My core research interest is the formation and evolution of planetary bodies. Collisions are a pervasive physical process that links the history of all planetary bodies, from the giant planets to grains of dust. I use a physical understanding of collisional processes to investigate problems in planetary sciences. My primary techniques are shock wave experiments and numerical simulations of collisions.

New! I have posted animations of planetary collisions.

I have developed several resources for other researchers.

Shock Processes in Ice and Cratering on Icy Planets

A major component of my research program is focused on understanding the effects of collisions on icy planets and satellites. Shock-induced phase changes in ice are a common occurrence throughout the history of the solar system. Melting and vaporization of ice during impact events redistribute volatile reservoirs and create transient, water-rich environments.

Isis crater on Ganymede

During accretion and subsequent erosive collisions, shock-induced devolatilization alters the composition of icy planetary bodies. Through laboratory experiments and numerical modeling, current work focuses on understanding the role of phase changes in the dynamics of impact events.

Left: Isis crater on the icy moon Ganymede. A 73-km diameter impact crater with a central pit.
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Cratering Mechanics

The morphologies of impact craters reflect variations in the composition and mechanical properties of the target materials. I use impact craters as tools to probe the subsurface of planets. My students and I have used observations of impact crater geometries and numerical simulations to study buried ice and structural heterogeneity in the Martian upper crust. Our results strongly support the presence of icy, layered terrains that may reflect recent glacial-interglacial periods. Our understanding of the mechanics of crater formation provides a means to investigate spatial and temporal variations of icy terrains using the cratering record.

I am also interested in the effects of giant impacts on planetary crusts and mantles (e.g., multi-ring basins). To that end, I have been improving numerical simulations and enhancing our understanding of cratering mechanics by studying progressively larger impact events. My current work focuses on the collapse of complex craters. During collapse, the bulk strength of the rock mass must be extremely low to reproduce the observed morphology of complex craters. The transient weakening mechanism remains an unsolved problem and current work focuses on the role of strain localization and temporary weakening along impact-generated faults.
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Small Bodies and Planet Formation

The dynamical and collisional histories of the solar system are recorded in populations of small bodies (e.g., the asteroid belt and Kuiper belt) and in the random properties of the planets (e.g., the large core of Mercury, Venus' retrograde spin, and Earth's moon). Zoƫ Leinhardt and I are investigating the interplay and evolution of the composition and internal structure of planetary bodies during repeated collisions. We developed a hybrid numerical technique combining a sophisticated shock physics codes (CTH, GADGET) with the fastest N-body gravity code (pkdgrav) to study large collisions on planetary bodies.

Collision Outcome Map

Our current work focuses on understanding the role of collisions during planet formation and evolution. We have defined the transition between major collision regimes: accretion, erosion, graze-and-merge, and hit-and-run as a function of impact velocity, impact angle, projectile-to-target mass ratio, and material properties.

Left: Example "map" of the major collision regimes during the end stage of planet formation. The area for each regime (different colors) is proportional to probability. The most common collision outcomes are partial accretion, hit-and-run, and graze-and-merge.
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Shock Processes in Natural Materials

The Harvard Shock Compression Laboratory conducts experiments to measure the complex response of natural materials to shock compression (primarily planar shock wave experiments). Our experiments are focused on fundamental shock processes in order to (i) increase our understanding of collisional processes in the solar system by improving numerical simulations and discovering new phenomena and (ii) strengthen our interpretation of observations of planetary materials.
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Rebound of the Harvard 40-mm gun during a shot.