A paper published in Geochimica and Cosmochimica Acta describes angrite meteorites’ role in bringing water to Earth. Angrites are rare basaltic meteorites that formed in the inner solar system 4.56 billion years ago. These meteorites could have delivered volatile compounds like water to a young Earth within the first two million years since the solar system’s formation.
Water is an important substance for carbon-based life. However, in the airless space between the planets of the inner solar system, it would have been easy for the water to boil off due to the intense heat from the Sun. Water’s volatility. The highly dynamic environment of the early solar system makes looking for an early asteroid that could have been the source of Earth’s water somewhere between a complicated three-dimensional Where’s Waldo and finding a needle in a really big haystack.
Besides, a lot of things were colliding with one another, which often caused bigger objects to throw off material or disintegrate into a lot of smaller pieces. There were a lot of meteorites hitting the “superheated to the point of being molten rock” surface of the proto-Earth right then. There’s even a theory that the Moon formed from material thrown off when another proto-planet collided with the proto-Earth. How do you find something that might have existed in that early environment — and possibly disintegrated in a violent collision?
“We’re looking at as many meteorite parent bodies as possible right now to figure out where they were in the early Solar System and how much water they had. We’re trying to build a map of the very early inner Solar System. Where was the water, where was it going and where did it come from?” says Adam Sarafian, the lead author of the paper.
Basalt forms when molten rock cools into a solid igneous rock. Basaltic meteorites like angrites typically contain olivine, a mineral that can tell scientists a lot about the volatile elements that the rock contained when it was still molten. In the early solar system, the protoplanets and asteroids were all blobs of molten rock, which can make normally non-volatile elements like carbon volatile. That means scientists could measure the amount of carbon, hydrogen, fluorine, and chlorine there was in the angrite meteorites.
“Once we know the melt composition, we can then calculate what a planetary body’s water content was,” Sarafian said.
Sarafian’s research team determined that about 20% of Earth’s water was delivered by angrite meteorites. This was at a time when Earth was about 20% of its current size and its surface was still cooling, so it had time to gather more material – including the remaining water from other sources like comets.
They haven’t found the asteroid that the angrites came from. However, they’re looking. Based on the pressure levels that the angrites would have had to have to retain to keep their hydrogen and carbon content, they probably came from an asteroid that was about 525 kilometers (325 miles) in diameter – about the same size as the asteroid Vesta.
They don’t think it was Vesta, though. How would they know? Well, they can compare ratios of hydrogen called deuterium (hydrogen atoms that have an extra neutron) and protium (“normal” hydrogen atoms that don’t have neutrons). Water molecules that contain deuterium are a little heavier than water molecules that contain protium because they’d effectively have two extra neutrons – which don’t have an electromagnetic charge and wouldn’t interfere with the bonds making water molecules possible.
They would see a difference in these ratios between Earth and asteroids that wouldn’t be the source of Earth’s water. Sarafian previously published a 2017 paper showing that the ratios between deuterium and protium in the angrite meteorites matched the ratios in Earth’s water. This suggested that Earth started accumulating water early in its formation.
“It’s a fairly simple assumption to say that Earth’s water at least started accreting to Earth extremely early, before the planet was even fully formed,” says Sarafian. “This means that when the planet cooled enough so that liquid water could be stable at the surface, there was already water here.”
Considering conditions in the early inner solar system, it could have been difficult to accumulate water so early. It could have evaporated before it had a chance to even reach Earth and blown away in the Sun’s solar wind. However, Earth managed it with help from angrite meteorites that could hold their water long enough to deliver it.