– The data was like nothing Margaret Kivelson and her team of physicists ever expected.

It was December 1996, and the spacecraft Galileo had just flown by Europa, an icy moon of Jupiter. The readings beamed back to Earth suggested a magnetic field emanating from the moon. Europa should not have had a magnetic field, yet there it was. “This is unexpected,” she recalled saying. “And that’s wonderful.”

It would be the most significant of a series of surprises from the Jovian moons. For Kivelson’s team, the mission should not have been this exciting. She and her colleagues had devised the magnetometer returning the anomalous data. The instrument’s job was to measure Jupiter’s massive magnetic field and any variations caused by its moons. Kivelson’s instrument was never supposed to change the course of space exploration.

And then it did. Kivelson and her team would prove that they had discovered the first subsurface, saltwater ocean on an alien world.

Kivelson, who turns 90 on Sunday, is professor emerita of space physics at the University of California, Los Angeles. For 40 years, she has been a part of almost every major NASA voyage beyond the asteroid belt. Her team transformed the way magnetometers are used on space missions, making them an essential tool of discovery.

Most recently, she has been an investigator working on the plasma instrument for the Europa Clipper, NASA’s next great voyage to the outer solar system. Scheduled to launch as early as 2022, the spacecraft will study the habitability of Jupiter’s ocean moon. Kivelson’s work will help answer whether life could be there by determining the ocean’s depth and salinity, and the thickness of the ice shell above it.

“I had the good fortune of starting my studies at a time when physics was regarded as the most exciting field,” she said. “This was right after World War II. Physicists had saved the world with the atom bomb and radar. And suddenly people noticed that physics was not only a wonderfully fundamental discipline, but that it was also useful.”

In 1955, she joined the RAND Corp., a firm founded to provide research to the Defense Department. She was assigned to work on an equation describing the state of hydrogen at a pressure equivalent to 1 million Earth atmospheres. “There are two places where you run into that kind of pressure in hydrogen,” she said. “One of them is in a hydrogen bomb, and the other is at the center of Jupiter.”

Her background in theoretical physics and eventual expertise in celestial science brought her to UCLA in 1967. Her RAND research made her the local expert on Jupiter, and she became well known in space physics for her work on some of the most fundamental ideas in the field.

The first major discovery by Kivelson and her team on the Galileo mission was an internal magnetic field on Ganymede, Jupiter’s largest moon.

Dr. Carol Paty, an associate professor of earth sciences at the University of Oregon, said nobody expected an object so small and cold to have the chemistry, thermodynamics and interior structure necessary to create its own magnetic field. “Its discovery reshaped the scientific understanding of the inner workings of planetary bodies,” she said.

Then came the encounters between Europa and Galileo. Something strange was happening, and Kivelson and her vexed team worked out several ways of explaining it.

Kivelson made the case for a flyby of Europa at a specific orientation — no small request given a spacecraft with limited resources flying on borrowed time. She prevailed, and the flyby in January 2000 found precisely what her team had predicted: definitive evidence of a global ocean.

“It’s one of the most fundamental discoveries ever in planetary science,” said Dr. Louise Prockter, director of the Lunar and Planetary Institute. “It spawned a revolution.”