Derby and District Astronomical Society

The Journal of the Derby and District Astronomical Society
September - December 2005

Rainbows on Titan?
Adpated from a NASA web article by Malcolm Neal

February 25, 2005: When the European Space Agency's Huygens probe visited Saturn's moon Titan, 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.

Christiaan Huygens (1629-1695) Christiaan 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 290 degrees 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 vapour in the atmosphere is many times that in Earth's atmosphere, so you could get very intense showers."

River channels and a shoreline on Titan from the Huygens probe.
River channels and a shoreline on Titan from the Huygens probe.  Image Credit: ESA.

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 colours. 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 49 degrees for methane versus 42.5 degrees for water. This is because the index of refraction of liquid methane (1.29) differs from that of water (1.33)." The order of colours, 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.

An infrared rainbow on Earth
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."
Image Credit:  NASA/Les Cowley

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. 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. 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."

Continuing this I recently read a science fiction book by a British author Stephen Baxter called Titan. It was published in 1997/8 and so written sometime before that. In it he uses many of the ideas that appear in this article, especially the idea of huge waves travelling very slowly in his hypothetical seas on Titan. Either he did some very serious research or he was a very good 'guesser'. I expect it was research as he also alludes to the idea that IR glasses would make features in the sky more visible e.g. rainbows. Reading this NASA article only a week after completing the book is like reading sections of the book again. Titan must be a very strange place and I do feel a little short changed by the Hygens lander. Though it produced some stunning information I was disappointed by the short time period it was active even ESA have yet to come up with a really effective long life battery.

To read Malcolm's book review of Titan by Stephen Baxter click here.

DDAS members met the Huygens Principal Scientist Professor John Zarnecki at the 2005 Flamsteed Lecture. Click here for the story.

ESA Cassini-Huygens website:-