CRUST: ROCKY SURFACES

AS NEWLY FORMED PLANETS MELT, ROCKY MATERIAL RISES TOWARD THE SURFACE, FORMING A MANTLE AND CRUST.

AS THE SOLAR SYSTEM FORMED MORE THAN FOUR BILLION YEARS AGO, primitive particles collided and clumped into larger and larger bodies. Many asteroids retain these ancient components virtually unchanged. But all the planets, the Moon and some large asteroids partially or completely melted. Dense molten iron-carrying other elements dissolved in it-sank to the cores of these bodies. Molten rock and silicate crystals hardened into rocky mantles above the cores. Further partial melting of mantles caused crusts to form, as on Earth.

Some meteorites come from differentiated planets and asteroids, which started as chondrites but melted and separated into iron and stony parts. Iron meteorites, for example, come from the cores of asteroids that broke up in collisions. And stony meteorites from once-molten crusts can reveal surprising details about how their parent planets formed.

A LOT IN COMMON

The meteorites below hail from the crusts of different planetary bodies in the inner solar system, but they have much in common. All are a rock type common in Earth's crust: basalt, a mixture of the minerals pyroxene, feldspar and olivine. Basalts are present in the rocky crusts of differentiated planets and asteroids throughout the solar system.

For example, Zagami (top) was blasted from the crust of the planet Mars by an impact about three million years ago. Camel Donga (middle) came from an asteroid, probably Vesta. The terrestrial basalt (bottom) is not a meteorite but basaltic lava from a Caribbean volcano-yet in composition and mineralogy, it is very similar to basaltic meteorites from the crusts of asteroids and other planets.

MATCHING METEORITES WITH POTENTIAL PARENTS

The Peña Blanca Springs meteorite belongs to a group, the aubrites, whose chemical composition has led to speculation that they came from the innermost asteroid belt, or possibly even the planet Mercury.

LESS MELTING

Kenna, a ureilite, comes from an asteroid that did not completely differentiate. Instead, it only partially melted, then cooled and crystallized before it could separate into iron and stony parts.

Some ureilites contain primitive particles such as microdiamonds and chondrules that survived partial melting of the surrounding rock. In more fully melted meteorites, no chondrules remain.

MORE MELTING

The Johnstown meteorite comes from a large asteroid, probably Vesta, that melted and differentiated. It is low in iron, nickel and other elements that sank to the asteroid core. It is also poor in aluminum and calcium, which seeped to the surface in volcanic basalts like the Camel Donga meteorite in this case.

This sample is the "main mass" of the Johnstown meteorite, meaning it is the biggest fragment of Johnstown ever found.

LOOK CLOSELY

The large crystals in Cumberland Falls formed inside a partially molten aubrite asteroid, while the two black clasts, or fragments, come from a chondritic asteroid that never melted. The odd mixture was probably produced when the chondrite collided with the aubrite.

WHY PLANETS MELTED

The larger a planet grows, the more likely it is to melt. Objects in space cool by radiating heat from their surfaces. Heat moves slowly through rocks, so larger planets trap more heat inside them. As rocky planets and asteroids collected undifferentiated chondritic material and grew larger, more and more heat built up inside them until they began to melt, leading to differentiation.

A major heat source in the early solar system was the decay of a radioactive isotope of aluminum. Heat was also generated by collisions, internal motion and possibly even by electric currents induced by the solar wind.

MELTING ON MARS

Olympus Mons, on Mars, is the largest volcano in the solar system. Immense volcanoes on Mars once sent molten basaltic rock pouring onto the planet's surface. Today, Mars is much cooler and its volcanic activity has ceased.