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As NASA’s Dawn spacecraft arrived at Ceres back in March 2015, a new crater came to the attention of mission controllers back in Houston.

Just under 10 km in diameter and embedded in an ancient, heavily cratered terrain darkened by thermal alteration and billions of years of falling micrometeorites, Oxo was in most way ways nothing spectacular. And yet its brightness made it one of the most striking features on the planet’s surface.

“The human eye would immediately see this difference,” says Andreas Nathues from the Max Planck Institute for Solar System Research, imagining himself crossing the Cerean terrain and coming across Oxo. “It would be like snow against a background which is about as bright as coal.”

Bright spots are a feature of Ceres. Occator Crater, the brightest of all was observed even before Dawn arrived, from the ground-based W. M. Keck Observatory on Mauna Kea earlier in 2015.

The exact origin of these brightest spots is still debated. One idea for Occator, put forward by Nathues back in 2015, was its formation impact triggered a carbonate rich brine to rise from 20-30 km down, reaching the surface through cryovolcanic eruptions.

However, Oxo poses new questions. Earlier this year a new paper reported evidence of water ice at the crater, a first for Ceres.

“Why did we find water ice at Oxo and not at other craters?” asked Nathues.

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