Mars may have experienced a giant tsunami
Evidence suggests a tsunami rolled across Mars, lending strong support to the theory that the planet was once covered by ocean.
A Martian tsunami would have been much larger than anything ever witnessed on Earth.
SHANNONSTENT/GETTY IMAGES
Scientists studying the complex topography of
a region of Mars known as Arabia Terra think they have identified the
source of a tsunami that may
have crashed into its shore billions of years ago, at a time when many
think Mars had ocean covering much of its northern hemisphere.
The
idea of a Martian ocean dates back to the 1990s, when Timothy Parker,
now at NASA’s Jet Propulsion Laboratory in Pasadena, California, used
Viking images to identify what appears
to be an ancient shoreline along the edges of the terrain dichotomy
that separates the Red Planet’s northern lowlands from its southern
highlands.
Since
then, scientists have realised that if one of the many asteroids that
pocked the Martian surface happened to crash into that ocean, a tsunami
was a likely possibility. One of
these teams, led by Alexis Rodriguez of the Planetary Science Institute
in Tucson, Arizona, has even found what it believes to be tsunami
deposits in flat-bottomed channels along the crustal dichotomy.
But
now, in research presented at the 2017 Lunar and Planetary Science
Conference in The Woodlands, Texas, and to be published this week in
the Journal
of Geophysics Research: Planets, a team led by Francois Costard, a
planetary geomorphologist from the Université Paris Sud in France,
believes it has identified the possible sources for one or more of these
tsunamis.
Costard
and colleagues began by looking for craters that might reveal the
locations of tsunami-producing impacts. Then they modeled how the
resulting tsunamis would propagate, using
what team member Stephen Clifford, a planetary scientist from the Lunar
and Planetary Institute in Houston, Texas, calls “well-verified
terrestrial models”.
Not
that tsunami processes on Mars would be exactly the same as on Earth.
Just to begin with, explains Costard, the lower gravity on Mars should
increase the amount of sediment a tsunami
can carry. That means that for any given-size tsunami, the resulting
sediment deposits would be relatively thicker.
Also,
a tsunami generated by an asteroid impact of the size examined by
Costard’s and Clifford’s team would have been huge – unlike anything
humans have ever witnessed.
According
to the team’s modeling, the waves near the point where the asteroid
struck would have been approximately 300 metres tall. When they hit the
coast they wouldn’t be quite that
big, but would still have reached 75 or 80 metres, Costard says.
The
most probable source of the tsunami, the scientists concluded, is a
60-kilometre impact crater located about 1000 kilometres off the
putative coast. But it is also possible that
the deposits could have been produced by the combined results of two
independent impacts, represented by smaller craters closer to the shore.
The
new research also explains bizarre features known as thumbprint
terrain, on the seaward ends of some of the tsunami deposits. Composed
of curving, concentric ridges 10 to 20 metres
high, these look for all the world like the ridges in a fingerprint.
The
explanation, Clifford says, starts with the fact that the tsunami would
have come in two pulses. The first would have been produced when the
asteroid hit, shoving tremendous amounts
of water out of its way. The second would have occurred when water
rushed back into the resulting depression from all sides.
The
onrushing water would have crashed together in the centre of the impact
depression in a giant splash, then rebounded outward in a second
tsunami, even larger than the first.
And
that’s just the beginning of the story. When the first wave, a few
minutes ahead of the second, hit the shoreline, part of it would have
been reflected back out to sea. There,
it would have met the oncoming second wave, where the turbulence would
have caused sediment to be dropped in patterns exactly like the
enigmatic thumbprint terrains.
But
the primary value of the study may not lie so much in localising the
source of the tsunami as in confirming the theory that Mars may once
have had an ocean capable of producing
one.
The
idea of an ocean on Mars has been controversial for many years, says
Jeff Andrews-Hanna, a planetary scientist from the Lunar and Planetary
Laboratory at the University of Arizona,
Tucson, who was not associated with the study. And while it may be that
no single piece of research will resolve the issue, he says, “this
study provides one more piece of evidence”.
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