Catherine Johnson

The MESSENGER mission from 2008-2015 yielded a wealth of information about our innermost planet, Mercury. For the first time, visible images of the entire planet, measurements of topography, global characterization of the gravity and magnetic fields as well as surface composition, were all acquired. From these observations, multiple enigmatic aspects regarding the geological evolution of Mercury emerged: The early evolution was dominated by crustal production and creation of the Intercrater Plains by extensive effusive volcanism covering the planet until ~4.0 billion years ago (Ga). This was followed by spatially localized effusive volcanism until ~3.5 Ga that created the Smooth Plains, by global contraction that resulted in a unique global distribution of thrust-fault related landforms called shortening structures, and an early internally-driven magnetic field at ~4.0-3.5 Ga producing crustal magnetization detectable from orbit. In this thesis, I focus on the origin and driving mechanism(s) of each these observations. First, I constrain and investigate the formation of shortening structures in the Smooth Plains. I compile a new comprehensive database that characterizes the morphology of shortening structures and provide the first statistical analysis of variations in morphological characteristics across the entire planet (Chapter 2). Second, areal strain estimates (Chapter 2), and numerical modelling of the surface morphology of Smooth Plain’s shortening structures to constrain the subsurface fault structure (Chapter 3), both require large compressional stresses. These stresses are attributed mainly to 5-10 km of global contraction due to secular cooling of the planet, rather than to bending stresses from volcanic loading by ~1-4 km of basalt as previously assumed. Lastly, I revisit the thermal evolution of Mercury (Chapter 4) and propose the first self-consistent model that matches the newly-constrained geologic record. I focus on a novel way to incorporate the thermal consequences of magmatism and crustal production to form the Intercrater and Smooth Plains. Here, I show that voluminous crustal production can drive a period of strong mantle cooling that both favors an ancient thermally driven dynamo, 5-10 km of radial contraction of the planet, and crustal production.

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Magnetic fields play a big role in the evolution of a planet and can be used as a tool to understandthe interior of it. Orbital spacecraft missions, Mars Global Surveyor (MGS) and Mars Atmosphereand Volatile EvolutioN (MAVEN), have acquired magnetic field data, collectively providingfull global coverage at different altitudes. Those data carry information about fields of internalorigin, specifically the crustal field, and of external origin, fields generated by the Sun andin the ionized upper atmosphere. Time variable external fields induce electric currents in thesubsurface, providing information about the electrical conductivity structure and thus, materialproperties of the martian interior. The locally strong static crustal field of Mars providesevidence for an ancient global dynamo field. I first explore the global structure of externalfields and what we can learn from this, in particular about the contribution of the ionosphereto large-scale magnetic fields. I then investigate how we can extract magnetically quiet orbits,e.g., orbits during which the external field is minimal, from MAVEN data to use in crustal fieldmodels. Such models are essential for predicting the field at the surface of the planet and themagnetization responsible for it; this is important for mission planning and we show predictionsfor the landing sites of Mars 2020 and InSight. Furthermore, such predictions in combinationwith satellite data provide insight into ancient Mars. I specifically address the timing of theancient dynamo, and the distribution of magnetization in Mars’ crust. Thus, in this thesis Iexplore internal and external aspects of the field, contributing to the understanding of past andon-going processes of Mars.

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This thesis is devoted to the study of Mercury’s magnetic field environment, to reveal the nature of the interaction between a weak planetary magnetic field and the interplanetary medium. Due to the lack of orbital spacecraft observations at Mercury prior to the MErcury Surface, Space Environment, GEochemistry, and Ranging (MESSENGER) mission, work in this thesis presents some of the first analysis and interpretation of observations in this unique and dynamic environment.The bow shock and magnetopause define the boundary regions of the planet’s magnetosphere, thereby representing the initial interaction of the planetary field with the solar wind. We established the time-averaged shapes and locations of these boundaries, and investigated their response to the solar wind and interplanetary magnetic field (IMF). We found that the solar wind parameters exert the dominant influence on the boundaries; we thus derived parameterized model shapes for the magnetopause and bow shock with solar wind ram pressure and Alfven Mach number, respectively.The cusp region is where solar wind plasma can gain access to the magnetosphere, and in Mercury’s unique case, the surface. As such, this area is expected to experience higher than average space weathering and be a source for the exosphere. Using magnetic field observations, we mapped the northern cusp’s latitudinal and longitudinal extent, average plasma pressure and observed its variation with the solar wind and IMF. From the derived plasma pressure estimates we calculated the flux of plasma to the surface.Mercury’s internal dipole field is not centered on the planet’s geographic equator but has a significant northward offset. We developed the technique of proton-reflection magnetometry to acquire the first measurements of Mercury’s surface field strength. Proton loss cones are evident in both the northern and southern hemispheres, providing confirmation of persistent proton precipitation to the surface in these regions. We used the size of the loss cones to estimate the surface magnetic field strength, which confirm the offset dipole structure of the planetary field. With additional proton-reflection magnetometry observations, we generated a global proton flux map to Mercury’s surface and searched for regional-scale surface magnetic fields in the northern hemisphere.

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Meteoroid impacts over hundreds of millions to billions of years can produce a highly fractured and heterogeneous megaregolith layer on planetary bodies such as the Moon that lack effective surface recycling mechanisms. The energy from seismic events occurring on these bodies undergoes scattering in the fractured layer(s) and this process generates extensive coda wave trains that follow major seismic wave arrivals. These long coda trains can obscure the secondary crustal, mantle or core phases that are often crucial in assessing the interior structure of these planetary bodies when using more traditional seismological analyses. However, the decay properties of these codas are affected by the interior velocity, intrinsic attenuation and scattering structure of the planet or moon. As such, these decay properties can contain valuable information regarding these aspects of interior structure. This thesis provides the first systematic analysis of scattering in the Apollo Passive Seismic Experiment dataset, demonstrating that scattering in the Moon occurs over a wide range of frequencies, and dominantly in the near-surface megaregolith that comprises many more small scale heterogeneities than large ones. I also present a new numerical modeling technique (referred to as PHONON1D) that models seismic energy propagation and integrates high levels of scattering. Using this method, I investigate the effects of various velocity, scattering and intrinsic attenuation structures on the scattered coda. Results show that the main controls on the coda generation and decay times are the seismic velocity profile, attenuation levels, and the number density of scatterers. Thus these properties can be assessed by comparing predicted synthetic seismic coda with those observed in the Apollo Passive Seismic Experiment data. Finally, I use the PHONON1D method to show that locations within young and large impact basins, away from the edges, have the potential to minimize the scattering observed in the recorded seismic signals. These locations would be ideal for the emplacement of future seismic surveys on the lunar surface.

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