An Asteroid of a Different Color … and Other Secrets of Bennu Unlocked
The UArizona-led OSIRIS-REx mission is gearing up for its first attempt to collect a sample from asteroid Bennu this month. But before even touching the surface, scientists are learning more than ever about the material that makes up the asteroid.
Scientists now know much more about the material NASA's first asteroid sample return mission will be collecting in just a few weeks.
In a special collection of six papers published today in the journals Science and Science Advances, scientists with the University of Arizona-led OSIRIS-REx mission present new findings on asteroid Bennu's surface material, geological characteristics and dynamic history. They also say that the sample delivered from Bennu may be unlike anything we have in the meteorite collection on Earth.
These discoveries complete the OSIRIS-REx mission's pre-sample collection science requirements and offer insight into an asteroid sample that scientists will study for generations to come.
Rocks in Rouge
One of the papers – published in Science and led by Dani DellaGiustina, lead scientist on the mission's image processing team and senior staff scientist at the UArizona Lunar and Planetary Laboratory – details a striking discovery at the mission's primary sample site, Nightingale, where OSIRIS-REx will make its first sample collection attempt on Oct. 20. The rocky debris covering the site has only recently been exposed to the harsh space environment. That means the mission will be collecting and returning some of the most pristine material on the asteroid.
Using spectroscopy – a technique that reveals a material's composition based on the pattern of reflected wavelengths of light – the scientists found that Nightingale is part of a population of young craters whose composition reflects mostly red light. Bennu's colors, as revealed through spectroscopy, 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.
While Bennu appears quite black to the naked eye, the authors illustrate the diversity of Bennu's surface by using false-color renderings of spectral data collected by the mission's 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 the 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.
"When a material is exposed to the space environment, its surface weathers, but in a very different way than things do on Earth," DellaGiustina said. "In space, weathering is caused by exposure to solar wind and the rain of tiny micrometeorites. On the moon's surface and many asteroids, we have observed that space weathering darkens and reddens surfaces. On Bennu, however, the opposite is true. We see that over time Bennu has become brighter and bluer in response to space weathering. This is an exciting finding because it tells us that something about Bennu is quite different than other planetary surfaces we've observed."
The paper led by DellaGiustina also distinguishes 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.
A 'Scientific Triumph'
Another paper published in Science, led by Amy Simon from NASA's Goddard Space Flight Center in Greenbelt, Maryland, shows that carbon-bearing, organic material is widespread on the asteroid's surface, including at the Nightingale site. These 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. Scientists are planning detailed experiments on these organic molecules and expect that the returned sample will help answer complex questions about the origins of water and life on Earth.
"The abundance of carbon-bearing material is a major scientific triumph for the mission. We are now optimistic that we will collect and return a sample with organic material – a central goal of the OSIRIS-REx mission," said Dante Lauretta, OSIRIS-REx principal investigator and UArizona planetary science professor, who co-authored all six papers in the collection.
History Hidden in Rocks
Another study in the collection, published in Science and led by Hannah Kaplan from Goddard, found that carbonate minerals – which are compounds containing special combinations of carbon, oxygen and metals – make up some of the asteroid's geological features and might be present in the returned sample. Because these carbonate minerals form under certain conditions, scientists theorize that Bennu's parent asteroid likely hosted an environment where water interacted with and altered the rock on Bennu's parent body.
A Science Advances paper led by Ben Rozitis from The Open University in the U.K. shows that the dark boulders are weaker and more porous, whereas the bright boulders are stronger and less porous. However, both boulder types are weaker than scientists expected. Rozitis and colleagues 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 asteroid Bennu will provide a missing link for scientists, as this type of material is not currently represented in meteorite collections.
A Surprising Shape
Bennu is a diamond-shaped pile of rubble floating in space, but there's more to it than meets the eye. Data obtained by the OSIRIS-REx Laser Altimeter, or OLA, has allowed the mission team to develop a 3D digital terrain model of the asteroid that, at nearly 8-inch resolution, is unprecedented in detail and accuracy.
In a Science Advances paper led by Michael Daly of York University, scientists 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 only became clear when the northern and southern hemispheres were split apart in the OLA data 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.
What's at the Center of Bennu?
Another Science Advances paper in the special collection, led by Daniel Scheeres of University of Colorado Boulder, examines 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 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.
All six publications in the special collection use global and local datasets collected by the OSIRIS-REx spacecraft from February through October 2019. The special collection underscores that sample return missions like OSIRIS-REx are essential to fully understanding the history and evolution of the solar system.
The mission is less than two weeks away from fulfilling its biggest goal – collecting a piece of a pristine, hydrated, carbon-rich asteroid. OSIRIS-REx will depart Bennu in 2021 and deliver the sample to Earth on Sept. 24, 2023.
NASA will provide live coverage of the Touch and Go, or TAG, event starting at 2 p.m. (PT) on Oct. 20, with sample collection schedule for 3:12 p.m. (PT). Viewers can tune in at https://www.nasa.gov/nasalive.
Hosted by Dante Lauretta, the mission's principal investigator and UArizona professor of lunar and planetary sciences, and Michelle Thaller, science communicator at NASA's Goddard Space Flight Center, the broadcast will cover milestones in the 90 minutes leading up to TAG and spacecraft back-away.
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