Because the asteroid Bennu comes into sharper focus, planetary scientists are seeing indicators of water locked up within the asteroid’s rocks, NASA group members introduced December 10.
“It’s one of many issues we had been hoping to seek out,” group member Amy Simon of NASA’s Goddard Area Flight Middle in Greenbelt, Md., stated in a information convention on the American Geophysical Union assembly in Washington, D.C. “That is proof of liquid water in Bennu’s previous. That is actually large information.”
NASA’s OSIRIS-REx spacecraft simply arrived at Bennu on December 3 (SN On-line: 12/three/18). Over the following yr, the group will seek for the proper spot on the asteroid to seize a handful of mud and return it to Earth. “Very early within the mission, we’ve came upon Bennu goes to supply the kind of materials we need to return,” stated principal investigator Dante Lauretta of the College of Arizona in Tucson. “It positively appears like we’ve gone to the suitable place.”
OSIRIS-REx’s onboard spectrometers measure the chemical signatures of assorted minerals primarily based on the wavelengths of sunshine they emit and soak up. The devices had been in a position to see signs of hydrated minerals on Bennu’s floor a few month earlier than the spacecraft arrived on the asteroid, and the sign has remained sturdy all around the asteroid’s floor because the spacecraft approached, Simon stated. These minerals can type solely within the presence of liquid water, and recommend that Bennu had a hydrothermal system in its previous.
Bennu’s floor can be covered in more boulders and craters than the group had anticipated primarily based on observations of the asteroid taken from Earth. Distant observations led the group to count on just a few massive boulders, about 10 meters vast. As an alternative they see a whole bunch, a few of them as much as 50 meters vast.
“It’s just a little extra rugged of an surroundings,” Lauretta stated. However that tough floor can reveal particulars of Bennu’s inside construction and historical past.
If Bennu had been one strong mass, for example, a serious affect may crack or shatter its complete floor. The truth that it has massive craters means it has survived impacts intact. It might be extra of a rubble pile loosely held collectively by its personal gravity.
Crater pockmarks on Bennu, like this 20-meter-wide melancholy photographed on December 2, might change into the most secure locations for the spacecraft to choose up a pattern of the asteroid. Craters additionally inform scientists that the asteroid shouldn’t be one strong mass however as a substitute totally different supplies clumped collectively. If Bennu had been a strong rock, massive impacts may crack or shatter its floor.
The asteroid’s density helps the rubble pile concept. OSIRIS-REx’s first estimate of Bennu’s density exhibits it’s about 1,200 kilograms per cubic centimeter, Lauretta stated. The common rock is about three,000 kilograms per cubic centimeter. The hydrated minerals go a way in direction of decreasing the asteroid’s density, since water is much less dense than rock. However as much as 40 p.c of the asteroid could also be filled with caves and voids as effectively, Lauretta stated.
A few of the rocks on the floor seem like fractured in a spindly sample. “Should you drop a dinner plate on the bottom, you get a spider net of fractures,” says group member Kevin Walsh of the Southwest Analysis Institute in Boulder, Colo. “We’re seeing this in some boulders.”
The boulders might have cracked in response to the drastic change in temperatures they expertise because the asteroid spins. Learning these fracture patterns in additional element will reveal the properties of the rocks.
The OSIRIS-REx group additionally must know what number of boulders of assorted sizes are strewn throughout the asteroid’s floor. Any rock bigger than about 20 centimeters throughout would pose a hazard to the spacecraft’s sampling arm, says Keara Burke of the College of Arizona. Burke, an undergraduate engineering scholar, is heading up a boulder mapping project.
A few of the rocks strewn throughout Bennu, like this 35-meter-wide boulder photographed on December 2, are fractured in a spindly sample. The fractures could also be the results of sudden, drastic temperature change. Learning the fracture patterns may assist scientists be taught extra in regards to the properties of the rocks.
“My major objective is security,” she says. “If it appears like a boulder to me, inside cheap tips, then I mark it as a boulder. We are able to’t pattern something if we’re going to crash.”
The group additionally must know the place the smallest grains of rock and mud are, as OSIRIS-REx’s sampling arm can decide up grains solely about 2 centimeters throughout. One technique to discover the small rocks is to measure how effectively the asteroid’s floor retains warmth. Larger rocks are slower to warmth up and slower to chill down, in order that they’ll radiate warmth out into area even on the asteroid’s night time aspect. Smaller grains of mud warmth up and funky down way more rapidly.
“It’s precisely like a seaside,” Walsh says. “Throughout the day it’s scalding sizzling, however then it’s immediately chilly when the solar units.”
Measurements of the asteroid’s warmth storage up to now recommend that there are areas with grains as small as 1 or 2 centimeters throughout, Lauretta stated, although it’s nonetheless too early to make sure.
“I’m assured that we’ll discover some fine-grained areas,” Lauretta stated. Some could also be positioned inside craters. The problem will likely be discovering an space vast sufficient that the spacecraft’s navigation system can steer to it precisely.