Limits of Standard Tidal Models at Quaoar: Matching Weywot’s Orbit, Missing the Spin

Abstract Weywot, Quaoar’s small satellite, follows a nearly circular orbit at a distance of 12.9 times Quaoar’s diameter and coexists with a compact ring system. Nevertheless, Quaoar’s flattening of 0.16, slow 17.7 hr rotation and Weywot’s low mass are difficult to reconcile with conventional tidal-evolution theory. We assess whether standard tides can reproduce the present-day architecture of the Quaoar–Weywot system and identify the initial conditions required. Orbit-averaged integrations spanning 4.5 Gyr were carried out with two formalisms: (i) a constant phase-lag and (ii) an Andrade creep-tide framework. With the nominal Weywot mass, both tidal prescriptions converge on Weywot’s observed orbital distance for a wide range of initial orbital distances and eccentricities; eccentricity is damped and present-day tidal torques are negligible, rendering the orbit quasi-stationary. Quaoar’s spin, however, remains essentially unchanged from its inferred primordial period based on its present-day flattening, and does not reproduce the observed value. A match is possible only if Weywot is ≳5–10× more massive than current estimates and if its initial eccentricity is finely tuned; such scenarios are inconsistent with occultation-derived masses and imply an implausibly dense satellite. Based on the best fitting viscoelastic parameters, the most plausible composition for Quaoar is found to be a partially differentiated dwarf planet containing roughly equal masses of silicate rock and H 2O -dominated warm (150–180 K) ices. Standard tidal models reproduce Weywot’s semimajor axis but cannot account for Quaoar’s slow 17.7 hr rotation without invoking an unrealistically massive satellite or external torques, suggesting that non-tidal processes—such as a largely primordial spin, early satellite loss, or a retrograde secondary giant impact—must have influenced Quaoar’s rotational evolution.