Planetary News: Earth (2009)
By Amir Alexander
May 29, 2009
Giant asteroid impact on early Earth
Credit: Don Davis/NASA
3.9 billion years ago the Earth was undergoing a cataclysmic convulsion. Over a period that lasted between 20 and 200 million years, the surface of our planet was battered by a storm of impactors that beat it to a pulp. Oceans were heated to the boiling point and bedrock was melted into lava, transforming and reshaping the Earth’s crust. Conditions were so hostile during this Late Heavy Bombardment (LHB), that scientists believed that no living thing could possibly survive them. Only in the calm that followed the storm, when the frequency of space rocks slamming into the Earth returned to its normal rate, did primitive life emerge on Earth. The oldest biological markers known to scientists date precisely to the end of the LHB.
How life appeared and flourished so suddenly on our planet when the bombardment ended is one of the great scientific mysteries of our time. Some scientists believe that the quick emergence of life indicates that life arises naturally and easily whenever conditions allow. This would suggest that we can confidently expect to discover living organisms on other worlds where minimal conditions exist. Other scientists believe that life arose on Earth gradually before the LHB began, in an era known as the Hadean age. In this scenario certain hardy strains of life survived the bombardment and thrived once it was over. Others yet speculate that life may have originated elsewhere in the solar system, and arrived on Earth with the bombardment itself. The Planetary Society’s LIFE experiment is designed to test crucial aspects of this “transpermia” hypothesis.
A new article published in the May 21 issue of Nature magazine may provide a crucial clue to this mystery. The study’s authors, Oleg Abramov and Stephen J. Mojzsis of the University of Colorado, used sophisticated computer modeling to reconstruct the Late Heavy Bombardment and its effect on the Earth. They used studies of lunar craters and computer modeling of the LHB estimate the total mass of space rocks that slammed into the planet. They considered the size distribution of the impactors, ranging from 1 meter to a thousand kilometers, which is known from the size distribution of rocks in the bombardment’s source – the asteroid belt. Finally, they used sophisticated computer simulations to determine the effects of the bombardment on the Earth. In particular they were interested in finding out how much of the Earth’s crust was heated by the bombardment, and to what extent.
The results were illuminating: contrary to the accepted scientific wisdom, the study showed that the Late Heavy Bombardment did not turn the Earth’s crust into an ocean of molten lava in which no life could survive. In fact, the authors argue, “most of the crust was not melted or thermally metamorphosed to a significant degree.” Only one tenth of the Earth’s crust was heated to as much as 500 degrees Celsius.
Pyrococcus furiosus, thermophilic archeon
This organism, whose name means “rushing fireberry,” is a heat-loving extemophile like those that could have survived the LHB. It is a member of the Archaea domain of life and is part of The Planetary Society’s LIFE experiment..
Admittedly, even habitats that had been heated to less than 500 degrees Celsius, say only 200 or 300 degrees, do not make for a life-friendly environment. As a result, “habitats at the immediate surface of the Earth were almost certainly repeatedly destroyed by the LHB” the authors write. But this is, nevertheless, far from the total extermination of life that is often attributed to this period. Life comes in many forms, including important thermophilic strains that are well-adapted to heated environments. While the surface of the Earth was struck repeatedly, the heat generated by the impacts could have produced new subterranean habitats, such as networks of cracks in deeply buried rocks and hydrothermal vents. In these environments life that had emerged in the Hadean age may have survived, or even thrived, during the LHB. It is even possible that life originated during the LHB in these unlkely habitats.
One of the most intriguing results of the study is that while some of the Earth’s crust was heated to a degree that only thermophilic microbes could survive there, much of the crust remained cool enough to sustain normal non-thermophilic life. This suggests that the extreme conditions of the LHB were not wholly detrimental to life, but may actually have helped it to diversify and adapt to differing conditions. “What doesn’t kill you makes you stronger” goes the saying, and it seems to apply not just to the human condition but to life as a whole. Life, it appears, is hardy, and capable of turning even the most hostile conditions to its own advantage.
All of which is good news for those of us looking for life in our solar system and beyond. If life can survive and even thrive under the hellish conditions of the Late Heavy Bombardment, who’s to say it could not survive in equally hostile conditions that prevail on other worlds? From the Hadean age to Mars, the search for life continues.