During the past year, there's been an unusual set of samples at the Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab): material gathered from the 4.5-billion-year-old asteroid Bennu when it was roughly 200 million miles from Earth.
Berkeley Lab is one of more than 40 institutions investigating Bennu's chemical makeup to better understand how our solar system and planets evolved. In a new study published today in the journal Nature, researchers found evidence that Bennu comes from an ancient wet world, with some material from the coldest regions of the solar system, likely beyond the orbit of Saturn.
The asteroid contained a set of salty mineral deposits that formed in an exact sequence when a brine evaporated, leaving clues about the type of water that flowed billions of years ago. Brines could be a productive broth for cooking up some of the key ingredients of life, and the same type of minerals are found in dried-up lake beds on Earth (such as Searles Lake in California) and have been observed on Jupiter's moon Europa and Saturn's moon Enceladus.
"It's an amazing privilege to be able to study asteroid material, direct from space," said Matthew Marcus, a Berkeley Lab scientist who runs the Advanced Light Source (ALS) beamline where some of the samples were studied and who wrote one of the programs used to analyze their chemical composition. "We have highly specialized instruments that can tell us what Bennu is made of and help reveal its history."
The samples from Bennu were gathered by NASA's OSIRIS-REx mission, the first U.S. mission to return samples from an asteroid. The mission returned nearly 122 grams of material from Bennu - the largest sample ever captured in space and returned to Earth from an extraterrestrial body beyond the Moon.
Marcus teamed up with Scott Sandford from NASA Ames Research Center and Zack Gainsforth from the UC Berkeley Space Sciences Laboratory to study the Bennu sample using scanning transmission X-ray microscopy (STXM) at the ALS. By varying the energy of the X-rays, they were able to determine the presence (or absence) of specific chemical bonds at the nanometer scale and map out the different chemicals found in the asteroid. The science team discovered that some of the last salts to evaporate from the brine were mixed into the rock at the finest levels.
"This sort of information provides us with important clues about the processes, environments, and timing that formed the samples," Sandford said. "Understanding these samples is important, since they represent the types of materials that were likely seeded on the surface of the early Earth and may have played a role in the origins and early evolution of life."
At Berkeley Lab's Molecular Foundry, researchers used a beam of electrons to image the same Bennu samples with transmission electron microscopy (TEM). The Foundry also helped prepare the samples for the experiments run at the ALS. Experts used an ion beam to carve out microscopic sections of the material that are about a thousand times thinner than a sheet of paper.
"Being able to examine the same exact atoms using both STXM and TEM removed many of the uncertainties in interpreting our data," Gainsforth said. "We were able to confirm that we really were seeing a ubiquitous phase formed by evaporation. It took a lot of work to get Bennu to give up its secrets, but we are delighted with the final result."
This is not the first time the ALS and Molecular Foundry have studied material from space. Researchers also used the two facilities to investigate samples from the asteroid Ryugu, building up our understanding of our early solar system. And there's still more to come, with additional studies of Bennu at both the STXM and infrared beamlines at the ALS planned for the coming year.
Berkeley Lab researchers also contributed to a second paper published today in Nature Astronomy that analyzed organic materials found on the asteroid. Within the Bennu sample, the science team identified 14 of the 20 amino acids that life on Earth uses to build proteins. They also found all five nucleobases, the ring-shaped molecules that form DNA and RNA, as well as ammonia, which on Earth might have helped spark the emergence of early life.
The results support the idea that asteroids like Bennu may have delivered water and essential chemical building blocks of life to Earth in the distant past. Based on the similarities between asteroid Bennu and the icy dwarf planets and moons of our outer solar system, these potential ingredients for life could be widespread.
