Space Weathering on Mercury: Laboratory Studies and Modelling

Bentley, Mark Stephen (2005). Space Weathering on Mercury: Laboratory Studies and Modelling. PhD thesis The Open University.



Mercury is one of the most mysterious objects in the Solar System. To date, Mercury has been visited only by the Mariner 10 spacecraft, which imaged less than half of the surface. The inner planet is now the target of two missions: NASA’s MESSENGER and the European Space Agency’s BepiColombo. One of the key measurement goals of both missions is global mapping of the surface composition. However, it is known from lunar research that regolith exposed to the space environment evolves in a way that obscures the mineralogical information otherwise derived from reflectance spectroscopy. This evolution process (space weathering), is the result of micrometeorite bombardment and solar wind sputtering, during which ferrous iron in lunar minerals is reduced to metallic iron. Such processes are also expected to operate on other airless bodies, including Mercury.

This thesis focuses on the use of laboratory simulations of space weathering, with particular emphasis on Mercury. A pulsed laser facility was established, capable of simulating impacts into regolith analogues at varying temperatures. A variety of analytical techniques were then evaluated to quantify the amount of metallic iron produced and its size distribution; space weathering is critically dependent on both of these parameters. Nearly all of the optical and magnetic properties observed in lunar space weathering were also observed in laboratory analyses.

Of the techniques used, measurements of magnetic susceptibility proved extremely useful. These were rapid and non-destructive measurements, and were diagnostic of the ferromagnetic iron produced during weathering. In addition, multi-frequency or temperature-dependent measurements could readily detect the very fine superparamagnetic particles that modified the reflectance spectra.

Having demonstrated the importance of these measurements, the possibility of including a magnetic susceptibility sensor on future planetary landers was investigated; this would provide useful data constraining mineralogy and regolith maturity for very low mass and power resources.

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