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’s 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?
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).
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.
—Shannon Hall, Freelance Writer; email: [email protected]
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.