Titan’s Sinuous Rilles
New radar images of Saturn’s moon Titan, taken by
the Cassini spacecraft, offer tantalizing clues about the
moon’s geologic history.
New images from the Cassini spacecraft offer further tests
of predictions made by electrical theorist Wallace Thornhill
a little more than a year ago.
The region called Xanadu displays dunes, hills, valleys and
mountains as high as the Appalachians. In fact, NASA
scientists suggest that the landscape looks a lot like
Earth’s surface, though it appears to have been carved
largely in ice.
"This land is heavily tortured, convoluted and filled with
hills and mountains," said Steve Wall, the Cassini radar
team's deputy leader at NASA's Jet Propulsion Laboratory. "Xanadu
has been washed clean. What is left underneath looks like
very porous water ice, maybe filled with caverns."
But this surface profile is a far cry from what NASA
scientists had expected prior to the descent of the Huygens
probe and prior to these more recent radar images. What
scientists expected to find were seas of methane, and the
expectation was based on “simple reasoning.” Great volumes
of methane are escaping from Titan’s atmosphere. Accepted
theory required that this methane be replaced continuously,
or it would have run out millions of years ago. But no
surface methane has been located.
NASA scientists are now dealing with a familiar puzzle – one
that we have observed again and again in the history of
comet investigations. If what theory requires is not seen on
the surface, it must be hidden under the surface – and in
great abundance! (Cf., the
futile search for sufficient surface water on comets to
sustain the “dirty snowball” theory.) So the theorists began
to envision periodic outbursts of methane from subsurface
reservoirs, subsequently raining down in vast floods. But no
such rains have been observed, nor has flowing methane been
found. Therefore, the scientists reason, these rains must be
Following this reasoning, the scientists even have another
observational fact to cite. Across the surface of Xanadu,
images reveal long winding channels (see picture above),
which the scientists identify as methane “river networks.”
The NASA press release states, “At the region's western
edge, dark sand dunes give way to land cut by river
networks, hills and valleys. These narrow river networks
flow onto darker areas, which may be lakes.”
But again, flowing methane has never been found on Titan.
Though no one can predict the actual state of things on
Titan with certainty, we are confident that neither rains of
methane nor rivers of methane nor lakes of methane will be
found on Titan’s surface. The entire line of reasoning
follows from a precarious assumption. We further predict
that if the NASA theorists will look at the topography with
sufficient care, they will see that the so-called rivers do
not follow terrain in the fashion of flowing liquids – which
of course is always downhill.
This expectation is not a wild guess but a cautious
prediction based on the observations of sinuous rilles on
other planets and moons, including the body Thornhill deemed
Titan’s “sister,” the planet Venus. As noted in our
Picture of the Day for Feb 03, 2005, on Venus we see
Baltis Vallis, a sinuous channel with many counterparts on
Venus, this one spanning some 6,800 kilometers. Planetary
scientists claim that Baltis Vallis was cut by flowing lava,
but the ravine, maintaining a constant width, rises and
falls again and again, with some 2 kilometers of elevation
separating its high and low points.
Evidence from both historical investigation and decades of
space exploration suggests that Venus and Titan share a
geologically recent birth in an electrically active phase of
solar system history. Electrical discharges during that
phase would have excavated narrow, sinuous channels as they
traveled uphill and downhill without regard for gravity.
This defiance of gravity is, in fact, one of the defining
features of electric rille formation. And together with
electrically carved rilles, Titan’s continuing loss into
space of a dense natal atmosphere of methane is an expected
consequence of recent birth.
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