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monsoon' describe by Arthur Clarke in his novel
Imperial
Earth
(1976) which is partly set on Titan.
Radar-bright delta-shaped features are apparent where
some channels debouche on to the lowlands; these are in-
terpreted as fan deposits (Lorenz
et al.
, 2008). Meandering
channel forms are also present (Lorenz
et al.
, 2008) and
the morphology of estuaries where some channels enter
lakes is strongly reminiscent of rias.
global radar coverage of the cloud-obscured surface is
available from the
Pioneer 12
,
Venera 15
and
16
, and
Magellan
missions, and from terrestrial radar astronomi-
cal studies the best resolution is 125 m. It is these images
that have provided our understanding of the larger-scale
geomorphology of Venus (Figure 5.9).
5.7.1
Surface-atmosphere interaction
5.6.7
Summary
The corrosive atmosphere and high temperatures and pres-
sures suggest that chemical weathering may be significant
on Venus (McGill
et al.
, 1983; Nozette and Lewis, 1982).
Arvidson
et al.
(1992) demonstrated that lava flows of dif-
ferent ages in Sedna Planita showed systematic changes
in radar backscattered characteristics that were greater
than could be explained by aeolian action alone. They hy-
pothesised that the flow modification was due in part to
weathering.
A wide range of trace volatiles are known to occur or are
likely to occur in the atmosphere of Venus (Brackett, Fe-
gley and Arvidson, 1995). Condensation of these phases
as metal sulfides and halides at higher altitudes, where
temperatures are almost 100 degrees cooler than the low-
lands, are hypothesised to produce weathering analogous
to terrestrial salt weathering. Therefore it may be signifi-
cant that topographic features on Venus more than 3 km
above datum are coated with a material of low emissivity
and high dielectric constant. This is consistent with the
condensing of heavy metal sulfides from the atmosphere
(Schaefer and Fegley, 2004). The most likely candidates
are galena (lead sulfide) and/or bismuthite (bismuth sul-
fide). Tellurium compounds have also been suggested.
Some of these condensed phases may well undergo melt-
ing, leading to the possibility not only of analogues to salt
weathering on Earth and Mars but also of the effects of
moisture.
Unfortunately all four landers that have imaged the sur-
face touched down at too low an elevation to observe this
phenomenon directly. However, if previous atmospheric
temperatures were lower, it is possible that these pro-
cesses may have operated at some of the landing sites
and rock breakdown features caused by condensed atmo-
spheric materials might still be preserved.
Titan remains enigmatic. With an active methane cycle,
large lakes, drizzle and apparently wet surfaces in osten-
sibly dry areas, the surface in some respects resembles
that of a terrestrial humid landscape. Conversely, the dry
lakebeds stained by dissolved residues from the evapo-
ration of liquid methane, evidence for major fluctuations
in lake levels, large dune fields, rare but intense rainfall
events and highly variable river flow, are all strongly rem-
iniscent of terrestrial aridity. Further complexity is intro-
duced by the slow pace of Titan's seasons - a full circuit
of the sun takes almost 30 years and observations of the
surface have to date comprised but a fifth of that period.
Combined with the lower insolation, the rate of environ-
mental variability of Titan may be much lower than of
Earth.
5.7
Venus: extreme aridity
Venus has an equatorial diameter of 94.9 % and a mass
of 81.5 % that of Earth's. The surface of Venus is ob-
scured by high-altitude clouds of sulfuric acid droplets.
The atmosphere is composed almost completely of CO
2
and the average surface pressure is
∼
60 bars, equivalent
to an ocean depth on Earth of
600 m. As a result of
the dense CO
2
atmosphere the greenhouse effect is ex-
treme and surface temperatures average
∼
400
◦
C. There
is considerable topographic variation, with pressures and
temperatures of 41 bars and 374
◦
C in the highlands and
96 bars and 465
◦
C typical of the lowlands. This range
means that impact, volcanic, aeolian and weathering pro-
cesses may operate very differently in the highlands and
lowlands (Greeley, 1994). Conversely, diurnal variations
from the slow, retrograde rotation of the planet are min-
imal because of the efficient equalization of latitudinal
temperature differences by atmospheric circulation.
Much of the surface of Venus remains enigmatic, de-
spite more than 20 missions to the planet and 13 probes
that successfully reached the surface, four of which re-
∼
5.7.2
Surface images
Black and white images of the surface of Venus were ob-
tained by
Venera 9
and
10
(Florenskii
et al.
, 1983) while
higher resolution colour images were obtained by
Venera
