MESSENGER Observations of Volcanism on Mercury: From Hokusai Quadrangle Down to Small Cones

Wright, Jack (2019). MESSENGER Observations of Volcanism on Mercury: From Hokusai Quadrangle Down to Small Cones. PhD thesis The Open University.

Abstract

This thesis addresses the regional geology of the planet Mercury as seen by the MErcury Surface, Space ENvironment, and Ranging (MESSENGER) mission. I aimed to investigate the existence and origin of volcanic photogeological units and landforms on Mercury not mappable at the global scale.

Using MESSENGER images, I have made the first 1:3,000,000-scale geological map of the Hokusai (H05) quadrangle. My map shows that this region has a similar history of effusive volcanism to the rest of Mercury, with widespread plains formation having given way to lower volume effusions that were ultimately confined to impact craters. However, I found that H05 contains plains that do not conform to the definitions of the two globally recognised plains units on Mercury, intercrater plains and smooth plains, which are thought to be volcanic in origin. I mapped such plains as intermediate plains, due to their intermediate roughness between the two global plains types, and I suggest that they represent incompletely volcanically resurfaced regions of Mercury’s crust.

The apparent absence of even small volcanic constructs on Mercury, emphasised to me during my mapping of the volcanologically diverse H05, led me to search for such constructs across Mercury. The two candidates I found, if volcanic, were probably built by low-volume eruptions that were uncharacteristic of most of Mercury’s volcanic history.
During my search for volcanoes, I investigated the circum-Caloris knobs. Through mapping, photogeological observations and topographic measurements, I have shown that these small (<15 km across) landforms are not volcanoes, but instead are Caloris ejecta blocks that have undergone long-lived, or even recent, modification into cones. My observations suggest that volatile-loss might have driven modification of the blocks.

The intermediate plains, candidate volcanoes, and circum-Caloris knobs will be important targets for the BepiColombo mission, which launched during the preparation of this thesis.

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