Saturn's magnetic shield has a surprising leak — and it's in the wrong place. Analyzing observations gathered by NASA's Cassini spacecraft between 2004 and 2010, an international team has found that the planet's magnetospheric cusp, the funnel-shaped opening near the poles through which solar wind particles stream into the atmosphere, sits far toward the afternoon side of the planet rather than near local noon, where it is found on Earth.
The finding, published in Nature Communications by researchers including Licia Ray and Sarah Badman of Lancaster University, confirms that giant planets operate under a fundamentally different magnetic regime. Saturn spins once every 10.7 hours — more than twice as fast as any point on Earth's surface — and its magnetosphere is also loaded with ionized material sprayed into space by its geyser moon Enceladus. Together, rapid rotation and that charged debris overpower the solar wind's influence, dragging the cusp away from noon.
On average, Saturn's cusp sits between 13:00 and 15:00 local time, and in some cases extends toward 20:00 — well into the planet's evening. On Earth, by contrast, the cusp is held close to local noon by the balance between solar wind pressure and the planet's magnetic field.
"This result allows us to move forward with new and improved theories on how planetary magnetospheres interact with the solar wind," said Ray. The displaced cusp has direct implications for where magnetic reconnection — the explosive process that accelerates particles to high energies — occurs, and for how Saturn's brilliant auroras form. It also demonstrates that data gathered by a mission that ended in 2017 still has rich science to give: "It highlights the rich science that can still be done with Cassini data more than eight years after the end of mission."
