502 Encyclopedia of the Solar System
Thus, within a few hundred million years after capture,
Triton in all probability went through an episode of run-
away melting. This is schematically illustrated in Figure 17,
where in the model labeled thin shells Triton melts sponta-
neously when enough energy has been accumulated to do
so (in reality the runaway occurs much earlier). Thereafter
Triton is a nearly totally molten, but still dissipative body.
Its tidal heating curve rises and falls sharply over the course
of∼100 million years.
During this epoch of extreme tidal heating Triton’s heat
flow is an amazing∼2–4 W m–^2 , equal or greater than that
measured today from Io. Its surface temperature is gov-
erned by this flux, and corresponds to a blackbody tem-
perature of 80–90 K. During and after this epoch there
would likely have been large chemical exchanges between
the global oceanic mantle with its dissolved volatiles and
the hot rock core below. Much of Triton’s volatiles may
have been driven into a massive atmosphere. Atmospheric
components plausibly include CO, CH 4 ,CO 2 , and NH 3 ,
or even H 2 (from photolysis of methane or ammonia or as
a minor component in Triton’s original ice). Conservative
assumptions yield an atmospheric greenhouse with surface
temperatures well above 100 K; more extreme possibilities
allow for surface temperatures greater than 200 K.
A most intriguing aspect of raising a massive greenhouse
atmosphere by tidal heating is that it may persist well after
the tidal heating input has tapered off and Triton’s interior
has begun to freeze. It may only collapse after enough of
it has been lost to space due to solar-UV-heating-driven
hydrodynamic escape, which could have taken in excess of
1 billion years. While the atmosphere existed it would have
kept Triton’s surface warmer, and enhanced the geological
mobility of the satellite’s surface layers. Unfortunately, there
are as yet no definitive indicators of the atmosphere’s former
presence (e.g., ancient aeolian or fluvial features, peculiar
crater shapes, etc.). If a thick atmosphere existed, Triton’s
continued geological activity has obscured the evidence.
Regardless, once tidal heating ended, Triton’s interior
should have begun to freeze. It would probably have taken
a few 100 million years to do so, but even today such freezing
would not be complete. Triton’s ice mantle is probably warm
enough, due to radiogenic heating from the core, that any
ammonia- and methanol-rich fluids are stable (perhaps in
an internal ocean), and Triton’s inner core of alloyed iron,
nickel, and sulfur should likewise be warm enough (more
than≈1250 K) to allow for a eutectic liquid mixture of those
elements.
The possible persistence of cryomagmas in Triton’s man-
tle due solely to radiogenic heating has raised the ques-
tion as to whether any of the geological observations in
Section 6 actuallydemandthat Triton was massively tidally
heated. Certainly, solar-powered plume models do not re-
quire Triton to be internally active at all. Triton’s surface,
on the other hand, is so peculiar (in the sense of being
unique or special). Furthermore, the extent and intensity
of the geological activity recorded there is only seen on satel-
lites that are undergoing active and substantial tidal heating
(Io, Europa, and Enceladus). While no ironclad argument
can be made, Triton’s geology and chemistry in all likeli-
hood indicate that it did indeed experience massive tidal
heating.
The proof of Triton’s history and provenance requires
further exploration of this extraordinary body. For exam-
ple, determination of the compositions of Triton’s icy lavas,
and terrains in general, would be key constraints. Detailed
exploration of the Neptune system by spacecraft is also a
technically feasible proposition, given recent and projected
technological advances. Instruments and electronics are
being increasingly miniaturized, thereby requiring smaller
launch vehicles. Missions to Triton can also take advantage
of innovative flight strategies, such as using aerobraking in
the Neptune atmosphere to go into initial Neptune orbit.
Thereafter a complement of advanced instruments can be
trained on Triton during repeated encounters, filling out
our picture of this amazing satellite.
As for Triton’s ultimate future, as a retrograde satel-
lite its orbit is actually decaying due to tides it raises on
Neptune. In the 1960s it was estimated that Triton would
closely approach Neptune and be torn apart by tides in a
geologically short time. Present estimates imply less peril:
Triton’s orbit will probably shrink by no more than 15%
over the next 5 billion years, giving Triton plenty of time for
further geological and atmospheric adventures.Bibliography
Beatty, J.K.,et al.(1999). “The New Solar System, 4th Ed.”
Sky Publishing, Cambridge, MA.
Cruikshank, D.P. (ed.) (1995). “Neptune and Triton.” Univer-
sity of Arizona Press, Tucson.
Greeley, R., and Batson, R. (1997). “NASA Atlas of the Solar
System.” Cambridge University Press, Cambridge, UK.
Littmann, M. (2004). “Planets Beyond: Discovering the Outer
Solar System.” Dover Publications, Mineola, NY.
Morrison, D., and Owen, T. (2003). “The Planetary System”
3d ed. Addison Wesley, San Francisco, CA. NASA Planetary Pho-
tojournal. http://photojournal.jpl.nasa. gov/index.html.
Rothery, D.A. (1999). “Satellites of the Outer Planets: Worlds
in Their Own Right: 2d ed. Oxford Univ. Press, NY.
Smith, B.A., and theVoyagerImaging Team (1989).Voyager
2 at Neptune: Imaging science results. Science 246 , 1422–
1449.