Carbonaceous chondrites CM
Synonyms: Mighei-like carbonaceous chondrites
General: The chondrites of this group are named for their type specimen Mighei, a meteorite that fell in Ukraine in 1889. The fact that CM chondrites contain water and complex organic compounds, such as amino acids, has caught the interest of science and the public, as well.
Description: Superficially, the CM chondrites resemble the CI chondrites and they also look a bit like a piece of tar on first sight. The CM members also exhibit a low density, and a high porosity, but they mostly belong to petrologic type 2, i.e., they still show some well defined chondrules troughout their black matrix. Besides that, they also often show white inclusions known as CAIs (calcium-aluminium-inclusions). These CAIs represent some of the oldest matter known, having crystallized in the early hours of the primordial solar nebula.
Mineralogy: With about 10% of water, CM chondrites contain less than the CI members and they show less aqueous alteration so that some chondrules have been well preserved. Those chondrules consist of olivine and are scattered throughout the black matrix. In that mixture of phyllosilicates and magnetite, similar to the matrix of the CI chondrites, one also finds light-coloured inclusions, such as CAIs. These high-temperature silicates are lacking in the CI group, and are quite typical for CM chondrites.
Origin and Formation: CM chondrites are known to contain a wealth of complex organic compounds. The well-studied meteorite of Murchison, a CM2 that fell in Australia in 1969, was found to contain more than 200 different amino acids, whereas on Earth only 20 different amino acids are known and used as the fundamental building blocks of life. Some of these extraterrestrial amino acids were found to exhibit strange isotopic signatures that might indicate that they don't have their origin within our solar system. These amino acids are believed to represent actual interstellar matter from other systems that were trapped into the CM chondrites more than 4.5 billion years ago. Because of this fact, some researchers have promoted the idea that the CM chondrites might be of cometary origin, but recent research indicates that certain dark asteroids within the main asteroid belt are the real source of the CMs. Comparisons based on the reflectance spectra of certain meteorite classes and main belt asteroids yielded a close match, making the asteroid 19 Fortuna a very good candidate to be the lost parent body of this peculiar class of carbonaceous chondrites.
Members: Excluding all probable pairings, about 100 CM group members are known. Some of them are famous witnessed falls, such as the above mentioned Murchison, Boriskino, Cold Bokkeveld, Mighei, Murray, and Nogoya. Only a few CM chondrites have been recovered from the hot deserts, so far, but more and more are being found in the ice fields of Antarctica, indicating that a cold climate is more likely to preserve these brittle, water-bearing rocks for several thousand years.