THE SOLAR SYSTEM: FROM HOT TO COLD

THE CHEMICAL COMPOSITION OF EACH METEORITE PROVIDES CLUES TO WHERE ITS PARENT BODY RESIDED IN THE SOLAR SYSTEM.

THE EARLY SOLAR NEBULA was a turbulent mixture of the chemical elements, including hydrogen, oxygen, carbon, iron and silicon. But how did these elements combine to form the Sun and planets? The answer is unclear—our knowledge of solar system evolution is incomplete.

One feature of the early solar system that is well understood, however, is that temperatures gradually decreased away from the young Sun. This change in temperature affected the distribution of elements in the solar system. Most of the hydrogen and other gases became part of the Sun. The rest of the material in the solar system combined in different proportions to form planetesimals. Close to the Sun, only rocky and metallic solids survived the intense heat. Farther out, ices could exist. These differences can be seen in the asteroid belt—and in the meteorites that fall to Earth.

ANATOMY OF THE SOLAR SYSTEM

Meteorites and the planets in our solar system share an important feature: their chemical makeup relates to distance from the Sun. The planets closest to the Sun (Mercury, Venus, Earth and Mars) are rocky, while the outer planets (Jupiter, Saturn, Uranus and Neptune) are made largely of gas and ices. The three rocks on display represent three distinct groups of meteorites. The chemical composition of meteorites in each group provides clues to how far from the Sun the rocks may have formed.

The meteorite Eagle belongs to a group of meteorites (enstatite chondrites) that could have formed relatively close to the Sun. Of the three samples, Eagle is richest in rock-forming elements.

Farmington belongs to a group (ordinary chondrites) that may have formed at an intermediate distance from the Sun.

Banten belongs to a group (carbonaceous chondrites) that may have taken shape relatively far from the Sun. This meteorite does not contain liquid water or solid ice today, but minerals in Banten have water molecules in their crystal structures. Water is a "volatile" compound that boils at relatively low temperatures and was typically not abundant in parent bodies that formed close to the Sun.

THE OUTER REACHES

The outer planets—Jupiter, Saturn, Uranus and Neptune—formed in the coolest part of the early solar nebula, which was rich in volatile gases and ices. The meteorite Allende also contains high levels of volatile elements. It belongs to a group of meteorites (carbonaceous chondrites) that may have formed relatively far from the Sun. The composition of Allende also matches that of asteroids in the far reaches of the asteroid belt.

COMETARY CORE

Studies suggest the meteorite Orgueil might come from the core of a comet, an object made of ice, dust and rock that orbits the Sun at high speed. Many comets originate in the Kuiper Belt, located in the cold outer fringes of our solar system, beyond the orbit of Neptune.

The comet Hale-Bopp was visible from Earth for weeks in the spring of 1997 before heading off to continue its orbit around the Sun. The dust and ices in the outer layers of comets eventually burn off after repeated close approaches to the Sun. The rocky cores of "dead" comets may occasionally fall to Earth as meteorites.

AN IRONCLAD CASE?

Iron will remain in a metallic state unless it is exposed to a substance like water, which "oxidizes" the metal. The iron in Abee is almost all in a metallic state—suggesting that the parent body from which this meteorite broke off never contained any liquid water.

Scientists speculate that Abee and other meteorites in the same group (enstatite chondrites) formed relatively close to the Sun, where parent bodies contained little or no water. Similar parent bodies appear to be concentrated in the inner asteroid belt.

MIDDLE OF THE ROAD

Like Renfrow (above), Melrose (right), Fisher and Wickenburg (left), Oakley (below) is an example of a meteorite that formed at an intermediate distance from the Sun (ordinary chondrites). These meteorites have a mixture of rock-forming elements and more volatile elements, such as carbon and oxygen. They also resemble asteroids found in the central region of the main belt.

THE ASTEROID BELT

The vast majority of meteorites were once part of asteroids that reside in the asteroid belt between Mars and Jupiter. Scientists have been able to link some types of meteorites with asteroids in certain regions of the asteroid belt. Like the planets, asteroids have chemical compositions that appear to vary depending on their distance from the Sun. The asteroids closest to Mars appear relatively rocky, while those closest to Jupiter are richer in volatile elements.

How do we know the chemical composition of the bodies in the asteroid belt, which is located more than 120 million kilometers (77 million miles) from Earth? Scientists study asteroids primarily by using spectroscopy, measuring how asteroids absorb and reflect different wavelengths of light. By using the latest telescopes—and even by sending space probes to orbit or land on asteroids—researchers have been able to learn a great deal about these objects.

Several spacecraft have orbited or landed on asteroids and sent pictures of these bodies back to Earth. The NEAR Shoemaker mission, launched in 1996, was the first spacecraft to land on an asteroid, touching down on Eros on February 12, 2001.