Nature2014-06-19 2:54 PM

小行星表面的“风化层”是由热循环造成的 Thermal fatigue as the origin of regolith on small asteroids

论文摘要 

小型小行星的表面覆盖着一层像尘埃或沙砾的东西,被称为“风化层”(regolith)。关于其怎样形成的标准观点一直是,它大部分是由微型陨石碰撞所产生的碎片,但这种观点与以下看法产生了冲突:这样的碰撞力量之大将足以使所产生的碎片远离小行星表面,而不是将其送回到小行星表面。在这项研究中,Marco Delbo等人证明,热疲劳是一个可能性更大的解释。他们进行了这样一些实验,在其中Murchison (CM2) 和Sahara 97210陨石的厘米大小的样本被暴露于一系列的温度循环。这些实验表明,这种岩石通过由日间温度变化所诱发的热碎裂而分解的速度要比因微型陨石碰撞而分解的速度更快。

Abstract 

Space missions and thermal infrared observations have shown that small asteroids (kilometre-sized or smaller) are covered by a layer of centimetre-sized or smaller particles, which constitute the regolith. Regolith generation has traditionally been attributed to the fall back of impact ejecta and by the break-up of boulders by micrometeoroid impact. Laboratory experiments and impact models, however, show that crater ejecta velocities are typically greater than several tens of centimetres per second, which corresponds to the gravitational escape velocity of kilometre-sized asteroids. Therefore, impact debris cannot be the main source of regolith on small asteroids. Here we report that thermal fatigue, a mechanism of rock weathering and fragmentation with no subsequent ejection, is the dominant process governing regolith generation on small asteroids. We find that thermal fragmentation induced by the diurnal temperature variations breaks up rocks larger than a few centimetres more quickly than do micrometeoroid impacts. Because thermal fragmentation is independent of asteroid size, this process can also contribute to regolith production on larger asteroids. Production of fresh regolith originating in thermal fatigue fragmentation may be an important process for the rejuvenation of the surfaces of near-Earth asteroids, and may explain the observed lack of low-perihelion, carbonaceous, near-Earth asteroids.

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