Tracking down a mistake

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How do we reconcile the idea of abundant liquid water with the Hell-on-Earth scenario traditionally theorized for the Hadean? According to David Morrison, senior scientist at NASA’s Astrobiology Institute, some of the things we first assumed about the Hadean may simply be wrong. Earth’s bombardment by asteroids and comets, for example, probably did not happen the way it is often illustrated, with multiple, simultaneous impacts streaking across a red-tinged sky. Based on the number and size of craters on the Moon, a hypothetical observer perched on a hilltop on Earth would probably not have seen more than one impact over a human lifetime.

   
  Craters on the Dark Side of the Moon

“The idea of a global magma ocean is also probably overblown,” says Morrison. “We know from experience on Earth that lava cools very rapidly on the surface. I have walked across a lava field in Hawaii within 10 hours of its emplacement with no more harm than slightly singed soles on my boots, and within a month, a lava flow is cool to the touch.” A planet with intermittent volcanism—even at much higher rates than we see on Earth today—is likely to have a relatively cool, solid crust most of the time, not a seething magma ocean. It is even possible for much of such a planet to be ice-covered.

Although the Australian zircons indicate average conditions less Hell-like that the name “Hadean” might suggest, there were still intervals of global catastrophe far more violent than anything we can experience today. The craters left on the Moon tell us that asteroids large enough to boil away the ocean probably hit Earth several times during the Hadean.

 

The density of craters on the moon allows scientists to estimate the rate of large impacts on the Earth during the early history of the solar system. During the intense asteroid bombardment that ended 3.8 billion years ago, roughly 20 impacts large enough to vaporize the oceans occurred over 20 to 200 million years. Between these strikes, the surface could have cooled enough to allow the oceans to re-form. (NASA photograph AS16-3021)

  Photograph of colorful thermophiles in Yellowstone National Park
 

Such impacts would be planet-sterilizing, at least for any life in the oceans. Thus life might have formed and been snuffed out several times in the Hadean. Eventually these sterilizing impacts ceased, and even the largest subsequent impacts left a few survivors in protected locations, such as ocean-floor hot springs. Evidence of this history appears in the organisms that sit at the root of the universal family tree of life: the most ancient common ancestors of life on Earth are heat-loving microorganisms that don’t need oxygen to survive. Such heat-loving microbes are the sort of creatures we would expect to survive the final, near-sterilizing impacts of the Hadean.

 

Archaea are organisms similar to bacteria that are often tolerant of hot, cold, acidic, or oxygen-free environments. These organisms live in extreme locales like Yellowstone hot springs. All life on Earth may have evolved from Archaea that were able to survive the final, large asteroid impacts of Earth’s early history, perhaps in hydrothermal vents in the ocean floor. (National Park Service photograph by J Schmidt)

  Micrograph of the Archaea Methanosarcina thermophila

Scientists are beginning to imagine a Hadean world that experienced far more dramatic swings in surface conditions than anything revealed in subsequent geologic history. Most of the time there appears to have been a stable, liquid water ocean, perhaps ice covered. Intermittently, however, massive volcanic eruptions or asteroid impacts destabilized conditions, creating something like the “Hell” scientists once thought prevailed then. It was in such conditions that life arose on our planet, and it is out of this maelstrom that our microbial ancestors emerged.

    References:
  • Morrison, D. (2006) Habitable Conditions on the Early Earth. Accessed February 27, 2006.
  • United States Geological Survey. (1999) Age of the Earth. Accessed February 27, 2006.
  • Watson, E.B., and Harrison, T.M. (2005) Zircon Thermometer Reveals Minimum Melting Conditions on Earliest Earth. Science, 308 (5723), 841-844.
  • Watson, E.B., and Harrison, T.M. (2006) Response to Comments on “Zircon Thermometer Reveals Minimum Melting Conditions on Earliest Earth”. Science, 311 (5762), 779.
 

Early Archaea may be the ancestors of all life on Earth. This specimen of Methanosarcina thermophila is an example of a methanogen, a type of Archaea capable of living in the harsh environment of the early Earth. (Micrograph copyright Stephen H. Zinder, Cornell University Department of Microbiology)

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