Other Dune Worlds - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 168

15

Other Dune Worlds

15.1 Triton

The last decade has seen a transformation in our understanding
of the abundance of planets in our galaxy. From a handful of
known exoplanets in the 1990s, the number of confirmed
candidates (albeit mostly gas giants) is approaching 1000.
Some of these planets have rocky surfaces, and it seems cer-
tain that our galaxy, and indeed the universe, is teeming
with worlds on which dunes may exist. However, for the
forseeable future, we can learn little about these planets and
their moons and must content ourselves with our own solar
system.
Beyond the terrestrial planets and Titan, which we
have already covered, two moons in our solar system are
presently known to have tenuous (and in fact rather vari-
able) atmospheres that may be thick enough to transport
particulate material: Triton and Io. We will discuss them
briefly in this section in order of decreasing pressure;
Triton also serves as a prototype for Pluto and perhaps
other similar worlds in the chilly Kuiper belt of bodies at
the edge of the solar system. However, as we discuss, it is
at best marginal to think of these as 'Dune Worlds'. We
may note that the acceleration of solid particles by gas is
a known (and spectacular) process on comets, and on
Saturn's moon Enceladus, but no details of horizontal gas
flows are known and thus, even if these fell under the
definition of aeolian transport (and they may not), there is
nothing at present to say.
It is conceivable that other worlds (such as the Galilean
moons) may have had atmospheres soon after they formed
but which are no longer present. If this is the case, aeolian
transport might have occurred, but there seems little hope
that traces of such processes are preserved to an observable
extent. Similarly, aeolian processes can occur in transient
volcanic 'atmospheres' (witness, say, the bedforms gener-
ated in ashfall deposits during maar eruptions on the Earth)
in which case perhaps some ripples or related features
may have formed even on our own moon. Again, however,
there

Triton is the largest moon (radius 1353 km) of Neptune, and
revolves around its planet in just under 6 days. Its density
suggests it has a rock or rock/iron core, but substantial
amounts of water and carbon dioxide ice. Unusually, its
orbit is retrograde, suggesting perhaps that Triton is a
'captured' moon, rather than having formed in a proto-
planetary disk. The orbital arrangement and Triton's rota-
tion lead to very large changes in the amount of sunlight on
different parts of Triton (its situation resembles the planet
Uranus, where during the course of a year the sun passes
nearly overhead each pole), with the result that there may be
substantial seasonal migration of volatiles.
Triton has an atmosphere that is quite thin, predomi-
nantly of nitrogen. The surface pressure determined by the
Voyager 1 radio occultation experiment was a mere
14 microbar: the gas appears to be in vapor pressure equi-
librium with nitrogen frost on the surface; this frost makes
Triton's surface very bright and thus it has an exceptionally
low surface temperature—about 40 K.
Aeolian transport on Tritan was considered by Sagan and
Chyba (1990). They found, given the pressure conditions
and the low Triton gravity of 0.78 m/s 2 , that even an
atmosphere as tenuous as Triton's 16 microbars could lift
5 micron or smaller particles into suspension if cohesion
between particles is small. They noted that, even though the
Triton surface pressure is 4009 smaller than Mars', the low
temperature on Triton (69 less) mitigates the atmospheric
density by that factor; also the Triton dust would be likely
organic (as on Titan) and thus less dense than Martian rock.
These factors, together with the low gravity, mean that the
threshold friction speed on Triton would be only *2.5
higher than on Mars.
Measurements of the atmosphere by stellar occultation in
1997 showed that the atmosphere had in fact thickened
somewhat (at these conditions, even the observed small rise
in surface temperature of *2 K can increase the vapor

is little more that can be said than to note the

possibility.