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Faces of the Foundation: Sean Solomon

For a geophysicist, Hertz Fellow Sean Solomon spends a lot of time looking up instead of down. Though much of his career has been spent probing beneath the ocean floor and deep within the Earth to understand how the surface of our planet is formed and moves, he has also sent instruments to the Moon, and to our three nearest neighbors in the solar system – Mercury, Venus, and Mars. The other three rocky planets, he says, are “nature’s experiments in how a planet like Earth became the planet it is today.” As the director of Columbia University’s Lamont-Doherty Earth Observatory, he is now applying a lifetime of experience studying these planets to the vital task of understanding and preserving our own.

Studying structures deep beneath the surface, or on other planets, requires the application of many different measurement systems – seismology, gravimetry, magnetometry – and different disciplines, says Solomon. “The roots of my work go back to when I was a Hertz Fellow,” Solomon adds.

He entered MIT at the right time to participate in two different geological revolutions. As the 1960s came to a close, the theory of plate tectonics was taking shape. As the scientific community was growing to understand how one plate sinking beneath another created the great trenches and deep earthquakes in the Western Pacific, Solomon was on a ship in that very region, operating a gravimeter to map the moving rock beneath him.

The second revolution was equally historic. With advisor M. Nafi Toksöz, Solomon helped interpret data from the seismometers that the Apollo astronauts took to the Moon, helping to probe the interior of a world other than Earth for the first time. (One of his papers with Toksöz and others drawing on these data is titled, simply, “Structure of the Moon.”)

“The Hertz Fellowship enabled my exploration,” he says. “Rather than focus on one narrow question, I could do research on geophysics, planetary seismology, and more.”

Since earning his PhD, much of Solomon’s terrestrial career has been devoted to seismology – at its simplest, measuring vibrations propagating through the Earth (or, occasionally, other planets). If you know enough about the vibrations’ source, you can tell a lot about the portion of the Earth or planet through which they have traveled to reach a detector. Solomon used this tool, alongside other sensing methods, to track how molten rock cools as it erupts along divergent plate boundaries such as the Mid-Atlantic Ridge. "We were able to learn how the crust forms and characterize the earthquakes that occur there," he says.

Different sensing methods were important for terrestrial research, but they were indispensable for the research Solomon has conducted about other planets “In exploring other planets, seismology has not, by and large, been one of the tools we use,” he says. Seismometers must make physical contact with the surface of a planet, creating design challenges and competing with other scientific equipment for valuable space, even on planetary lander and rover missions.

Solomon’s interdisciplinary thinking has allowed him to take leadership roles in missions to other worlds, including as principal investigator for NASA’s recently completed MESSENGER mission to orbit and characterize Mercury. It was through neutron spectrometry, laser altimetry, and imaging, not seismology, that MESSENGER confirmed that even Mercury – the closest planet to the Sun – has water ice hiding in the permanent shadows of its cratered surface. This and other MESSENGER discoveries were cited as among Solomon’s key contributions when President Barack Obama awarded him the National Medal of Science in the physical sciences in 2014.

Solomon says exploring the inner planets is fascinating because they provide a very diverse range of outcomes for how rocky planets like ours evolve. "In some respects, they don't look like they came from the same family, and yet they did."

These differences illustrate just how lucky we are that Earth’s surface environment can support life, he says. From the magnetic field generated in the molten core providing magnetic shielding to tectonic activity providing nutrient cycling that could have been especially important for early life in the ocean, "Earth is a good place for life's experiment to work out."

As director of Lamont, Solomon sees his role as working across disciplines to help understand and protect “life’s experiment.” “I was attracted by the passion of the scientists here,” he says. As the geologists, biogeochemists, atmospheric scientists, and others at Lamont learn more about the challenges facing Earth’s changing climate and the response of biological systems to those changes, Solomon says "the work Lamont scientists are doing is really at the core of how to grapple with some of society’s most pressing issues.”