Zachary Geballe aims to better understand the Earth’s core by electrical and calorimetric experiments of iron alloys through their melting temperatures at pressures up to 330 GPa. He is also working to improve our understanding of the Earth’s lowermost mantle by measurements of thermal conductivity of the mantle minerals at the pressure and temperature of the lowermost mantle.
The Earth’s core is a massive ball of metal located 3000 km beneath our feet. Improved knowledge of its basic physical properties will help Earth scientists better understand the way heat and material move inside the solid Earth’s many layers (inner core, outer core, mantle, and crust).
For example, motion of liquid metal in the outer core generates a powerful magnetic field that shields life at the surface from harmful radiation, but the magnitude of forces driving the motion are uncertain. Another example is that heat from the outer core might melt some minerals in the rocks at the base of the mantle, but only if the temperature of the core is high enough.
One notable reason for these uncertainties is that a basic fact about the Earth’s core remains highly uncertain: its temperature.
- Modulated Joule heating and calorimetry can be used to reveal phase transitions in high-pressure phases of H2O by jumps in total heat capacity and in thermal conductance.
- Pulsed Joule heating and identification of latent heat can be used to reproducibly and unambiguously determine melting of metals up to at least 100 GPa (1 Mbar)
- The low noise of photomultiplier tubes may allow a 10x improvement of the precision of latent heat-based melting temperature in diamond anvil cells.
To infer the core temperature, Geballe assumes that the interface between liquid outer core and solid inner core is most likely the temperature of the mostly iron liquid alloy that constitutes the outer core.
Hence, the starting point for understanding the core’s temperature, and its effect on the entire Earth, is to accurately determine the melting temperature of iron when subjected to the enormous pressure that exists at the inner core-outer core boundary: 3.3 million atmospheres.
Geballe's work uses a new, conceptually simple method of identifying melting in highly compressed samples: absorption of latent heat. His team runs tens of amperes of current through tiny iron and iron-alloy samples compressed between the tips of diamond anvils and measure the temperature every tenth of a microsecond for several microseconds. They identify melting as the moment that the temperature plateaus due to the latent heat of melting.
In practice, this requires a combination of cutting-edge electronics, high-pressure loading techniques, optics, and optoelectronics, meaning he spends months engineering experiments that last microseconds!
- B.S. Physics, University of Michigan, Ann Arbor, 2008
- Ph.D. Earth and Planetary Science, University of California, Berkeley, 2014
- Teaching Assistant for Planetary Astrophysics, UC Berkeley, Spring 2013
- Teaching Assistant for Computer Simulations in Earth Science, UC Berkeley, Fall 2009
- 2015-2018 DC STEM science fair judge
- 2013-2014 Presenter/tutor at elementary schools through BASIS program (Bay Area Scientists in Schools)
- 2014-2019, Postdoc, Carnegie Institution for Science
- 2019-present, Research Scientist, Carnegie Institution for Science