GEO 287 (Feb. 16, 1996):
EVOLUTION OF ORBITAL RESONANCES WITH TIME:
THE URANIAN SATELLITES EXAMPLE
* Equations describing evolution of satellite mean motion
The mean motion (n) describing a satellite orbit changes with time t
as a result of tidal dissipation:
where n = satellite's mean motion = 2/P (P = orbital period)
(also called "orbital frequency" or "orbital angular velocity")
k2 = planet's Love number =
Q = tidal dissipation parameter of planet
= rigidity of satellite
M,m = mass of planet, satellite
R,r = radius of planet, satellite
density, gravity of satellite
As Q increases, tidal dissipation lessens, and dn/dt decreases.
* Orbital Resonances
For Io (J1), Europa (J2), and Ganymede (J3):
n1 - 3n2 + 2n3 = 0
describes the Laplace resonance, which forces Io's eccentricity and gives
rise to that satellite's volcanoes.
Miranda and Ariel are relatively small bodies, but they show evidence for
past endogenic activity. The heat for this activity likely was tidal. Also,
Miranda has a high eccentricity, and Miranda, Ariel, and Umbriel all have high
inclinations, suggesting present or past orbital interactions.
For the Uranian satellites Miranda (U5), Ariel (U1), Umbriel (U2), Titania
(U4), and Oberon (U5), there are currently no resonances of the form:
j5n5 + j1n1 + j2n2 + j3n3 + j4n4 + j5n5 = 0
where the sum of the coefficients must be zero for a stable resonance (see:
Squyres, S.W., R.T. Reynolds, and J.J. Lissauer 1985, The enigma of the Uranian
satellites' orbital eccentricities, Icarus 61, 218-223).
* How can Uranian satellites be explained?
The satellites may have moved through temporary resonances as their
orbits evolved outward due to tidal dissipation.
Peale, S.J. 1988, Speculative histories of the Uranian satellite systems,
Icarus 74, 153-171:
Peale (1988) finds that Ariel and Umbriel once may have been involved in a
2:1 resonance. If Miranda's mass is near the upper limit of its error, then
Miranda, Ariel, and Umbriel may have been involved in a Laplace-type resonance.
The actual capture into and escape from resonance, however, is likely complex
Dermott, S.F., R. Malhotra, and C.D. Murray 1988, Dynamics of the uranian
and saturnian satellite systems: A chaotic route to melting Miranda?, Icarus
Increase in the satellite orbital radii over time could have caused past
resonances of the uranian (and saturnian) satellites' orbits. These resonances
would not have been well separated, so chaotic motion would have resulted, and
has been simulated numerically. A 3:1 resonance between Miranda and Umbriel
may explain Miranda's unusual orbital parameters and geology.
* How might Miranda have been heated?
Greenberg, R., S.K. Croft, D.M. Janes, J.S. Kargel, L.A. Lebofsky,
J.I. Lunine, R.L. Marcialis, H.J. Melosh, G.W. Ojakangas, and R.G. Strom 1991,
Miranda, in Uranus (J.T. Bergstralh, et al., eds.), pp. 693-735:
Miranda likely formed in a nebula of temperature ~80 K. How can its
temperature have risen high enough to have caused its endogenic
- Accretional heating:
can raise temperature only ~30 K.
- Radiogenic heating:
only ~10 - 20 K temperature rise for an H2O Miranda. But if Miranda is
composed primarily of low-thermal conductivity ice clathrate, then the
temperature rise could have been much greater, ~140 K.
- Heating during tidal spindown:
spindown was likely fast, resulting in only ~4 to tens of degrees
- Heating during initial eccentricity damping:
possibly a ~10 - 20 K temperature rise, but uncertain initial eccentricity
makes this explanation somewhat ad hoc.
- Heating during chaotic rotation:
an episode of chaotic rotation may have raised temperatures as much as
~200 K, but if the time scale of decay instead were short, the temperature rise
would be much less.
- Orbital resonance (such as 3:1 with Umbriel):
of similar magnitude as radiogenic heating, only ~20 K for an H2O Miranda
(but much more for a clathrate satellite composition).
- Tidal heating of a blocky satellite:
tidal heating concentrated along the boundaries of 10 - 100 km blocks
could produce greatly enhanced tidal heating. But this model seems
non-geological for a satellite-sized body in which deep fractures may anneal.