Geoscience Reference
In-Depth Information
(a)
(b)
(c)
(d)
Figure 5.7
Titan as seen by
Cassini
during encounters: (a) probable methane/ethane lakes (dark) surrounded by uplands (bright)
on Titan; note ria-like indented shoreline;
Cassini
radar image, width
∼
300 km; (b) radar-dark channels (possibly filled by flowing
methane/ethane) with delta features along shore of probable hydrocarbon lake;
Cassini
radar image, width
∼
200 km; (c) radar-
dark longitudinal dunes adjacent to radar-bright highlands;
Cassini
radar image, image width
∼
400 km; (d) radar-bright channel
sediments and outwash plain;
Cassini
radar image, width
∼
350 km. Images NASA/ESA/University of Arizona.
provided detailed in situ data on atmospheric properties
and surface composition (Lebreton
et al.
, 2005). It also
imaged the surface (or ranging over) of several kilometres
as it descended through the atmosphere. The final images
returned by the
Huygens
probe were of the surface itself.
Huygens
Lander encountered methane drizzle during its
descent (Tomasko
et al.
, 2005) and the surface it landed on
appeared to be saturated with liquid methane, even though
visually dry (Zarnecki
et al.
, 2005). Transient events in
the surface images that are consistent with methane rain-
drops were observed (Karkoschka
et al.
, 2007). Rainfall
generates runoff and methane rivers (Soderblom
et al.
,
2007a; Jaumann
et al.
, 2008; Lunine and Lorenz, 2009),
which flow into methane lakes in the north polar region,
evaporate or recharge 'methanifers'.
The methane cycle is not completely closed as ir-
reversible ultraviolet-driven photochemical reactions in
the atmosphere form complex hydrocarbons and nitriles.
These make up the atmospheric haze of Titan and are
removed by methane rainfall (Lunine and Lorenz, 2009;
Lunine and Atreya, 2008).
Seasonal variations are expected over the 29.5 year or-
bital period of Saturn. Those observed to date include the
frequency of clouds. Modelling suggests that the shore-
5.6.1
Methane cycle
Methane on Titan plays an analogous role to water on
Earth (Lunine and Atreya, 2008). Methane is evaporated
from long-lasting and deep (probably tens of metres or
more deep) lakes in the north polar region and condenses
to form clouds, which in turn precipitate methane rain
to the surface. The relative humidity of methane at the
tropics is high (45 %). This would be enough to trigger
convective rainfall on Earth but does not do so on Titan to
the same extent because Titan receives
∼
1 % of the solar
energy that Earth does. The methane cycle is therefore
