Monte-Carlo simulation of Mercury's exosphere
Monte-Carlo simulation of Mercury's exosphere
复制标题
水星外逸层的蒙特卡罗模拟
DOI：
10.1016/s0019-1035(03)00123-4
复制
发表时间：
2003
期刊：
影响因子：
3.2
通讯作者：
H. Lammer
中科院分区：
文献类型：
--
作者：
P. Wurz;H. Lammer
由于它离太阳很近，而且体积很小，水星无法保留它的大气层，只有一个很薄的外圈环绕着这颗行星。根据水手10号掩星实验，行星表面的大气层压力约为10−10 mbar。水手10号已经证实了气态物种H、He和O的存在。此外，还用地面仪器观测到了Na、K和Ca。由于已知物种的总压力几乎比外圈压力小两个数量级，预计在水星的外圈中还会发现其他元素。它打算利用欧空局比皮科伦坡水星行星轨道器(MPO)航天器上的仪器现场测量这些外层粒子密度。由于预期的外层密度非常小，我们开发了一个蒙特卡洛计算机模型来研究在MPO航天器轨道上进行这种测量是否可行。我们模拟了粒子在表面的能量和抛射角分布，发射过程决定了实际的分布函数。我们的模型通过数值积分跟踪每个粒子的轨迹，直到粒子再次撞击水星表面或逃离计算域。利用这些轨迹中的一大组，得到了外层气体的体积参数，例如，各种原子和分子物种的粒子密度。我们的研究表明，质谱学测量是可行的，至少在MPO的周缘，将观察到从表面释放的所有物种。
Because of its proximity to the Sun and its small size, Mercury has not been able to retain its atmosphere and only a thin exosphere surrounds the planet. The exospheric pressure at the planetary surface is approximately 10−10mbar, set by the Mariner 10 occultation experiment. The existence of gaseous species H, He, and O has been established by Mariner 10. In addition Na, K, and Ca have been observed by ground based instrumentation. Other elements are expected to be found in Mercury's exosphere since the total pressure of the known species is almost two orders of magnitude less than the exospheric pressure. It is intended to measure these exospheric particle densities in situ with an instrument on board of ESA's BepiColombo Mercury Planetary Orbiter (MPO) spacecraft. Since the expected exospheric densities are very small we developed a Monte-Carlo computer model to investigate if such a measurement is feasible along the MPO spacecraft orbit. We model energy and ejection angle distributions of the particles at the surface, with the emission process determining the actual distribution functions. Our model follows the trajectory of each particle by numerical integration until the particle hits Mercury's surface again or escapes from the calculation domain. Using a large set of these trajectories bulk parameters of the exospheric gas are derived, e.g., particle densities for various atomic and molecular species. Our study suggests that a mass spectrometric measurement is feasible and, at least at MPO's periherm, all species that are released from the surface will be observed.