Estimation of the space weathering timescale on (25143) Itokawa: Implications on its rejuvenation process

Vol. 667
10. Planets and planetary systems

Estimation of the space weathering timescale on (25143) Itokawa: Implications on its rejuvenation process

Space weathering designates the ensemble of processes by which surfaces of airless bodies are physically and optically affected by their exposition to various sources of energy from outer space. This may include the impacts of cosmic rays, irradiation by the solar wind, and meteoritic or micrometeoritic bombardment. Space weathering is observed to occur on lunar rocks, asteroid surface, and meteorite samples, but the dominant mechanism and its characteristic timescale are often difficult to assess, with proposed timescales ranging from ~10^3 to 10^8 years in the case of asteroids. Focusing on asteroid (25143) Itokawa, which was visited by the Hayabusa probe, Jin and Ishiguro propose a novel method that makes use of a statistical evaluation of the bright mottles that occur on some boulder surfaces of the asteroid. These mottles are thought to consist of fresh material exposed by impacts of millimeter- to centimeter-sized interplanetary dust particles (IDPs). As the asteroid surface is permanently darkened and reddened by space weathering, the number and size distribution of the bright mottles is controlled by the balance between space weathering and the IDP impact frequency. The authors identify ~400 such mottles and determine their cumulative size-frequency distributions. The latter is then compared to expectations from an IDP flux model. The slope of the size distribution (with power index q ~ -3.7) agrees with expectations from the IDP flux model, lending confidence to the proposed mechanism, and the absolute number of bright mottles indicates a space weathering timescale of ~10^3 years, with a likely uncertainty of a factor of 10 at most. Compared with expectations from laboratory experiments, this timescale is consistent with space weathering by light ions from the solar wind, as opposed to irradiation by heavy ions (timescale ~ 10^6 years) or micrometeorite impact (~10^8 years). With such a short timescale, the occurrence of bright regions elsewhere on Itokawa implies the existence of another rejuvenation process, which the authors tentatively identify as seismic shaking triggered by a recent impact that created the Kamoi crater.