Could life arise twice on the same planet?
That’s the question I posed to both Oleg Abramov, a research space scientist at the USGS Astrogeology Science Center in Flagstaff and Steve Mojzsis, a geologist at the University of Colorado in Boulder.
“Life evolves, adapts and spreads pretty quickly on timescales of millions of years,” said Abramov. “So, it’s pretty difficult to sterilize a planet.”
However, because earth was habitable long before the era of massive, surface-sterilizing asteroidal and cometary impacts had ended, a few researchers think there’s a chance that life might have even evolved more than once.
Aside from a highly-anomalous moon-forming type impactor, once life takes hold on a given planet, it’s actually more resilient than commonly thought. So, impactors of the sort that would actually sterilize a life-rich planet in order for it to emerge all over again are actually few and far between.
That’s because microbes can survive deep in our crust. So-called thermophile bacteria are arguably our most distant ancestors. It’s thought that these high-temperature loving micro-organisms could ride out massive bombardments by surviving in very warm environments; like near deep-sea hydrothermal vents, or even kms beneath earth’s crust.
Abramov says that even impacts during the Late Heavy Bombardment (or LHB), the putative spike in the number of impacts from Main Belt Asteroids some 3.9 billion years ago would probably not have been enough to vaporize the oceans.
Thus, Mojzsis says the commonly held view is that life on earth emerged sometime at or before the LHB. That’s well “after” the cataclysmic impact event that formed our moon, some 4.53 billion years ago.
Hence the “window” for the emergence of life on our planet, says Mojzsis, is in earth’s geological “dark ages” between 4.51 and 3.9 billion years ago.
“I strongly suspect that the origin of life happened on [earth], not much later than 4.4 billion years ago,” said Mojzsis. “That’s when we have the first direct evidence for chemically evolved crust interacting with liquid water in the form of the oldest terrestrial minerals from Western Australia.”
Abramov points out that living microbes have been found kms below the surface deep in the earth’s crust. Thus, he reasons that if those areas were already colonized at the time of the Late Heavy Bombardment, then it would have been exceedingly difficult to sterilize such deep subsurfaces.
“I’m running computer models that simulate the thermal conditions in the entire crust of the earth during various bombardment scenarios,” said Abramov. “What I’m seeing from the vast majority of my simulations is that it’s quite difficult to sterilize the planet using impacts.”
If a bombardment-type scenario is not enough to sterilize a planet once life had started, what would be enough to do it?
An extremely large impact, like the impact that formed the moon, says Abramov.
“Essentially, you would have to melt most of the crust and heat the remainder of the crust to a temperature that’s not survivable by any kind of micro-organism,” said Abramov.
What are we learning about how life evolved and held on here on earth that can be applied to astrobiology in general?
Impacts and planetary bombardments, Abramov says, may in fact have a net positive effect on the processes leading to the origin and evolution of early life. They not only deliver essential elements for life, he says, but also create hydrothermal systems that may have provided a site for life’s origin.
Mojzsis says no one knows how life originates, but geologists can answer the question “when could life have emerged?” using geochemical tools and physical models.
He notes that after the moon-forming event about 30-80 million years after the solar system formed, it took something like 2-3 million years for the earth to cool sufficiently to have a rocky surface rind (proto-crust) upon which liquid water could condense.
“Once you have that at the global scale,” said Mojzsis, “then the planet’s pre-biotic chemical reactor could perform its work.”
Could earth life have arisen before the moon-forming impact?
“If there was life on earth before the giant impact that formed the moon, it was completely destroyed,” said Mojzsis. “But [since] Mars did not experience a moon-forming impact on the scale of earth’s, perhaps Mars is where we can answer [this] question.”
Martian subsurface hotspots that still generate occasional hydrothermal activity might provide habitats for life even in the present day, says Abramov. Even so, he says, we’d have to drill several kms down to get to them.
But from what we know now, is microbial life likely to be ubiquitous in the galaxy?
“That’s still a big open question,” said Abramov. “We just don’t know if what happened on earth was unique or, given the right conditions, life starts easily.”