Aug
15,
2006
Titan Update—Our
Prediction Stands
Recent weeks have seen
growing confidence by NASA scientists that they have
observed methane
“lakes” on Saturn’s moon Titan. But we stand by our
prediction that no methane lakes will be found.
On July 27,
Nature magazine published a study by R. Hueso and A
Sanchez-Lavega, suggesting that surface features on Titan
include dry river channels cut by periodic downpours of
methane and regional floods. This study, combined with
earlier stories concerning supposed “lakes” of methane at
Titan’s north pole, has provoked many optimistic headlines
in scientific journals.
“At Last, Methane Lakes on Saturn’s Icy Moon Titan,” reads a
headline in Science magazine. According to Science News, the
radar images give the “first
compelling evidence for bodies of liquid on the surface of
any object besides Earth.” And the New Scientist speaks of
rivers on Titan “that are probably liquid ethane or
methane.”
In the issue of Nature discussing “methane storms” on Titan,
the journal also published another study under the direction
of lead author Tetsuya Tokano, from the University of
Cologne, Germany. According to this study, based on the
Huygens probe dropped to the surface of Titan, “the data
indicate an upper methane ice cloud and a lower, barely
visible, liquid methane-nitrogen cloud, with a gap in
between.” The report concludes that, by wetting Titan’s
surface globally, the drizzle “plays an active role in the
surface geology of Titan.”
The scientific reports have set up an ideal opportunity to
test the predictive ability of two models. In our Picture of
the Day for
December 28, 2004, following the lead of
Wallace Thornhill, we suggested that Titan’s surface
features should be compared to those of Venus. In
Thornhill’s interpretation, Titan’s recent birth is the
reason for its dense but diminishing methane atmosphere. In
this hypothesis a similarity of Titan’s surface to that of
Venus would be expected. We noted:
“In the first close-up image of Titan’s surface by Cassini a
“Venusian-type” dome was tentatively identified. We can also
expect many flat-bottomed valleys bordered by steep cliffs
with scalloped edges—a common signature of high-energy
surface machining by electricity. And while planetary
scientists puzzle over the absence of craters, we predict
that, as higher resolution images of the surface are
returned, many regions will reveal channels formed of
overlapping smaller craters and parallel grooves that
can be expected of discharge streamers raking across the
surface.”
When NASA scientists looked at radar images showing channels
appearing to “feed” into flat bottomed depressions, they
thought they were seeing methane lakes fed by episodic
methane rivers. But for obvious reasons NASA scientists have
not noticed the similarity between radar images of Venus’
surface and those of Titan’s surface. They have no reason to
associate these two bodies so remote from each other. In the
radar image of Venus above, we see both flat-bottomed
depressions and river-like channels, presenting a parallel
to Titan’s “lakes” too obvious to miss. In fact our
prediction of “flat-bottomed valleys bordered by steep
cliffs with scalloped edges” turns out to be a perfect
description of what we actually observe in the radar images
of Titan. So we are not deterred by the confidence of NASA
scientists: When Cassini takes a closer look at the north
polar region in question, they will not find what they are
looking for, except by straining credulity further.
And really, how credible is the claim of methane rivers
feeding methane lakes on Titan? Thornhill contends that
while a dense methane fog can “dampen” the ground, it cannot
generate the torrential rains that various NASA scientists
have depicted in response to the Titan radar images. It will
not create rivers and lakes. According to Christopher McKay,
a scientist at NASA Ames Research Center in California's
Silicon Valley and second author of the Tokano study noted
above, "The rain on Titan is just a slight drizzle, but it
rains all the time, day in, day out. It makes the ground wet
and muddy with liquid methane. This is why the Huygens probe
landed with a splat. It landed in methane mud," said.
"We determined that the rain on Titan is equal to about two
inches (about 5 centimeters) a year," McKay said. "This is
about as much rain as Death Valley (receives). The
difference is (that) on Titan, this rain is spread out
evenly over the entire year."
Could two inches of methane precipitation, spread out over a
year, contribute significantly to the carving of surface
relief? Would it cut “riverbeds” on Titan that some have
compared to the action of flash floods, creating large lakes
of standing methane? In fact, the marginal precipitation
from a fog, expected by Thornhill, appears to be exactly
what the Huygens probe detected.
Despite the headlines and self-congratulations there is
every reason for skepticism about NASA’s claims. Given
Thornhill's solo success in predicting the nature of Titan's
surface before the Huygens Probe saw beneath its
thick clouds, we are confident that no methane rivers or
lakes of any size will be found on Titan. Both the channels
and the flat-floored depressions were cut by electrical
discharge in the tumultuous early history of the moon.
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