Asteroids are generally regarded as the solar system’s scrap heap, the battered bits that broke off and were left behind when the planets were forming. But the lumpy asteroid 21 Lutetia may be a whole, unbroken building block left nearly untouched since the solar system’s birth.
“We think planets were built of things like Lutetia,” says Ben Weiss of the Massachusetts Institute of Technology in Cambridge. “We’re getting a chance to see one of the building blocks of the solar system up close.”
The European spacecraft Rosetta zipped past Lutetia at 50,000 kilometres per hour in July 2010, snapping photos of a cratered world about 121 kilometres long. That makes it the second-largest asteroid ever visited by a spacecraft, next to 560-kilometre-wide Vesta.
Most of the asteroids to get visits from spacecraft are rubble piles, chunks of debris that were loosely held together by gravity. But Lutetia is so dense that it appears to have survived the violent mêlée of collisions in the early solar system intact.
“The real new thing is that it’s not a rubble pile, it’s a solid block of rock,” says Holger Sierks of the Max-Planck Institute for Solar System Research in Germany, lead author of a new paper reporting the observations. “It’s really a remnant from the early days.”
The rock has one of the highest asteroid densities ever measured, at 3.4 tonnes per cubic metre. That is denser than granite and suggests Lutetia might have heavy metals in its core.
For that to have happened, it must have melted in the past, allowing the heavy elements to sink to the centre. It could have done that if it formed within the solar system’s first million years, when there was enough radioactive aluminium-26 to melt the space rock.
Curiously, Rosetta’s optical instruments showed an unmelted surface covered in craters – providing no hint that its interior might be full of heavy metals. That hidden history of melting might explain a “seeming contradiction between the meteorite suite and the asteroids we see”, says Weiss.
Most asteroids look unmelted, but most meteorites that have fallen to Earth – thought to be fragments of asteroid cores – look like they have melted significantly. So it is possible that the melted meteorites are chips from space rocks that formed very early in the solar system, before the bulk of the radioactive aluminium – which has a half-life of 700,000 years – had decayed.
If Lutetia is a remnant from the solar system’s earliest days, what can we learn from it? Sadly, says Sierks: “We can’t really say much about the composition of the material. For that we really have to analyse in situ [with a lander] – or better, grab a piece and carry it back home for in-depth lab analysis.”
Journal reference: Science, DOI: 10.1126/science.1207325, 10.1126/science.1209389, 10.1126/science.1204062;
Planetary and Space Science, DOI: 10.1016/j.pss.2011.09.012