Quaoar's moon is darker and stranger than expected

Cameron Collyer and colleagues combined a June 2023 stellar occultation by Weywot with eleven Hubble orbits to separate the moon's rotational light curve from its photometric phase behavior. They derive a volume-equivalent radius of 73 plus or minus 5 kilometers and three unequal principal axes of roughly 82, 73, and 65 kilometers.

Weywot's geometric albedo is only about 0.036, more than an order of magnitude darker than the minor moons of Pluto and Haumea. Its elongated shape is compatible with a friction-bearing rubble pile in rotational and tidal equilibrium. The authors suggest the dark surface may mean Weywot did not form entirely from the icy mantle of a differentiated impact progenitor, but that origin remains an interpretation rather than a measured composition.

Why it matters

Small outer-solar-system moons preserve clues to giant impacts. Weywot's combination of shape and darkness challenges the assumption that these satellites are uniformly bright, ice-rich fragments.

What the source establishes

Fresh preprint submitted October 8. Size, shape, and albedo combine occultation geometry with Hubble photometry; the rubble-pile structure and impact origin are physical interpretations.