Formation of asteroids

(ORDO NEWS) — During the formation of the Sun, the conditions in the protoplanetary disk were, of course, not the same at different distances from the Sun and changed over time.

Matter remained cold only away from the Sun. It was very warm near it and the dust was subjected to complete or partial evaporation. Only later, when the gas cooled, did it condense again, but most of the volatile substances contained in interstellar dust grains were lost and did not enter the new dust.

The evolution of the protoplanetary disk led to the formation of planetesimals in it, from which the planets later grew. The composition of planetesimals formed at different heliocentric distances was different due to the different composition of the dust that went into their construction.

It just so happened that asteroids are planetesimals formed on the border of the hot and cold zones of the protoplanetary disk, which have survived to this day. Although the asteroid ring has a small extent (only about 1 AU), the difference in conditions in it was apparently sufficient to form S- and C-asteroids unlike each other.

It is quite logical to think that S-asteroids formed in a warmer zone, at smaller heliocentric distances than C-asteroids, and are now slowly mixing.

However, since only those bodies that were formed in the most stable orbits survived, their complete mixing did not occur over the past 4.5 billion years. That is why, until now, C-asteroids gravitate towards the outer part of the ring, and S-asteroids – towards the inner. But facing each other

Asteroids formed in the protoplanetary cloud as loose aggregates. A small force of gravity could not compress the planetesimals condensed from dust. Due to radioactive heat, they warmed up. This heating, as shown by the calculations of J. Wood, was very effective: after all, loose bodies retain heat well.

Warming up began at the stage of growth of asteroids. Their substance in the central parts was heated, sintered, and, perhaps, even melted, and dust still continued to fall out on the surface of asteroids, replenishing the loose, heat-insulating layer.

The main source of heating is now considered to be aluminum-26, the same aluminum-26, which, a million years before the formation of asteroids, was injected together with the substance of a supernova into the protosolar nebula.

Collisions of asteroids among themselves at first also led to the compaction of their matter. Asteroids became compact bodies. But in the future, disturbances from the grown planets led to an increase in the speeds at which collisions occurred.

As a result, already more or less compact bodies were broken. Collisions were repeated repeatedly, crushing, shaking, mixing, welding fragments, and crushing again. That is why modern asteroids are most likely poorly packed blocks.

To the earth’s orbit, small asteroid fragments come, of course, from the asteroid ring. This happens due to the mechanism of successive resonant buildup of orbits under the influence of planetary disturbances, which is not yet completely clear in details.

But the buildup occurs only in some areas of the ring. Asteroids from different parts of the ring do not arrive equally efficiently, and debris in the vicinity of the Earth’s orbit may not at all be representative of those objects that move beyond the orbit of Mars.

And in the Earth’s atmosphere, only the slowest and strongest of them survive, which leads to further selection.

Therefore, many varieties of asteroidal matter are undoubtedly absent from our collections, and it is possible that the idea of ​​asteroidal matter as a dense and compact substance is nothing but an outdated delusion inspired by meteorites.


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