Triton 501
FIGURE 16 (a) Example evolution of Triton’s semimajor axis,
aT, as a function of time,t, due to tidal dissipation within Triton.
(b) Evolution of Triton’s minimum and maximum periapse
distance,rp(the closest point to Neptune in its orbit), as a
function of semimajor axis due to the combined influence of
semiannual solar perturbations and tidal dissipation. The
periapse distance oscillates between the two curves shown.
[Adapted from P. Goldreichet al.(1989).Science 245 , 500–504.]
or greater, or it may have begun closer in, such as in the
example shown in Figure 15. The important point is that
early on Triton’speriapse(the closest point to Neptune in
its orbit) would lie as low as half its present semimajor axis.
Triton’s tidal evolution probably took 100 million years or
longer, so there would have been sufficient time for Triton’s
orbit to evolve through and interact with any preexisting
satellites.
This point is emphasized in Figure 16b, which includes
the periodic effects of solar tides on Triton’s evolving orbit.
If Triton’s initial capture orbit was very large and eccentric,
its periapse would have fluctuated, and may have period-
ically been as low as 5RN! Triton would have had ample
opportunity for collisions with Neptune’s original satellites
(if they were like Uranus’ today), possibly accreting them
in the process. It may also have scattered original satellites
into distant orbits, caused them to crash into Neptune, or
perhaps even ejected them from Neptune altogether. There
is now nothing left of Neptune’s original system (if it indeed
existed) other than the inner satellites and Nereid. The in-
ner satellites all lie within 5RN, however, which is perfectly
consistent with this capture scenario. Nereid may also be a
survivor of this orbital mayhem. Little is known about this
distant moon, save its size (∼340 km in diameter), reflec-
tivity (∼20%), and presence of surface water ice, but these
facts make Nereid more akin to a regular satellite than a
dark captured asteroid or comet.
The end state of Triton’s orbital evolution is an extremely
circular orbit. As such, the orbital energy potentially dissi-
pated by tides within Triton represents an absolutely enor-
mous reservoir, about 10^4 kJ kg–^1. It is sufficient to com-
pletely melt all the ice, rock, and metal within Triton ten
times over. The magnitude of Triton’s temperature change,
however, depends on the heating rate, and somewhat on
the size of the initial capture orbit. Two such models are
FIGURE 17 Power dissipated per unit mass and surface heat
flow for Triton as its post-capture orbit shrinks and circularizes.
Two models are shown. One assumes Triton remains a uniform,
undifferentiated sphere, while the second allows for melting. In
both cases the time scales are longer than in the calculations in
Figure 16a, due to updated parameters for Triton, but the
periapse variations as a function of semimajor axis in Figure 16b
are unchanged. The thin shells model is more realistic than the
elastic sphere, but even here the meltdown of Triton has been
artificially suppressed; in reality a thermal runaway probably
occurs much earlier. [From W.B. McKinnonet al.(1995).In
“Neptune and Triton” (D.P. Cruikshank, ed.). University of
Arizona Press, Tucson.]illustrated in Figure 17. Tidal heating after capture in ei-
ther model is at first modest, as the satellite spends most
of its time far from Neptune. As its semimajor axis shrinks
and its orbital period decreases, the average heating rate
begins to rise. The epoch of greatest heating occurs when
the relative change in semimajor axis is the greatest (be-
cause orbital energy is inversely proportional to semimajor
axis), roughly when the semimajor axis drops below 100
RN. Because the orbit can only evolve as fast as the tides
can convert orbital energy to heat, the response of Triton
to tidal flexing is crucial. If Triton responds as a dissipative
elastic sphere, then the semimajor axis drops continuously
(Fig. 16a) and the tidal heating rises and then falls smoothly
as the orbit becomes more circular (Fig. 17, elastic sphere
model). The calculations in Figures 16a and 17 are actually
for two different elastic sphere models, but are shown here
to illustrate a range of possible time scales.
A dissipative elastic sphere is clearly an idealized and
oversimplified model for Triton. Triton is in reality a com-
plex rock, metal, organic matter, water-ice, and volatile ice
body. The volatile ices especially should be melted and mo-
bilized within Triton early in its history (e.g., ammonia),
with or without tidal heating. A partially molten body is a
particularly dissipative body, so when capture occurs and
tidal heating begins, heat concentrates in the partially liq-
uid regions. This causes more melting, which makes the
body more dissipative, which results in greater tidal heating.