Habitable Conditions on the Early Earth

January 23, 2006 / Posted by: Shige Abe

Some of the “conventional wisdom” about the Hadean may be wrong. The heavy bombardment by asteroids and comets, for example, did not happen the way it is often illustrated, with multiple simultaneous impacts streaking across a red-tinged sky. A modest extrapolation from the lunar cratering record suggests that a hypothetical observer perched on a terrestrial hilltop would not have seen more than one impact over a human lifetime. The idea of a global magma “ocean” is also probably overblown. We know from experience on Earth that magma (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 of my boots. Within a month a lava flow is cool to the touch. Thus 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 red magma ocean. It is even possible for much of such a planet to be ice-covered.

When we turn to the origin of life, our attention focuses on liquid water – the one common ingredient that seems to be required for all life on this planet. Therefore, the prime question for astrobiologist is when the Earth first supported a liquid water ocean. New data on this question have been published by E. B. Watson and NAI-member T. M. Harrison, writing in Science for 6 May 2005. Their paper is titled “Zircon Thermometer Reveals Minimum Melting Conditions on Earliest Earth”. Zircons are mineral fragments that retain some evidence of their crystalization even when they have been recycled in generations of rock. These authors assert that zircons from Western Australia’s Jack Hills preserve a record of conditions that prevailed on Earth not long after its formation.

Knowledge of the crystallization temperatures of the Hadean zircons is key to this debate. A thermometer based on titanium content revealed that these zircons cluster strongly at 700°C, which is indistinguishable from temperatures of much more recent zircons formed in granites. They also provide evidence of reworked continental crust as old as 4.1 billion years ago. The temperatures substantiate the existence of wet, minimum-melting conditions within 200 million years of solar system formation, 4.55 billion years ago. They further suggest that Earth had settled into a pattern of crust formation, erosion, and sediment recycling as early as 4.35 billion years.

These authors assert that the crystallization temperatures for the Hadean zircons provide definitive evidence about the state of the early Earth, especially when considered in the context other work. The simplest scenario is melting in crustal environments not unlike those of today under conditions at or close to water saturation. The water saturation indicates the presence of liquid water at that time.

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. Again, the cratering record preserved on the Moon tells us something about what was happening on its sister world, Earth. Even though we cannot identify individual impacts from so long ago, we expect that the Earth was hit several times during the Hadean by asteroids large enough to boil away the ocean. 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 as the biggest projectiles were exhausted, and even the largest subsequent impacts left a few survivors in refugia such as ocean-floor hot springs. We see evidence of this history in the prevalence of anaerobic thermophiles near the root of the universal phylogenetic tree of life. Such heat-loving microbes are sort of creatures we would expect to survive the final, near-sterilizing impacts.

We are beginning to imagine a Hadean world that was highly dynamic, experiencing dramatic swings in surface conditions far greater than those preserved in subsequent geologic history. Most of the time there was a stable liquid water ocean, perhaps ice covered. Intermittently, however, massive volcanic eruptions or asteroid impacts destabilized conditions, creating something like the “hell” we 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.