Hi’iaka: Haumea’s Rapidly Spinning Moon

Recent observations of Hi’iaka, the largest satellite of the dwarf planet Haumea, reveal that the moon is spinning much more rapidly than expected. What could this tell us about how Haumea and its moons formed?

A Distant Dwarf

The dwarf planet Haumea orbits beyond Neptune and has a mass of roughly 1/3 that of Pluto. Like Pluto, Haumea also has companions: two satellites of roughly 0.5% and 0.05% of Haumea’s mass, orbiting at rather large distances of 36 and 70 Haumea radii (roughly 26,000 and 50,000 km).

In a recently published study, a team led by Danielle Hastings (UC Los Angeles and Florida Institute of Technology) explored Hubble and Magellan observations of Hi’iaka — Haumea’s larger, outer satellite — to determine the rate at which it rotates on its axis.

Rapid Rotation

Nominally, we’d expect Hi’iaka to be rotating synchronously — its rotation period should be the same as its orbital period of 49.5 days. We expect this because the amount of time needed for tidal forces to despin Hi’iaka to synchronous rotation should be much shorter than the time needed for these forces to produce Hi’iaka’s observed low eccentricity and large semimajor axis.

Therefore it was quite the surprise when Hastings and collaborators analyzed Hi’iaka’s light curve and found that the moon revolves on its axis once every 9.8 hours! That’s roughly 120 times faster than the expected synchronous rate.

Formation Theories

What does this discovery reveal about Hi’iaka’s formation? Hastings and collaborators propose three possible scenarios. They then use analytic calculations and numerical simulations to try to constrain them based on Hi’iaka’s orbital and spin properties.

1. Hi’iaka formed close in and then migrated outwards
   The authors show that the time needed to despin a satellite depends strongly on its initial spin rate and semimajor axis. If Hi’iaka formed with the right initial conditions and moved outward from Haumea very quickly, it would have been possible for it to maintain the high spin rate we observe.
2. Hi’iaka formed in place
   Hi’iaka’s spin rate is also shown to be consistent with a model in which the satellite formed at its current location from an especially large proto-satellite disk around Haumea.
3. Hi’iaka was spun up by a recent impact
   What if Hi’iaka was rotating synchronously, but a recent impact spun it back up again? The authors show that a glancing impact/merger with a hypothetical third satellite of Haumea could have spun up Hi’iaka to its current rate without affecting its circular, low-obliquity orbit.

The upshot of the authors’ analysis is that Hi’iaka’s rotation rate is faster than we expected, but this discovery is not enough to discriminate between the different hypotheses for how the moon formed. Future observations of other parameters of Haumea and its satellites are our best bet for understanding the origin of this system.

Citation

Danielle M. Hastings et al 2016 AJ 152 195. doi:10.3847/0004-6256/152/6/195