One of the most shocking discoveries of the past 10 years is how much
the landscape of Saturn's moon Titan resembles Earth. Like our own
blue planet, the surface of Titan is dotted with lakes and seas; it has
river channels, islands, mud, rain clouds and maybe even rainbows. The
giant moon is undeniably wet.
The "water" on Titan is not, however, H2O. With a
surface temperature dipping 290 degrees F below zero, Titan is far too
cold for liquid water. Instead, researchers believe the fluid that
sculpts Titan is an unknown mixture of methane, ethane, and other
hard-to-freeze hydrocarbons.
A new ScienceCast video ponders the mystery of the missing waves on Titan. Play it
This glint of sunlight detected by Cassini in 2009 is widely thought to
be a reflection from the mirror-like surface of one of Titan's northern
lakes. More
The idea that Titan is a wet world with its own alien waters is
widely accepted by planetary scientists. Nothing else can account for
the observations: NASA's Cassini spacecraft has flown by Titan more than
90 times since 2004, pinging the Moon with radar and mapping its lakes
and seas. ESA's Huygens probe parachuted to the surface of Titan in
2005, descending through humid clouds and actually landing in moist
soil.
Yet something has been bothering Alex Hayes, a planetary scientist on the Cassini radar team at Cornell University.
Researchers have toyed with several explanations. Perhaps the
lakes are frozen. Hayes thinks that is unlikely, however, "because we
see evidence of rainfall and surface temperatures well above the melting
point of methane." Or maybe the lakes are covered with a tar-like
substance that damps wave motion. "We can't yet rule that out," he adds.
Here on Earth, bodies of water are rarely still. Breezes blowing
across the surface cause waves to ripple and break; raindrops striking
sea surfaces also provide some roughness. Yet on Titan, the lakes are
eerily smooth, with no discernable wave action down to the millimeter
scale, according to radar data from Cassini.
"We know there is wind on Titan," says Hayes. "The moon's magnificent sand dunes [prove] it."
Add to that the low gravity of Titan—only 1/7th that of Earth—which offers so little resistance to wave motion, and you have a real puzzle.
The answer might be found in the results of a study Hayes and
colleagues published in the July 2013 online edition of the journal
Icarus. Taking into account the gravity of Titan, the low viscosity of
liquid hydrocarbons, the density of Titan's atmosphere, and other
factors, they calculated how fast wind on Titan would have to blow to
stir up waves: A walking-pace breeze of only 1 to 2 mph should do the
trick.
This suggests a third possibility: the winds just haven’t been
blowing hard enough. Since Cassini reached Saturn in 2004, Titan’s
northern hemisphere (where most of the lakes are located) has been
locked in the grip of winter. Cold heavy air barely stirs, and seldom
reaches the threshold for wave-making.
But now the seasons are changing. In August 2009 the sun crossed
Titan’s equator heading north. Summer is coming, bringing light, heat
and wind to Titan's lake country.
"According to [climate models], winds will pick up as we approach
the solstice in 2017 and should be strong enough for waves," he says.
If waves appear, Cassini should be able to detect them. Radar
reflections from wavy lake surfaces can tell researchers a great deal.
Wave dimensions, for instance, may reveal the viscosity of the
underlying fluid and, thus, its chemical composition. Also, wave speeds
would track the speed of the overlying winds, providing an independent
check of Titan climate models.
Hayes is excited about "bringing oceanography to another world. All we need now," he says, "are some rough seas."
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