Andrew J. Dombard
I am a planetary scientist.
Here I am, as far south as you can get. My research centers on the study of the processes that shape the surfaces of the bodies in our solar system. Specifically, I am a geodynamicist who examines the response of an outer, mechanically stiff layer of a planet (known as the lithosphere) to various loads. The thickness of this lithosphere is tied directly to the thermal state of the body, which provides clues to its long-term evolution. Recent projects, done in collaboration with a host of talented colleagues, include:
1. Crater relaxation: craters are the most common landform in the solar system, formed from the very high-speed impact of a meteoroid on the surface of a body. The topography of an impact crater may change (or relax) over long time scales, because solid geologic materials may flow, just like glaciers flow on Earth. The icy moons Ganymede and Callisto, the two largest satellites of Jupiter, show a range of crater forms, from completely unrelaxed to fully relaxed.
This view of Marius Regio on Jupiterís satellite Ganymede shows unrelaxed (black arrows) and strongly relaxed (white arrows) craters. These craters are about 25 km across. (Figure 1 from Dombard and McKinnon, 2006) My work was the first to demonstrate this range using the expected mechanical parameters of ice and assuming reasonable thermal states. In addition, my most recent efforts have explored for the first time the consequences of leftover heat from the impact itself on the long-term relaxation of the crater topography. The extreme central peaks found in some large, old craters on the satellites of Saturn are likely produced by relaxation augmented by this remnant heat.
Crater relaxation, however, is not limited to the icy satellites of the outer solar system. I have shown that crater topography on the nuclei of comets may relax, a process initially thought impossible because of the cold temperatures in comets. A variant of crater relaxation may also occur on the rocky worlds of the inner solar system. A large impact here will not only produce topography on the surface but also on the interface between the crust and the underlying mantle. While temperatures near the surface are low enough that the surface topography won’t relax over geologic time scales, temperatures at this crust-mantle boundary could conceivably be high enough, especially when augmented by the leftover impact heat, to allow this topography to relax. I will soon begin a project to use this process to trace the thermal evolution of Mars and the Moon.
2. Flexure of the lithosphere: when a large load (for instance, a volcano) is emplaced on the surface of a body, the lithosphere will warp in response to this load. How this flexure is expressed depends on the thickness of the lithosphere, which again records the thermal state of the planet/satellite. Thus, flexural modeling of the lithosphere is a common tool for planetary scientists. Most studies, however, have assumed that the thickness of the lithosphere is constant. My work has begun to highlight the effect of lateral variations in the thickness of the lithosphere caused by localized thermal anomalies. A thermal anomaly arising from the volcanic plumbing system beneath the Tharsis Montes (the large shield volcanoes in the center of the Tharsis province of Mars) and coronae (a type of volcanic structure exclusive to Venus) might explain the presence of a distinct ring of fractures observed around these features.
This perspective view, with radar imagery draped over topography, shows the 400-km wide Fatua Corona on Venus. The ring of fractures around the central high may indicate a locally thin lithosphere. (Figure 1 from Dombard et al., 2007)Similarly, a localized thermal anomaly may explain the observation of flexural deformation at double ridges on Europa, another icy satellite of Jupiter, without having to invoke an implausibly hot satellite.
These topics are just of sampling of the projects I have tackled, which span the solar system (including the Earth!). Most of this work is done sitting in front of a computer; sadly, there are not too many opportunities for field work in planetary science. But like many geoscientists, I enjoy getting my hands dirty on occasion. Thus, I go to the field whenever I can. I have joined expeditions to Yellowstone National Park, was a crew member on research cruises to the south Pacific, and was a member of the 2003-2004 ANSMET (Antarctic Search for Meteorites) team (http://geology.cwru.edu/~ansmet/), which explains the above portrait.
Selected Recent Publications
- Dombard, A.J., and S.A. Hauck, II, Despinning plus Global Contraction and the Orientation of Lobate Scarps on Mercury, Icarus, 198, 274-276, 2008.
- Dombard, A.J., Cracks under Stress (News & Views), Nature, 447, 276-277, 2007.
- Dombard, A.J., C.L. Johnson, M.A. Richards, and S.C. Solomon, A Magmatic Loading Model for Coronae on Venus, Journal of Geophysical Research, 112, E04006, doi:10.1029/2006JE002731, 2007.
- Dombard, A.J., and W.B. McKinnon, Folding of Europa’s Icy Lithosphere: An Analysis of Viscous-Plastic Buckling and Subsequent Topographic Relaxation, Journal of Structural Geology, 28, 2259-2269, 2006.
- Cheng, A.F., and A.J. Dombard, Viscous Relaxation on Comets, Icarus, 184, 584-588, 2006.
- Dombard, A.J., and W.B. McKinnon, Elastoviscoplastic relaxation of impact crater topography with application to Ganymede and Callisto, Journal of Geophysical Research, 111, E01001, doi:10.1029/2005JE002445, 2006.
- Dombard, A.J., M.L. Searls, and R.J. Phillips, An alternative explanation for the “buried channels” on Mars: The gravity signal from a sharp boundary on partially compensated, long-wavelength topography, Geophysical Research Letters, vol. 31, L05701, doi:10.1029/2003GL019162, 2004.
- Dombard, A.J. and A.M. Freed, Thermally induced lineations on the asteroid Eros: Evidence of orbit transfer, Geophysical Research Letters, vol. 29, no. 16, cit. no. 1818, DOI 10.1029/2002GL015181, 2002.
- Dombard, A.J. and W.B. McKinnon, Formation of grooved terrain on Ganymede: Extensional instability mediated by cold, superplastic creep, Icarus, vol. 154, no. 2, p. 321-336, 2001.