Life on Earth took a licking, kept on ticking

Early organisms may not have had to restart after a long spell of asteroid impacts

An extended period of asteroid bombardment early in Earth’s history may not have wiped out the planet’s nascent life after all. Indeed a report in the May 21 Nature suggests that the pummeling those extraterrestrial impacts delivered may have cracked Earth’s crust and created large numbers of hydrothermal environments where some forms of life could have thrived.

About 3.9 billion years ago, shifts in the orbits of Jupiter, Saturn and other planets in the outer solar system disturbed the orbits of many asteroids, several of which plunged sunward (SN: 2/14/09, p. 26). As a result, the rocky bodies of the inner solar system — including Earth, the moon and Mars — were repeatedly pounded by asteroids during a multimillion–year interval dubbed the Late Heavy Bombardment, says Stephen Mojzsis, a geochemist at the University of Colorado at Boulder. Scientists have previously thought that any life on Earth during that era was snuffed out time and time again as major impacts sterilized the planet. However, Mojzsis and colleague Oleg Abramov report, some forms of life could have survived — and even thrived — throughout the bombardment.

Mojzsis and Abramov used a computer model to estimate how asteroid impacts of the Late Heavy Bombardment would have affected Earth’s habitable zones. Today, life covers Earth’s surface and inhabits its seas, and some microbes can dwell inside rocks of the planet’s crust at depths of at least 4 kilometers. Previous studies have shown that microbes can survive and even reproduce at temperatures of 110° Celsius, so Mojzsis and Abramov used that temperature as the threshold at which an ecosystem became uninhabitable.

Other studies hint that about 2 trillion trillion (2 X 1018) metric tons of material fell on Earth during the bombardment, an interval that was between 20 million and 200 million years long, says Mojzsis. By modeling how that mass would distribute among asteroids similar to those orbiting the sun today, the researchers figure that the largest asteroid to strike Earth during the bombardment would have been about 300 kilometers across. Almost three dozen would have had diameters larger than 100 kilometers, and around 12 million would have been between 100 meters and 1 kilometer across, the researchers estimate.

The small impacts — those made by objects measuring 1 km across or less — would have wiped out life in no more than 1 percent of Earth’s habitable zone. The largest impacts, those by bodies more than 100 km across, would have wiped out life in no more than 13 percent of hospitable areas, they add. Even if all of the bombardment’s impacts happened at once, life could have survived, Mojzsis notes. “Although we tried to kill everything, we were unable to,” he adds.

Many of the bombardment’s impacts would have snuffed out any photosynthetic organisms, which depend on light to survive. However, Mojzsis notes, the events also would have fractured Earth’s crust, creating large hydrothermal systems where temperatures remained below 110° C and where chemosynthetic life — microbes that derive their energy from chemical reactions, not from sunlight — could have thrived, Mojzsis notes.

The new study “is a nice piece of work,” says Mark Harrison, a geologist at the University of California, Los Angeles. The results “may break the knee-jerk reaction that the Late Heavy Bombardment would have annihilated life on Earth,” he adds. “Actually, it would have created some cozy homes for thermophiles” — microbes that can survive at high temperatures.

If the team’s findings are true, “the likelihood of finding life on planets with an equally strong impactor record, such as Mars, … has just improved,” Lynn J. Rothschild of NASA’s Ames Research Center at Moffett Field, Calif., comments in the May 21 Nature.

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