Planet Mercury: volatile release on a contracting world

Thomas, Rebecca (2015). Planet Mercury: volatile release on a contracting world. PhD thesis The Open University.



This thesis investigates the geological processes by which materials containing high concentrations of volatile substances have been delivered to the surface of the planet Mercury throughout its history, despite global contraction, which could be expected to impede the replenishment of these materials from depth. With the aid of high-resolution data from the MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) spacecraft, I perform detailed analyses of the two types of geological features most indicative of the action of volatiles at Mercury’s surface: flat-floored depressions known as ‘hollows’, and pits and deposits thought to be the products of explosive volcanism. For hollows, I seek to clarify the nature of the substance lost to form them and the mechanisms by which this has been introduced to, and lost from, the surface recently enough to explain their pristine appearance. I produce a global catalogue of hollows, study their local associations, and investigate their spectral character, finding evidence that they form primarily by sublimation of a moderately volatile substance, potentially a sulfide, from a specific low-reflectance substrate exhumed and exposed by meteorite impacts. To better understand explosive volcanism, I investigate its longevity and the processes promoting it at specific locations. Through identifying, dating and characterising proposed pyroclastic deposits and vents, I show that putative explosive volcanism was more long-lived than voluminous effusive eruptions, that its occurrence is tectonically controlled, and that its eruptive style indicates magma storage prior to eruption, at greater depths than on the more tectonically-neutral Moon. This indicates that, rather than precluding it, global contraction favoured volcanic explosivity by promoting magma storage during which volatiles could become concentrated in the magma. Furthermore, because processes concentrating volatiles have been important in the genesis of both hollows and explosive volcanism, their occurrence does not necessarily indicate a high planetary bulk volatile fraction.

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