Earth & Space
Incoming: New fellow will probe moon’s magnetic past for clues about its formation and interior
51 Pegasi b Fellow Sarah Steele will spend three years at UC Santa Cruz working with Earth and planetary scientists to advance her work to decode ancient magnetic records preserved in rocks
Sarah Steele also studies the process by which a churning molten core or magma ocean generates a global magnetic field.
Photo by Kristen Chiama
While all eyes are currently on NASA’s Artemis II lunar mission, Harvard Ph.D. graduate Sarah Steele will spend the next three years looking deeply at our moon’s magnetic history for clues that could shed light on the cosmic collision that first formed it.
Steele will do this as a postdoctoral researcher in the Earth & Planetary Sciences Department at the University of California, Santa Cruz. Ultimately, she aims to clarify when and how the moon formed, and how its interior evolved over time. This objective aligns strongly with the planetary science research of Professor Francis Nimmo, who will be Steele’s mentor.
Throughout her time at UC Santa Cruz, Steele will be supported by a highly coveted 51 Pegasi b Fellowship, which was awarded by the Heising-Simons Foundation and provides outstanding postdoctoral scientists with the opportunity to conduct theoretical, observational, and experimental research in planetary astronomy. Steele is one of eight new fellows in the 2026 cohort that was announced on April 7.
Steele said she hopes her work will inform upcoming missions to icy moons, including the Jupiter Icy Moons Explorer (Juice), Dragonfly, and Europa Clipper. Additionally, she aims to expand access to planetary science programs like the one at NASA that first opened her eyes to “paleomagnetism”—the study of ancient magnetic fields fossilized in minerals and rocks.
“I’m interested in the evolution of planetary interiors because magnetic fields aren’t just about shielding atmospheres,” Steele said. “These magnetic records are the only direct information that we have about the deep interiors of planets.”
During her doctoral research, 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 conflicting observations and reshaping the narrative of the planet’s interior and climate evolution.
In addition to exploring how early episodes of tidal heating on the moon might be recorded in its magnetic history, as a second project while at UC Santa Cruz, Steele 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.
“How and when the lunar dynamo died is subject to a lot of debate right now,” Nimmo said. “Looking at the magnetic field recorded by different impact basins of different ages will help resolve that debate.”
Now in its 10th year, the 51 Pegasi b Fellowship provides early-career postdoctoral scientists the opportunity to lead transformative research in planetary astronomy. The fellowship is named after the first exoplanet discovered orbiting a Sun-like star and is one of the most postdoctoral awards in the field. The three-year grant covers salary and benefits, along with highly flexible research-related discretionary spending, and the opportunity to extend the fellowship for a fourth year.
The fellowship is awarded by the Heising-Simons Foundation’s science program, which supports scientific advancements in astronomy and cosmology that illuminate our basic understanding of the universe, its celestial objects, and processes. Steele will begin her research at UC Santa Cruz in September 2026.