Keller L P, Thompson M S, Seifert L B, Melendez L E, Thomas-Keprta K L, Le L, Snead C J, Welten K C, Nishiizumi K, Caffee M W, Masarik J, Busemann H, Krietsch D, Maden C, Rahman Z, Dukes C A, Cloutis E A, Gainsforth Z, Sandford S A, DellaGiustina D N, Connolly H C, Lauretta D S
NASA Johnson Space Center, Houston, TX USA.
Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, IN USA.
Nat Geosci. 2025;18(9):825-831. doi: 10.1038/s41561-025-01745-w. Epub 2025 Aug 22.
The OSIRIS-REx mission deployed contact pad samplers to collect regolith from the uppermost surface of the asteroid Bennu that was exposed to the space environment. Space weathering processes, dominated by micrometeoroid impacts and solar irradiation, modify the mineralogy and chemistry of exposed surfaces to produce solar wind-amorphized layers on clays, metallic whiskers associated with high temperature melts and Fe nitride created by the reaction of indigenous N-bearing gases with space-weathered surfaces. Here, we use cosmogenic noble gases and radionuclides to suggest that the upper metre of Bennu's regolith has been exposed to cosmic rays for 2-7 million years, consistent with remote sensing observations indicating that the asteroid's surface is dynamic and regularly modified by mass movement. Solar energetic particle track and microcrater densities constrain the space weathering spectral changes observed in Hokioi crater to <50,000 years. These spectral changes are driven largely by the accumulation of impact melt deposits on particle surfaces, although compositional or grain size effects may also occur. Comparison of Bennu samples with those collected from the asteroids Ryugu and Itokawa suggest that micrometeoroid impacts might play a more active and rapid role in the space weathering of asteroidal surfaces than was initially suggested, particularly for carbonaceous bodies.
奥西里斯-雷克斯任务部署了接触垫采样器，以从小行星贝努暴露于太空环境的最上层表面收集风化层。以微流星体撞击和太阳辐射为主导的太空风化过程，改变了暴露表面的矿物学和化学性质，在黏土上产生了太阳风非晶化层、与高温熔体相关的金属须，以及由原生含氮气体与太空风化表面反应生成的氮化铁。在此，我们利用宇宙成因惰性气体和放射性核素表明，贝努风化层的上层一米已经暴露于宇宙射线200万至700万年，这与遥感观测结果一致，即该小行星表面是动态的，且经常因质量迁移而改变。太阳高能粒子轨迹和微陨坑密度将在霍基奥伊陨石坑观测到的太空风化光谱变化限制在<50000年。这些光谱变化主要由撞击熔体沉积物在颗粒表面的积累驱动，不过也可能出现成分或粒度效应。将贝努样本与从小行星龙宫和糸川收集的样本进行比较表明，微流星体撞击在小行星表面的太空风化中可能比最初认为的发挥更积极、更快速的作用，特别是对于碳质天体。
