Zoom on near-earth asteroids
1er janvier 2010
In general, the surface of the atmosphere-free small bodies of the solar system looks older through micrometeorite impacts, solar wind and cosmic rays. The asteroids that cross the orbit of the Earth, on the contrary, appear to us with a rejuvenated surface, different from those of the Main Belt. A franco-american team, including researchers from Paris Observatory, proposes that seismic waves generated by the close passage to the Earth are able to re-arrange rocks and therefore rejuvenate the surface of the asteroid.
Within the Solar System, surfaces of the atmosphereless bodies are strongly influenced by factors often associated with the expression "space weathering (*). Recent scientific evidence has shown that the aging process surface due to space weather has a very strong momentum (of the order of one million years) relative to the age of the solar system (about 4.5 billion years).
In the case of small bodies, there is a dichotomy between the asteroids that cross the orbit of the Earth and those of the Main Belt. The spectral response surfaces of a class of Near-Earth Asteroids (the taxonomic class Q) shows minerals whose physical properties are less affected by the space weathering(**). Mechanisms such as close approaches with the Earth, Mars or Venus, or collisions with other small asteroids are proposed to produce the re-surfacing of Near-Earth Asteroids. The french-american team examined the issue of proximity between NEOs and the terrestrial planets. The spectra of a sample of one hundred asteroids have been analyzed and the results were corroborated with their orbital dynamics.
The conclusion of this research is that the Earth can produce a significant impact for the re-surfacing processes, if the asteroid passes at least a distance of 16 Earth radii from the planet. This distance is approximately one quarter of the Earth-Moon distance. Seismic waves generated by the close passage are able to "shake up" the surface of the object so that rocks and regoliths are reorganizing. The reorganization of the surface of the asteroid is revealed by the spectral measurements. Having suffered less exposure to space weather, minerals resulting from the re-surfacing will show spectra which are in better agreement with laboratory spectra of ordinary chondrite meteorites.
In 2029, the asteroid 99942 Apophis, whose diameter is estimated at about 270 meters, will pass close to Earth. This passage will take place at a distance of about 42,000 km and will not affect our planet. However, during this close passage, the asteroid will be located inside the perimeter of the theoretical limit discussed above, and vibrations will be strong enough to produce its re-surfacing. Spectroscopic observations of the close approach of 99942 Apophis will be possible in 2029, will confirm these assumptions and will contribute to the validation of these results. See also the IMCCE site.
(*) Space weather includes several factors such as the solar wind, the cosmic rays, and the impacts with micro-meteoroids. (**) The spectrum of Q-type asteroids is the best analogue for most meteorites that exist in collections (ordinary chondrite meteorites).
Reference
R. Binzel, A. Morbidelli, S. Merouane, F. DeMeo, M. Birlan, P. Vernazza, C. Thomas, A. Rivkin, S. Bus and A. Tokunaga Earth encounters as the origin of fresh asteroid surfaces, Nature du 21 janvier 2010
Contact
Mirel Birlan (Observatoire de Paris, IMCCE, et CNRS)
Dernière modification le 4 mars 2013