Chronicles of Lutetia

Asteroid 21 Lutetia isn’t just another pebble in a big pile of space rocks. Scientists now think it is a leftover planetary seed, booted into the main belt by the planetary bullies growing around it.

Lutetia and its asteroid cousins are thought to be relics from the early solar system, rocky fossils that have recorded a history of the solar system’s early days in their pits and fractures. In July 2010, the European Space Agency’s Rosetta spacecraft flew within 3,200 kilometers of Lutetia, peered at the asteroid and attempted to read its stony story.

Using data gathered by Rosetta, three reports describe Lutetia’s surprising composition and terrain. They appear in the Oct. 28 Science.

“If you have visited one asteroid, you have not visited them all,” says Lindy Elkins-Tanton of the Carnegie Institution for Science in Washington, D.C. “We can still learn some amazing new things about planetesimals, primitive materials, solar system dynamics and [asteroid] composition.”

Scientists estimate that Lutetia is 121 kilometers long, 101 kilometers tall and 75 kilometers wide. Data suggest that it is what’s known as an enstatite chondrite — a rare form of asteroid that makes up around 2 percent of the meteorites that have fallen to Earth. “It’s pretty uncommon,” says planetary scientist and study author Pierre Vernazza of the European Southern Observatory. “Our understanding is that this kind of meteorite is the starting composition of the terrestrial planets, from Mercury to the Earth.”

Among the other characteristics betraying Lutetia’s identity as a planetesimal — or planet precursor — is the asteroid’s abnormally high density. At 3.4 grams per cubic centimeter, Lutetia is denser than most asteroids measured, and comparable in density to the giant asteroid Vesta, says study author and planetary scientist Holger Sierks of the Max Planck Institute for Solar System Research in Germany.

A density that high suggests that the asteroid is not a rubble pile, or collection of fragments produced by violent collisions. Rather, the rocky body has probably maintained its primordial state, and might have a differentiated interior, with a metallic core, mantle and surface that never melted, says Elkins-Tanton.

Indeed, Lutetia’s ancient and complex surface — marked with landslides, enormous craters, faults and fractures — supports the finding that the asteroid is primitive and undisturbed, and suggests that it formed within the solar system’s first 3 million years, she says.

But the question of where Lutetia formed is still open. A team of scientists, including Vernazza, proposes in a paper to be published by the journal Icarus that Lutetia grew near the sun, in the terrestrial planet region, and was subsequently pushed outward. Whether the four inner planets or a migrating Jupiter nudged the planetesimal toward the asteroid belt is unclear, the team reports.

“I would say the jury is out on this one,” Elkins-Tanton says. “But … if we begin to learn some of those answers from our solar system, we can begin to understand why it is that all the exoplanet systems we found don’t look like ours. There’s so much we need to learn about movement during planet formation.”