Oceans of the Solar System - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 239

more than 400 kilometres per hour. Titan is ferociously cold too. Its
surface temperature reaches down to −179 degrees Celsius, much
colder than the coldest place on Earth and cold enough to allow its
methane seas. 147
These hydrocarbons, falling as rain on the surface and being chan-
nelled through streams and rivers, accumulate as lakes and small
inland seas: Titan is the only place in the solar system, other than
Earth, where liquid persists at the surface (see Plate 8). This liquid is
made of mixtures of ethane, propane, methane, and butane mixed
with nitrogen, all these being stable under the ambient atmosphere
of Titan. Of these seas, the wonderfully named Kraken Mare is 1,000
kilometres across, but a journey across this sea by any future human
explorers would be fraught with danger. Compared with the liquid
water oceans on Earth, the hydrocarbon seas of Titan are much less
dense (only half that of water). Ships manufactured on Earth would
sink helplessly if they tried to navigate this particular sea.
Titan is a world of two oceanic levels. Deep below its surface, there
is a water ocean that was detected by Cassini . 148 As Titan orbits Saturn
it is squeezed by the gravity of that giant planet. The way Titan dis-
torts is revealed by changes to its gravitational field, and this reveals
its internal structure. These changes were detected via changes in the
speed of Cassini as it made flybys of the moon. The measurements
show that Titan is susceptible to tidal effects, indicating a layer of
liquid about 100 kilometres below its surface. This water below the
surface may be the source of the methane in Titan's atmosphere.
Indeed, something must be replenishing the atmospheric methane,
as evidence from isotopes of nitrogen around it suggest that Titan
has lost something like five times the present amount of its atmos-
phere to space over its life. Where is the methane coming from?
Titan's atmosphere contains a telltale signal in the gas argon, more
specifically the isotope 14 Ar, which originates from the decay of