A new international study led by Western University shows that Mars' first 'real chance' at developing life started very early, 4.48 billion years ago, when giant, life-inhibiting meteorites stopped striking the red planet.

These findings, published online in Nature Geoscience, suggest that conditions under which life could have thrived may have occurred on Mars from around 4.2 to 3.5 billion years ago. This predates the earliest evidence of life on Earth by up to 500 million years.

It is known that the number and size of meteorite impacts on Mars and Earth gradually declined after the planets formed. Eventually, impacts became small and infrequent enough that the near-surface conditions could allow life to develop.

However, when the heavy meteorite bombardment waned has long been debated. It has been proposed that there was a 'late' phase of heavy bombardment of both planets that ended around 3.8 billion years ago.

For the study, Desmond Moser from Western's Departments of Earth Sciences and Geography and his students and collaborators analyzed the oldest-known mineral grains from meteorites that are believed to have originated from Mars' southern highlands. These ancient grains, imaged down to atomic levels, are almost unchanged since they crystallized near the surface of Mars.

In comparison, analysis of impacted areas on Earth and its Moon shows that more than 80 per cent of the grains studied contained features associated with impacts, such as exposure to intense pressures and temperatures. The analyses of Earth, Mars and Moon samples were conducted at Western's nationally unique Zircon and Accessory Phase Laboratory, which is led by Moser.

The results suggest that heavy bombardment of Mars ended before the analyzed minerals formed, which means, the Martian surface would have become habitable by the time it is believed that water was abundant there. Water was also present on Earth by this time so it is plausible that the solar system's biological clock started much earlier than previously accepted.

"Giant meteorite impacts on Mars between 4.2 and 3.5 billion years ago may have actually accelerated the release of early waters from the interior of the planet setting the stage for life-forming reactions," says Moser. "This work may point out good places to get samples returned from Mars," Moser said.

Research Report: "Decline of Giant Impacts on Mars by 4.48 Billion Years Ago and an Early Opportunity for Habitability"

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