The solar system's "late heavy bombardment" blasted our planet – but might also have delivered our water, and created nurseries for life
The adolescent Earth's travails were not over with the impact that created the moon. Trouble was brewing further out among the giants of the solar system. Those rumblings eventually precipitated a calamity that once again provided life with an opportunity.
The craggy features of the "man in the moon" are familiar from childhood stories. Prosaically, though, they are impact craters scarring the moon's surface.
Rock samples brought back by Apollo astronauts reveal an odd fact: the big impact craters all seem to date to the same time, around 3.9 billion years ago. This is concrete evidence of a violent period in the solar system's history known as the late heavy bombardment. The moon was unlikely to have been the only target. Being bigger, Earth was probably pummelled even more intensely, though its more active geology has since erased most of the evidence.
The cause of this impromptu game of planetary bagatelle is still not entirely clear. In 2005, however, came the suggestion that it was triggered by a tussle between the solar system's four giant planets, Jupiter, Saturn, Uranus and Neptune (Nature, vol 435, p 459). Slight drifts in Saturn and Jupiter's orbits eventually led to Saturn's orbital period becoming exactly twice that of Jupiter. This gravitational "resonance" shook up the orbits of all four giant planets and sent nearby comets and asteroids shooting off towards the inner solar system.
Lucky for us? Where Earth formed close in to the sun it would have been too hot for water to condense and be incorporated into our planet. Comets and asteroids formed further out where water ice would have been plentiful. It seems plausible, then, that Earth's first water was delivered by bombardment.
The late heavy bombardment could have had a more direct impact on life's origins, too. Initially, it created extremely harsh conditions on Earth. "Imagine pools of molten rock at the surface the size of the continent of Africa," says Stephen Mojzsis, a geologist at the University of Colorado in Boulder. But once they had cooled, the impact craters would have been ideal sites in which to start life, says Charles Cockell of the Open University in Milton Keynes, UK, with residual heat driving chemical reactions in warm water circulating through the rock (Philosophical Transactions of the Royal Society B, vol 361, p 1845).
Alternatively, if life had already begun, the event would have altered the course of evolution, eliminating all but the most heat-tolerant microbes, says Mojzsis. "This is the story of life - mass extinction leading to new styles of life," he says. It was a story that still had a few chapters to run.
Author: David Shiga | Source: New Scientist [September 30, 2010]
