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A period of intense meteorite assaults on the inner solar system may have stopped far earlier than we thought.

Now there’s evidence to suggest that giant asteroid and comet strikes on Mars stopped 4.48 billion years ago, allowing it to develop conditions favourable to life as early as 4.2 billion years ago.

It overturns a previous suggestion that the inner solar system, including Earth and the moon, continued to be heavily hit by meteorite impacts – a period known as the Late Heavy Bombardment – until around 3.8 billion years ago.

Desmond Moser at the University of Western Ontario, Canada, and colleagues have analysed meteorites thought to come from Mars’s southern highlands. The specimens are pieces of Mars’s crust which were knocked into space by a collision, and have since fallen to Earth as meteorites. Around 120 of these have been recovered to date.

The team analysed the oldest-known mineral grains from these meteorites, zircon and baddeleyite, aged between 4.43 and 4.48 billion years old.

Zircon contains tiny amounts of radioactive uranium that decay to lead over time, which enables the age of the rock to be accurately measured. They also contain microfeatures that reveal whether they have been exposed to the high pressure typical of a big impact event, says Moser.

“We found none of these bombardment signatures in the Mars zircon and baddeleyite grains,” he says. The finding suggests that the asteroid assault of Mars ended before the analysed specimens formed.

“We know there was a giant impact on Mars, but it has to be older than 4.48 billion years ago,” says Moser. “The implication is that there could have been platform hosting life as much as half a billion years earlier than previously thought it was possible in the inner solar system.”

Diminishing evidence

The massive impact on Mars would have been a “globally sterilising” event, he says. But it may also have helped to establish habitable conditions by accelerating the release of water from the planet’s interior.

“We’re refining our understanding of the history of the solar system in terms of what took place in these earliest times,” says Michele Bannister of Queen’s University Belfast.

“Impacts would take place because of things getting scattered around the system in the process of forming planets and rearranging the architecture of the system to what we see today,” she says.

But the evidence for the Late Heavy Bombardment – a specific period of heavy asteroid strikes – is diminishing, she says.

“The Late Heavy Bombardment is an idea that was originally put together because of the way that the crater record on the moon was interpreted,” she says.

It was suggested that a specific period of heavy bombardment of the inner planets was required to make sense of the moon’s craters – but measurements in the past decade made from moon rocks collected during the Apollo missions suggest otherwise.