Young Mars' blitz by comets and asteroids hundreds of kilometres in diameter likely made the planet more habitable, not less, a new study suggests.
A pair of geologists in the US modelled the Red Planet during the Late Heavy Bombardment around four billion years ago and found the heat generated by the collisions warmed the cold little planet to life-supporting temperatures.
The work, by Oleg Abramov at the United States Geological Survey and the University of Colorado's Stephen Mojzsis, was published in Earth and Planetary Science Letters.
Mars orbits around half as far again from the Sun as Earth, within what's known as the "Goldilocks zone" where conditions are just right for liquid water to form on the surface. And while we know water flows on Mars, we're yet to find evidence that it could to nurture life.
Besides orbiting further from a solar heat source, there are a few reasons Mars is the cold barren desert we see today.
It's much smaller than Earth, so shed heat faster over time. By cooling and solidifying, it lost its protective magnetic field, allowing energetic particles from the Sun to whisk its atmosphere away.
But when the Solar System was a mere 500 million years old or so, it copped a blitz of comets and asteroids that smashed into the planets – including Mars and Earth – for more than 100 million years.
Massive craters in Earth's crust, over time, were folded back into the underlying mantle, thanks to plate tectonics. But on static Mars, the craters remain. Some 60% of the planet's crust is older than 3.7 billion years.
They found that 100-million-year blitz would have melted and fractured the Martian crust to melt subsurface ice, even if the planet was as cold and barren as it is today.
To see how did the Late Heavy Bombardment may have shaped the Red Planet, Abramov and Mojzsis used a 3-D computer model. Running the model across the whole surface of Mars – which is around 28% that of Earth's – took a supercomputer cluster two weeks.
They found the 100-million-year blitz would have melted and fractured the Martian crust to melt subsurface ice, even if the planet was as cold and barren as it is today.
As heavenly bodies slammed into the planet, huge amounts of hot material ejected from the craters would have draped over nearby areas.
Underneath, molten rock became volcanoes, generating hydrothermal regions such as those in Yellowstone National Park, which house heat- and acid-tolerant bacteria. And massive impacts might have temporarily increased atmospheric pressure – thus increasing temperature.
"On Mars, impacts giveth more than impacts taketh away," they write.
But all good things must come to an end. Their model also showed just a few million years after the bombardment ceased, the planet cooled to its desert-like state.
“None of the models we ran could keep Mars consistently warm over long periods,” Mojzsis says.
Neighbouring Earth, though, did stay warm and wet. It didn't escape the bombardment, but a 2009 study in Nature also by the pair showed the deluge simply did not have the force to stamp out potential early life on Earth. In fact, it may have even given life a boost if it was already present.
“What really saved the day for Earth was its oceans,” Mojzsis says. “In order to wipe out life here, the oceans would have had to have been boiled away.
"Those extreme conditions in that time period are beyond the realm of scientific possibility.”
Looking for life in salty Martian streams
Belinda Smith is a science and technology journalist in Melbourne, Australia.
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