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kilometres in a year (Hayes
et al.
, 2008; Mitri
et al.
, 2007).
Over longer timescales, the hemispheric dichotomy be-
tween lake-filled basins in the north polar region and
almost completely liquid-free basins in the south invite
explanations involving orbitally forced climatic cycles
(Aharonson
et al.
, 2009) with surface liquids accumu-
lating first at one pole and then the other.
tures seen further away in the descent images (Tomasko
et al.
, 2005). The mechanical properties of the surface are
consistent with sediment saturated with a liquid, proba-
bly methane (Zarnecki
et al.
, 2005). Methane rain was
encountered during
Huygen
s' descent (Tomasko
et al.
,
2005); the heat of the Lander also appears to have evap-
orated methane from the surface. Whether this relatively
'wet' environment (in the sense of the presence of liq-
uid methane) is the norm, or the exception, remains to be
determined.
5.6.2
Surface images
The
Huygens
probe landed on the margins of Xanadu Re-
gio close to a boundary between light highland areas and
dark lowland areas. Descent and surface images by the
probe (Soderblom
et al.
, 2007a) showed that it landed in a
lowland area that appears swept by episodic floods (Fig-
ure 5.8). The landing site itself is about
5.6.3
Lakes
Lakes of methane or similar hydrocarbons such as ethane
have long been proposed as present on Titan's surface,
based on the extant surface temperature and pressure
and the presence of hydrocarbon gases in the atmosphere
(Lunine, Stevenson and Yung, 1983). Initial observations
by
Cassini
indicated that any lakes covered only a tiny
fraction of the surface (Mitri
et al.
, 2007). Radar im-
ages of the north polar region in 2006 uncovered more
than 75 liquid methane-filled lakes at latitudes above 70
◦
N (Stofan
et al.
, 2007) plus a large number of appar-
ently dry lake bed depressions. Lakes are almost absent
from southern circumpolar latitudes (Aharonson
et al.
,
2009).
40 % covered
(Karkoschka
et al.
, 2007) by rounded pebbles of icy mate-
rial on a darker and finer grained substrate. The pebbles are
13-16 cm in diameter (Tomasko
et al.
, 2005). Rock abun-
dance for those
∼
5 cm varies, in the foreground (within
80 cm of the Lander), rocks are more common than in the
middle distance (80-160 cm). Rock abundance increases
again further away. This variation may indicate a possible
stream bed. Elongated dark sediment trails behind some
of the larger rocks are aligned with the possible stream bed
and is consistent with the orientation of channel-like fea-
>
(a)
(b)
Figure 5.8
Titan as seen by
Huygens
during its descent to the surface of Titan: (a) two types of channels formed by flowing
methane/ethane, composite image from an altitude of
∼
40 km, varying resolution due to atmospheric haze and different altitude
of composite components; channels cut through optical bright terrain and filled with optically dark material and terminate in an
optically dark plain; stubby channels (left) possibly cut by sapping methane/ethane; contrast with dendritic channels, possibly
formed by runoff of methane/ethane rain; (b) ground view from
Huygens
Lander showing optically bright cm-scale pebbles on
optically dark depositional plain; note tails of fine-grained sediment to pebbles in distance and low relief ridge on skyline. Images
