PARENT BODIES: A METEORITE FAMILY TREE

SOME SEEMINGLY UNRELATED METEORITES CAME FROM THE SAME SOURCE, OR "PARENT BODY."

IN JUST A FEW THOUSAND YEARS, THE SOLAR SYSTEM EVOLVED from a collection of small particles sticking together to an assortment of larger bodies known as planetesimals—precursors of the planets. Today, fragments of this primordial material fall to Earth as meteorites, although pieces of the same object do not necessarily fall at the same time. Scientists have identified groups of meteorites with the same composition—and have concluded that members of the same group were probably once part of the same "parent body." Evidence suggests that the tens of thousands of meteorites discovered to date are fragments of fewer than 200 parent bodies.

Today most parent bodies exist in the asteroid belt, located between the planets Mars and Jupiter. But initially, these bodies could be found throughout the solar system. As parent bodies formed in the early solar system, some incorporated more of certain ingredients than others. These differences are crucial to our ability to trace family ties among groups of meteorites.

MEET THE PARENTS

Meteorites were once part of larger objects, or parent bodies, that formed when chondrules, calcium-aluminum inclusions (CAIs), dust grains and other components accreted, or stuck together. The chemical makeup of each meteorite depends in large part on how much of each of these ingredients was present when and where the parent body came together.

Scientists rely on detailed laboratory analysis to determine whether two meteorites came from the same parent body. One of the most common techniques is to look at the different forms, or isotopes, of oxygen present in a meteorite. Researchers have grouped meteorites into categories based on their oxygen-isotope "signatures". If two samples have the same signature, they probably came from the same parent body. In contrast, the three samples shown above and below appear similar but actually have quite different oxygen isotope signatures. As a result, scientists think these meteorites originated from three different parent bodies.

Scientists distinguish groups of meteorites by measuring the ratios of three forms, or isotopes, of oxygen in meteorites—oxygen-16, oxygen-17 and oxygen-18. The relative amounts of these isotopes produce an isotope "signature." When these signatures are plotted on a graph, the data fall in discrete regions. Meteorites with signatures that fall in the same region may come from the same parent body.

Sometimes the groups are named for the first meteorite sample discovered with that signature—for example, the Ivuna-type chondrites. Isotope signatures for other bodies in our solar system have also been determined: all samples from Earth, including the atmosphere, ocean and mantle, fall along the dotted white line. Mars and the asteroid Vesta also reflect a unique mixture of primordial oxygen-bearing material.

DIVIDED THEY FALL

The meteorites Kunashak, Kyushu and Suizhou fell in three different countries—Russia, Japan and China—over a span of 100 years. Nevertheless, they are virtually identical in composition, so they may have originally belonged to the same parent asteroid. Billions of years ago, these fragments broke off after the parent body collided with another asteroid.

A DIVERSE FAMILY

At first glance, meteorites don't look too much alike. Yet researchers have concluded that Bath and Hammond Downs were once part of the same parent body. Richardton (right) was probably part of the same original body as well, although the evidence is less certain.

SUBTLE DIFFERENCES

The two meteorites belowshare certain features in common—they are both carbon-rich, or carbonaceous, chondrites and their oxygen-isotope signatures overlap. Yet they are different enough in their texture and composition that scientists think they come from unrelated parent bodies. These differences are visible when the samples are examined under a microscope. Ornans (below, left) has relatively small chondrules, while Vigarano (below, right) has notably larger chondrules. (Both images have been enlarged 30 times.)