Oceans of the Solar System - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 238

carving valleys between mountains of water ice that are strangely
reminiscent of rocky valleys on the surface of Earth.
As the rain falls through Titan's atmosphere it must create rain-
bows through the prism effect of methane droplets, which are as
transparent as the rain on Earth. These rainbows will rarely be within
the spectrum that humans see, as it is difficult for light to penetrate
Titan's thick, hazy atmosphere. Future explorers from Earth might
take with them night vision glasses, to see rainbows in the infrared
spectrum—for radiation at this wavelength penetrates Titan's atmos-
phere more easily. The spacecraft Huygens itself saw the Titan land-
scapes in infrared, to help penetrate the haze.
After 2 hours and 27 minutes Huygens completed its descent and
settled on to the surface of Titan, encountering a damp sandy surface—
the damp sand here comprising grains of water ice moistened by
liquid methane—and being rocked by gentle winds. For 90 minutes,
peering through that thin haze, in the dim light (to an astronaut, it
would be akin to a moonlit night) it sent back pictures of eroded,
stream-transported ice pebbles stretching into the distance, and infor-
mation on the atmosphere. It remains the most distant landing craft
ever sent by humans.
Titan is the second largest moon in the solar system after Ganymede.
It may have a complex, multi-layered water ice skin and a rocky
inner core. Its skin includes a layer of water and ammonia that
remains liquid at temperatures as low as −97 degrees Celsius. This
deep-lying fluid is perhaps the source of much of the methane in the
thick atmosphere of Titan, an atmosphere that is uniquely dense for
a planetary moon at a little short of 1.5 times the atmosphere of
Earth. Titan rotates slowly, in tandem with its circuit around Saturn
of 15 days and 22 hours. But its atmosphere is rotating much more
rapidly. Despite the gentle breezes of its surface, at an altitude of
120 kilometres the winds in the atmosphere are speeding along at