Extraterrestrial arid surface processes - Arid Zone Geomorphology - page 73

The lakes appear to be bounded by bedrock with
bedrock islands and complex shorelines. How the lake de-
pressions formed is not known, but none are obviously as-
sociated with impact craters. Bourgeois et al. (2008) sug-
gested that dissolution of the substrate by liquid methane
was one possible process, analogous to karst developed
on calcretes beneath playa lakes of Namibia. Further sim-
ilarities with terrestrial playa lakes were noted by Lorenz,
Jackson and Hayes (2010).
Dendritic drainage is visible at depths of at least 10 m
below the surface of lakes owing to the transparency of
methane liquids at the relevant radar wavelengths (Lunine
and Lorenz, 2009). This indicates alternating lake levels
and, together with the rias, indicates periods of low lake
level with fluvial incision that are followed by high lake
levels and drowning of the fluvial features.

abundant sand and at least episodically strong winds im-
plies that, at least locally, wind may be a major process in
rock disintegration.
The third Cassini flyby of Titan revealed the presence
of dark, linear features. Their distribution and configu-
ration indicated that they are large, longitudinal dunes
similar to those in many terrestrial deserts such as in Aus-
tralia and Namibia (Rubin and Hesp, 2009). The dunes are
radar dark, occur in lowland areas along the equator, are
oriented east-west and appear to have been formed by a
predominantly westerly wind. Like terrestrial longitudinal
dunes, those on Titan show tuning fork junctions, diverge
around some topographic highs and terminate against oth-
ers. Where terminated by bedrock rises they pick up again
in the downwind direction (Lunine and Lorenz, 2009).
The dunes appear to be composed of organic-rich materi-
als (Soderblom et al. , 2007b). The origin of the sand-sized
particles is presently unknown; the sand may have been
eroded from the highlands, formed by aggregation of air-
borne hydrocarbon particles (Lunine and Lorenz, 2009),
from some as yet unknown process, or a combination of
all of these.

5.6.4 Rock breakdown: process and form

Prior to the Cassini mission, Lorenz and Lunine (1996)
postulated that one of the most effective means of rock
breakdown on Titan was by differential solubility. Titan's
crust was considered to be composed largely of water, am-
monia and water-ammonia ices with solid hydrocarbons.
These materials have different solubilities with respect
to methane-rich liquids. This proposal may have been
borne out by the apparent morphological similarity of
Titan's lakes to solution-generated lakes in arid environ-
ments on Earth (Bourgeois et al. , 2008), a suggestion that
requires further investigation. Dissolution of ices by liq-
uid methane also implies subsequent deposition through
evaporation of the methane. This is likely to leave be-
hind a residue of solid hydrocarbons and crystallisation
of ices. These deposits are potentially analogous to ter-
restrial salts. The ability of such deposits to cause rock
breakdown is a subject for further research.
Lorenz and Lunine (1996) also dismissed diurnal tem-
perature variations as a likely mechanism for rock break-
down. This position is probably still valid because of the
low diurnal temperature range on Titan.

5.6.6 Fluvial systems

Lorenz and Lunine (1996) raised the possibility of flu-
vial erosional features on the surface of Titan in the pre-
Cassini era. Modelling studies (Burr et al. , 2006) showed
that liquid methane is potentially even more effective at
transporting sediments under titanian conditions than wa-
ter is on Earth. Thus it is not surprising that a range of
fluvial landforms are visible on Titan. In the region of
the Huygens landing site two distinct types of channels
are visible (Soderblom et al. , 2007a): deeply incised den-
dritic drainage networks up to fourth order and short,
stubby low-order drainages that follow apparent struc-
tural features. The dendritic, high-order channels are at-
tributed to runoff and the stubby, low-order channels to
discharge from methanifers. Valley slopes of up to 30 de-
grees occur on the sides of both channels, indicating a
competent substrate. River channels elsewhere on Titan
are typically hundreds of kilometres in length and in some
cases are more than a thousand kilometres long (Jaumann
et al. , 2008). Channel morphology is consistent with rare
(decadal or century scale intervals) but intense (20-50 mm
per hour) rainfall events, analogous to those that occur
in terrestrial desert regions. The drizzle observed during
Huygens ' descent (Tomasko et al. , 2005) is not sufficient
to generate runoff events, although it appears sufficient to
moisten the surface (Tomasko et al. , 2005). Lorenz (2005)

5.6.5 Aeolian landforms

Likewise, Lorenz and Lunine (1996) dismissed aeolian
processes as a likely cause of rock breakdown because
of the modelled low wind velocity. However, the discov-
ery of large linear dune fields was one of the first major
discoveries of the Cassini mission (Elachi et al. , 2006).
Although no ventifacts are visible in the Huygens descent