Highly porous rocks responsible for Bennu's surprisingly craggy surface


Date:
October 6, 2021
Source:
University of Arizona
Summary:
Using data from NASA OSIRIS-REx mission, scientists concluded that
asteroids with highly porous rocks, such as Bennu, should lack fine-
grained material on their surfaces.



FULL STORY ========================================================================== Scientists thought asteroid Bennu's surface would be like a sandy beach, abundant in fine sand and pebbles, which would have been perfect for
collecting samples.Past telescope observations from Earth's orbit had
suggested the presence of large swaths of fine-grain material called
fine regolith that's smaller than a few centimeters.


==========================================================================
But when the spacecraft of NASA's University of Arizona-led OSIRIS-REx
asteroid sample return mission arrived at Bennu in late 2018, the mission
team saw a surface covered in boulders. The mysterious lack of fine
regolith became even more surprising when mission scientists observed
evidence of processes capable of grinding boulders into fine regolith.

New research, published in Natureand led by mission team member Saverio Cambioni,used machine learning and surface temperature data to solve
the mystery. Cambioni was a graduate student at the UArizona Lunar
and Planetary Laboratory when the research was conducted and is now a postdoctoral distinguished fellow in the Department of Earth, Atmospheric
and Planetary Sciences at the Massachusetts Institute of Technology. He
and his colleagues ultimately found that Bennu's highly porous rocks
are responsible for the surface's surprising lack of fine regolith.

"The 'REx' in OSIRIS-REx stands for Regolith Explorer, so mapping
and characterizing the surface of the asteroid was a main goal,"
said study co- author and OSIRIS-REx principal investigator Dante
Lauretta, a Regents Professor of Planetary Sciences at the University
of Arizona. "The spacecraft collected very high-resolution data for
Bennu's entire surface, which was down to 3 millimeters per pixel at
some locations. Beyond scientific interest, the lack of fine regolith
became a challenge for the mission itself, because the spacecraft was
designed to collect such material." To collect a sample to return to
Earth, the OSIRIS-REx spacecraft was built to navigate within an area
on Bennu roughly the size of a 100-space parking lot.

However, because of numerous boulders, the safe sampling site was reduced
to roughly the size of five parking spaces. The spacecraft successfully
made contact with Bennu to collect sample material in October 2020.

A Rocky Start and Solid Answers "When the first images of Bennu came
in, we noted some areas where the resolution was not high enough to see
whether there were small rocks or fine regolith. We started using our
machine learning approach to separate fine regolith from rocks using
thermal emission (infrared) data," Cambioni said.



==========================================================================
The thermal emission from fine regolith is different from that of larger
rocks, because the former is controlled by the size of its particles,
while the latter is controlled by rock porosity. The team first built a
library of examples of thermal emissions associated with fine regolith
mixed in different proportions with rocks of various porosity. Next,
they used machine learning techniques to teach a computer how to "connect
the dots" between the examples. Then, they used the machine learning
software to analyze the thermal emission from 122 areas on the surface
of Bennu observed both during the day and the night.

"Only a machine learning algorithm could efficiently explore a dataset
this large," Cambioni said.

When the data analysis was completed, Cambioni and his collaborators found something surprising: The fine regolith was not randomly distributed on
Bennu but instead was lower where rocks were more porous, which was on
most of the surface.

The team concluded that very little fine regolith is produced by Bennu's
highly porous rocks because these rocks are compressed rather than
fragmented by meteoroid impacts. Like a sponge, the voids in rocks
cushion the blow from incoming meteors. These findings are also in
agreement with laboratory experiments from other research groups.

"Basically, a big part of the energy of the impact goes into crushing the
pores restricting the fragmentation of the rocks and the production of new
fine regolith," said study co-author Chrysa Avdellidou, a postdoctoral researcher at the French National Centre for Scientific Research (CNRS)
-- Lagrange Laboratory of the Co^te d'Azur Observatory and University
in France.



========================================================================== Additionally, cracking caused by the heating and cooling of Bennu's rocks
as the asteroid rotates through day and night proceeds more slowly in
porous rocks than in denser rocks, further frustrating the production
of fine regolith.

"When OSIRIS-REx delivers its sample of Bennu (to Earth) in September
2023, scientists will be able to study the samples in detail," said
Jason Dworkin, OSIRIS-REx project scientist at NASA Goddard Space Flight Center. "This includes testing the physical properties of the rocks to
verify this study." Other missions have evidence to confirm the team's findings. The Japanese Aerospace Exploration Agency's Hayabusa 2 mission
to Ryugu, a carbonaceous asteroid like Bennu, found that Ryugu also lacks
fine regolith and has highly porous rocks. Conversely, JAXA's Hayabusa
mission to the asteroid Itokawa in 2005 revealed abundant fine regolith
on the surface of Itokawa, an S-type asteroid with rocks of a different composition than Bennu and Ryugu. A previous study by Cambioni and his colleagues provided evidence that Itokawa's rocks are less porous than
Bennu's and Ryugu's, using observations from Earth.

"For decades, astronomers disputed that small, near-Earth asteroids could
have bare-rock surfaces. The most indisputable evidence that these small asteroids could have substantial fine regolith emerged when spacecraft
visited S-type asteroids Eros and Itokawa in the 2000s and found fine
regolith on their surfaces," said study co-author Marco Delbo, research director with CNRS, also at the Lagrange Laboratory.

The team predicts that large swaths of fine regolith should be uncommon on carbonaceous asteroids, which are the most common of all asteroid types
and are thought to have high-porosity rocks like Bennu. In contrast,
terrains rich in fine regolith should be common on S-type asteroids,
which are the second-most common group in the solar system, and are
thought to have denser, less porous rocks than carbonaceous asteroids.

"This is an important piece in the puzzle of what drives the diversity
of asteroids' surfaces. Asteroids are thought to be fossils of the
solar system, so understanding the evolution they have undergone in
time is crucial to comprehend how the solar system formed and evolved,"
said Cambioni. "Now that we know this fundamental difference between carbonaceous and S-type asteroids, future teams can better prepare sample collection missions depending on the nature of the target asteroid." ========================================================================== Story Source: Materials provided by University_of_Arizona. Original
written by Mikayla Mace Kelley. Note: Content may be edited for style
and length.


========================================================================== Journal Reference:
1. Cambioni, S., Delbo, M., Poggiali, G. et al. Fine-regolith
production on
asteroids controlled by rock porosity. Nature, 2021 DOI:
10.1038/s41586- 021-03816-5 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/10/211006160046.htm

--- up 4 weeks, 6 days, 8 hours, 25 minutes
* Origin: -=> Castle Rock BBS <=- Now Husky HPT Powered! (1:317/3)