A simulation of the thermal and physical state of the Earth at the end of Late Heavy Bombardment. Dark circles denote crater locations, and colors represent temperatures: red is hot, blue is cool.
Credit: Oleg Abramov
SYDNEY: Intense asteroid bombardment nearly four billion years ago may not have sterilised early Earth as previously thought. The impacts may have even provided a boost for early life, says a new study.
The research focussed on a particularly cataclysmic occurrence known as the Late Heavy Bombardment, or LHB. This event occurred approximately 3.9 billion years ago and lasted 20 to 200 million years.
In this week's edition of the U.K. journal Nature astrobiologists at the University of Colorado, in boulder, U.S., report on the results of a computer modelling project designed to study the heating of Earth by the bombardment.
Subsurface habitats
Their results show that while the bombardment might have generated enough heat to sterilise the Earth's surface, microbial life in subsurface and underwater environments almost certainly would 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 appeared," said study co-author Stephen Mojzsis.
Surface habitats for microbial life on early Earth would have been destroyed repeatedly by the bombardment. However, at the same time, impacts could have created subsurface habitats for life, such as extensive networks of cracks or even hydrothermal vents.
Any existing microbial life on Earth could have found refuge in these habitats. If life had not yet emerged on Earth by the time of the bombardment, these new subsurface environments could have been the place where terrestrial life emerged.
A growing scientific consensus is that during our Solar System's formation, planetary bodies were pummelled by debris throughout the LHB. A visual record of the event is preserved in the form of the scarred face of our Moon.
Recycled rock
On Earth, all traces of the bombardment appear to have been erased by rock recycling forces like weathering, volcanoes or other conditions that cause the crust to move or change.
"Even under the most extreme conditions we imposed on our model, the bombardment could not have sterilised Earth completely," said Oleg Abramov, lead author of the paper.
"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," he said. "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."
"Exactly when life originated on Earth is a hotly debated topic," commented astrobiologist Michael New, manager of the Exobiology and Evolutionary Biology Program at NASA Headquarters in Washington DC. "These findings are significant because they indicate that if life had begun before the LHB or some time prior to four billion years ago, it could have survived in limited refuges and then expanded to fill our world."
With NASA.
