498 Encyclopedia of the Solar System
7.2.1 PLUMES AS JETS
The energy needed to drive the plumes is determined by
how much gas is involved and how fast it has to be erupted.
The worst case assumption is that the nitrogen does not
condense as it rises. Instead of becoming buoyant and ac-
celerating, it is denser than its surroundings because of any
inert dust entrained in it and the small amount of N 2 (several
percent by mass) that crystallizes immediately upon erup-
tion. The plume is therefore slowed both by gravity and by
interaction with the atmosphere around it. How high it will
rise depends on both the size of the eruption and its speed,
and can be calculated based on laboratory simulations. As
an example, a jet with a diameter of 20 m, a velocity of
230 m sec–^1 , and 5% solids by mass will reach the observed
altitude of 8 km on Triton. The plumes might be this small,
but they could be as big as 1–2 km in diameter, in which
case they could be somewhat slower. As discussed, plumes
erupting more slowly could also reach 8 km if condensation
continues after eruption, but in either case the plume will
stop at about 8 km because of the increasing atmospheric
temperature (buoyancy) above this altitude.
7.2.2 ERUPTION VELOCITY AND TEMPERATURE
Both the initial velocity of the gas and the amount of solid ni-
trogen that will condense can be calculated from the initial
and final temperatures and the thermodynamic properties
of nitrogen. The example given above (5% solids, 230 m
sec–^1 ) is attained for nitrogen expanding freely (no change
in entropy) and cooling from 42 K to 38 K. Thus, the sub-
surface gas must be heated about 4 K to power the geyser
to the right altitude. We also learn from this calculation
that the 10-to 20-kg sec–^1 of solids estimated to be feeding
the plumes is accompanied by as much as 400 kg sec–^1 of
gas. Given the latent heat of sublimation of nitrogen, about
100 megawatts of power is needed to convert solid to gas at
this rate.
7.2.3 TEMPERATURE OF A SOLID-STATE GREENHOUSE
The “greenhouse effect” usually describes heating of the
Earth’s atmosphere (or that of another planet) when sun-
light at visible wavelengths penetrates the atmosphere be-
fore being absorbed, but longer wavelength thermal radi-
ation is absorbed by the atmosphere and cannot escape to
space as easily. A similar effect can take place in a transpar-
ent solid, for example, nitrogen ice on Triton. The amount
of solar energy is not great at Triton’s distance from the
Sun, but nitrogen is an excellent thermal insulator and the
deeper the sunlight is absorbed the warmer the subsur-
face will get. A 6-m thick layer of clear nitrogen ice over a
dark subsurface layer would actually melt at the base, while
even a 4-m layer would blow itself apart because the hot
ice would produce gas at a pressure higher than the weight
of the solid above. (This cannot be how plumes originate,
however, because the production of gas would cease very
quickly as chunks of the ruptured layer cooled.) Heating by
4 K can be achieved with a greenhouse layer only 1–2 m
thick.7.2.4 SUBSURFACE ENERGY TRANSPORT
What happens after sunlight is absorbed below Triton’s sur-
face and before hot gas is erupted? As just estimated, 100
megawatts are needed to heat the gas in a typical plume.
This is the amount of power deposited by sunlight on a re-
gion of Triton about 10 km in diameter, much bigger than
the 1- to 2-km size of the plume sources. We can therefore
conclude that gas (or energy to produce gas by sublima-
tion) is stored over time and then released quickly, or is
transported horizontally from the larger area to the geyser,
or both. Somewhat counterintuitively, gas is not mainly
“stored” in voids in the nitrogen ice, but is produced on
demand from hot ice, while heat transport is mainly carried
by flowing gas rather than ordinary thermal conduction. Ni-
trogen ice can give off more than 100,000 times its own vol-
ume of gas as it cools just 4 K. If there are voids in the solid
nitrogen, this gas will flow to colder areas and recondense,
warming them by releasing its latent heat. Depending on
the size of such void spaces, the gas flow can transport en-
ergy hundreds of times more efficiently than conduction.
Not only could flow between meter-sized blocks of solid
readily supply a geyser, but when a path to the surface was
first opened eruption would be vigorous at first and decline
over a period of about a year, roughly the estimated life-
time of the plumes. Energy transport by production of gas,
its flow through pores, and recondensation at colder points
is known on Earth: “heat pipes” containing a condensable
gas (with a wick to return the liquid to the hot end) conduct
heat better than metal and are used for baking potatoes from
the inside out and for controlling the temperature of space-
craft, includingVoyager! How a suitably fractured layer of
nitrogen ice, overlain by a clear, gas-tight greenhouse layer,
might form on Triton is discussed in the next section.
The idea of solar-powered geysers thus seems extremely
promising, though much work remains to take the separate
pieces that have been modeled so far and make sure that
they fit together. Internally powered geysers (more simi-
lar to their terrestrial counterparts) have not been studied
nearly as thoroughly, but other possibilities exist. As dis-
cussed below, the nitrogen “polar caps” on Triton may be
so thick near their center (over a kilometer) that they begin
to melt at the base. Liquid N 2 finding its way to the surface
could erupt as a boiling geyser, with more than enough en-
ergy to power the plumes. Gases other than nitrogen could
also be erupted from deeper in Triton’s water-ice mantle,
driven by internal heating. Most recently, a similar solar-
powered geyser model has been proposed for the formation
of dark spots, “spiders,” and fans at high southern latitudes