Meteorite Classification

How do we classify meteorites?

In order to better understand the origin of these rocks from space, we want to know more precisely where they came from. The variety in what they are made of tells us that they are not all from the same place, but how many different places do they come from? The majority of meteorites are from the asteroid belt, but we seek to understand which and how many parent body asteroids might be responsible for the rocks we have in our collection today.

By studying the similarities and differences in what the rocks are made of, we can group together rocks with similar characteristics that could have originated from the same parent body, and group together other meteorites that must have come from a different parent body. A different parent body may be required, for example, if one rock has minerals that could only have formed at high temperature, while another has minerals that mean it could not have been hotter than a certain temperature. Sometimes a parent body could be of sufficient size that both sets of conditions could have co-existed with a cooler surface and hotter interior, but other constraints, such as the age of the rock or the timing of heating events may mean it is not possible for them to be from the same parent body.

Over the past 100 years our techniques for analysing, comparing and contrasting the compositional differences between space rocks have become increasingly sophisticated. As our abilities to analyse features at ever smaller scale have improved, we have gradually sub-divided groups of rocks into more categories. Further research can then compare and contrast the composition and history of these rocks.

“Undifferentiated” meteorites are chondrites, which contain chondrules. Chondrules are spherical droplets, thought to have cooled extremely quickly in the very early stages of the Solar System formation. The parent bodies of chondrites may have experienced limited alteration from heat or water processing, but as chondrules can still be identified, the rocks have never been fully molten or part of a large body. Thus they provide some of the earliest examples of material formed in our Solar System.

“Differentiated” meteorites originate from larger planetary bodies that were big enough to have enough internal heat from radioactive materials, to melt and form separate layers, with iron-rich cores and silicate-rich surface layers. This includes meteorites from the Moon and Mars, as well as large asteroids such as Vesta in the asteroid belt, iron meteorites and stony iron meteorites that represent the deep interior of larger asteroids.

The main classification tree can be seen below outlining how we group different meteorites into different categories.

Generalized meteorite classification scheme into undifferentiated and differentiated groups of samples. Image KJoy, adapted from NASA.
How meteorite groups relate to different parent bodies. Image by KJoy adapted from LPI/E&SS/NASA/Earth interior graphic adapted from Gary Hincks/Science Photo Library