Ground zero for the impact that caused a Mars mega-tsunami more than 3 billion years ago may have been found.
The meteor that spawned that ancient flood probably blasted out Lomonosov Crater, a 120-kilometre (75-mile)-wide hole in the ground in the icy plains of the Martian Arctic, a new study reports.
Lomonosov’s large size suggests that the impactor itself was big – similar in scale to the 10-kilometre (six-mile)-wide asteroid that hit Mexico’s Yucatan Peninsula 66 million years ago, sparking a mass extinction that killed off 75% of Earth’s species, including the dinosaurs.
Such big space rocks don’t hit the Red Planet (or Earth) very often. So, the new study provides some important clues about Mars’ ancient northern ocean, and the planet’s past potential to host life as we know it, team members says.
“The implication is that the ocean would have retained a liquid component for a very long time,” study co-author Alexis Rodriguez, a senior scientist at the Planetary Science Institute in Tucson, Arizona, told Space.com. He offered four million to five million years as a representative figure, but stressed that the number is just an estimate.
A cold and mysterious ocean
Mars’ big, salty northern ocean likely formed about 3.4 billion years ago. The ocean’s existence is widely accepted by Mars researchers, Rodriguez says, but there is considerable debate about its nature.
For example, some scientists believe the ocean was relatively long-lived, if quite cold. But others don’t think the ancient Martian climate could have supported stable bodies of surface water for long, and therefore argue that the ocean froze over very quickly – perhaps in a few thousand years or less.
The new study, which was published in late June 2019 in the Journal of Geophysical Research: Planets, bolsters the former viewpoint.
Rodriguez and his colleagues, led by François Costard of the French National Centre for Scientific Research, built upon several years of previous research into the ocean and its imprints on the landscape of ancient Mars.
For example, Rodriguez led a 2016 study that identified huge lobes in the northern plains – features that strongly resemble marks left by tsunamis here on Earth. The team determined that the lobes were carved out by two different mega-tsunamis, which flooded the region more than three billion years ago.
Mars does not have significant plate-tectonic activity, so the big waves were probably unleashed by impacts. So, Costard, Rodriguez and their colleagues hunted for craters left behind by the cosmic culprits, narrowing the search over the next few years.
That search may now be over, at least for one of the two impactors. Multiple lines of evidence point to Lomonosov, the scientists report in the new study. For example, Lomonosov is in the right place, it’s the right age (as determined by crater counts), and it looks a lot like marine craters here on Earth.
Lomonosov fits the bill in other ways as well. For instance, the crater is about as deep as scientists think the shallow northern ocean was at the time of impact. And part of Lomonosov’s rim is missing, which is consistent with a mega-tsunami; the displaced water may have knocked this big chunk free as it raged.
While this evidence is suggestive, however, it does not yet rise to the level of a smoking gun, Rodriguez says.
“This crater is a candidate,” he says. “I would not go so far as to say this is definitely the crater that produced the tsunami.”
That tsunami, by the way, is probably the first of the two big floods that Rodriguez and his colleagues identified back in 2016. That earlier mega-tsunami featured both runoff and backwash flows, the latter of which are caused by water returning to the sea. Lomonosov seems to have been carved by both types of flows.
The second mega-tsunami caused runoff but not backwash, suggesting that Mars, and the ocean, may have been colder at the time. It’s possible the northern ocean had a significant amount of ice cover when this other impactor came crashing down, he added.
Boosting the case for Mars life?
Lomonosov is interesting enough on solely geological grounds.
“This is possibly the first time that a potential marine crater associated with a tsunami has been investigated outside Earth,” Rodriguez says.
And then there are the astrobiological implications. As noted above, Lomonosov’s size suggests the northern ocean – a potentially habitable environment – persisted for a relatively long time. It’s statistically unlikely, after all, that the Lomonosov impact occurred right after this liquid ocean formed.
And even if the ocean were largely frozen at the time, the impact would have created an environment favourable to life as we know it: The tremendous energy unleashed would have melted lots of ice and created a hydrothermal system at Lomonosov, Rodriguez says.
The crater is therefore a tantalising target for future life-hunting missions. Robotic explorers probably aren’t up to the task, however, because the Lomonosov area is covered by an ice layer about 10 metres (33 feet) thick, Rodriguez says.
But human explorers could probably drill down through the ice and access sediments on the crater floor. And these pioneers could use the abundant water ice for life support, providing an exploration twofer.
“That would be very interesting,” Rodriguez says.
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