Did a vast ocean once cover Mars' northern plains? The idea has been hotly debated among scientists for the past 20 years, ever since Viking Orbiter images revealed possible ancient shorelines near the pole. Later findings even suggested that the primordial ocean—dubbed Oceanus Borealis—could have covered a third of the planet.
But even if the evidence has mounted steadily, fostering our hopes of finding signs of past life on the Red Planet, the case for an ancient Martian ocean remains unsettled.
Now a new study by Lorena Moscardelli, a geologist at the University of Texas, Austin, puts forward yet another line of evidence.
Today, large fields of boulder-size rocks blanket parts of Mars' northern plains. By pointing to analogue geological features on our Earth, Moscardelli suggests that the boulders were delivered to their current locations by catastrophic underwater landslides—bolstering evidence for an ancient Martian ocean.
The boulders were spotted by the HiRISE camera on the Mars Reconnaissance Orbiter a while ago. So Moscardelli is not reporting their presence as something new, but rather a new interpretation of the processes behind their origin. The paper was published this month in a journal of the Geological Society of America.
In the past, geoscientists thought of ocean sediments as mostly fine-grained, floating in the water column and settling like a slow "rain" on the sea floor, Moscardelli explained. But we now know it's not the only possible scenario.
Boulder-size rocks in Arcadia Planitia, northern lowland of Mars (HiRISE ESP_019853_2410). Credit: NASA (Moscardelli 2014) "We know that 'submarine landslides' can transport big boulders—sometimes as big as a house—for hundreds of kilometers into the deep-water of the Earth oceans," she said. "Imagine a huge landslide affecting the entire state of Texas, but happening in the ocean."
In her new study, Moscardelli documents several sites where these events have occurred on Earth, such as the Pennsylvanian Jackfork Group of south-central Arkansas; the outcrops of the Guandacol Formation in the Pangazo Basin, Argentina; or in the Santos Basin, offshore Brazil ( via phys.org ).