Early meteorites brought enough water to Mars to create a global ocean

The meteorites that bombarded Mars during the early days of the inner solar system may have carried enough water to create a 300-metre-deep ocean on the planet.

Martin Bizzarro at the University of Copenhagen in Denmark and his colleagues have analysed the concentration of a rare chromium isotope, known as chromium-54, in samples of meteorites that have come to Earth from Mars to estimate how much water was deposited on the Red Planet by asteroids.

The uppermost layer of Mars contains the chemical signatures of carbonaceous, or C-type, meteorites that bombarded it as its crust solidified some 4.5 billion years ago.

Because Mars isn’t made up of large tectonic plates that move around, causing material in the planet’s interior and surface to churn, this chemical signature from the meteorites should be preserved in the rocks of the planet’s crust. But the rocks from the mantle below should still show what Mars was like before the bombardment.

“It’s a bit like DNA,” says Bizzarro. “Carbonaceous-type asteroids have a very distinct chromium isotope composition relative to the inner solar system.”

By looking at the difference between the amount of chromium-54 in samples of meteorites on Earth that have come from either the surface or mantle of Mars, the researchers could estimate the total mass of the asteroids that originally collided with Mars.

If the original bombarding asteroids were just 10 per cent water, the lower limit for C-type meteorites, they would have deposited enough of the molecule to create a global ocean, say the researchers. If spread out over the whole planet, the water would form a layer 300 metres deep.

“I think this is the first time where we have a smoking gun,” says Bizzarro, and we can finally say with certainty that water-rich asteroids hit Mars’s surface.

C-type asteroids also contain elements that are essential to life. This means that two of the most important ingredients necessary to life – organic molecules and water – were present on Mars during a time before Earth’s moon even formed, say the researchers.

The paper gives good evidence for the presence of C-type meteorites in the Martian mantle, but the meteorite samples might not be representative of the bulk of the planet’s mantle material, says Simon Turner at Macquarie University in Sydney, Australia.