LPI | Science

I study the internal structure and geophysical evolution of Venus, the Moon, Mars, Io, and differentiated meteorite parent bodies. I am a member of the science teams for three upcoming spacecraft missions, DAVINCI, EnVision, and Europa Clipper.

Education and Current Employment

Ph.D., Planetary Science and Geophysics, California Institute of Technology, 1990

Thesis: Models for the Formation of Highland Regions on Venus

B.S., Physics and Astronomy, Texas Christian University, 1984, summa cum laude

Honors Thesis: Fourier Transform Infrared Spectroscopy of Clay Minerals and Tar Sands

Lunar and Planetary Institute, Houston TX (since 1993)

Currently Associate Director (since 2021) and Senior Staff Scientist

Selected Professional Service

• NASA Planetary Science Advisory Committee, 2021-2024
• Chair or Co-chair, Lunar and Planetary Science Conference Program Committee, 2016-
• Co-convener for the Venus as a System Conference, 2023
• NASA Venus Landed Platform Science Working Group, Geophysics Group Leader, 2018-2021
• Member of the Focus Groups for revising the “Goal, Objectives, and Investigations for Venus Exploration” and “Roadmap for Venus Exploration” for NASA’s Venus Exploration Analysis Group, 2018-2019
• Lead convener for Differentiation: Building the Internal Architecture of Planets, 2018
• Guest Associate Editor, Meteoritics and Planetary Science, special issue on Volatiles in the Martian Interior (published November 2016)
• Science adviser for Year of the Solar System: Digital Media for Planetary Science, WGBH/PBS, Boston MA, 2012-2014

Spacecraft Mission Experience

• Deep Atmosphere Venus Investigation of Noble gases, Chemistry, and Imaging (DAVINCI), Co-Investigator for noble gas studies of Venus volcanism rates and descent imaging studies of tessera tectonics, 2021-
• EnVision, Co-Investigator on the VenSAR radar science team, 2022- , and NASA representative on the European Space Agency Phase A/B Science Study Team, 2018-2024
• Europa Clipper, Gravity and Radio Science Team Co-Investigator, 2020-
• Gravity Recovery and Interior Laboratory (GRAIL), Guest Scientist, 2012-2016

Planetary Geophysics Research

My research activities are in the field of planetary geophysics. I develop computer simulations of mantle convection and heat transport in the interiors of planets, and I analyze observations of planets made by NASA spacecraft, particularly measurements of planetary gravity and topography and images of surface features. I also participate in lab studies of the physical properties of rocks, such as their densities and melting temperatures, which help to constrain the computer models. My goal is to combine models and observations to better understand the current internal structure of Venus, Mars, the Moon, Io, and differentiated meteorite parent bodies, as well as the processes that have controlled the evolution of these objects.

Venus: My studies of Venus focus on the relationship between mantle convection and surface features such as rift zones, large volcanos, and mountain belts. The motivating factor behind both the observational studies and the numerical mantle convection models is to understand the processes that led to the divergent evolution of Venus and Earth. My recent work with Matt Weller strongly suggests that climate-driven loss of water increased fault friction on Venus, which caused Venus to evolve from an Earth-like body with a mobile surface in the past to its present state in which mantle convection occurs beneath a stagnant or sluggish lithosphere. The resulting transition is both spatially and temporally complex and can explain many details of the geologic history of Venus. I hope to test this hypothesis with data from the forthcoming DAVINCI and EnVision missions to Venus.

Moon: I have used gravity and topography observations of the Moon to better understand the structure and magmatic plumbing of volcanic systems such as the Marius Hills. I used laser scanning and helium pycnometry to measure the density and porosity of lunar rocks help to constrain the rock densities used in the gravity models.

Mars: My models of mantle convection on Mars emphasize both magma generation in present-day mantle plumes as well as the long-term thermal evolution of Mars. These studies primarily involve computer models but also incorporate laboratory data about the melting of the martian mantle. Hot, upwelling mantle plumes from the core-mantle boundary provide the best explanation for long-lived, point-like volcanic sources beneath volcanos such as Olympus Mons. Loss of water from the mantle via volcanic outgassing provides a strong-feedback loop controlling the rate of later volcanism and also provides a mechanism for terminating the magnetic dynamo about 4 billion years ago.

Io: I have tested models of tidal dissipation inside Io using observations of volcanic heat flux made by the Galileo spacecraft and by ground-based telescopes to constrain the distribution of silicate melt and the depth of tidal heating in Io’s mantle. Our results are most consistent with silicate melt concentrations in the rheologically critical range (25-30%) that extends for hundreds of kilometers into the mantle and are inconsistent with models of Io that have a thin, shallow asthenosphere.

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