Bob Pappalardo has spent much of his life thinking about a world covered in ice.
Europa first caught his attention at Cornell, where Carl Sagan allowed him to sit in on a graduate seminar called Ices and Oceans in the Outer Solar System. Bob had grown up watching the Apollo launches, landings, and splashdowns, and visiting the Hayden Planetarium with his father. At home, he suspended handmade planets from his bedroom ceiling. Their moons were bits of masking tape pressed onto toothpicks. At Cornell, that childhood fascination became a life’s direction.
Bob trained as a planetary geologist and earned his doctorate at Arizona State University. His research examined the ridges and grooves of icy moons, especially Miranda, one of the moons of Uranus. Ice, in his work, is not a blank or lifeless surface. It bends, breaks, flows, and keeps a record of forces moving deep below.
He later helped plan observations of Europa and Ganymede for NASA’s Galileo spacecraft. The images revealed long ridges, dark bands, broken plains, and pieces of crust that appeared to have shifted and rotated. Europa also had remarkably few large impact craters. Its surface had somehow been renewed, suggesting a geologically young and perhaps still active world.
Galileo supplied strong evidence for a global saltwater ocean beneath the ice. Scientists estimate that it may contain about twice as much water as Earth’s global ocean. As Europa travels around Jupiter, the planet’s gravity stretches and relaxes the moon, generating heat inside it. Scientists are confident that the ocean has a rocky floor. If water and warm rock interact there, the resulting chemistry could provide a source of energy. Europa appears to have water, useful chemistry, energy, and immense stretches of time. That combination makes it one of the most compelling places to investigate habitability beyond Earth.
Bob joined NASA’s Jet Propulsion Laboratory in 2006 to help turn the long discussed Europa mission into something that could fly. He is now the project scientist for Europa Clipper, responsible for its scientific objectives and scientific quality. The work also requires him to connect scientists, engineers, NASA leaders, and the wider research community. Bob calls the role that of a “science diplomat.”
Europa Clipper launched on October 14, 2024. It passed Mars on March 1, 2025, and is scheduled to return past Earth on December 3, 2026. Earth’s gravity will increase the spacecraft’s speed and redirect it toward Jupiter, where it is expected to arrive in April 2030.
Clipper will orbit Jupiter and make 49 close passes of Europa. Its instruments will study the icy shell, the ocean beneath it, the moon’s composition and geology, and any evidence of water vapor plumes. The mission is not designed to detect life. It will ask the question that must come first: Does Europa have places where life could exist?
Bob first encountered Europa in a Cornell classroom roughly four decades ago. Now a spacecraft carrying questions he helped shape is crossing the solar system to meet it.
