VESTA: CHIPS OFF AN OLD ROCK

METEORITES FROM THE ASTEROID VESTA REVEAL ITS DYNAMIC HISTORY.

BETWEEN THE ORBITS OF MARS AND JUPITER, THERE IS A WIDE GAP. In the early 1800s, astronomers searching for the "missing" planet in this gap instead discovered several smaller objects, which they named asteroids. Today, the known asteroids, or minor planets, number in the tens of thousands.

Asteroids range from less than a kilometer to hundreds of kilometers in diameter. Similar objects were once common throughout the solar system. Most eventually merged into full-fledged planets, but in the asteroid belt between Jupiter and Mars, Jupiter's gravitational pull kept them from combining. The surviving asteroids—and broken pieces that reach Earth as meteorites, such as these samples of Vesta—offer a rare glimpse of the early stages of planet formation.

VOLCANISM ON VESTA

When searching for a meteorite's source, experts can sometimes link a group of related meteorites to a type of asteroid or a region in the asteroid belt. Just one group, the HED meteorites—composed of the howardites, eucrites and diogenites—has been traced back to a specific asteroid, Vesta.

How do we know these meteorites came from Vesta? The composition of an asteroid's surface can be determined by the way it reflects sunlight. Vesta is the only large asteroid whose "light signature" matches the basaltic rock of the HED meteorites; each meteorite type matches a different part of Vesta's surface.

The three types of HED meteorites tell the story of the sometimes violent processes that shaped Vesta. The eucrites are hardened lava that flowed onto Vesta's surface; the diogenites come from rock buried deeper down; and the howardites are a mixture of the other two, created by impact mixing.

Vesta is the second-largest asteroid in the solar system, with a diameter of 525 kilometers (325 miles). Covering most of one side is a giant crater with a central uplift. The huge impact that made this crater knocked off more than enough material to account for all the HED meteorites.

SURFACE LAVA

Eucrites resemble lava from Hawaiian volcanoes. They formed much the same way, when molten basaltic rock flowed onto Vesta's surface. This lava cooled and hardened so quickly that only very small crystals had time to form.

The rapid cooling on Vesta's surface also prevented iron and magnesium from spreading evenly through the larger pyroxene crystals, causing the shifting colors in the magnified photo at right.

When a thin slice of Pasamonte is photographed in cross-polarized light, the minerals feldspar and pyroxene appear in different colors. The small needle-shaped crystals are feldspar, a mineral common in eucrites but not in diogenites.

BURIED CRYSTALS

Diogenites are made of molten rock that was trapped beneath Vesta's surface. As a result, they cooled much more slowly than the eucrites, so their crystals had more time to grow.

This photo of a diogenite is magnified 40 times—exactly the same amount as the eucrite. The eucrite's smaller crystals indicate that it hardened closer to Vesta's surface.

This magnified photo of Johnstown shows three large crystals of pyroxene, the main mineral of diogenites. Dark lines show cracks and cleavage.

MIXED BY IMPACTS

Howardites were created by impacts that crushed and mixed different parts of Vesta and melted enough rock to cement them together. In both Kapoeta and Bholghati, the contrast between different colored materials is visible to the naked eye.

LOOK CLOSELY

Ibitira contains thousands of bubbles, or vesicles. These bubbles formed when molten lava flowed onto Vesta's surface, where the sudden drop in pressure caused gases dissolved in the lava to form bubbles-just as bubbles form in soda when a bottle is opened.

HOW DO WE KNOW?

THE PATH TO EARTH

Although the meteorites in this case were long suspected to have come from Vesta, for many years no one could figure out how they got to Earth. Vesta orbits more than twice as far from the Sun as Earth does—and both orbits are nearly circular, so their paths never come close to crossing. How, then, could a piece of Vesta ever reach Earth?

The mystery was solved when scientists discovered a group of small asteroids whose light signatures showed they were once part of Vesta. Some of these "Vestoids" appear to be drifting toward a gap in the asteroid belt. Objects in these gaps are pulled into new orbits by Jupiter's gravity, which could eventually send them flying toward Earth. Similar journeys in the past could account for the rocks from Vesta that have already found their way to Earth.

Asteroids do not all orbit the Sun in one large belt, but in a series of belts with distinct gaps between them. These gaps are areas that have been cleared out by Jupiter's gravity.

Several Vestoids—small asteroids that broke off of Vesta-appear to be drifting out to a gap in the asteroid belt. Objects in this gap complete exactly three orbits for every one orbit of Jupiter, causing them to pass Jupiter at the same point on every third orbit. Repeated gravitational tugs from Jupiter eventually pull them into longer, narrower orbits—which could send them toward Earth.