Within the Solar System, surfaces of the atmosphere less 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.
A 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.
(*) 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).
French team: Francesca DeMeo, Sihane Merouane (Observatoire de
Paris), Alessandro Morbidelli (Observatoire de la Cote d'Azur),
Pierre Vernazza (ESTEC and Observatoire de Paris), Richard Binzel
(MIT IMCCE, Observatoire de Paris), Mirel Birlan (IMCCE, CNRS 8028,
Observatoire de Paris).
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, published January 21, 2010.
Contact: Dr. Mirel Birlan, Institut de Mecanique Celeste et de
Calcul des Ephemerides (IMCCE),
Observatoire de Paris, Email: Mirel.Birlan at imcce.fr