Rainbows on Titan

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

http://science.nasa.gov/headlines/y2005/25feb_titan2.htm?list577962

February 25, 2005: 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.

Right: River channels and a shoreline on Titan. Credit: ESA Huygens probe.
[more]

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.

Left: Sunlight + raindrops = rainbows. [more]

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.

Below: An 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."



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 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," 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."

Forwarded By Yours : Dr. BHUDIA.- Science Group Of INDIA.

President:"Kutch Science Foundation".
Founder :"Kutch Amateurs Astronomers Club - Bhuj - Kutch".
Life Member:"kutch Itihaas Parishad".

kutchscience@..., kutchscience@...,

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