Formation and Evolution of Lunar Regolith

Introduction: The formation of regolith on all airless bodies is a complex interaction of the components of space weathering and the surfaces of these objects, be they asteroids (e.g., 4-Vesta), moons (e.g., Phoebus), small planets (e.g., Mercury), or our unique neighbor, the Moon. In fact, it has been the Apollo Missions to the Moon that have provided the data basis and opportunities to study just such a regolith. The geologic term “regolith” (Greek: rock) refers to a layer of loose, heterogeneous material covering solid rock. With planetary studies, it is often used synonymously with “soil”, which has been defined as that portion of the regolith <1 cm in particle size [1]. The term in this planetary setting is where biologic activity is absent; it is not organicbearing soil in terrestrial usage, but merely a size designation for a particular range of regolith particles. In addition, in recent years, “dust” has been defined as the <20 μm portion of the soil. For excellent reviews of the formation of lunar regolith/soil, see [1-2]. Weathering: With the practical absence of an atmosphere (10 -10 to 10 -12 torr) and total lack of water on the Moon, the major weathering and erosional processes are totally alien to those on Earth. Instead, the flux of meteorites and micrometeorites (<1 mm) is the major regolith-, soil-, and dust-forming agent. Other processes, of less importance, are due to galactic/cosmic/solar particle irradiation, which causes spallation, vaporization, and radiation damage. Other factors are mass wasting, with downhill landslides on various scales, and electrostatic transport, which occurs along the lunar terminator, although the density of such particle levitation is minute. Lastly, pyroclastic soils (e.g., Apollo 17 orange soil) have been formed largely by volcanic activity. Lunar soil is an ever-changing blanket of crushed, pulverized, and melted materials, which evolves in direct response to continued reworking by the micro-meteorite flux. The two main dynamic processes responsible for this developmental sequence are (1) comminution (crushing and pulverizing) and (2) agglutination (the aggregation of soil particles by impact-produced glass. The textural maturity of the lunar soil is determined almost entirely by the balance between these two opposing processes, one destructive, the other constructive, with respect to particle size. During time, larger impact events penetrate the soil blanket in a stochastic process and effectively mix soils from below with the latest surficial soil. This mixing and turn-over of the soils takes place on all scales. A 2.98 m deep core revealed that mare soils do not vary systematically in maturity as a function of depth, except for a distinct array of very-mature soil in the upper 5 cm. It has been estimated to take a few million years for the formation of a single cm of soil [2]. The particle size distribution (PSD) of lunar soil decreases as the exposure age and maturity increase. As shown in Fig. 1, the average size of lunar soil is 50-60 μm, and this constitutes about 50 wt% of the soil [1]. Notice also that lunar dust at <20 μm makes up ~20 wt%. It is this abundant dust that has the toxicologists concerned for its possible effects on human respiratory and pulmonary systems. Until recently, little was known of the size distribution and general physical nature of lunar dust. However, studies of the PSD and morphologies and shape analysis have demonstrated the unusual nature of the dust with the maximum number of particles/mass is at 100-200 nm (0.1-0.2 μm) [3-6].