月壤中纳米金属铁的太空风化成因及模拟方法分析

The Origin and Simulation of Nanophase Iron in Lunar Soil

月壤中普遍存在着大量由太空风化作用产生的纳米金属铁,这些纳米金属铁在一定程度上改变了月球表面的物理、化学和光学特征。纳米金属铁在月壤中主要赋存于胶结质玻璃相中和月壤颗粒的表面,胶结质玻璃相的纳米金属铁起源于微陨石轰击富含太阳风氢粒子的月壤产生的高温熔融还原作用,颗粒表面的纳米金属铁来自微陨石轰击引起的蒸发沉积作用和太阳风、宇宙射线粒子的溅射沉积作用的共同作用。根据月壤中纳米金属铁的成因和特征,分析了采用微波加热技术和磁控溅射技术分别模拟胶结质玻璃相中和颗粒表面纳米金属铁的可行性研究,初步的模拟实验表明这两种方法模拟生产的样品中纳米金属铁的特征和真实月壤的相近,模拟样品对于月球遥感光谱解译、月球科学研究和月球探测工程都具有重要的意义。

Unprotected by atmosphere and magnetosphere,exposed lunar surface has been subjected to several harsh space processes.Among them,meteoroids and energetic charged particles are the dominant forces that shape the lunar surface.Collectively,these processes are known as ＂space weathering＂.Nanophase iron（np-Fe0） particles produced by space weathering have been widely observed in the lunar soil,which could have important effects on the physical,chemical and optical properties of lunar soil.In studies of lunar samples,it is found that np-Fe0 occurs in two main petrographic settings in lunar soil： as inclusions within agglutinitic glass and as inclusions in thin amorphous rims surrounding soil grains.Meteorites bombardment,especially micrometeorites bombardments could locally melt the lunar surface materials which saturat with hydrogen implanted from solar wind,and then reduce the ferrous minerals to metallic irons in agglutinatic glass.In previous studies,the size of np-Fe0 particles was found to be about 3~33 nm.But recent studies show that the np-Fe0 particles in agglutinatic glass are much larger,and most of them are about 50~200 nm in size and finely dispersed.Vapor deposition from micrometeorites bombardments and sputtering deposition from solar wind and cosmic rays produce np-Fe0 particles in the amorphous rims around the lunar grains surfaces.The size of np-Fe0 particles in the rim is ranged from several nanometers to tens of nanometers.Most of them are about 10nm.To investigate the relationship between np-Fe0 and various properties of lunar soil,simulation of the production process of np-Fe0 by space weathering is necessary because of the scarcity of lunar samples for research purpose and the complexity of the lunar soil.At present,there are three main methods to simulate the lunar soil analogs with the np-Fe0 particles.Laboratory simulation of np-Fe0 production in lunar space weathering deserves a more realistic approach to model the production process of np-Fe0 in lunar soil.Combining the formation and characteristics of np-Fe0 in lunar soil,we discuss the feasibility of two new experimental methods,which use microwave heating and magnetron sputtering to simulate np-Fe0 in the glass and on the grains surfaces respectively.Microwave heating can heat materials such as ilmenite which have a large dielectric loss to high temperature at a very fast heating rate,which might be a good method to simulate the production of np-Fe0 in the glass phase by reducing ferrous materials to metallic iron.Magnetron sputtering technique is a physical vapor deposition technique,which is similar to the deposition process of vapor deposition and sputtering deposition in lunar space weathering.The primary results of these two experimental simulations show that the characteristics of np-Fe0 produced in the simulations are in consistent with those of lunar samples documented in literature.Simulation of np-Fe0 is very important to remote sensing,lunar science and lunar exploration.