月表静电作用对月球尘埃外逸层的贡献

Lunar dust exosphere has been verified by many observations, but how it is generated has not been successfully explained. There are two main explanations: electrostatic levitation and micrometeoroid-impact ejecta. The former was preferred in the Apollo era, since the observed dust cloud was too bright to be interpreted as secondary ejecta from a micrometeoroid impact. However, recent observations from Clementine, LRO and LADEE/LDEX favor the latter explanation, along with an empirical model for the micrometeoroid-impact ejecta. Whereas the empirical model is still not enough to explain the observations from LADEE/UVS and Apollo missions. Generally speaking, the dust exosphere should be contributed by both effects, and the actual contribution seems to be related to the size of dust grains: the electrostatic effect contributes more to smaller grains and the micrometeoroid-impact effect contributes more to larger grains. This program is aimed to quantitatively calculate the dust density caused by the electrostatic effect with SPIS-dust tool, along with some experiments to determine the simulation parameters. Moreover, we’ll compare the calculated result with the empirical model to determine the relative contribution of these two effects to the dust exosphere, and finally explain the related observations.

月球尘埃外逸层已被大量观测所证实，但其产生机制仍然没有统一定论。主流解释有两种：一是静电悬浮机制，二是微流星体撞击溅射机制。前者在Apollo时期占优势，因为后者不足以解释当时所观测的尘埃云亮度。不过，最近Clementine、LRO和LADEE/LDEX的观测结果却又更多地支持微流星体撞击溅射机制，并建立了相应经验模型。但是，这个经验模型又不能解释LADEE/UVS和 Apollo的观测结果。总的来看，两种机制都对尘埃外逸层有贡献，只是二者的贡献与尘埃尺寸相关：即静电悬浮作用主要针对小颗粒尘埃，而微流星体撞击溅射作用主要针对大颗粒尘埃。本项目旨在利用SPIS-dust数值模拟工具，结合实验测量所得的模拟参数，定量计算由静电作用造成的尘埃空间分布，并与微流星体撞击溅射经验模型进行比较，确定两种作用对月球尘埃外逸层的相对贡献，最终解释相关观测结果。

阿波罗时期看到的月球地平辉光现象表明月面附近有大量漂浮尘埃，这些形状不规整、尺寸在微米级的细小颗粒会对探测仪器及人类造成伤害，是月面探测活动必须要面对的重要环境问题。然而，尘埃是如何离开月表的以及月面附近的尘埃是如何分布的目前仍然不清楚。本项目借助SPIS-dust数值模拟软件，计算得到了不同太阳风条件下不同太阳天顶角处月面电势和尘埃密度随高度的分布，这些结果有助于理解正常情况的月尘分布。此外，我们还研究了地形起伏（石头或坑）月尘分布的影响，发现地形遮挡效应会带来比较强的负电势，从而使尘埃更容易离开月面。通过分析嫦娥三号任务的岩石照片，我们发现对于较年轻的月面，其尘埃颗粒尺寸相对较大（微米级），尘埃迁移的高度较低（<28 cm）。通过分析LADEE任务的尘埃观测数据，我们发现在晨昏线附近的大撞击坑周围存在局部的尘埃云，其特征高度为50 km，对应的尘埃大小约为0.1 μm、密度最高可达103 m-3。通过理论分析，发现这些尘埃云是由地形遮挡效应带来的尘埃静电喷泉现象引起的。我们的研究结果表明月球并没有全球性的浓密尘埃外逸层，而只是在晨昏线附近的撞击坑旁边有静电喷泉效应带来的局部尘埃云，这可能就是阿波罗任务中只零星观测到几次地平辉光现象的原因。我们的结果可为评估未来月面探测活动的尘埃环境提供科学依据，同时也为研究其他缺气天体的尘埃环境提供参考。