Privileged Planet: what meteorites bring (Introduction)

by David Turell , Friday, February 11, 2022, 20:36 (1017 days ago) @ David Turell

Analysis of an asteroid shows mainly carbonaceous chondrite:

https://phys.org/news/2022-02-asteroids-sample-earth-reveals-solar.html

"The solar system is full of asteroids: chunks of rock much smaller than a planet. By looking at asteroids through telescopes and analyzing the spectrum of light they reflect, we can classify most of them into three groups: C-type (which contain a lot of carbon), M-type (which contain a lot of metals), and S-type (which contain a lot of silica).

"When an asteroid's orbit brings it into a collision with Earth, depending on how big it is, we might see it as a meteor (a shooting star) streaking across the sky as it burns up in the atmosphere. If some of the asteroid survives to reach the ground, we might find the remaining piece of rock later: these are called meteorites.

"Most of the asteroids we see orbiting the sun are the dark-colored C-types. Based on their spectrum, C-types seem very similar in makeup to a kind of meteorite called carbonaceous chondrites. These meteorites are rich in organic and volatile compounds such as amino acids, and may have been the source of the seed proteins for making life on Earth.

"However, while around 75 percent of asteroids are C-types, only 5 percent of meteorites are carbonaceous chondrites. Until now this has been a conundrum: if C-types are so common, why aren't we seeing their remains as meteorites on Earth?

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"The Ryugu samples (and presumably meteorites from other C-type asteroids) are too fragile to survive entering Earth's atmosphere. If they arrived traveling at more than 15 kilometers per second, which is typical for meteors, they would shatter and burn up long before reaching the ground.

"But the Ryugu samples are even more intriguing than that. The material resembles a rare subclass of carbonaceous chondrite called CI, where C is carbonaceous and the I refers to the Ivuna meteorite found in Tanzania in 1938.

"These meteorites are part of the chondrite clan, but they have very few of the defining particles called chondrules, round grains of predominantly olivine apparently crystallized from molten droplets. The CI meteorites are dark, uniform, and fine grained.

These meteorites are unique in being made up of the same elements as the sun, and in the same proportions (besides the elements that are normally gases). We think this is because CI chondrites formed in the cloud of dust and gas that eventually collapsed to form the sun and the rest of the solar system.

"But unlike rocks on Earth, where 4.5 billion years of geological processing have changed the proportions of elements we see in the crust, CI chondrites are largely pristine samples of the planetary building blocks of our solar system.

"What are the chances, then, of the first C-type asteroid we visit being so similar to one of the rarest kinds of meteorite?

"It is likely the rarity of these CI meteorites on Earth is indeed related to their fragility. They would have a hard time surviving the trip through the atmosphere, and if they did reach the surface the first rainstorm would turn them into puddles of mud."

Comment: Perhaps when our Earth did not have its current atmosphere the meteorites got here and did supply amino acid compounds. Current analyses of meteorites found here do not show that. Life requires formed amino acids, all left-handed, and that issue plagues origin of life studies.