[meteorite-list] Impacts Might Have Made Ancient Mars Briefly Hospitable to Life

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed, 4 May 2016 16:36:23 -0700 (PDT)
Message-ID: <201605042336.u44NaNKY001213_at_zagami.jpl.nasa.gov>

https://eos.org/articles/impacts-might-have-made-ancient-mars-briefly-hospitable-to-life


Impacts Might Have Made Ancient Mars Briefly Hospitable to Life

A bombardment of the Red Planet 4 billion years ago could have created
hot springs that allowed life to flourish.
  
Comets or asteroids striking the Martian surface long ago might have unleashed
intense heat that lingered in the bottoms of impact craters and supported
microbial life. This relatively recent impact crater photographed last
year spans a little more than a kilometer in the Sirenum Fossae region
of Mars. Credit: NASA/JPL/University of Arizona

By Shannon Hall
EOS - Earth & Space Science News
28 April 2016

Hydrothermal springs like those in Yellowstone National Park - pools of
blue steaming water with concentric rings of green, yellow, orange, and
red mats of thriving bacteria - might have once dotted Mars' young
surface.

New research suggests that when comets and asteroids bombarded the Red
Planet roughly 4 billion years ago, the heat from the largest impacts
transformed the craters they forged into hydrothermal springs, which are
well known for being microbial oases.

Microbial life could have inhabited these hot spots for the geologically
brief interval - a few million years - during which the springs would
have existed, proposed Oleg Abramov from the U.S. Geological Survey in
Flagstaff, Ariz., and Stephen Mojzsis from the University of Colorado,
Boulder.

For ancient microbes, hydrothermal springs would have provided a cornucopia
of nutrients. Those in Yellowstone, for example, typically dine on sulfur
and release a gas, which gives the park its aroma of rotten eggs.
"I like to draw the analogy to a buffet bar in Vegas," says Mojzsis.
 "[Hot springs give] lots of food and it's practically for free."
He and Abramov will publish their findings in the 15 May issue of Earth
and Planetary Science Letters. The paper was posted on the journal's
website in March.

Creating Hydrothermal Springs from Impacts

Scientists dub the period during which these Martian hydrothermal springs
would have formed the Late Heavy Bombardment. At the time, swarms of comets
and asteroids pelted the inner moons and planets of the developing solar
system. A few of the targets - like Mercury, the Moon, and Mars - have
abundant battle wounds still visible today.

On Mars, "some of [these impacts] were large enough - hundreds of kilometers
across - to make a thousand-kilometer-diameter basin and drape the entire
surface of Mars in a hundred meters or so of molten rock," says Mojzsis.
"It's like pouring very hot molten chocolate on top of your vanilla
ice cream. It melts after a short time."

Any impact large enough to melt the rock would have also melted huge amounts
of the ice that lay frozen in the Martian crust, allowing water to flow
into newly formed craters. Indeed, evidence of ancient river valleys and
lake sediments on Mars often coincides with the oldest crater surfaces,
says Mojzsis. The hot rock below those newly formed rivers or lakes would
have then heated the water enough to create a hot spring - lasting as
long as the rock remained hot.

Analogues on Earth?

Steam rises from the hydrothermal Grand Prismatic Spring in Yellowstone
National Park, Wyoming. Credit: Paul Racko, CC BY-NC-ND 2.0
Two massive impact craters on Earth provide definitive proof that crater
bottoms can cradle hydrothermal springs. Both the Sudbury crater in Canada
and the Vredefort crater in South Africa show signs of hydrothermal alteration,
in which the hot water chemically changed the nature of the rock - a signature
that remains visible long after the water has disappeared. To boot, the
Haughton crater in the Arctic shows evidence of not only hydrothermal
alteration but also the fossilized microbial communities that once lived
there.

Although the craters on Mars show similar hydrothermal alteration (but
no microbial communities have been found...yet), scientists can't be
sure if that alteration resulted from impacts or previous volcanism. Mojzsis
thinks the best evidence comes from the ages of the crater bottoms.

Samples measured in situ by the Curiosity rover currently roaming around
one of the largest impact craters show that the rock dates from the onset
of the Late Heavy Bombardment. Only an impact large enough to melt the
surface and create a hydrothermal hot zone could have erased prebombardment
age signatures that otherwise would have been left in the rock by decay
of radiometric isotopes, Mojzsis explained.

Further evidence could come from the Curiosity rover or perhaps the Mars
2020 mission, if its team chooses a site within another massive impact
crater.

Cooking Up a Thick Atmosphere

To explore the effects of the bombardment on ancient Mars, Abramov and
Mojzsis used a supercomputer cluster to model every impact that struck
the Red Planet's surface throughout that time. Although scientists can't
know exact details of the population of asteroids and comets that pummeled
the Red Planet back then, they think that those objects likely resembled
the denizens of the current asteroid belt (i.e., a few massive objects
interspersed with many small objects).

With so many impacts, there could have been enough thermal energy to cook
the surface of the planet and release plenty of water vapor into the atmosphere.
In the end, Abramov and Mojzsis modeled four different bombardment scenarios.
All suggest that with so many impacts, there could have been enough thermal
energy to cook the surface of the planet and release plenty of water vapor
into the atmosphere. The higher temperature, thicker atmosphere, and newly
flowing water might have even created a global environment conducive to
life - at least for a time.

Are We Martians?

Norman Sleep, a geophysicist at Stanford University who was not involved
in the research, thinks the possibility that Martian conditions could
have temporarily supported life planet-wide strengthens a hypothesis put
forth a few years ago that the seeds of life actually arose on Mars. Subsequent
asteroid impacts on the Red Planet would have then spewed chunks of rock
into space, potentially carrying life onto the surface of Earth.

Steven Benner, an astronomer at the Westheimer Institute of Science and
Technology in Gainesville, Fla., proposed the idea 3 years ago. If life
on Mars had survived the bombardment, said Sleep, its extended survival,
thanks to lingering hot springs, makes this intriguing hypothesis more
probable.

There's also no reason to think that life couldn't have arisen in
these hot watery settings. "It's widely hypothesized that life on
Earth got started in a hydrothermal environment, in hot springs," says
Henry Melosh, a geophysicist at Purdue University in West Lafayette, Ind.,
who was also not involved in the research. This hypothesis is based on
the tree of life - a diagram outlining the evolution of all things - which
points back to a common ancestor: an organism endemic to these scalding
environments.

Although the study doesn't prove life began on Mars, let alone in hydrothermal
springs, says Melosh, it might expand scientists' thinking about where
life could have started.

Citation: Hall, S. (2016), Impacts might have made ancient Mars briefly
hospitable to life, Eos, 97, doi:10.1029/2016EO051515. Published on 28
April 2016.
Received on Wed 04 May 2016 07:36:23 PM PDT