Colloquium: Paul K. Byrne

Paul K. Byrne

Department of Terrestrial Magnetism, Carnegie Institution of Washington, & Lunar and Planetary Institute

Title: "The Global Contraction of Mercury"

Abstract: The surface of Mercury is replete with tectonic landforms interpreted to be products of horizontal shortening that accompanied planetary cooling and contraction, but the number and distribution of such structures and their relation to large-scale variations in topography have not been well understood. Additionally, prior estimates of the amount of global contraction from photogeological studies of shortening structures were far lower than those predicted by interior thermal history models. In this talk I show a global synthesis of deformational structures on Mercury derived from orbital imaging and topographic measurements by the MESSENGER spacecraft. Lithospheric shortening on Mercury has been accommodated by a substantially greater number and variety of landforms than previously recognized, including by long fold-and-thrust belts as on Earth. These new observations show that Mercury contracted radially by as much as 7 km, well in excess of the 0.8–3 km previously reported from photogeology. This new measure of Mercury's planetary radius change provides a critical constraint for future studies of the planet's thermal history, bulk silicate abundances of heat-producing elements, mantle convection, and the structure of its large metallic core, and our observations offer fresh insight for investigating Mercury's tectonic and volcanic development.