Many of the asteroids in our solar system are largely piles of rubble, torn apart by collisions and only weakly pulled back together by gravity. But the larger asteroids, the ones 100km and up, will generally survive most collisions with smaller objects intact. As a result, it has been suggested that these larger asteroids will generally have been unchanged since the formation of the solar system. As such, they can provide us with a picture of the early solar system's conditions at the time the first planetesimals condensed, and before they merged to form the rocky inner planets.
In July of 2010, the ESA's Rosetta mission performed a close flyby of the large asteroid 21 Lutetia. For a year afterwards, it was the largest asteroid we've visited (NASA's Dawn mission has since gone into orbit around 4 Vesta. Rosetta sent home fantastic pictures at the time, and now, over a year later, the scientific analysis has been completed. Three papers on Lutetia will be published in today's issue of Science.
One of the papers is devoted to estimating the asteroid's mass. As the probe approached, the gravitational pull of Lutetia accelerated it, creating an additional Doppler shift of Rosetta's signals; in the same way, Rosetta was slowed down once it shot past. All this was picked up by NASA's Deep Space Network, allowing the authors to plug in the values to a model of Rosetta's flyby, and calculate the strength of the gravitational pull, and thus Lutetia's mass. It's 1.7 x 1018 kg, give or take one percent. (Compare that with a bit under 1023 kg for the Moon.)
With that in hand, figuring out the volume occupied by Lutetia would allow the density of its material to be determined. Lutetia is pretty oblong, having major axes of 121, 101, and 75km. But a program called KOALA (Knitted Occultation, Adaptive-optics and Light-curves Approach) could use these dimensions and the images of the asteroid to estimate its volume. That produced a density of 3,400 kilograms per cubic meter, "one of the highest bulk densities known for asteroids."
That reinforces the impression that Lutetia formed as a solid object, and doesn't have the sort of porous internal structure that you'd expect from a pile of debris. It also indicates that its composition must be quite different from that of stony asteroids—it probably has a high metal content, and is thus likely to be iron rich.
That solid internal structure doesn't mean that Lutetia hasn't taken some hits over the years. There are a number of surface grooves that indicate strains caused by major collisions, and its largest crater is about 55km in diameter, about half of the body's typical width. Several areas of its surface show fewer signs of impacts, suggesting they are younger, and the asteroid's surface has been reworked a couple of times in its past. The authors estimate that the crater was caused by an impactor 8km in diameter which fractured it internally, but didn't shatter it.
Even though the gravity on the body would be incredibly weak by Earth standards, the images show features that are clearly driven by a gravitational attraction. Large boulders, blasted out of the interior, are scattered within other craters. And small landslides of surface materials have tumbled down crater walls.
But small craters are rare on the body's surface. The authors propose that, although the interior of Lutetia is dense, the surface now consists of a dusty material, termed regolith, similar to that found on the Moon. This swallows the smaller impactors; larger ones reach the solid material underneath and blast out a crater. The presence of this regolith reinforces the idea that Lutetia is ancient by solar system standards, since the material is formed over time by the shattering of rock by cosmic rays and micrometeorites.
Rosetta is now in deep space, but NASA's Dawn mission entered orbit around 4 Vesta this year. In many ways, the two missions nicely complement each other. Vesta appears to be the next step up in planet formation, a proto-planet, formed by the combination of multiple planetesimals and with a differentiated interior. Between the two asteroids, we should have a clearer picture of the sorts of bodies that merged to form the inner planets.
Science, 2011. DOI: 10.1126/science.1209389, 10.1126/science.1204062, 10.1126/science.1207325 (About DOIs).
Listing image by Photograph by ESA
reader comments
13