Thermal Evolution and Global Tectonics on Mercury with 3D Structure

California Institute Of Technology, Pasadena CA

Investigators

Abstract

AST 0709151 Aharonson Mercury holds the key to answering several basic questions related to the formation and evolution of solid planets and satellites. It is the smallest planet in the solar system, yet it possesses a global magnetic field. If the field is generated by a dynamo, part of the core must remain molten and convecting. It has the largest uncompressed density of any other planet, from which it is inferred to have the largest fractional core size. The tectonic features observed on the surface of the planet provide a record of the global contraction as a result of secular cooling of the interior. This record has been obliterated or obscured on the other terrestrial planets. Because of these attributes, Mercury represents an invaluable resource for studying the origin and comparative evolution of the terrestrial planets. This research project, led by Dr. Oded Aharonson, will characterize the internal structure of Mercury and its thermal history, and its relation to the energetics and lifetimes of planetary dynamos. Further, the tectonic expression of the planet's thermal history will be explored taking into account the inhomogeneous lithosphere predicted from the planet's rather unique 3:2 spin-orbit resonance with the sun. The study will make predictions about the formation and timing of a possible dynamo, the corresponding internal structures, the magnitude of global contraction, and the resulting distribution and orientation of tectonic features from predicted stresses resulting from the laterally variable lithospheric structure. This is well timed as the upcoming MESSENGER spacecraft flybys of Mercury will provide information of significantly greater detail than did Mariner 10 over thirty years earlier. This study will then bridge the gap between modeling and data by incorporating new results derived from the spacecraft into the modeling effort and thus, provide a framework for drawing conclusions from the data sets. This study will specifically implement three objectives that follow in logical order. First, an improved thermal evolution model will be developed for Mercury. An exploration of the internal structure of the planet and the conditions in which a dynamo is energetically feasible will be carried out. The primary question addressed here is whether a present-day dynamo is possible and the implications for the structure and composition of the interior. Second, a 3-D viscoelastic finite-element model will be adapted to Mercury in order to explore the tectonic history. The model will be driven by the thermal evolution model and will explore the effects of a variable lithosphere thickness caused by a nonuniform solar insolation history, which itself is inherited from the planet's 3:2 spinorbit resonance and large orbital eccentricity. Third, the distribution of tectonic features observed by MESSENGER will be used to determine the stress pattern recorded at the surface. This, along with other published results from the mission, will be folded back into the simulation to further constrain the model and conclusions. In addition to the broad consequences of understanding the overall evolution of terrestrial planets in general, the three dimensional nature of this investigation lends itself well to visualization. This study will take advantage of this to create educational products for general use for the public and the classroom. Small printed posters will be generated along with a complimentary webpage that will provide an overview of Mercury, and with a focus on visualizing the interior of the planet. ***