Catherine Johnson


Relevant Thesis-Based Degree Programs


Graduate Student Supervision

Doctoral Student Supervision

Dissertations completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest dissertations.

Mercury's global evolution: elucidating the tectonic, volcanic, and early magnetic field history through observations and numerical modeling (2022)

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.

View record

Mars' external and internal magnetic fields from orbital observations (2019)

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.

View record

Investigation of Mercury's Magnetospheric and Surface Magnetic Fields (2014)

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.

View record

Seismic Energy Propagation in Highly Scattering Environments and Constraints on Lunar Interior Structure from the Scattered Signals of the Apollo Passive Seismic Experiment (2014)

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.

View record

Master's Student Supervision

Theses completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest theses.

Metre-scale roughness of asteroid (101955) Bennu from the OSIRIS-REx Laser Altimeter (2024)

Asteroid (101955) Bennu is a near-Earth, potentially hazardous asteroid and was the primary target of the NASA OSIRIS-REx mission. Bennu is a rubble pile asteroid: a highly porous, gravitationally bound aggregate of boulders, formed after the catastrophic disruption of its parent body. The asteroid’s rugged surface is dominated by the expression of boulders and has been heavily modified by impact cratering. Here, I analyze surface roughness, calculated using data from the OSIRIS-REx Laser Altimeter, to investigate spatial variations in boulders and fine-grained material across Bennu globally. Surface roughness is a statistical measure of change in surface height over a given baseline (horizontal spatial scale) and can be used to gain insight into the geologic processes that modify the surface over different scales. Here, I performed a surface roughness assessment of asteroid Bennu at baselines of 0.25 m to 25 m using the root-mean-square (RMS) deviation. Studies of surface roughness provide an excellent opportunity to compare asteroids for which detailed surface roughness analyses exist, and I compare my results with those of (25143) Itokawa and (433) Eros. I find that the surface roughness of Bennu is self-affine, spatially varying, and is dominated by the spatial density of boulders on the asteroid surface. A moderate statistical correlation was found between roughness and Bennu’s shape, most pronounced in the mid to high latitudes. At the longest baselines, I find that roughness is produced by the prominent equatorial ridge, the topographic relief of Bennu’s largest boulders, and occasionally by large boulders in local topographic minima such the interiors of large craters. At baselines between 0.25 m and 2.0 m, I find that the interiors of craters with diameters
View record

Potential ice sheet and glacial modulation of volcanism in West Antarctica: constraints on the cadence of melt delivery into the crust (2024)

The rate of change of orbitally-driven variations in continental ice massmodulates volcanism by inducing variations in the compressional (lithostatic)stress regime of the wall rock surrounding hot magma reservoirs.At West Antarctica, as crustal temperatures rise in response to the deliveryand storage of melt since 36 Ma, and to the insulating effects of a thickWest Antarctic Ice Sheet emplaced ∼34 Ma, the effective viscosity of wallrocks decreases. Pressures in excess of lithostatic related to the injectionof magma are increasingly relieved through wall rock creep at the expenseof volcanism and stress changes imparted by orbitally-forced deglaciationare consequently reduced in magnitude and delayed in time. Theoretically,maximally rapid creep and a negligible volcanic response to orbital forcingcorresponds to a time lag of 1/4 of the period of the glacial forcing. Clustersof tephra layers near Mount Berlin corresponding to peaks in the rate ofchange of volcanism occur at eight and 110 ka, but only the eight ka clusterlags a maximum in the rate of change of deglaciation by less than 1/4 ofan obliquity-driven glacial cycle (∼6 kyr), suggesting the crust underwent arheological transition before eight ka. Using a 1D numerical model of heatdiffusion we study the effects of ice sheet insulation and time-dependentmagma supply on the viscoelastic response of the crust to glacial cycles.Long-term ice insulation increases the crust’s proclivity for viscous creep byincreasing the temperature by ∼50 K. However, a time-dependent magmasupply causes the crust to alternately cool and warm. Through the exponentialdependence of crustal viscosity on temperature, thermal oscillationsand long-term insulation force the crust into transient behavior in whichmagma is preferentially stored during warmer intervals and erupted duringcooler periods. We argue that the change in the response of the crustbetween 110 and eight ka is a consequence of such oscillations. Our dataanalysis applied with inferred geothermal heat fluxes and peak magma productionrates in Marie Byrd Land constrain the magnitude and period ofoscillation for mantle magma production to 0.01-0.1 km³/year and ∼10⁶-10⁸years, respectively.

View record

Heat flow and late-stage volcanism at Aramaiti Corona, Venus (2020)

In this thesis, an investigation of volcanism associated with Aramaiti Corona, Venus, is presented. Two features were identified in images and altimetry data collected during the NASA Magellan mission, which orbited Venus 1990–1994, and are studied here using higher resolution topography later derived from stereo SAR images. The first is Narina Tholi, a steep-sided volcanic dome located on the western fracture annulus of Aramaiti Corona. Around this dome is a topographic moat characteristic of a lithospheric flexural signature. By modelling lithospheric flexure on terrestrial planets and icy moons, subsurface heat flow, which is an important metric in investigating a thermal history, can be estimated. The topographic signature is compared with models for the flexure of a thin elastic shell to estimate the lithospheric thickness and the associated heat flow. The results for Narina Tholi indicate a thinner lithospheric thickness and correspondingly higher heat flow than previously estimated regionally from flexure around Aramaiti Corona. This implies a locally higher heat flow, consistent with the emplacement of Narina Tholi late in the corona’s evolution. The second feature is associated with the north portion of Aramaiti Corona and is inferred to be the result of volcanism associated with the corona fracture annulus. This suggests a different style of volcanism at Aramaiti Corona than has been previously observed, one that is possibly late-stage.

View record

News Releases

This list shows a selection of news releases by UBC Media Relations over the last 5 years.

Membership Status

Member of G+PS
View explanation of statuses

Program Affiliations


If this is your researcher profile you can log in to the Faculty & Staff portal to update your details and provide recruitment preferences.


Get key application advice, hear about the latest research opportunities and keep up with the latest news from UBC's graduate programs.