Lutetia: A rare survivor from the birth of the Earth

A team of astronomers from French and North American universities have studied the unusual asteroid Lutetia in detail at a very wide range of wavelengths to deduce its composition. Data from the OSIRIS camera on ESA's Rosetta spacecraft, ESO's New Technology Telescope (NTT) at the La Silla Observatory in Chile, and NASA's Infrared Telescope Facility in Hawaii and Spitzer Space Telescope were combined to create the most complete spectrum of an asteroid ever assembled.

This spectrum of Lutetia was then compared with that of meteorites found on Earth that have been extensively studied in the laboratory. Only one type of meteorite — enstatite chondrites — was found to have properties that matched Lutetia over the full range of colours.

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This animation is an artist’s impression of a possible scenario to explain how the unusual asteroid Lutetia came to now be located in the main asteroid belt, between Mars and Jupiter. The recently formed asteroid is seen passing close to one of the very young rocky planets about four billion years ago and having its orbit drastically altered. The rare spectral properties show that Lutetia started life as a fragment of the material that was forming the inner planets but it is now found to be an unusual interloper much further from the Sun. Credit: ESO/M. Kornmesser, Nick Risinger

"But how did Lutetia escape from the inner Solar System and reach the main asteroid belt?" asks Pierre Vernazza (ESO), the lead author of the paper.

Astronomers have estimated that less than 2% of the bodies located in the region where Earth formed, ended up in the main asteroid belt. Most of the bodies of the inner Solar System disappeared after a few million years as they were incorporated into the young planets that were forming. However, some of the largest, with diameters of about 100 kilometres or more, were ejected to safer orbits further from the Sun.

Lutetia, which is about 100 kilometres across, may have been tossed out from the inner parts of the young Solar System if it passed close to one of the rocky planets and thus had its orbit dramatically altered. An encounter with the young Jupiter during its migration to its current orbit could also account for the huge change in Lutetia's orbit.

"We think that such an ejection must have happened to Lutetia. It ended up as an interloper in the main asteroid belt and it has been preserved there for four billion years," continues Pierre Vernazza.

Earlier studies of its colour and surface properties showed that Lutetia is a very unusual and rather mysterious member of the asteroid main belt. Previous surveys have shown that similar asteroids are very rare and represent less than 1% of the asteroid population of the main belt. The new findings explain why Lutetia is different -- it is a very rare survivor of the original material that formed the rocky planets.

"Lutetia seems to be the largest, and one of the very few, remnants of such material in the main asteroid belt. For this reason, asteroids like Lutetia represent ideal targets for future sample return missions. We could then study in detail the origin of the rocky planets, including our Earth," concludes Pierre Vernazza.

More information: This research was presented in a paper, “Asteroid (21) Lutetia as a remnant of Earth’s precursor planetesimals”, to appear in the journal Icarus.

Journal reference: Icarus

Provided by ESO