What formed these dunes that
stretch for thousands of kilometers?
Dune
formations have been found in four
locations in the Solar System:
Earth, Mars, Venus, and Titan. In
one instance the average temperature
ranges from between 15 and 17
degrees Celsius, with an environment
rich in liquid water. In another
case, the average temperature is
minus 50 degrees Celsius with no
liquid water at all, while an
average of 460 degrees Celsius on a
scalded lava planet makes up the
third scenario.
The
surface temperature on Titan
averages minus 180 degrees Celsius.
It is so cold that if water were
present it would be more like rock
and would not contribute to any
chemistry on the frigid world. What
force can create patterns of fine
particles heaped into regular piles,
some hundreds of meters high,
despite the disparity in
environments?
On
Titan, the so-called "dunes"
are quite large, being visible from
the Cassini orbiter thousands of
kilometers away. They are also
well-defined, almost solid-looking
waves that pass over craters and
around what are called on Mars "yardangs."
The dunes appear to follow the
prevailing wind patterns on Titan
that blow a mere 8 kilometers per
hour, but they also have some
unusual characteristics
that may mean they are not
wind-generated in the conventional
sense.
Many
of the
dunes
look like fingerprint patterns—they
have whorls and arches that are
crisscrossed
by other ripples in a perpendicular
arrangement, looking almost exactly
like the dune fields found along the
coast of
Namibia.
On
Earth, dunes are found only in the
driest places. The sand grains and
dust must be able to slide freely
over one another for the dunes to
pile up and move across the
landscape. Strong winds are usually
required to move the sand. In some
places, such as Australia's Simpson
Desert, sand dunes have become
fossilized
and immobile, although they cover
thousands of square kilometers.
Whatever deposited the sand there
was unable to move it again, so they
have crusted over, plants have taken
root, and gullies have formed.
It
has been speculated by mission team
members that Titan is "wet" with
hydrocarbons. In fact, when the
Huygens probe
landed on Titan, it sank slightly
into a somewhat friable surface.
NASA planetary scientists described
it as "like wet snow," or "loose
clay," or more significantly, "dry
sand." There was detectable methane
in the area surrounding the probe,
but it quickly dissipated—presumably
because of the heat emanating from
the lander. In such deep cold, the
temperature of an ice cube would act
like a cigarette lighter on the
surroundings.
If
Titan were wet, despite its low
gravity, the winds (really no more
than gentle breezes) would not be
able to accumulate material into
long dunes. Any moisture would cause
the dust to become sticky and any
larger grains would be mired in the
slush. With such minimal wind speeds
recorded by the Huygens lander, it
stands to reason that Titan is dry,
despite claims from NASA that there
are "lakes" of
liquid ethane
and methane on its surface.
So,
if Titan is dry and there are
craters
with folded rims and wide, flat
bottoms (often with more than one
tier or
concentric basin),
parallel fractures, large domes
similar to those on Venus, and
Lichtenberg figures
(called river channels by the
Cassini team) incised into the
terrain, then perhaps a better
explanation for what happened to
Saturn's planet-sized moon might be
found by considering Mars.
On
Mars
the dunes have been identified with
electric arcs.
When the discharges excavated
material from the strata, they blew
it upward along the path of current
flow where it then fell away,
forming what are called "wind
streaks."
Because the dust is most likely
charged on Mars, the particles can
be attracted or repelled from one
another, depending on their
polarity. Therefore, they will align
themselves in much the same way that
iron filings will align themselves
with the magnetic field of a bar
magnet. It is possible that that is
the mechanism behind the dune
structures on Titan, as well.
In
the equatorial latitudes of Titan is
a great "sand sea" called Belet. If,
like Earth, Venus, and Mars it
represents the fragments of rock
strata that were destroyed by
electric discharge activity and then
ionically deposited in masses of
sand larger than the State of
Michigan, it is probably the source
for the dunes and streaks of fine
dust.
Streaks and
dunes
have also been discovered on
Venus.
Most of the time, they are
associated with craters and sinuous
rilles; the edges of both formations
are often dusted with dark or light
colored grains. Titan's dunes are so
similar that we predict
"blueberries," or other spherical
pebbles and stones, may be found if
a rover-type lander ever pays a
visit there.
Dry,
cold, covered with sinuous rilles,
gigantic craters, chaotic
topography, and thousand-kilometer
ridges of piled-up dust 500 meters
high, Titan is one more example of
catastrophic devastation among the
Sun's family in the recent past. We
should breathe a sigh of relief that
those processes no longer take place
in this time of calm and peace
between the storms.
Stephen Smith