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Formation of a rubble pile asteroid inferred from study of the Chelyabinsk meteorite — Origin of rubble pile asteroids (Itokawa, Ryugu, and Bennu) through comet capture and sublimation

1. Introduction


Nakamura et al. (2012) conducted initial analyses of particles collected from the asteroid Itokawa by the spacecraft Hayabusa at the Pheasant Memorial Laboratory. It was found that Itokawa was a rubble pile asteroid and the components were dominated by fragments of a pre-existing asteroid (100 km radius), which had been heated at some point in the past. Since the size of the collected particles was very small, 50—100 micron, we could not apply age determination techniques to the particles. We could not tell how and when the fragments accumulated to form the rubble pile asteroid.

On the 15th February (2013), a meteorite impacted Chelyabinsk state in Russia. Because of damages to local people, it was reported as a natural incident on worldwide news. It is referred to as the Chelyabinsk meteorite and was categorized as an ordinary chondrite. It has a genetic relationship to the asteroid Itokawa, which is also composed from fragments of ordinary chondrite.

The Chelyabinsk meteorite was disassembled by a shockwave during entry into the atmosphere of the Earth, and fell down as little black bodies on the ground covered by white snow. Even if the chemical composition is identical, meteorites discovered on the Earth may not necessarily have the same origin. They could have visited the Earth from different asteroids and at different timings. It is not the case for the Chelyabinsk meteorite. The small bodies collected from the surface of snow soon after the fall were from a single 20-m-sized boulder (before the entry to the Earth). Since Chelyabinsk was collected soon after the fall, the terrestrial modification is considered to be minimum and the sample is appropriate to understand the origin of the meteorite by applying systematic analyses.

We have initiated the research to reveal the formation processes of asteroids, which are the source of meteorites.


2. Observation and age determination


The observation of the cross-sectional surface tells us that (1) the meteorite consists of white colored mineral phases, (2) there are ubiquitous cracks, and (3) some of cracks are filled with black colored quenched glass. The observation suggests that a sub portion of the meteorite was heated because of hyper velocity collision, quenched, and solidified rapidly.

The concentration of water in the meteorite is 30—100 ppm (in hydrogen). Electron microscopy and secondary ion mass spectrometry were used to identify the career of water. The results tell us that most of the hydrogen is located in the cracks, which originated from the impact event, instead of inside the mineral phases that formed prior to this event. This observation implies that water sintered into cracks after their formation, by the impact event. In other words, the 20-m-sized object was submerged in water, within a space environment that lacks gravity and is under ultra-high vacuum.

Let’s summarize the history of the Solar system. In the early Solar system 4.56 billion years ago, gas in the nebula was condensed into solid dust as the nebula cooled down. The dust was accreted to form asteroids and the asteroids were gathered to form planets. It took only a few million years to form the framework of the current planetary system. When did the Chelyabinsk meteorite get impacted, melted and subsequently aqueous altered?

We extracted quenched glass that formed by the impact event from the Chelyabinsk meteorite and applied age determination technique to it. It turned out that the impact event occurred as recent as 30 million years ago instead of an early period of the Solar system.


3. Link between an asteroid and a comet


The Chelyabinsk meteorite, as a 20 m sized boulder interacted with water after being shocked, whilst hot and within the last 30 million years (Figure a) somewhere in the Solar system. What are the main Solar bodies that contain water as their main component in the current Solar system? A comet is a water-dominated body, which has an icy core and consists of 80% water. As comets get closer to the Sun, water was sublimated from the core and the phenomena is observed as a jet, even from the Earth.

How the Chelyabinsk boulder reacted with water inside of the icy core? A snow ball collects sand from the ground on creation of a snowman, in a similar way comets coming from Oort cloud or small bodies around Jupiter collect fragments of rocks and dust and transport them around the Solar system. When the comet captured the fragment, the kinetic energy converted into heat and raised the temperature of the ice temporarily. Under high pressure conditions, ice is turned into a super critical fluid. The viscosity of the super critical fluid is as low as vapor and reactive. Therefore, it is possible that the fluid would sinter into the cracks and react with preexisting minerals (Figure b).

As the comet got closer to the Sun, the ice was sublimated (Figure c) and in the end only non-volatile materials such as rock fragments and organic materials remained (Figure d). The organic materials that were contained in the cometary core and rock fragments and dust collected during the trip across the Solar system were loosely and weekly bound to each as an aggregate. This is like the rubble pile that we saw on the asteroid Itokawa. We infer that a comet is involved in the formation of a rubble pile asteroid, playing the important roles of collection and concentration of materials with different origins. The comet also evolves organic and inorganic materials, assisted by fluid.


Figure: How a comet turns into a rubble pile asteroid (a) By collision, an asteroid gets shocked and melted within the last 30 million years. (b) A comet captures the fragments of rock. Because of the relative velocity, the kinetic energy is turned into heat. On impact, ice turns into water and the rock fragment gets submerged in water. (c) Ice on the comet is sublimated by radial heat from the Sun. (d) At the end, only rock fragments, dust, and organic materials remain. They are weakly bound by gravity and the aggregate looks like a rubble pile asteroid.．

4. Closing remarks


The Pheasant Memorial Laboratory plans to conduct initial analyses of organic and non-organic materials collected from the asteroid Ryugu by spacecraft Hayabusa 2. Ryugu is also a rubble pile asteroid and we expect that Ryugu also is originated interaction with a comet. Thus we predict that the materials collected include rocks with different origins and organic materials inherited from the comet. We are preparing for the analyses of the materials that are coming back to the Earth in the winter of 2020.

This article is based on following manuscript published in Proceedings of the Japan Academy, Series B. The samples and datasets inferred in the manuscript are all accessible in the depository DREAM.