Crater population on asteroid (101955) Bennu indicates impact armouring and a young surface
Crater population on asteroid (101955) Bennu indicates impact armouring and a young surface
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DOI：
10.1038/s41561-022-00914-5
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发表时间：
2022-04-07
影响因子：
18.3
通讯作者：
Lauretta, D. S.
中科院分区：
文献类型：
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作者：
Bierhaus, E. B.;Trang, D.;Lauretta, D. S.
能够估计行星表面年龄的撞击器与陨石坑尺寸的比例关系依赖于撞击器-目标物理的准确表述。当冲击器的直径与目标表面颗粒（例如，巨石）相当时，就会出现特定于碎石堆表面的装甲状态。建议采用装甲来减小陨石坑直径，或防止小行星表面在小陨石坑直径下形成陨石坑。在这里，通过对碎石堆小行星 (101955) Bennu 上 1,560 个陨石坑的测量，我们表明，巨石群控制着陨石坑直径为 -2-3 m 时从陨石坑形成到装甲的过渡，低于该直径，大块表面的陨石坑形成越来越罕见。通过将撞击器通量的估计与装甲比例关系相结合，我们发现Bennu的陨石坑保留年龄（由陨石坑丰度得出的表面年龄）范围从小于几米的陨石坑的1.6-2.2 Myr到直径>100 m的陨石坑的-10-65 Myr，相对于之前的估计，最大表面年龄减少了>15倍。陨石坑保留年龄的范围，以及大陨石坑空间密度的纬度变化，表明持续的表面重修过程使表面比大块小行星年轻许多倍。
The impactor-to-crater size scaling relationships that enable estimates of planetary surface ages rely on an accurate formulation of impactor-target physics. An armouring regime, specific to rubble-pile surfaces, has been proposed to occur when an impactor is comparable in diameter to a target surface particle (for example, a boulder). Armouring is proposed to reduce crater diameter, or prevent crater formation in the asteroid surface, at small crater diameters. Here, using measurements of 1,560 craters on the rubble-pile asteroid (101955) Bennu, we show that the boulder population controls a transition from crater formation to armouring at crater diameters -2-3 m, below which crater formation in the bulk surface is increasingly rare. By combining estimates of impactor flux with the armouring scaling relationship, we find that Bennu's crater retention age (surface age derived from crater abundance) spans from 1.6-2.2 Myr for craters less than a few meters to -10-65 Myr for craters >100 m in diameter, reducing the maximum surface age by a factor of >15 relative to previous estimates. The range of crater retention ages, together with latitudinal variations in large-crater spatial density, indicate that ongoing resurfacing processes render the surface many times younger than the bulk asteroid.