The underlying physical and chemical processes relevant to the study of space-weathering in the solar system have been reviewed as well as the many advances of laboratory experiments and our understanding as far as interpretation and reproducing observational results. The exposure of rocky and icy materials to the space-weathering agents revealed the extensive chemical and morphological transformations undergone by the planetary bodies as they synergistically interact within the space environment. The action of the weathering agents on the surfaces of airless bodies increases the molecular complexity and alters the morphology of the surface itself whilst concomitantly causing the destruction and desorption of molecular species into the exosphere. This release yields a source of energetic charged and neutral atoms and molecules that can be further involved in the processing of surfaces either of the body from which they were born or the surface of other bodies if transport within the vicinity, planetary, or solar system is efficient. Therefore, all the processes may be intertwined, and all are necessary to accurately simulate space-weathering. The relative contributions of different space-weathering agents acting in different regions of the solar system are currently a subject of intense debate. For example, on Mercury the MESSENGER mission has recently been able to map the spatial distribution of sodium, potassium, and calcium. While it is generally agreed that these species are sputtered from the surface, there is currently a debate about the relative contribution of micrometeoroid bombardment, sputtering from ESD/PSD, and ions from the solar wind (e.g., Schmidt et al.,346 Figure 39).763,764 It appears the sodium correlates with a mechanism which is enhanced at the poles or equator compared to calcium which demonstrates the reverse trend. This could be explained by the fact that the accessible regions of Mercurys surface differ for each space-weathering agent. For example, while PSD processes are able to release the alkali metals from surface analogs which will be more effective at the equator, the desorption of calcium/magnesium alkaline earth metals require the more energetic electrons which may have more access at the poles, as shown by experiments of Yakshinky and Madey606 and McLain et al. 530 Laboratory experiments simulating the known space environment (e.g. solar wind, micrometeoroid bombardment) were performed in the intent of determining the observed surface age of asteroids, inferred from the degree of reddening observed from the spectral slope over the visible region (from section 6.3.3). However, surface ages determined in the laboratory vary considerably, depending on the space-weathering agent and the parameters used to simulate the reddening. Therefore, the question of the relative contributions of the various space-weathering agents has been debated. 376 Willman and co-workers380,765 have extensively studied the results from laboratory groups in order to compare the results to their photometric observations of dynamically young asteroids. When solely laboratory-determined timescales are used, the time required for the observed reddening effects varied from as long as 1 Gyr based on the results of pulsed-laser irradiation experiments,15 to as short as 5 kyr based on the results of ion-irradiation experiments on the same mineral (e.g., Figure 40). The authors also emphasize the potential role of gardening that can influence the derived surface ages considerably. Research groups around the world are becoming more capable of reproducing these results using a synergistic approach, and more aware of the complexity of the situation. This methodology should enable future experiments to more accurately represent the actual astrophysical environments of these bodies, leading to the elucidation of the relative contributions of each space-weathering agent. This, in turn, allows for the possibility to trace back through time to the primordial compositions from which our solar system emerged. Moreover, the extent that these weathering agents could be responsible for enriching these primitive environments with more complex molecular building blocks from which life on Earth arose may be deduced (e.g., Brack,157 McClendon,766 Ehrenfreund and Charnley767).
