Today, Mars is a cold, dry world, home to dust devils and robotic explorers. But many scientists suspect it was once waterlogged.
A new study, published last month in the Journal of Geophysical Research: Planets, suggests that a 75-mile-wide impact scar in the Martian northern lowlands is to the red planet what the Chicxulub crater is to Earth: the mark of a meteor that generated a mega-tsunami when the planet was relatively young. If accurate, the finding adds evidence to the hypothesis that Mars once had an ocean, and would have implications for our search for life there.
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Whether Mars was ever warm and wet enough to retain a long-lived liquid water ocean has long been debated by planetary scientists. Several climate models have indicated that it was probably too cold. But other researchers point to ancient river deltas and other geological evidence of a northern ocean some 3.7 billion years ago.
Additional evidence includes hints of mangled, buried coastlines visible from orbit; these suggest that mega-tsunamis with skyscraper-high waves inundated parts of Mars’s northern shores around three billion years ago. On a world believed to have lacked Earthlike plate tectonics, any tsunamis were probably triggered by a meteor slamming into a huge body of water.
The area where the northern ocean was thought to exist also contains what are called thumbprint terrains, found on the landward side of those suspected ancient shorelines. These terrains may have been etched by the chaotic and violent flow of water in the aftermath of the mega-tsunamis. But which craters were tied to the impacts responsible for those mega-tsunamis was long a mystery.
In 2017, a team led by François Costard, a planetary geomorphologist at the French National Center for Scientific Research, used computer modeling to reproduce the mega-tsunamis most likely to have created those thumbprint terrains. That narrowed it down to a handful of impact craters.
For the new paper, Dr. Costard and his colleagues examined these candidate craters and ultimately zeroed in on Lomonosov, which was likely forged by a 9-by-12 mile meteor in just the right place at just the right time.
Its rim is also roughly the same height as the estimated depth of the ocean. Crucially, it physically resembles Earth’s marine craters, suggesting — if not definitively demonstrating — that it was created in a shallow ocean.
Another clue is the hole in the crater’s southern section. The plains there are tilted up toward the southern highlands. It’s possible that the ocean, displaced by the impact, would have rushed back most aggressively from this direction, bursting through the crater’s southern rim.
Alexis Rodriguez, a Mars geomorphologist at the Planetary Science Institute in Tucson, Ariz. and co-author of the new study, say the remnants of this Martian ocean may be a good target in the search for evidence of life on the red planet.
The ocean may have been fed by catastrophic floods from underground caches of liquid water. If so, sediments in the north “may be a window into the subsurface habitability of Mars,” Dr. Rodriguez said. If they contain geochemical signatures of ancient microbiology, then the liquid aquifers thought to exist beneath Mars’ surface may still be reservoirs for life today.
Of course, all this depends on whether a northern ocean actually existed — a conclusion challenged by studies of the red planet’s ancient climate. Dr. Rodriguez calls this one of the “core paradoxes” of Martian planetary science.
Paul Byrne, a planetary geologist at North Carolina State University who was not involved with the study, agreed: “It’s fair to say that we don’t yet fully understand the history of Mars’s climate, and certainly, the climate models we use will continue to be improved.”
Despite compelling geological evidence, he said, scientists still don’t have unambiguous proof of a northern ocean. “So the climate models might not be wrong,” he added — but more data is needed to say one way or the other.
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