Early Earth could have been habitable

Hell on Earth: Despite being bombarded by 1000-kilometre wide asteroids, the environment of Earth could have been suitable for life, say researchers (Simone Marchi )

Stuart Gary
ABC

Isolated pockets of liquid water may have existed on the infant Earth even while it was being smashed by giant asteroids that boiled the oceans and created vast seas of magma, a new study suggests.

This means there could have been habitable regions on the Earth during its violent early period, say the authors in today's issue of the journal Nature.

But, they add, any life emerging during Earth's first half billion years would need to have been resistant to extreme conditions, and capable of spreading from the few stable niches existing at that time.

The constant mixing and burial of the Earth's crust by the unrelenting bombardment of asteroids, comets and meteors, during this Hadean epoch, means the geological history of this time -- and whether life existed then -- is poorly understood.

Dr Simone Marchi of the Southwest Research Institute, in Boulder Colorado, and colleagues, have developed a computer model that provides the most detailed picture yet of the Hadean epoch.
Some simulations show up to four large impacts involving 1000-kilometre wide asteroids capable of causing global sterilisation of any life existing at the time.

The simulations also found up to seven asteroids over 500 kilometres wide would have collided with the Earth, each capable of causing global ocean vaporisation, producing a steam atmosphere, and magma oceans, with the most recent occurring four billion years ago.

The researchers found every major part of the Earth would have been affected at one point or another.
Despite all this, the researchers say, there is evidence that there could have been habitable environments at the time.

"We found that the magma oceans were likely to be regional events, so at any given time there were some locations that were calm," says Marchi.

"That means liquid water could have existed in one place or another throughout the 500 million years of the Hadean period."

Marchi says the bombardment by 1000-kilometre wide asteroids would have completely wiped out any life existing at the time and, if that was the case, then life must have started over again after those large collisions.

Moon craters

Marchi and colleagues found the peak bombardment of the early Earth occurred soon after the formation of the Moon and gradually tapered off until the simulation ended 3.5 billion years ago, by which time the number of impacts was negligible.

They determined Earth's early impact history by examining the heavily cratered surface of the Moon which provides a record of the number of impact events and the size of the objects that caused them.

"The moon's surface is on average much older than the surface of the Earth, because the Moon is basically a dead body in which geological evolution is reduced to a minimal level," says Marchi.

"We found the populations of different sized impactors hitting the Moon is very similar to the size distribution of asteroids in the main asteroid belt today. This was also true for Mercury and the oldest surfaces on Mars."