The UA-led OSIRIS-REx mission announced Monday that two onboard instruments have detected widespread evidence of water locked within the minerals of the ancient asteroid Bennu.
The announcement — the first major news since the spacecraft met up with Bennu a week ago — came during a presentation at the annual fall meeting of the American Geophysical Union in Washington, D.C.
Scientists said the news was important because to find hydrated minerals, or clays, on Bennu, water must have interacted with regular minerals.
“It is very exciting to see these hydrated minerals distributed across Bennu’s surface, because it suggests they are an intrinsic part of Bennu’s composition, not just sprinkled on its surface by an impactor,” Ellen Howell, senior research scientist at the UA’s Lunar and Planetary Laboratory and member of the OSIRIS-REx science team, said in a statement.
This finding may provide an important link between meteorites that fell to Earth that scientists study and what scientists think happened to asteroids in space like Bennu, Howell said.
The mission team targeted Bennu because it thought they’d find “water-bearing minerals,” said Dante Lauretta, University of Arizona professor and the mission’s principal investigator.
“It looks like we went to the right place,” he said.
Bennu, which is about three city blocks wide and weighs about 80 million tons, can reveal what materials were delivered to planets as they were forming in the early history of the solar system.
Additionally, “Our work at Bennu brings us a step closer to the possibility of asteroids providing astronauts on future missions into the solar system with resources like fuel and water,” said UA President Robert Robbins in a statement.
Bennu’s surface more rugged than thought
OSIRIS-REx arrived within 12 miles of Bennu on Dec. 3. While there was no landing, that day marked the beginning of the preliminary survey to understand Bennu’s immediate surrounding and any potential hazards.
What scientists have found so far is a surface much more rugged than they expected, although the computerized model of the asteroid’s approximate shape was highly accurate, said Michael Nolan, science team chief and UA senior research scientist.
Until a sample of dust and pebbles can be collected in July 2020, the spacecraft will use onboard instruments to map and analyze the asteroid in increasing detail. The sample will be returned to Earth in 2023.
The OSIRIS-REx science team was also surprised to find the density of the asteroid was lower than predicted.
The low density “tells us it is made of hydrated minerals,” Lauretta said, but he also speculates that Bennu must have interior caves. To get such a low density, he estimated that 40 percent of Bennu could be empty space.
Understanding the density and mass of Bennu is important.
The navigation team has been given months to learn how to fly in this micro-gravity environment, Lauretta said.
On New Year’s Eve, OSIRIS-REx will begin orbiting Bennu, making it the smallest object ever orbited by a spacecraft.
The low gravity also makes sample collection a delicate dance.
“It’s more akin to docking than landing when we get a sample,” Lauretta said.
Peering back into the darkness
Lauretta hopes to also find organic molecules, the building blocks of life, but none have been confirmed.
“We’re looking at rock older than the Earth,” he said.
“We’re peering back to the dawn of the solar system and trying to understand the processes that made the Earth habitable.”
Bennu is too small to have hosted liquid water on its own.
From the data garnered from the mission so far, Lauretta speculates that Bennu was originally part of a larger asteroid that was more than 62 miles across.
Bennu’s parent asteroid, he said, accumulated dry minerals and ice particles when it formed. Then, about 5 to 10 million years later, the ice melted and reacted to form the clay that is preserved to this day.
He estimates that Bennu broke off from the larger asteroid about a billion years ago.
“This is a hypothetical timeline for this asteroid,” Lauretta said.
“We’re going to test all of that. The sample will tell us (Bennu’s) history in great detail.”
