Earth Asteroid Bennu

NASA’s OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer) mission to the near-Earth asteroid Bennu is designed to return a carbon-rich sample of the asteroid to Earth. This 500-m-diameter body was chosen as the mission target due to its spectral similarity to primitive and organic-rich carbonaceous chondrite meteorites. In a series of six papers published this week in the journal Science and the journal Science Advances, a team of scientists on the OSIRIS-REx mission presents new findings on the surface material, geological characteristics and the dynamic history of Bennu.

NASA’s OSIRIS-REx spacecraft at asteroid Bennu. Image credit: NASA Goddard Space Flight Center.

NASA’s OSIRIS-REx spacecraft at asteroid Bennu. Image credit: NASA Goddard Space Flight Center.

One of the studies, led by Dr. Amy Simon of NASA’s Goddard Space Flight Center, shows that carbon-bearing, organic material is widespread on Bennu’s surface, including at the mission’s primary sample site, Nightingale, where OSIRIS-REx will make its first sample collection attempt on October 20, 2020.

The findings indicate that hydrated minerals and organic material will likely be present in the collected sample. This organic matter may contain carbon in a form often found in biology or in compounds associated with biology.

“The abundance of carbon-bearing material is a major scientific triumph for the mission,” said OSIRIS-REx principal investigator Dr. Dante Lauretta, a researcher at the University of Arizona.

“We are now optimistic that we will collect and return a sample with organic material — a central goal of the OSIRIS-REx mission.”

“It’s been such thrill and honor to be part of the OSIRIS-REx team,” said Dr. Joshua Emery, a researcher at Northern Arizona University.

“As lead of the thermal analysis working group, it has been very exciting for me to be very involved in planning the observations the spacecraft has made in preparation for sampling and then figuring out from the data what the surfaces is like.”

“The rocks on Bennu look strange, and we found from the thermal data that they are so weak that we could easily crush them in our hands. Still, they have existed on this asteroid for over a billion years!”

“These rocks also contain complex organic molecules that form naturally in space, and asteroids like Bennu could have brought these organic molecules to Earth billions of years ago to seed the beginnings of life.”

“When the sample is returned to Earth, scientists will be able to study these molecules in exquisite detail.”

“Our recent studies show that organics and minerals associated with the presence of water are scattered broadly around Bennu’s surface, so any sample returned to Earth should contain these compounds and minerals,” said Dr. Vicky Hamilton, a researcher at Southwest Research Institute.

“We will compare the sample’s relative abundances of organics, carbonates, silicates and other minerals to those in meteorites to help determine the scenarios that best explain Bennu’s surface composition.”

In another study, Dr. Hannah Kaplan of NASA’s Goddard Space Flight Center and colleagues found that carbonate minerals make up some of the asteroid’s geological features. These minerals often precipitate from hydrothermal systems that contain both water and carbon dioxide.

A number of Bennu’s boulders have bright veins that appear to be made of carbonate — some of which are located near the Nightingale crater, meaning that carbonates might be present in the returned sample.

“Bennu’s parent asteroid likely had an extensive hydrothermal system, where water interacted with and altered the rock on the parent body,” the authors said.

“Although the parent body was destroyed long ago, we’re seeing evidence of what that watery asteroid once looked like here — in its remaining fragments that make up Bennu.”

“Some of these carbonate veins in Bennu’s boulders measure up to a few feet long and several inches thick, validating that an asteroid-scale hydrothermal system of water was present on Bennu’s parent body.”

Dr. Dani DellaGiustina from the University of Arizona and colleagues made another striking discovery at the Nightingale site: its regolith has only recently been exposed to the harsh space environment, meaning that the mission will collect and return some of the most pristine material on the asteroid.

Nightingale is part of a population of young, spectrally red craters identified in a study led by.

Bennu’s ‘colors’ (variations in the slope of the visible-wavelength spectrum) are much more diverse than originally anticipated. This diversity results from a combination of different materials inherited from Bennu’s parent body and different durations of exposure to the space environment.

The findings are a major milestone in an ongoing debate in the planetary science community — how primitive asteroids like Bennu change spectrally as they are exposed to space weathering processes, such as bombardment by cosmic rays and solar wind.

While Bennu appears quite black to the naked eye, the scientists illustrate the diversity of Bennu’s surface by using false-color renderings of multispectral data collected by the MapCam camera.

The freshest material on Bennu, such as that found at the Nightingale site, is spectrally redder than average and thus appears red in these images.

Surface material turns vivid blue when it has been exposed to space weathering for an intermediate period of time.

As the surface material continues to weather over long periods of time, it ultimately brightens across all wavelengths, becoming a less intense blue — the average spectral color of Bennu.

Dr. DellaGiustina and co-authors also identified two main types of boulders on Bennu’s surface: dark and rough, and (less commonly) bright and smooth. The different types may have formed at different depths in the parent asteroid of Bennu.

Not only do the boulder types differ visually, they also have their own unique physical properties.

 

In a separate study, Dr. Ben Rozitis from the Open University and colleagues show that the dark boulders are weaker and more porous, whereas the bright boulders are stronger and less porous.

The bright boulders also host carbonate minerals, suggesting that the precipitation of carbonates in cracks and pore spaces may be responsible for their increased strength. However, both boulder types are weaker than scientists expected.

The authors suspect that Bennu’s dark boulders (the weaker, more porous, and more common type) would not survive the journey through Earth’s atmosphere.

It’s therefore likely that the returned samples of Bennu will provide a missing link for scientists, as this type of material is not currently represented in meteorite collections.

In another study, Dr. Michael Daly of York University and colleagues explain how detailed analysis of the asteroid’s shape revealed ridge-like mounds on Bennu that extend from pole-to-pole, but are subtle enough that they could be easily missed by the human eye.

Their presence has been hinted at before, but their full pole-to-pole extents only became clear when the northern and southern hemispheres were split apart for comparison.

The digital terrain model also shows that Bennu’s northern and southern hemispheres have different shapes. The southern hemisphere appears to be smoother and rounder, which the scientists believe is a result of loose material getting trapped by the region’s numerous large boulders.

Dr. Daniel Scheeres from the University of Colorado Boulder and colleagues examine the gravity field of Bennu, which has been determined by tracking the trajectories of the OSIRIS-REx spacecraft and the particles that are naturally ejected from Bennu’s surface.

The use of particles as gravity probes is fortuitous. Prior to the discovery of particle ejection on Bennu in 2019, the team was concerned about mapping the gravity field to the required precision using only spacecraft tracking data.

The natural supply of dozens of mini gravity probes allowed the team to vastly exceed their requirements and gain unprecedented insight into the asteroid interior.

The reconstructed gravity field shows that the interior of Bennu is not uniform. Instead, there are pockets of higher and lower density material inside the asteroid.

It’s as if there is a void at its center, within which you could fit a couple of football fields.

In addition, the bulge at Bennu’s equator is under-dense, suggesting that Bennu’s rotation is lofting this material.

“We were hoping to find out what happened to this asteroid over time, which can give us better insight into how all of these small asteroids are changing over millions, hundreds of millions or even billions of years,” Dr. Scheeres said.

“Our findings exceeded our expectations,” he added.

_____

Amy A. Simon et al. Widespread carbon-bearing materials on near-Earth asteroid (101955) Bennu. Science, published online October 8, 2020; doi: 10.1126/science.abc3522

H.H. Kaplan et al. Bright carbonate veins on asteroid (101955) Bennu: Implications for aqueous alteration history. Science, published online October 8, 2020; doi: 10.1126/science.abc3557

D.N. DellaGiustina et al. Variations in color and reflectance on the surface of asteroid (101955) Bennu. Science, published online October 8, 2020; doi: 10.1126/science.abc3660

B. Rozitis et al. 2020. Asteroid (101955) Bennu’s weak boulders and thermally anomalous equator. Science Advances 6 (41): eabc3699; doi: 10.1126/sciadv.abc3699

M.G. Daly et al. 2020. Hemispherical differences in the shape and topography of asteroid (101955) Bennu. Science Advances 6 (41): eabd3649; doi: 10.1126/sciadv.abd3649

D.J. Scheeres et al. 2020. Heterogeneous mass distribution of the rubble-pile asteroid (101955) Bennu. Science Advances 6 (41): eabc3350; doi: 10.1126/sciadv.abc3350

This article is based on press-releases provided by the National Aeronautics and Space Administration, the American Association for the Advancement of Science, the University of Colorado at Boulder, Northern Arizona University and Southwest Research Institute.

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