Last year’s close-up photos of Pluto from the New Horizons probe were a revelation, but don’t forget the dwarf planet’s proportionally sizable moon Charon. The surface of that world presented its own puzzles of geology and history.

For starters, Charon sported a dark and dusty red cap at its illuminated northern pole. A later image taken looking back at the moon’s dark southern pole, dimly lit by “Pluto-shine,” showed that the pole was also darker—perhaps due to a similar reddish cap. The early hypothesis was that, similar to dark regions of Pluto’s surface, this cap was a thin residue that solid organic compounds formed from reactions of gases catalyzed by incoming solar radiation and charged particles.

There’s just one problem with this idea: Pluto is the one with the gases, not Charon…