Water arrived on Earth earlier than thought

Water might have arrived on Earth as the planet was forming, according to new measurements of water in ancient space rocks. The findings could help planetary scientists piece together the development of asteroids and rocky planets.

About 4.6 billion years ago, the solar system began as a spinning disk of searing gas and dust around the newborn sun. The dust stuck together, gradually growing into pebbles and rocks that would go on to become asteroids and planets. Researchers know that Earth acquired its water when the solar system was less than about 200 million years old, based on measurements of moon rocks brought back by the Apollo astronauts. But until now, no one knew whether water fell to Earth during its molten, fragmented stage or arrived much later as the planet cooled and solidified.  

To narrow down the time of water’s arrival, a geologist at the Woods Hole Oceanographic Institution in Massachusetts, studied ancient meteorites called eucrites. Researchers suspect that eucrites are shrapnel blasted off the asteroid Vesta, based on mineral similarities. Measurements of radioactive isotopes in eucrites reveal that Vesta formed within the solar system’s first few million years, so water trapped in eucrites must have been there around the same time.

Sarafian hoped to see if the water in eucrites shared a common origin with water on Earth. Planetary scientists track water in the solar system by measuring the amount of deuterium, a heavy form of hydrogen. If two samples have similar ratios of deuterium to regular hydrogen, then the samples’ water probably came from the same place.

Sarafian’s team reports in the Oct. 31 Science that the deuterium-to-hydrogen ratio in the eucrites, and by extension Vesta, closely matches the ratio in Earth’s oceans. The similarity means Vesta and Earth have a common source of water. And because Vesta formed before Earth, that water was available when the planet started taking shape. “Earth wasn’t waiting for water,” Sarafian says. Instead, it seems, the water was waiting for Earth.

Conel Alexander, a cosmochemist at the Carnegie Institution for Science in Washington, D.C., praises Sarafian’s work. “It’s one thing to have a hunch,” he says, “but it’s quite another to build up some evidence.”

Scientists don’t have a lot of data from the solar system’s formative years. By figuring out when water-bearing material arrived, researchers can test ideas about Earth’s development, including when water-dependent geological processes such as plate tectonics began (SN: 7/12/14, p. 9). Sarafian’s discovery also means that water was ubiquitous in the early inner solar system and was most likely absorbed by numerous asteroids and planets such as Mars.

Early water doesn’t necessarily mean an early start for life, says Alexander. Toward the end of Earth’s formation, several Mars-sized interlopers smashed into the planet, one of which led to the birth of the moon (SN: 6/29/13, p. 8). “Those sterilized the entire Earth,” he says.