Scientists make step towards solving mystery of where Earth’s water came from

It is not from meteorites, study suggests

Planets with water are more common than previously thought, research suggests (Owen Humphreys/PA) (PA Archive)

Scientists have made a step towards solving one of Earth’s biggest mysteries: where did all the water come from?

Some 71 per cent of our planet is covered in water, but scientists still do not know how it got here, or why we have so much of it.

The mystery not only covers our planet but the search for alien life elsewhere, too. By understanding how our planet became habitable, we might understand how other planets could support life.

Now researchers have made a step towards getting an answer, by ruling out one of the most common explanations: that water made it here by coming from melted meteorites.

Melted meteorites are also known as achondrite meteorites, which are relatively uncommon forms of meteorite that have not been melted during their journey through space.

Scientists analysed melted meteorites that had been floating in space since the solar system formed some 4.5 billion years ago.

They found that they had extremely low water content – so dry, in fact, that they had less water than almost anything ever studied in space.

“We wanted to understand how our planet managed to get water because it’s not completely obvious,” said University of Maryland assistant professor of geology Megan Newcombe, who led the research. “Getting water and having surface oceans on a planet that is small and relatively near the sun is a challenge.”

In an attempt to understand how that happened, scientists looked at seven melted meteorites that crashed into Earth recently. The asteroids were created after they splintered off planetesimals, or the objects that collided to eventually create the planets that surround us.

Measuring such objects is difficult because they can be easily tainted by water on Earth. So scientists find baked them in a vacuum oven that removed surface water and then dried them out using a pump.

They found that afterimages less than two millionths of the mass of the samples. That is in comparison with the wettest meteorites, which can be up to 20 per cent water.

That indicates that such meteorites lose almost all of their water when they are heated and melted, no matter where they started out their life or how much water they began with.

It means that water likely came to Earth on chondrite meteorites, or those that are not melted.

That finding could also help tell us more about how and whether water made it to other planets – and in turn help inform the search for alien life.

“Water is considered to be an ingredient for life to be able to flourish, so as we’re looking out into the universe and finding all of these exoplanets, we’re starting to work out which of those planetary systems could be potential hosts for life,” Professor Newcombe said in a statement. “In order to be able to understand these other solar systems, we want to understand our own.”

The findings are reported in a new study, ‘Degassing of early-formed planetesimals restricted water delivery to Earth’, published in Nature.