Water is over 70 percent of the surface of the earth but how that came to be, through what mix of random chance and extraterrestrial involvement, has been a debate. Earth is a relatively small planet and relatively near its star so creating large surface oceans is difficult.

A new study analyzed melted meteorites that had been floating around in space since the solar system’s formation four and a half billion years ago and found they had extremely low water content, among the driest extraterrestrial materials ever measured. This led them to conclude that water was likely delivered to Earth via unmelted, or chondritic, meteorites. 

The seven melted, or achondrite, meteorites crashed into Earth billions of years after splintering from at least five planetesimals heated up by the decay of radioactive elements in the early solar system’s history. Because these meteorites fell to Earth only recently, this experiment was the first time anyone had ever measured their water contents. The team used an electron microprobe to measure their levels of magnesium, iron, calcium and silicon and then water contents were measured with a secondary ion mass spectrometry instrument. 

Illustration by Jack Cook, Woods Hole Oceanographic Institution

To reduce contamination, researchers first baked their samples in a low-temperature vacuum oven to remove any surface water. Before the samples could be analyzed in the secondary ion mass spectrometer, the samples had to be dried out once again, which took a month using a turbo pump.

Some of the meteorite samples came from the inner solar system, where Earth is located and where conditions are generally assumed to have been warm and dry. Other, rarer samples came from the colder, icier outer reaches of our planetary system. While it was generally thought that water came to Earth from the outer solar system, it has yet to be determined what types of objects could have carried that water across the solar system.

After analyzing the achondrite meteorite samples, researchers discovered that water comprised less than two-millionths of their mass. For comparison, the wettest meteorites—a group called carbonaceous chondrites—contain up to about 20% of water by weight, or 100,000 times more than the meteorite samples studied in this paper.

This means that the heating and melting of planetesimals leads to near-total water loss, regardless of where these planetesimals originated in the solar system and how much water they started out with.