|Introduction: Space weathering is a generic term for the effects on atmosphereless solid bodies in the solar system from a range of processes associated with direct exposure to the space environment. The classic example of space weathering is the formation of the lunar spectral red slope associated with the production of nanophase Fe (npFe0) in the lunar regolith.
But our understanding of the processes and products of space weathering has been limited by our access to pristine samples like the lunar soils and our necessarily limited view of surfaces provided by telescopic remote sensing. We have primarily focused on the most obvious aspects of weathering, such as the lunar red slope, but our limited data has also limited our view of this essentially physical and chemical phenomena.
However, there is another way to explore space weathering that is not limited by observations or avail-able pristine samples. Space weathering can be viewed as the response of surface materials to inputs that drive the surface composition away from equilibrium, resulting in chemical reactions can be understood from the underlying thermodynamic driving forces. Using techniques and insight developed by materials science physics, especially related to surface science, we can assess the environment of the common asteroidal and planetary materials and forward model the expected results of the weathering reactions. This approach can help us understand the formation processes of known weathering products, predict the formation of other products, and identify already well-known materials as the products of weathering reactions.
A General Theory of Space Weathering: Space weathering can be viewed as driven by a combination of the chemical environment of space (hard vacuum, low oxygen fugacity, solar wind implantation of hydrogen) along with thermal energy supplied by micrometeorite impacts. The forward modeling of space weathering as thermodynamically-driven decomposition of common rock-forming minerals suggests the production of a range of daughter products:
(1) The silicate products typically lose oxygen, other volatile elements (i.e. sulfur and sodium), and metallic cations, producing minerals that are typically more disordered and less optically active than the original parent materials.
(2) The decomposed metallic cations form in nano-sized blebs including npFe0, on the surfaces or in condensing rims of mineral grains. This creates the powerful optical component seen in the lunar red slope and also creates an environment for catalyzing further reactions.
(3) The liberated volatile elements and gases (O, S, Na) may form an observable exosphere if sufficient quantities are available, and can either escape from the body or recombine with solar wind implanted hydrogen to form trace amounts of water and OH.
Mineral decomposition can be thought of as the first stage of space weathering. It produces weathered surfaces somewhat depleted in volatile elements, creates a predictable set of minor or trace minerals, and leaves the surfaces with catalytic species, primarily npFe0. However, a second stage of further reactions and weathering depends upon the presence of “feedstock” components that can participate in catalyzed chemical reactions on exposed surfaces.