How do you read the history of a place that keeps burying its past?
Katherine de Kleer faces that problem on Io, Jupiter’s volcanic moon. Eruptions cover old ground with fresh material. Evidence disappears beneath the surface. The moon has been around for billions of years, but much of the terrain that might explain its history is gone from view.
Katherine looks for what survives.
I photographed her at Caltech on December 4, 2022. Barefoot and at ease, she spends her working life investigating places of extraordinary physical violence. On Io, Jupiter’s gravity repeatedly stretches and compresses the interior. Neighboring moons help maintain the orbit that makes this possible. The resulting heat drives the most intense volcanic activity in our solar system.
Her questions are wonderfully concrete. Where is the molten rock reaching the surface? How hot is it? How long has this been happening?
Answering them requires considerable ingenuity. You cannot pick up a rock or walk to the edge of a lava lake. Almost everything must be inferred from light collected at a distance, often under conditions that allow only a brief opportunity to observe.
When Europa passes in front of Io, for example, it gradually blocks the heat coming from the volcanic surface. Katherine and her collaborators have used those changes to map temperatures within individual volcanic regions. At Loki Patera, an enormous volcanic feature, their observations revealed evidence of two waves of resurfacing moving through different areas. One moon obscuring another gave them a better understanding of the ground below.
Katherine studied mathematics and physics at MIT, then earned her doctorate in astrophysics at Berkeley in 2017. She joined Caltech’s faculty in 2019. Her work brings astronomy into close contact with geology. The measurements arrive through a telescope, but they concern the behavior of rock.
In 2024, she and her colleagues published research addressing the deeper question of Io’s age as a volcanic world. They used the ALMA telescope array in Chile to measure different forms of sulfur in its atmosphere. Lighter sulfur escapes into space more readily than heavier sulfur. Over time, that loss changes the proportions left behind.
Those proportions carry a record. The findings supported a history of eruptions extending back billions of years. Where the surface had concealed the past, the atmosphere retained evidence of it.
The implications reach beyond Io. Heat generated by gravity also matters on icy moons, where it can help sustain water beneath the surface. Understanding that heat, and how long it lasts, helps scientists assess the conditions in which an ocean might persist.
I have spent much of my life traveling to remote places. Being there matters to me. I want to feel the cold and see what lies beyond the next ridge. Katherine’s work asks me to consider another kind of exploration, one that depends on making sense of the faint evidence that reaches us.
There is a volcano out there. Something about it has changed since the last observation. With enough care, she can begin to tell us what happened.
