The image came down to the NASA Deep Space Network on May 31, eleven hours after Juno's 67th perijove, and showed Callisto in higher resolution than any previous spacecraft had managed.

Callisto is the outermost of Jupiter's four large moons, an ancient cratered world roughly the size of Mercury, long considered the least geologically interesting of the Galileans.

It was supposed to be a calibration target. Juno's mission was extended in 2021 to include flybys of the Galilean moons, and Callisto was scheduled, almost as an afterthought, for a 1,540-kilometer pass.

What the JunoCam image showed, just inside the rim of the Asgard multi-ring impact structure, was a bright reflective feature roughly four kilometers across.

The feature does not appear on Voyager imagery from 1979 or on Galileo imagery from the late 1990s.

Mission scientist Dr. Aiyana Whitehorse of the Southwest Research Institute confirmed at a press briefing on June 2 that the spot is real, is on the surface, and is not an instrument artifact.

Mission scientist Dr. Aiyana Whitehorse of the Southwest Research Institute confirmed at a press briefing on June 2 that the spot is real, is on the surface, and is not an instrument artifact.

What it is, however, remains an open question.

The leading hypotheses are, in rough order of plausibility, a recent small impact that exposed fresh subsurface ice, a localized outgassing event, or a slope failure that exposed a clean ice face beneath darker overlying material.

Each hypothesis has problems. Callisto is geologically dead, by every model. Its surface is one of the oldest in the solar system. Recent impacts of the required size are statistically rare, and outgassing has never been confirmed on the moon.

Whitehorse is careful in her phrasing. "We are not announcing activity on Callisto," she said. "We are announcing a feature that we cannot yet explain, on a body where we did not expect to find unexplained features."

The spectral signature of the feature, derived from JIRAM infrared data taken on the same pass, is consistent with water ice substantially purer than the surrounding regolith.

That alone does not require recent activity. But it does narrow the field.

Juno's orbit will bring it within imaging range of Callisto again in early September, at which point a deliberate observation campaign is planned.

The European Space Agency's JUICE mission, currently en route to the Jovian system and scheduled for arrival in 2031, has also been alerted, and its science team is reviewing observation priorities to include the Asgard region.

Whether the feature will still be visible in 2031 is, of course, unknown. If it is a fresh exposure, it may darken on timescales of years as it accumulates dust and is processed by the local radiation environment.

Whitehorse, asked whether she found the discovery exciting or frustrating, paused before answering.

"Both," she said. "That is, in my experience, what discovery usually feels like."