A new theory about the origin of boulder fields strewn across the northern Martian landscape may add to a body of evidence that an ocean once covered a large portion of the planet.
Lorena Moscardelli, a geologist at the University of Texas, published a study in the Geological Society of America’s journal, GSA Today, this month suggesting that the boulders were dragged into place by landslides on the ocean floor.
This phenomenon has been studied across Earth’s landscape. For example, large sandstone blocks known as the Jackfork Group remain in what was once an ocean basin in south-central Arkansas.
Moscardelli contests the alternative theory that the boulders on Mars were deposited by meteorites. She noted that there are no impact craters in some areas. “While the meteorite impact hypothesis can certainly explain the occurrence of some of the boulders observed on the northern plains of Mars, especially those that occur in close proximity to impact craters, it does not provide a comprehensive mechanism to understand the wide distribution of boulders in these regions,” she wrote.
Her theory about the boulders should be taken in the context of mounting evidence for a Martian ocean, and not as a singular proof, she said.
In July 2013, researchers at Caltech found what they said could be the most convincing evidence for a Martian ocean to date. Studying a riverbed in the north with detailed topographical mapping technology, they found the river likely fed into a delta.
A delta forms when water flows into a standing body of water, such as an ocean or lake. While evidence of deltas had been found on Mars before, they were usually fed into a crater or other geological boundary. The water would have filled the crater, forming a lake, not a wide, open ocean.
“This is probably one of the most convincing pieces of evidence of a delta in an unconfined region—and a delta points to the existence of a large body of water in the northern hemisphere of Mars,” Roman DiBiase, a postdoctoral scholar at Caltech and lead author of the paper said, according to a Caltech article.
Moscardelli cited previous studies that suggested deposits around the Martian volcano Olympus Mons were also evidence of underwater landslides; they are known as submarine mass-transport deposits (MTDs). Those deposits were described in terms of their similarities to MTDs in the Hawaiian Islands.
“This interpretation would imply that the total volume of water occupying the northern plains of Mars would have been equivalent to [217 million miles cubed, 350 million kilometers cubed],” she wrote.
By comparison, the Atlantic Ocean is about 192 million miles cubed (310 million kilometers cubed), according to the U.S. National Oceanic and Atmospheric Administration.