None of us would have wanted to be around during the Late Heavy Bombardment, that frenetic bashing of our planet as the young Solar System worked out its debris problems between 4.1 and 3.8 billion years ago. The Hadean period was a time when enormous asteroids pummeled our world over a span lasting as long as 200 million years, an ongoing series of events one would have assumed lethal for whatever organisms may have evolved by then.
But was the Late Heavy Bombardment really the deadly rain we’ve always assumed? A new paper in Nature questions the idea, basing its results on computer modeling of the Earth’s heating during the bombardment. Oleg Abramov and Stephen J. Mojzsis (University of Colorado) argue that our planet’s surface would likely have been sterilized during this period, but microbial life below the surface or in underwater conditions would almost certainly have survived.
“Our new results point to the possibility life could have emerged about the same time that evidence for our planet’s oceans first appears,” said Mojzsis, principal investigator of the project.
If so, it’s conceivable that life has been through a continuous process of development here, rather than one marked by sudden extinctions and re-starts. The theory is given weight by our growing understanding of the role of hydrothermal events deep beneath the oceans, where early life might well have lingered. Abramov thinks the finding has implications for life elsewhere:
“Even under the most extreme conditions we imposed on our model, the bombardment could not have sterilized Earth completely. Our results are in line with the scientific consensus that hyperthermophilic, or ‘heat-loving,’ microbes could have been the earliest life forms on Earth, or survivors from an even more ancient biosphere. The results also support the potential for the persistence of microbial biospheres on other planetary bodies whose surfaces were reworked by the bombardment, including Mars.”
It takes but a glance at the Moon through a small telescope to see the extent of the bombardment on a surface that has not, because of internal activity, resurfaced itself in billions of years. And it’s worth speculating, as we look at nearby stars like Tau Ceti, that the abundance of impactors that may be present in a system like this may not preclude the continuing development of life. That’s a conclusion that’s cheering to this writer, for life once started seems determined to last.
The paper is Abramov and Mojzsis, “Microbial Habitability of the Hadean Earth during the Late Heavy Bombardment,” Nature 459 (21 May 2009), pp. 419-422 (abstract).