. New 3-D maps of the seafloor, which were taken as part of oil and gas prospecting, gave the researchers a more detailed look at the remnants than had been available before.

Moscardelli suggests that the same process could have formed the teardrop-shaped islands, with impact craters instead of mud volcanoes guiding fast-flowing sediment into a comet-like shape.

"Most analogies between Earth and Mars are made using continental (onland) data sets or environments," because that data is easier to gather, Moscardelli said. "I am providing for the first time, as far as I know, a deep-water terrestrial analog."

The streaky islands on Mars are between 3 and 30 miles long and cover 1.5 to 150 square miles, big enough that they more closely resemble the erosional remnants in deep water than land-based features carved by air. Lab experiments suggested the islands' triangular shapes only happen underwater, too, providing more support for the idea of an ancient standing ocean on Mars.

Planetary Scientist Devon Burr of the University of Tennessee thinks the analogy holds up, but it doesn't necessarily mean the Martian islands formed underwater. Instead, Mars' lower gravity could make islands carved by air look like islands carved by water on Earth.

"That is an option," Moscardelli said. "We will be sure when we put a geologist there with an old fashioned hammer to take samples and make direct observations."

Image: 1) NASA/JPL/Arizona State University. 2) Lorena Moscardelli.

See Also: