title: Investigating Mercury's Tectonic Structures, and Mapping of the Neruda Quadrangle (H13)
Creators: name: Man, Ben
Publisher: name: The Open University
Year: 2024

description: With Mariner 10 imaging ~45% of Mercury’s surface in 1974, the rest of the planet remained shrouded in mystery for nearly four decades. In 2011, NASA’s MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission arrived in orbit about Mercury and was able to return images of the planet’s globe in its entirety. This thesis uses MESSENGER mission data to explore a previously uncharted region of the planet and to investigate tectonic landforms globally.

In this document I present my geological map of the Neruda quadrangle (H13) a southern hemisphere region that was imaged for the first time by the MESSENGER spacecraft. My map is made at a scale of 1:3,000,000 as part of a series of geological maps that are being constructed in preparation for ESA-JAXA’s BepiColombo mission to Mercury. In my map I identify three plains units believed to be of mostly volcanic origin. I also identify and map craters and their materials using two different crater degradation schemes (three-class and five-class). My map also includes geomorphological units specific to the Rembrandt impact basin, the second largest well-preserved impact structure on Mercury, which straddles the H13-H14 quadrangle boundary. I also distinguish superficial deposits including crater rays, hollows and bright spots termed faculae. As part of the map discussion, I examine the Alvin–Altair thrust system that spans from the Caloris basin in H04/H08 through the H13 quadrangle and into the south polar H15 quadrangle.

Investigation of the Alvin–Altair thrust system led to the serendipitous discovery of small grabens; secondary extensional structures found associated with larger compressional structures. These grabens are small scale, shallow landforms, typically 10s to 100s of metres deep, 10s of kilometres in length and often less than 1 km wide. As a consequence, they are not expected to survive for hundreds of millions of years because impacts and impact gardening would quickly mask their signature. With their discovery I undertook a global survey for these structures where I first re-mapped all shortening structures on the planet and then investigated all Narrow Angle Camera images with spatial resolution of 150 m/pixel or better that intersected each shortening structure in my database. In total I identified 727 grabens of which I am confident of 190. These 190 confident grabens are found on 48 distinct shortening structures that are widely distributed across the surface of Mercury. By considering the depths of the grabens calculated from shadow measurements in addition to calculating their maximum initial depth from displacement/length scaling ratios, (a relationship well documented for the Earth and Moon) I calculated that in order to have remained visible in the present day, despite Mercury’s rate of surface degradation, a majority of the grabens I discovered must be ~300 Ma or younger. This demonstrates that the parent shortening structures on which these grabens are found must have continued to be active into geologically recent times, indicating that global contraction on Mercury is ongoing.