Analyses of meteorites and theoretical models indicate that some carbonaceous near-Earth asteroids may have been thermally altered due to radiative heating during close approaches to the Sun^1,2,3. However, the lack of direct measurements on the subsurface doesn’t allow us to distinguish thermal alteration due to radiative heating from parent-body processes. In April 2019, the Hayabusa2 mission successfully completed an artificial impact experiment on the carbonaceous near-Earth asteroid (162173) Ryugu^4,5, which provided an opportunity to investigate exposed subsurface material and test potential effects of radiative heating. Here we report observations of Ryugu’s subsurface material by the Near-Infrared Spectrometer (NIRS3) on the Hayabusa2 spacecraft. Reflectance spectra of excavated material exhibit a hydroxyl (OH) absorption feature that is slightly stronger and peak-shifted compared with that observed for the surface, indicating that space weathering and/or radiative heating have caused subtle spectral changes in the uppermost surface. The strength and shape of the OH feature suggests that the subsurface material experienced heating above 300 °C, similar to the surface. In contrast, thermophysical modelling indicates that radiative heating cannot increase the temperature above 200 °C at the estimated excavation depth of 1 m, even at the smallest heliocentric distance possible for Ryugu. This supports the hypothesis that primary thermal alteration occurred on Ryugu’s parent body.

对陨石和理论模型的分析表明，一些碳质近地小行星可能由于接近太阳时的辐射加热而发生了热变化^1,2,3。然而，由于缺乏对地下的直接测量，我们无法区分由于辐射加热引起的热变化与母体过程。2019年4月，隼鸟2号任务成功完成对碳质近地小行星（162173）龙宫^{4,5的人工撞击实验}，这为研究暴露的地下材料和测试辐射加热的潜在影响提供了机会。在这里，我们报告了隼鸟 2 号航天器上的近红外光谱仪 (NIRS3) 对 Ryugu 地下物质的观察。挖掘出的材料的反射光谱表现出羟基 (OH) 吸收特征，与在表面观察到的相比，该特征略强且峰移，表明空间风化和/或辐射加热已导致最上表面的光谱发生细微变化。OH 特征的强度和形状表明，地下材料经历了 300 °C 以上的加热，类似于地表。相比之下，热物理模型表明，在估计开挖深度为 1 m 时，辐射加热不能将温度提高到 200 °C 以上，即使在 Ryugu 可能的最小日心距离上。这支持了原始热变化发生在 Ryugu 的母体上的假设。