The complex history of asteroid Bennu

The more scientists learn about near-Earth asteroid Bennu, now being orbited by NASA’s OSIRIS-REx spacecraft, the more it appears that NASA chose well when it picked it as the mission’s target.

Previous research had found Bennu to be a “rubble pile” of rocks assembled from the breakup of a larger body. Many of these rocks appear to be rich in hydrated minerals, a sign that its parent body was rich in water.

Now, scientists are also finding signs that the asteroid is rich in carbon, including minerals that must have formed in hydrothermal systems beneath the surface of the parent asteroid.

That means that it not only had carbon, but was hot enough to have liquid water, says Hannah Kaplan, a spectroscopist at NASA Goddard Space Flight Center, and first author of one of six papers describing the latest findings in the journals Science and Science Advances.

Not that the water was on the surface. “We are talking about something occurring in the interior of the body,” she says.

The telltale minerals, Kaplan says, are carbonates, which her team discovered by looking for their spectroscopic features in close-up images of Bennu’s surface. “I found places where I was quite convinced that this was carbonate,” she says.

At the same time, other scientists were noticing bright streaks in high-resolution images of some of the asteroid’s boulders that appear to be veins of carbonates, up to a metre long and several centimetres wide.

These veins, Kaplan says, appear to be relics of the parent body’s internal plumbing, now broken up, jumbled together, and visible on Bennu. They also appear to date from very early in Solar System history, when the parent body had enough internal heating from short-lived radioisotopes to still be hot enough for hydrothermal systems to form.

201009 bennu blue
False-colour red-green-blue (RGB) composite of Bennu. Average and bluer than average terrain looks blue, surfaces that are redder than average appear red. Bright green areas correspond to the instances of mineral pyroxene, which likely came from a different asteroid. Black areas near the poles indicate no data. Credit: NASA/Goddard/University of Arizona

Nor are carbonate minerals the only carbon-bearing materials scientists are seeing on Bennu. There are also organic materials, discovered from the spectroscopic signature of their carbon-hydrogen bonds, says Amy Simon, a planetary scientist also at NASA Goddard, and first author of another of the new papers.

What exactly these materials are is uncertain, but Kaplan notes that a lot of meteorites collected on Earth are rich in what she calls “organic gunk”. You can extract it from them and get a solid dark substance, somewhat like asphaltite, a naturally occurring mineral akin to asphalt, she says.

This is important because asteroids are thought to be a possible source for the Earth’s organic materials, including important precursors to life, such as amino acids.

OSIRIS-REx’s primary mission is to return a sample of the asteroid to Earth for study in the world’s finest laboratories, so finding such materials on Bennu could be extremely useful to understanding how our planet became habitable.

And happily, says Simon, these materials appear to be scattered all across Bennu’s surface, rather than clustered in small areas.

That means anywhere OSIRIS-REx can safely land to pick up a sample is likely to be a good site, without mission controllers having to choose between getting the materials scientists want and risking an excessively dangerous landing.

Another study in the new set of papers found that Bennu’s boulders behave unusually as they cool when Bennu’s rotation cycles them from day into night.

Oddly, says Humberto Campins, a planetary scientist at the University of Central Florida, Orlando, who was a co-author on three of the new papers, the largest boulder on Bennu’s surface appears to respond to this daily heating and cooling more like beach sand than a normal boulder.

On Earth, he says, if you were to go to the beach shortly after sunset and lean against a big rock, you would find it to be pleasantly warm while the sand is cool – a hallmark of the difference between the rates at which small particles and large rocks warm and cool. “[But] on Bennu,” he says, “the biggest rock behaves like loose sand, and the smaller [ones] behave like rocks.”

Why is an open question. “Maybe the big ones are porous and the small ones are more compact,” he says. “The point is the thermal behaviour is different from what we expected.”

All told, he says, Bennu is proving to be a treasure trove of information. Though, he adds, “of course, we will know more when we bring back that sample”.

And that while that sample can’t get back to Earth before 2023, collecting it is the next goal, planned for later this month, on 20 October.

If it works, expect a big cheer from mission control. If not, NASA has backup plans to attempt as many as three sample-collection efforts.

Global map of Bennu showing peak surface temperature at 12:30 pm local solar time, ranging from 46 to 78 degrees Celsius. Credit: Simon et al., Science (2020)

Other papers

Variations in Color and Reflectance on the Surface of Asteroid (101955) Bennu

Hemispherical differences in the shape and topography of asteroid (101955) Bennu

Asteroid (101955) Bennu’s weak boulders and thermally anomalous equator

Heterogeneous mass distribution of the rubble-pile asteroid (101955) Bennu

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