Asteroids help trace water's journey across the solar system

A pair of missions to two asteroids near Earth are beginning to provide new information about when, where and in what form water first existed in the solar system.

Why it matters: Water played a key role in the formation of planets — and the emergence of life that evolved on at least one world thereafter.

• "Asteroids at some level were carriers of water throughout the solar system," says Kevin Walsh, a scientist at the Southwest Research Institute who studies asteroids.
• The "children and grandchildren" of these asteroids that have drifted closer to Earth can be studied to infer more about primordial asteroids that first transported water, he says.
• Signs of water can be spotted in asteroid material chemically altered when the rocks and dust interact with water.

Driving the news: NASA's OSIRIS-REx spacecraft collected about 250 grams (roughly half a pound) of rock and dust from the surface of the near-Earth asteroid Bennu in October 2020, a team of scientists that includes Walsh reports today in the journal Science. The sample will be returned to Earth next year.

• Bennu is about 500 meters (1,640 feet) wide, orbits the Sun at an average distance of 105 million miles — and sometimes drifts relatively close to Earth.
• The spacecraft created a plume of debris and a 9-meter-long (30-foot) crater as it collected the sample. Analyzing both the particles kicked up in the process and the forces experienced by the spacecraft, the researchers found the material is finer, less dense and more loosely packed than the surface, Walsh and his colleagues report in a separate paper in the journal Science Advances.
• The particles landed on the spacecraft's spectrometer, giving the team an opportunity to study their infrared signals. Earlier research posited that the material is likely rich in magnesium-containing minerals formed by interactions with water, similar to some "chemically primitive" meteorites found on Earth. They report the latest analysis substantiates that.

The big picture: Small, rocky bodies like Bennu are thought to be fragments of the solar system's earliest asteroids.

• Evidence suggests these early-stage asteroids formed mere tens of millions of years after the solar system's start 4.5 billion to 4.7 billion years ago when a cloud of gas and dust collapsed and formed the Sun.
• As rocks orbiting the Sun accreted in the outer solar system, ices and gases were also pulled into their mix. As the bodies grew bigger, there was more heat, more pressure, and more squeezing of rocks and fluids.
• And "very quickly these bodies became geologically active," says Harold Connolly, a professor of geology at Rowan University and co-investigator and mission sample scientist on OSIRIS-REx. Water was "an agent for change" in the early solar system that helped make planets the geologically active worlds they are today — "something life can actually evolve on," he adds.

As asteroids bashed together in the early outer solar system, bits of rocks broke off and reaccumulated to form "rubble pile" asteroids like Bennu.

• It's current composition is thought to have been established on a larger carbon-rich parent asteroid within 10 million years of the solar system's start, per NASA.
• "The whole history of the solar system is tucked away in these small little rubble piles," Walsh says. "You can walk your way back to the original asteroids."

A separate mission by the Japanese space agency, JAXA, published an analysis of a sample returned from the asteroid Ryugu in late 2020.

• It matched a rare type of meteorite that landed in Tanzania in 1938.
• Radioisotope dating indicates the material was altered when ice melted to water that circulated through the asteroid about 5.2 million years after the solar system formed, the team reported.

The intrigue: For Bennu, "we don’t know where it came from or who it might be related to," Walsh says.

• OSIRIS-REx can provide images of the asteroid down to its individual boulders and craters. The random scattered boulders spotted on the asteroid are unlike the rest of the rocky material.
• "This tells us it interacts with something that wasn’t primarily its main parent. It speaks to the wild history of the belt — and probably to collisions of [Bennu's] own."
• Connolly, who is also a co-investigator on the JAXA mission, says Bennu and Ryugu could be cousins, but that hypothesis can't be confirmed until the bits of Bennu are on Earth.

What to watch: JAXA's two sample return missions — Hayabusa1 and Hayabusa2 — brought back samples that turned out to match the most abundant and the rarest meteorites on Earth.

• The big question is where Bennu fits in, Connolly says.