The solar wind is the cause of rapid ageing of the asteroids
Publiée le 2009-04-22
The solar wind is the cause of rapid ageing of the asteroids
A European team including researchers of Paris Observatory has just highlighted the influence of the solar wind on young surfaces of asteroids. The solar wind in less than one million years ages their surface giving them a very old appearance. They also showed that the observation of near Earth asteroids having a young surface can be explained by a renewal of materials of their surface due to the gravitational tidal forces at the time of their passage close to the Earth, thus compensating for the ageing effect of the solar wind. These results are published in the Nature magazine of April 23, 2009.

Why the materials constituting the surface of the asteroids reflect in a different way the light when they are in space or in laboratory, appearing thus redder and darker in space? This different behavior is mainly due to the interaction of the interplanetary medium with the surface of the asteroids. This surface is deteriorated by the solar wind and the micrometeorites present in the interplanetary medium.

This team studied young families of asteroids formed from violent collisions and having thus a young surface. These asteroids submitted to the impacts of the interplanetary medium see their surface evolving very quickly, in approximately 1 million years, to arrive at surfaces having average colors identical to those of old asteroids. This rapid evolution supports the role of the solar wind as principal element in the transformation of their surfaces. The composition plays also a part, the asteroids including olivine evolving more quickly.

Figure 1 : Evolution of the color of asteroids as a function of their age, assuming that these objects have the same composition. The initial color of an asteroid when it has just undergone a catastrophic collision (which breaks it up into various fragments) should be similar to the color of the meteorites measured in laboratory. The observation of young asteroids (age < 1 million years) reveals that their color has been strongly modified in this short time, giving them the appearance of old surfaces. Only the solar wind (red arrow) can act so quickly on their color, as demonstrated by laboratory experiments. In a second time, micrometeorites impacts could age surfaces with a longer time scale. © Nature.

This team was also interested in the asteroids crossing the orbit of the Earth (NEA: Near-Earth Asteroids) which for some have a color which reveals a surface virgin of any deterioration. All these objects were formed by collisions since more than 100 million years. Thus, recent collisions cannot explain their apparent youth and it is necessary to find another process being able to renovate the matter on the surface. These asteroids could undergo gravitational effects with the approach of the Earth or another telluric planet (tidal effects) tending to dredge up on the surface the internal material, which was not exposed to the solar wind and thus to give this effect of ~renovation~. It will be necessary to confirm this hypothesis in observing the spectral color of the asteroids of the same type present in the principal belt, between Mars and Jupiter, which should then be redder. Reference

Solar wind as the origin of rapid reddening of asteroid surfaces
P. Vernazza, R. P. Binzel, A. Rossi, M. Fulchignoni & M. Birlan
Nature 23/04/2009.


Pierre Vernazza (ESA, et Observatoire de Paris, LESIA)
Marcello Fulchignoni (Observatoire de Paris, LESIA, et Paris VII)
Mirel Birlan (Observatoire de Paris, IMCCE, et CNRS)

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