Achondrites

Achondrite meteorites resemble typical igneous rocks found on Earth. They formed by crystallizing from a

Meteorites Plate Tectonics

Computer artwork of main asteroid belt of the solar system (not to scale), between orbits of Mars and Jupiter (Mark Garlick/Photo Researchers, Inc.)

silicate magma and are remnants of larger bodies in the solar system that were large enough to undergo differentiation and internal melting. These meteorites do not contain chondrules or remnant pieces of the early solar system since they underwent melting and recrystallization.

Some achondrites have been shown to have origins on the Earth's moon and on Mars. They formed by crystallization from magma on these bodies, and were ejected from the gravitational fields of these bodies during large-impact events. The debris from these impacts then floated in space until being captured by the gravitational field of Earth, where they fell as meteorites. These meteorites are named for the places they have fallen on Earth, and include shergottites (Shergotty [Shergahti], India), nakhlites (El Nakhla, Egypt), and chassignites (Chassigny, France) (collectively named SNC meteorites after these three falls). SNC achondrites have ages between 150 million and 1.3 billion years, billions of years younger than other meteorites and the age of the solar system. These ages mean that the SNC meteorites must have come from a large planet that was able to remain hot and sustain magma for a considerable time after formation at 4.56 billion years ago. Chemical analysis of the SNC meteorites revealed that most match the bulk chemistry of Mars, confirming the link. Analysis of the damage done to the surface of these meteorites by cosmic rays as they were in space has yielded estimates for the time of transit from the ejection during impact on Mars to the landing on Earth at fewer than 2 million years. Thus most of the SNC meteorites originated from meteorite impacts on Mars in the past 1-20 million years. An estimated billion tons of material has landed on Earth that was originally ejected by meteorite impact on Mars. A smaller number of SNC meteorites have been shown to come from the Earth's moon.

Other achondrites formed on other bodies that have been destroyed by giant impacts. For example, the howardites, eucrites, and diogenites are thought to have formed in one body, the largest remnant of which is the asteroid 4 Vesta, currently orbiting the Sun in the asteroid belt. The eucrites and diogenites represent basaltic magma produced on this early protoplanet and destroyed by a large impact at 4.4 billion years ago, within a hundred million years of the formation of the solar system. Eucrites are basalts that contain the minerals clinopyroxene and plagioclase, diogenites contain orthopyroxene that formed layers of dense crystals called cumulates, and howardites are breccias of these rocks that formed during the giant impact that destroyed the parent achondrite body.

A number of unusual achondrites have no known parent bodies. These include the acapulco-ites, angrites, brachinites, lodranites, and urelites. Some of these are relatively primitive—for instance, the urelites formed early in the solar system evolution, were heated inside a large parent body and crystallized at 2,300°F (1,250°C), were destroyed in a massive impact, then cooled at 50°F (10°C) per hour in the cold vacuum of space. one arubite shows a remarkably fast cooling rate of 1,800°F (980°C) per hour, probably coming from deep within the parent body then being suddenly frozen in space. Brachinites show some of the earliest igneous activity known from any asteroid body, showing the earliest time at which planets may have begun accreting in the early solar system. The crystallized magmas from these bodies have given ages of 4.564 billion years, meaning that the accretion of planetesimals to a size that could partially melt from the solar nebula happened within 5 million years.

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