research
a short tour and motivation of my research across the solar system
My research lies at the intersection of Geophysics, Geodesy, and Geodynamics to investigate the structural, compositional, and thermal state of planetary interiors. Why should we care about planetary interiors? Because they help us understand why each planet and moon look and behave so differently from one another! “Comparative Planetology” is a central theme across my research. It basically means that understanding differences in the observed properties of a planet or moon can help us understand others. The most obvious comparison is to Earth. Why is Earth special? Why does life (as far as we know…) only happen here? Why is Earth the only rocky body with plate tectonics? It seems Earth’s geodynamic evolution plays a role, but how? This idea extends past Earth towards all the terrestrial planets/moons (Mercury, Venus, our Moon, and Mars). Why doesn’t Mercury have an atmosphere? Why does Venus have such a thick atmosphere and no magnetic field? Why is Tharsis (a large volcanic region on Mars) so damn large? ETC….
To make sense of all of this, I’ve partaken in research along four(ish) themes that investigate connecting what we currently understand about the interior states of planets and moons today, the initial conditions these bodies started in, and the evolutionary quagmire in between. Below are short summaries of each of these themes which link to separate pages where I ramble more about work colleagues and I have done across my career.
Tidal Tomography →
Tidal Tomography is a relatively new technique in the world of geodesy that offers a way to directly use measurements of very precise changes in the surface height and gravity of a planet or moon to infer how its internal structure varies laterally. This last phrase is the crux of why this method is revolutionizing our understanding of planetary interiors. At its core, it aims to answer “How can the changing shape and gravity field of a planetary body reveal its deep internal structure?” Traditional geodesy uses data, such as tidal $k_2$ and $h_2$, that assume a spherically symmetric interior. In other words, it assumes the planet these data are measured from is like an onion and only varies as a function of its radius. This was a great approximation for decades, but as we have started to ask more complicated questions about planetary interiors and data got better, it was realized we need to consider how lateral variations can affect things. If you want to know more, please click the link above!
Lithospheric Flexure & Gravity →
My Ph.D. dissertation was titled “Deformation of the Martian Lithosphere Through Time” so perhaps it’s not a surprise that this theme is present in my research portfolio. We can distill this entire theme into the question: “How do planetary lithospheres support loads, and what can gravity and topography tell us about their structure and strength?” The lithosphere of rocky (and icy, literally) bodies behaves a lot like icebergs in the ocean. The density difference between ice and water is slightly negative, and thus for any amount of an iceberg sitting above the surface, there is a proportional amount sitting just below the water supporting it. There is a mathematical tool we use called “isostasy” that represents a balance of forces. In our iceberg analogy, the pressure across two columns must be equal (called quite creatively equal-pressures or “Airy” isostasy). For an iceberg, this means that the entire iceberg thickness ($H$) must produce the same pressure at depth underneath the iceberg as a column of only water sitting right beside it. Mathematically, if the density of the ocean is $\rho_{\mathrm{ocean}}$, the density of the iceberg is $\rho_{\mathrm{ice}}$, and the column of water is $W$, then the equivalence between these two columns must be exactly: \begin{equation} \rho_{ice}H = \rho_{ocean}W \end{equation} and the exact amount our little iceberg sticks above the waves (let’s call $D$) is simply $D = H - W$, and thus \begin{equation} D = H\left(1-\frac{\rho_{\mathrm{ice}}}{\rho_{\mathrm{ocean}}}\right). \end{equation}
If we assume ice to have a density of 900kg/m$^3$ and water to be 1000kg/m$^3$ then with a total iceberg of 100 meters, then only 10 meters of that entire iceberg will be visable above the surface of the water! This is a simple illustrative example, but mountains, valleys, impact craters, and any other variation in topography or density interface on or in a planet or moon also obey this simple idea Of course there are ways to make this more complex (see the “equal masses” method of Hemingway & Matsuyama (2017) or “Pratt” isostasy), but the basic principle can allow us to probe the near-subsurface of various planetary bodies by only knowing how topography and gravity vary across their surfaces. I haven’t even discussed where gravity comes into the picture here, but I’ve rambled enough in this “short” summary of this theme.
If you want to know more, please click the link above!
Planetary Geodynamics & Evolution →
My research concerns itself mainly with modern day observations of the state of the interior of planetary bodies. Obviously this begs the question: How did we get here? Furthermore, how do planetary interiors and lithospheres deform and evolve through time? A long time ago in a galaxy not far away our solar system started as a primordial slurry of hot dust and gas that coalesced into the planets we see today. Between that initial formation and today, over 4 billion years of stuff happened. By understanding this stuff, we can peak at what the initial conditions of our solar system might have been.
Most of the methods involved with these ideas involve large geodynamic simulations, which I do not use directly. This is an area I’d like to learn more about, but for now I use simplied ideas about how heat dissipates through time, how topography is preserved (or erased), and how this stuff may obscure early solar system processes in the present. A short example of this concerns itself with determining how topography is “frozen in”. When, for instance, a volcano forms, the state of the lithosphere determines how this volcano is compensated at depth, directly related to the previous ideas about flexure mentioned earlier. There are some analytical expressions to connect the amount the volcano is compensated, through the elastic thickness of the lithosphere, to the heat flow through the surface of the planet. All else held equal planets cool over time and thus determine the local surface heat flow when topography, our volcano in this instance, formed. This idea forms a very crude approximation of the age of certain features, but its been quite successful in studying the spatial and temporal thermal evolution of Mars (see McGovern et al. (2002)).
If we go deeper and think about how using tidal tomography can constrain mantle variations, we can directly interpret inferred density and rigidity variations in terms of temperature and composition. I’ve been using equation of state modeling to connect these two ideas and give us some sense on what is the current state of interior bodies, and what the state of the planet was in order to get to where we are now.
If you want to know more, please click the link above!
Mission Science & Development →
Data collection in planetary science tends to take the form of a stair step pattern, because most of our data comes in the form of planetary missions that only happen every X years. Thus, planetary mission design is essential for designing needed measurements to answer outstanding scientific questions. In this theme it is essential to define measurements that are needed to distinguish between competing models of planetary interiors and geological evolution. My first paper in my Ph.D. was born from discussions about what we could learn about the interior of Mars if we had a certain measurement, and I went on to be involved with the Planetary Science Summer School hosted at the Jet Propulsion Laboratory, which taught me about how to design a mission, but I think also made me a sharper scientist in my ability to propose and investigate interesting scientific questions.
If you want to know more, please click the link above!
References
- Hemingway, D. J., & Matsuyama, I. (2017). Isostatic equilibrium in spherical coordinates and implications for crustal thickness on the Moon, Mars, Enceladus, and elsewhere. Geophysical Research Letters, 44(15), 7695–7705. https://doi.org/10.1002/2017GL073334
- McGovern, P. J., Solomon, S. C., Smith, D. E., Zuber, M. T., Simons, M., Wieczorek, M. A., Phillips, R. J., Neumann, G. A., Aharonson, O., & Head, J. W. (2002). Localized gravity/topography admittance and correlation spectra on Mars: Implications for regional and global evolution. Journal of Geophysical Research E: Planets, 107(12), 19–11. https://doi.org/10.1029/2002je001854