“I’m interested in the evolution of planetary interiors because magnetic fields aren’t just about shielding atmospheres. These magnetic records are the only direct information that we have about the deep interiors of planets.”

Sarah Steele, Ph.D.

Long before spacecraft orbited other worlds, planets were writing their histories in stone. Paleomagnetism—the study of ancient magnetic fields fossilized in minerals and rocks—allows scientists like Sarah Steele, Ph.D., to investigate that record. What began as curiosity in a NASA outreach program during high school became a career devoted to decoding the magnetic fingerprints of planetary interiors. 

During her doctoral research, Dr. Steele broke new ground as the first to use Harvard University’s quantum diamond microscope—a high-resolution magnetic imaging tool—to analyze slices of the Martian meteorite ALH 84001. The resulting magnetic maps revealed some surprising evidence. Mars’ dynamo—the process by which a churning molten core or magma ocean generates a global magnetic field—may have lasted longer than previously thought. Combined with models of Martian impact basins, the measurements also suggested the dynamo periodically reversed direction, causing the planet’s north and south poles to switch places. This may explain why large depressions on Mars’ surface are far less magnetic than expected, reconciling previously conflicted observations and reshaping the narrative of the planet’s interior and climate evolution. 

More broadly, Dr. Steele studies magnetic fields as the only direct records of early interiors of planets and moons. By constraining the strength and longevity of ancient dynamos, she aims to reconstruct how planetary interiors formed and changed over billions of years.  

As a 51 Pegasi b Fellow, Dr. Steele will turn her attention to Earth’s Moon. In one project, she will use geodynamical models to explore how early episodes of tidal heating—heating caused through friction as the Moon was repeatedly stretched and squeezed by Earth’s gravity—could be recorded in the Moon’s magnetic history. These magnetic clues could shed light on the impact that formed the Moon. In a second project, she will use modeling of lunar impact basins to test hypotheses on why the Moon’s magnetic field appears to have flickered on and off later in its history. By combining these approaches with other lunar observations, she aims to clarify when and how the Moon formed and how its interior evolved over time. 

Dr. Steele hopes her work will inform upcoming missions to icy moons, including the Jupiter Icy Moons Explorer (Juice), Dragonfly, and Europa Clipper. Just as important, she hopes to expand access to planetary science programs like the one that first opened the world of paleomagnetism to her. 

Dr. Steele received her Ph.D. in planetary sciences from Harvard University in Fall 2025.