Fans of the Hollywood blockbuster “Armageddon” will recall the extraordinarily rough surface of its civilization-ending asteroid, “about the size of Texas” as Billy Bob Thornton tells the fictional president. It’s hardly the stuff of fine dust and smooth pebbles. And until now, it was totally contrary to what planetary scientists used to think about the surfaces of most carbonaceous asteroids, the most common out there.
But when NASA's OSIRIS-REx spacecraft arrived at the asteroid Bennu in late 2018, says NASA, it found a surface covered in boulders. Now, new research indicates that many carbonaceous asteroids, may be much more rocky than previously thought. Or so says a paper appearing this week in the journal Nature.
Previous studies found that, if a rock is highly porous (that is, it has a lot of voids), it is harder to break into pieces during an impact than a rock with lower porosity, Saverio Cambioni, the paper’s lead author and a distinguished postdoctoral fellow at MIT, told me. This is because a big part of the impact energy goes into crushing the voids instead of into producing finer rocks and pebbles, he says. Hence, where rocks are more porous on asteroids, one should expect less regolith to be present, says Cambioni, which is exactly what we found on Bennu.
“This finding provides an answer to the debate about the nature of asteroids' surfaces that was going on since the 1980s,” said Cambioni.
This news is important for planetary science because we need to sample asteroids to answer fundamental questions such as how the solar system formed and how life came to be on Earth, says Cambioni. Our findings will help future teams better prepare sample collection missions depending on the nature of the target asteroid, he says.
Between April and June 2019, the Origins, Spectral Interpretation, Resource Identification, and Security Regolith Explorer (OSIRIS-REx) Thermal Emission Spectrometer (OTES) measured thermal infrared emission spectra from the surface of Bennu at different local times of day, Cambioni and colleagues write in their paper. The team then used machine learning to analyze 122 areas on the surface of Bennu, that were observed both during the day and the night.
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“We demonstrated the central role of rock porosity in driving the diversity of asteroid surfaces,” said Cambioni.
By contrast, Cambioni and colleagues predict terrains rich in fine regolith to be common on S-type asteroids, the second-most populous type of asteroids observed in the solar system, says NASA. S-type asteroids are expected to have denser, less porous rocks than carbonaceous asteroids.
This finding has also been born out by Japan’s Hayabusa2 mission to Ryugu, a carbonaceous asteroid like Bennu, says NASA. Hayabusa2 found that Ryugu also lacks fine regolith and has high-porosity rocks, says the agency. In contrast, A 2005 Japanese mission to the asteroid Itokawa, an S-type asteroid with rocks of a different composition than Bennu and Ryugu, found it to have a smooth, less rocky surface.
The authors therefore infer that regolith blankets are uncommon on carbonaceous asteroids, which are the most numerous types of asteroid.
As for “Armageddon”?
The asteroid depicted in the film has a surface that’s extraordinarily treacherous; full of sharp ice structures, more like the surface of an ever-mutable comet than a rocky body.
The asteroid in Armageddon was about the size of Texas, says Cambioni. I would expect an asteroid of that size to be like asteroid Ceres (which is some 550 miles in size), he says. However, the surface of Ceres was imaged by the NASA Dawn mission, and it does not have sharp ice structures.
So, did Hollywood get it wrong after all?
“The texture of that [“Armageddon”] asteroid seems very extreme to me,” said Cambioni.
Source : https://www.forbes.com/sites/brucedorminey/2021/10/22/most-asteroids-likely-have-rough-rocky-surfaces-like-bennu-says-new-paper/1070