Mars water comes from space

Mars water comes from space

Asteroids bombarding Mars early in its history could have delivered organic molecules and large amounts of water – enough to cover the entire planet at 300m depth.1 The discovery offers insight into where Mars water came from as well as how it may have once contained the ingredients necessary for life to emerge.

Mars orbiters had already revealed signs that an ocean once existed in the northern hemisphere of Mars, 4.3 billion years ago. However, the amount of water had remained uncertain, with estimates based on features such as suspected ancient shorelines. The origins of water, too, have remained elusive. Some models propose that the water comes from gas trapped inside the planet, accumulating on the surface as the mantle cools and the crust forms.

But it seems that even larger amounts of water actually came from the outer solar system. ‘Our study is the first to demonstrate that a large amount of water-rich carbonaceous asteroids must have bombarded the planet early in its evolution,’ says Martin Bizzarro of the University of Copenhagen, Denmark. “This firmly establishes that organic molecules relevant to life must have been present in the early evolution of the planet along with water.”

Researchers analyzed 31 Martian meteorites found on Earth. These meteorites come from the eruption of lava across the surface of the planet, which caused the assimilation of part of its crust. Since Mars has no plate tectonics, its crust is as old as the planet. This means that meteorites may contain traces of the asteroids that have already hit Mars, in the form of isotopic signatures of chromium.

Chromium isotopes are a good criterion for identifying the different types of space rocks. Bizzarro and his team have developed a way to measure chromium isotopes that is five times more accurate than other methods. They found that the isotopic signatures of some meteorites matched those of water-rich asteroids known to originate from the outer solar system.

A photo of an unremarkable brown and black rock held in the palms of someone's hands

“The magnitude of the isotopic shift between the original mantle-derived chromium and that of lavas contaminated with exotic chromium allows us to determine the mass of asteroids that must have been delivered to the surface of Mars,” says Bizzarro. “This, in turn, allows us to estimate the amount of water and organic material that was delivered to Mars by these asteroids.” He says the findings are consistent with Mars crater records.

“This adds to the growing body of evidence that planets, including Earth and Mars, do not necessarily start out with enough water to support the evolution of life, but rather must bring it in from outer parts of the system. solar,” comments planetary scientist Martin Lee at the University of Glasgow, UK. “In addition, an efficient water delivery mechanism may be another factor to consider when assessing the potential for life in exoplanetary systems in other parts of the galaxy.”

“It would be great to have access to the samples currently collected by Perseverance,” adds Bizzarro. “This would allow us to directly determine the chromium isotope of the primordial crust that was impacted by the bombardment and confirm our results and interpretations.”

“It’s likely that at least some of Earth’s water was also brought in by this late carbonaceous asteroid bombardment,” Bizzarro speculates. Indeed, another study2 led by Luke Daly at the University of Glasgow, UK, supports this.

Daly and his team showed that the hydrogen isotopic composition of the Winchcombe meteorite, which crashed into a family’s driveway in England in early 2021, is comparable to that of Earth’s water. Since the meteorite came from an asteroid, this suggests that they played an important role in the Earth’s water supply.

‘Both [studies] come to very similar conclusions that water and organic matter – the building blocks of life – were delivered to Earth and Mars by carbonaceous chondrite asteroids early in those planets’ histories,” says Daly. . “It’s a very good double whammy.”

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