Was Mars the Cradle of Life?
The problem of life’s origin remains one of the great outstanding challenges to science. Ever since Charles Darwin mused about a “warm little pond” incubating life beneath sunny primeval skies, scientists have speculated about the exact location of this transforming event. Nearly a century and a half later, we remain almost completely ignorant of the physical processes that led from a nonliving chemical mixture to the first autonomous organism. However, some progress at least has been made on tracking down where and when life first established itself on Earth. Fossil evidence suggests that the biological record extends back at least 3.5 billion years, pointing to a still earlier origin1. But this presents a problem. The cratering record of the moon implies that Earth was subjected to intense bombardment by large comets and asteroids over an extended duration until about 3.8 billion years ago. The largest of these impacts would have released enough energy to swathe the planet in incandescent rock vapour, boiling the oceans and sending sterilizing heat pulses a kilometre into the exposed crust2. This unpromising setting – hardly a secure one for warm little ponds – has prompted some astrobiologists to conjecture that life began somewhere else and came to Earth readymade3. Favourite among extraterrestrial originating locations is the planet Mars4.
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