Dive to survive!
Drive your time machine back 3.9 billion years.
When the calendar reads 3,899,997,991 BC, you'll want to scurry down to a fallout shelter in the basement, as you have entered a challenging time called the late heavy bombardment (LHB).
Earth is being pummeled by a rockfall of asteroids from the far reaches of the Solar System. No record of the LHB survives on Earth, because the ancient rocks have long since melted back into the molten interior. But the faces of the moon and Mercury record a serious series of impacts that lasted 20 million to 200 million years.
Thus planetary scientists assume Earth was similarly battered during the same period.
By analyzing isotopes in ancient crystals, some researchers say that life existed on Earth 3.83 billion years ago, right after the bombardment ceased. So did this life start right after the asteroid assault abated, or did it originate earlier and survive the bombardment?
The question seems to answer itself, because the rockfall was a blockbuster: About 2 x 1017 tons of rock fell to the planet, enough to cover the surface with a 150-meter layer of rock.
Long live life
But in a new computer study, two University of Colorado researchers calculate that the entire planet did not get hot enough to obliterate all life in the crust, where bacteria and the other group of single-celled organisms, archaea, are found in holes drilled 4 kilometers into the crust.
To explore how the rain of asteroids affected temperatures in Earth's ancient crust, post-doctoral researcher Oleg Abramov coded a computer to simulate the heavy bombardment. His model included 90 giant strikes by objects at least 50 kilometers in diameter.
This larger-than-a-ping-pong-ball stone would make a crater at least 1,000 kilometers across, and distribute enough steam and blazing rock dust into the atmosphere to sterilize the planet's surface zone. A much smaller impact is blamed for exterminating the dinosaurs about 61 million years ago.
"There had been several previous studies saying that life could not have survived on the surface ... but it was still possible that life could have survived in the subsurface," says Abramov. "When you look at habitable environments on present day Earth, there is always some kind of life there. So long as the temperature is tolerable, life will find a way to colonize."
And so Abramov and Stephen Mojzsis, an associate professor of geological sciences at Colorado, looked a little deeper: literally.
If bacteria had a sense of humor, maybe they were laughing off the long airborne assault, because Abramov and Mojzsis found that some places on Earth always remained comfortably below 110° C -- an upper limit for the most heat-tolerant bacteria known.
Natural fallout shelter
Because microbes are regularly found in the top 4 kilometers of the crust, Abramov directed the computer model to calculate temperatures in that zone, and calculated that the impacts warmed the top 4 kilometers by least 10° C.
At no time did the entire zone exceed 110° C.
Habitable volume of Earth's crust
The surface of the moon shows that asteroids came in many sizes, and Abramov says size matters. "If you are looking at the top 100 meters of the crust, the huge, basin-forming impacts caused more sterilization." Deeper down, the more numerous smaller impacts were a bigger dealer of death.
Overall, the large impacts may actually have promoted life, because they formed hydrothermal systems - hot water networks that could have circulated microbes through the crust. In fact, the volume of hot pools and streams of water that were tolerable to the most heat-tolerant bacteria actually increased 10-fold during the heavy bombardment, says Mojzsis.
Thanks to the fallout shelter
Until recently, nobody seriously proposed that life existed during the "Hadean" (hellish) conditions before and during the late heavy bombardment, when water was supposedly scarce and temperatures were blazing hot. But Mojzsis says scientists are revising that picture. "In the last dozen years or so, how we view the early Earth has undergone a fundamental shift. Work from this lab and others shows that the oceans were established within 150 million years of the formation of our moon" about 100 million years after Earth's formation.
Analyses of zircons, durable, ancient crystals that provide some of the only evidence for the Hadean period, "reveal an early planet that was far more benign than theorists had previously promoted," Mojzsis says.
Life is the ultimate striver...
The new study, Mojzsis stresses, does not prove that life survived the bombardment, but "simply evaluates the thermal history of the top 4 kilometers of crust. There is always some place cool enough for life to survive."
But if life did predate the LHB, then the new study could explain why, as many scientists now believe, all organisms on Earth descended from heat-loving bacteria. If the wimpier microbes were killed by the bombardment, the heat-tolerant ones must be everybody's ancestors, Mojzsis says. "In hindsight, it makes sense. The impact may have created a genetic bottleneck, and the life that thrived, the hyperthermophiles, lived in the deep hydrothermal environment; that's the life that passed on its genetic inheritance to all subsequent life forms."
Life, Mojzsis says, is not some wilting flower, but a force of nature that adapts and survives, even if it occasionally must duck into the fallout shelter. "Life as we know it is entrepreneurial. It takes advantage of any place where there is liquid water, an energy source and something to eat."
- David J. Tenenbaum
• Microbial habitability of the Hadean Earth during the late heavy bombardment, Oleg Abramov and Stephen J. Mojzsis, Nature, Vol 459, 21 May 2009, doi:10.1038/nature08015.