Saturn's moon Titan
is wet, according to the ESA's Huygens probe, but Titan's "water"
is not like Earth's.
by Dr Tony Phillips
When the European Space
Agency's Huygens probe visited Saturn's moon Titan in January 2005,
the probe parachuted through humid clouds. It photographed river
channels and beaches and things that look like islands. Finally,
descending through swirling fog, Huygens landed in mud.
To make a long story
short, Titan is wet.
Christian Huygens wouldn't
have been a bit surprised. In 1698, three hundred years before the
Huygens probe left Earth, the Dutch astronomer wrote these words:
certain that Earth and Jupiter have their Water and Clouds, there
is no reason why the other Planets should be without them. I can't
say that they are exactly of the same nature with our Water; but
that they should be liquid their use requires, as their beauty
does that they be clear. This Water of ours, in Jupiter or Saturn,
would be frozen up instantly by reason of the vast distance of
the Sun. Every Planet therefore must have its own Waters of such
a temper not liable to Frost."
channels and a shoreline on Titan.
Titan in 1655, which is why the probe is named after him. In those
days, Titan was just a pinprick of light in a telescope. Huygens
could not see Titan's clouds, pregnant with rain, or Titan's hillsides,
sculpted by rushing liquids, but he had a fine imagination.
is liquid methane, CH4, better known on Earth as natural
gas. Regular Earth-water, H2O, would be frozen solid
on Titan where the surface temperature is 290o F below
zero. Methane, on the other hand, is a flowing liquid, of "a
temper not liable to Frost."
Jonathan Lunine, a
professor at the University of Arizona, is a member of the Huygens
mission science team. He and his colleagues believe that Huygens
landed in the Titan-equivalent of Arizona, a mostly-dry area with
brief but intense wet seasons.
"The river channels
near the Huygens probe look empty now," says Lunine, but liquids
have been there recently, he believes. Little rocks strewn around
the landing site are compelling: they're smooth and round like river
rocks on Earth, and "they sit in little depressions dug, apparently,
by rushing fluids."
The source of all this
wetness might be rain. Titan's atmosphere is "humid,"
meaning rich in methane. No one knows how often it rains, "but
when it does," says Lunine, "the amount of vapor in the
atmosphere is many times that in Earth's atmosphere, so you could
get very intense showers."
And maybe rainbows,
too. "The ingredients you need for a rainbow are sunlight and
raindrops. Titan has both," says atmospheric optics expert
On Earth, rainbows
form when sunlight bounces in and out of transparent water droplets.
Each droplet acts like a prism, spreading light into the familiar
spectrum of colors. On Titan, rainbows would form when sunlight
bounces in and out of methane droplets, which, like water
droplets, are transparent.
[requires] that they be clear...."
+ raindrops = rainbows
"A methane rainbow
would be larger than a water rainbow," notes Cowley, "with
a primary radius of at least 49o for methane vs 42.5o
for water. This is because the index of refraction of liquid
methane (1.29) differs from that of water (1.33)." The order
of colors, however, would be the same: blue on the inside and red
on the outside, with an overall hint of orange caused by Titan's
One problem: Rainbows
need direct sunlight, but Titan's skies are very hazy. "Visible
rainbows on Titan might be rare," says Cowley. On the other
hand, infrared rainbows might be common.
Bob West of NASA's Jet Propulsion Laboratory explains: "Titan's
atmosphere is mostly clear at infrared wavelengths. That's why the
Cassini spacecraft uses an infrared camera to photograph Titan."
Infrared sunbeams would have little trouble penetrating the murky
air and making rainbows. The best way to see them: infrared "night
All this talk of rain
and rainbows and mud makes liquid methane sound a lot like ordinary
water. It's not. Consider the following:
The density of liquid
methane is only about half the density of water. This is something,
say, a boat builder on Titan would need to take into account. Boats
float when they're less dense than the liquid beneath them. A Titan-boat
would need to be extra lightweight to float in a liquid methane
sea. (It's not as crazy as it sounds. Future explorers will want
to visit Titan and boats could be a good way to get around.)
infrared rainbow on Earth, photographed by Prof.
Robert Greenler. Reference: Science 173,1231
(1971). "A rainbow on Titan might look like this,"
notes Les Cowley. "It would be larger than the visible
methane 'bow' with a radius slightly more than 49-52 degrees."
Liquid methane also
has low viscosity (or "gooiness") and low surface tension.
See the table below. Surface tension is what gives water its rubbery
skin and, on Earth, lets water bugs skitter across ponds. A water
bug on Titan would promptly sink into a pond of flimsy methane.
On the bright side, Titan's low gravity, only one-seventh Earth
gravity, might allow the creature climb back out again.
|index of refraction
(#1) NIST Chemistry Webbook. Reference temperature: 40o
F for water, -290o F for methane. Reference pressure
1.5 atm; (#2) AIChE Journal, Volume 42, No. 5, pp. 1425-1433, May
1996; (#3) Les Cowley.
Back to boats: Propellers
turning in methane would need to be extra-wide to "grab"
enough of the thin fluid for propulsion. They'd also have to be
made of special materials resistant to cracking at cryogenic temperatures.
And watch out for those
waves! European scientists John Zarnecki and Nadeem Ghafoor have
calculated what methane waves on Titan might be like: seven times
taller than typical Earth-waves (mainly because of Titan's low gravity)
and three times slower, "giving surfers a wild ride,"
Last but not least,
liquid methane is flammable. Titan doesn't catch fire because the
atmosphere contains so little oxygen - a key ingredient for combustion.
If explorers visit Titan one day they'll have to be careful with
their oxygen tanks and resist the urge to douse fires with "water."
towering waves, seas beckoning to sailors. Huygens saw none of these
things before it plopped down in the mud. Do they really exist?
is no reason why the other Planets should be without them."