Small ‘pancakes’ indicate that some asteroids may remain active

On the night of April 23, 2019, the calm of the rainforest near Aguas Zarcas, Costa Rica, was shattered by a streak of bright light in the sky, a thunderous explosion, and a storm of rocks — the remnants of a small asteroid that exploded in the atmosphere. One of the fragments punched a hole the size of a grapefruit in the roof of a house, while the second hit the kennel.

Within days, the silence broke again as scientists and collectors from around the world began searching for shards that were worth more than gold – in the literal and scientific sense.

One team of geologists Suggestion that Aguas Zarcas meteorite It shows that at least one class of asteroids may be busier than expected, with tiny pebbles jumping around their surfaces like popcorn kernels in a microwave — an act already observed on the surface of asteroid Bennu.

“Aguas Zarcas…contains pre-Arctic star dust, some of the oldest mineral crystals in the Solar System, and bio-organic molecules. The organics gave it an interesting scent—a little tar, a little sweet—almost like vanilla.”

“Asteroids were very active in the early days of the solar system, but today most people think they are dead,” he said. Shen Yangwho is a graduate student in University of Chicago and lead author of the study. “This tells us that carbonaceous asteroids may not be dead; they have a low level of activity.”

The study began shortly after Field Museum In Chicago, she received a 1.9-kilogram piece of the meteorite, which was classified as A carbonaceous chondrites. It consists of small pieces of material embedded in a matrix of fine-grained rocks.

“These meteorites are very rare,” he said. Philip Heck, curator of meteorology and polar studies at the museum and co-author of the study. “It contains primitive materials formed in the early solar system and still preserved today. Aguas Zarcas is a type of very pure primitive stuff. It contains star dust, some of the oldest mineral crystals in the solar system, and vital organic molecules. The organic materials gave it an interesting scent— A little tar, a little sweet – almost like vanilla.”

Small pebbles resist destruction

Yang, who also works at the Field Museum with Heck, began processing a 79-gram portion of the meteorite, which had been subjected to a computerized tomography (CT) scan to map its interior for mineralogy study. The sample was collected before any rain fell on it and then stored in nitrogen, which limits ground pollution.

The first step was to subject it to an alternate cooling and heating cycle in liquid nitrogen and water, which eventually crushed the rock. After more than 100 cycles, there are pebble-sized inclusions known as chondrocytes remained unbroken. It also proved difficult to destroy with mortar and pestle. “We thought this was interesting, so we said, ‘Let’s not break it,'” Heck said.

They did an additional CT scan and looked at the original scans again, which revealed that the alien cartilages were flat, pancake-like, rather than spherical, with the cartilages (including many found in the same piece of Aguas Zarcas) appearing spherically natural. (Chondroles are spherical because they form as tiny beads of molten rock that cool and solidify in the zero-gravity environment of space. They are then fused into an asteroid as they coalesce, so they are ahead of the host asteroid.)

Additionally, the scans showed that all of the individual flattened spheres were aligned in the same direction within the meteorite, indicating that they were all crushed when another space rock collided with the original asteroid and later scattered, allowing them to mix with undistorted cartilage.

This was the first observation by OSIRIS-REx of pebbles ejected from the surface of Bennu. attributed to him: NASA / Goddard / University of Arizona / Lockheed Martin

Heck said the results remind him Determinethe asteroid visited Osiris Rex (Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer) from 2018 to 2021. In addition to collecting a few dozen grams of surface material to bring back to Earth next year, the craft will discover pebbles They flow into space and either fall back or enter orbit. The pebbles were similar in size to aguas Zarcas – generally 0.5 to 1 centimeter for a Beno, compared to a few millimeters to about a centimeter for aguas Zarcas.

“The events ranged from quite explosive, with many particles releasing, to like popcorn, with just a few small particles popping up.”

OSIRIS-REx recorded more than 300 ejection events, or an average of about one event per day. “The events ranged from quite explosive, with many particles releasing, to like popcorn, with some small particles popping up,” he said. Harold Connolly Jr.He is a professor of geology at Rowan University in Glassboro, NJ, and science sample team leader for OSIRIS-REx, who was not involved in the study. “Some have gone into orbit but usually no more than a few days, so what goes up has to go down.”

Mission scientists have devised a range of explanations for the emissions, including shocks, faulting caused by the extreme temperatures of the Bennu cycle’s day and night, and small eruptions caused by ice evaporation just below the surface.

The cycle of day and night may be devoid of pebbles

The dynamics of the ejected pebbles suggest that the asteroid that gave birth to Aguas Zarcas must have been like Bennu, Yang said, as the cartilage appeared in space and then settled back into different locations, mixing with other cartilage that was also transported from elsewhere. (The process shouldn’t work on larger asteroids because their surface gravity will limit the distance an ejected piece of rock can travel.)

Large impacts can etch a lot of material, spraying it all over the surface of the asteroid, but the cartilage that has been transported should be similar – all deformed and aligned similarly. Yang said because Aguas Zarcas has a mix of shapes and trends, however, that scenario doesn’t seem to work. Instead, “this deformation appears to be localized,” with less forceful effects of compressing the cartilage across a smaller area and leaving many buried beneath the surface.

In this illustration of the process of mixing pebbles taken from the original body of Aguas Zarcas, pebble-sized fragments are ejected and re-deposition on the surface of the asteroid. Credit: April I. Neander, NASA/Goddard/University of Arizona (asteroid image)

Heck suggested that this observation favors the day-night cycle as the reason for the scattering of both the Bennu and Aguas Zarcas pebbles. Brittle rock may crack, turning it into a loose pit of rock and dust and possibly removing some debris. Additionally, small impacts can cause some rocks to scatter away, scattering them across the asteroid. Another effect can cause the mixed materials to smash together again, forming hard rocks like Aguas Zarcas. This effect can also cause parts to explode in space to drift through the solar system – and eventually land on Earth.

The Chicago team is examining additional carbonaceous chondrites, as well as reviewing old CT scans from previous studies, to verify the scenario identified in the study. In addition, Heck is a member of the team that will analyze approximately 60 grams of Bennu collected by OSIRIS-REx, which may shed some light on the formation and redistribution of gravel.

“It will contribute to answering parts of their hypothesis,” Connolly said. “And it will undoubtedly open up new questions we never thought of. Such is the beauty of looking at a new rock for the first time – you will never get bored because you always find something you don’t understand.”

—Damond Benningfield, science writer

the quote: Benningfield, D. (2022), Small ‘pancakes’ suggest some asteroids may remain active, Eos, 103, https://doi.org/10.1029/2022EO220460. Posted on September 22, 2022.
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