The Solar System

518 PART 4^ |^ THE SOLAR SYSTEM

b Visual-wavelength image

a

d

c

Drainage channels probably were

cut by flowing liquid methane.

Icy grapefruit-size “rocks”

on Titan are bathed in

orange light from its hazy

atmosphere.

At visual wavelengths,

Titan’s dense atmosphere

hides its surface.

Possible methane fog

Cassini radar image

of methane lakes that look

dark because they

do not reflect radio waves.

though the liquid had temporarily evaporated. If you visit Titan,
you may get caught in a shower of methane rain, but it may not
rain often at your landing site.
When the Huygens probe landed on Titan’s frigid surface,
it radioed back measurements and images. Th e surface is
mostly frozen water ice with some methane mixed in. Th e
sunlight is orange because it has fi ltered down through the
orange haze. Rocks littering the ground are actually steel-hard
chunks of supercold water ice smoothed by erosion. Some rest
in small depressions, suggesting that a liquid has fl owed
around them. You can see these depressions around the rocks
in Figure 23-17c.
Radar observations made as the Cassini probe orbited past
Titan have revealed lakes of liquid methane in its polar regions,
confi rming the earlier hypotheses. Some of those lakes are as

■ Figure 23-17

As the Huygens probe descended by parachute through Titan’s smoggy atmo-
sphere, it photographed the surface from an altitude of 8 km (5 mi). Although
no liquid was present, dark drainage channels lead into the lowlands. Radar
images reveal lakes of liquid methane and ethane around the poles. Once the
Huygens probe landed on the surface, it radioed back photos showing a level
plain and chunks of ice smoothed by a moving liquid.
(ESA/NASA/JPL/USGS/University of Arizona)

large as Lake Superior. Evaporation from the lakes can maintain

the 1.6 percent methane gas in the atmosphere, but sunlight

eventually destroys methane, so Titan must have a large supply

of methane ice. Ice volcanoes on Titan may occasionally vent

methane into the atmosphere.

By the way, before you go to Titan, check your spacesuit for

leaks. Nitrogen is not a very reactive gas, but methane is used as

cooking gas on Earth and is highly fl ammable. Of course, there

is no free oxygen on Titan, so you are safe so long as your space-

suit does not leak oxygen.