The bombardment of Earth by asteroids 3.9 billion years ago may have enhanced early life, according to a new University of Colorado study. Credit: NASA/JPLSelections from the NASA Press Release...
A NASA-funded study indicates that an intense asteroid bombardment nearly 4 billion years ago may not have sterilized the early Earth as completely as previously thought. The asteroids, some the size of Kansas, possibly even provided a boost for early life.
The study focused 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 a letter published in the May 21 issue of Nature magazine titled "Microbial Habitability of the Hadean Earth during the Late Heavy Bombardment," Oleg Abramov and Stephen J. Mojzsis, astrobiologists at the University of Colorado's Department of Geological Sciences, report on the results of a computer modeling project designed to study the heating of Earth by the bombardment.
Results from their project show that while the Late Heavy Bombardment might have generated enough heat to sterilize Earth's surface, microbial life in subsurface and underwater environments almost certainly would have survived.
"Exactly when life originated on Earth is a hotly debated topic," said Michael H. New, the astrobiology discipline scientist and manager of the Exobiology and Evolutionary Biology Program at NASA Headquarters in Washington. "These findings are significant because they indicate that if life had begun before the LHB or some time prior to 4 billion years ago, it could have survived in limited refuges and then expanded to fill our world."
"Even under the most extreme conditions we imposed on our model, the bombardment could not have sterilized Earth completely," said 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. 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."
NASA's Astrobiology Program's Exobiology and Evolutionary Biology Program and the NASA Astrobiology Institute at NASA's Ames Research Center at Moffett Field, Calif., through its support of NASA's Postdoctoral Program, provided funding for this research. The Astrobiology Program supports research into the origin, evolution, distribution and future of life on Earth and the potential for life elsewhere.
Selections from the news article...
Impact evidence from lunar samples, meteorites and the pockmarked surfaces of the inner planets paints a picture of a violent environment in the solar system during the Hadean Eon 4.5 to 3.8 billion years ago, particularly through a cataclysmic event known as the Late Heavy Bombardment about 3.9 million years ago.
No such record exists for Earth because tectonic processes have folded ancient craters back into the interior, but scientists assume our planet took the same pummeling.
Although many believe the bombardment would have sterilized Earth, the new study uses a computer model to show it would have melted only a fraction of Earth's crust, and that microbes — if any existed in the first 500 million years or so of Earth's existence — could well have survived in subsurface habitats, insulated from the destruction.
"These new results push back the possible beginnings of life on Earth to well before the bombardment period 3.9 billion years ago," said CU-Boulder Research Associate Oleg Abramov. "It opens up the possibility that life emerged as far back as 4.4 billion years ago, about the time the first oceans are thought to have formed."
Because physical evidence of Earth's early bombardment has been erased by weathering and plate tectonics over the eons, Abramov and his colleagues used data from Apollo moon rocks, impact records from the moon, Mars and Mercury, and previous theoretical studies to build three-dimensional computer models that replicate the bombardment.
The researchers plugged in asteroid size, frequency and distribution estimates into their simulations to chart the damage to the Earth during the Late Heavy Bombardment, which is thought to have lasted for 20 million to 200 million years.
The 3-D models allowed the researchers to monitor temperatures beneath individual craters to assess heating and cooling of the crust following large impacts in order to evaluate habitability, said Abramov. The study, detailed in the May 21 issue of the journal Nature, indicated that less than 25 percent of Earth's crust would have melted during such a bombardment.
The team even cranked up the intensity of the asteroid barrage in their simulations by 10-fold — an event that could have vaporized Earth's oceans.
"Even under the most extreme conditions we imposed, Earth would not have been completely sterilized by the bombardment," Abramov said.
Instead, hydrothermal vents may have provided sanctuaries for extreme, heat-loving microbes known as "hyperthermophilic bacteria" following bombardments, said study team member Stephen Mojzsis. Even if life had not emerged by 3.9 billion years ago, such underground havens could still have provided a "crucible" for life's origin on Earth, he added.
The modeling work was supported by the NASA Astrobiology Program's Exobiology and Evolutionary Biology Department and the NASA Postdoctoral Program.
The researchers concluded subterranean microbes living at temperatures ranging from 175 degrees to 230 degrees Fahrenheit (79 degrees to 110 degrees Celsius) would have flourished during the Late Heavy Bombardment. The models indicate that underground habitats for such microbes increased in volume and duration as a result of the massive impacts.
Some extreme microbial species on Earth today — including so-called "unboilable bugs" discovered in hydrothermal vents in Yellowstone National Park — thrive at 250 F (120 C).
Geologic evidence suggests that life on Earth was present at least 3.83 billion years ago, Mojzsis said.
Link: LiveScience Article
Link: NASA News Release
Link: 40th Lunar and Planetary Science Conference (2009) Abstract
Link: Article ("Earth science: Life battered but unbowed") in Nature 459, 335-336 (21 May 2009)
Link: Nature 459 Editor's Summary (21 May 2009)
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