Rainbows on Titan

http://science.nasa.gov/headlines/y2...feb_titan2.htm

Rainbows on Titan
NASA Science News
February 25, 2005

Saturn's moon Titan is wet, according to the ESA's Huygens probe, but
Titan's "water" is not like Earth's.

When the European Space Agency's Huygens probe visited
Saturn's moon Titan last month, 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:

"Since 'tis 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."

Huygens discovered 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.

Titan's "water" 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
Les Cowley.

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.

"Their beauty [requires] that they be clear...."

"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 orange sky.

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.

Atmospheric scientist 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
vision" goggles.

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.)

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.

Physical Data: Liquid methane vs liquid water

liquid water liquid methane methane/water
ref.
density 1 g/cc 0.45 g/cc 0.45 #1

surface tension 70 dyne/cm 17 dyne/cm 0.24 #2
viscosity 1.54 cP 0.184 cP 0.12 #1

index of refraction 1.33 1.286 0.97 #3


Sources: (#1) NIST Chemistry Webbook. Reference temperatu 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," says Ghafoor.

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."

Infrared rainbows, towering waves, seas beckoning to sailors. Huygens
saw none of these things before it plopped down in the mud. Do they
really exist?

"...there is no reason why the other Planets should be without them."