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In-Depth Information
14
Venus Dunes
14.1
Introduction
that the lower atmosphere of Venus is relatively clear, and
the greenhouse effect is due primarily to carbon dioxide and
water vapor.
The Venus probes revealed the atmospheric pressure at
the surface to be some 90 bar: this translates, given the
molecular weight of the predominantly CO
2
atmosphere and
the high temperatures, to an air density of about 64 kg/m
3
,
or about 50 times that of sea-level air on Earth. This high
density might lead one to expect aeolian transport to be
rather easy. However, in contrast to the abundant sand drifts
seen in images from the surface of Mars by the Viking
landers in 1976, images returned from the torrid Venusian
surface by several Soviet Venera probes
1
showed
(Fig.
14.2
) relatively little fine-grained material (compared
with the Moon or Mars), although there were some indi-
cations that the landings kicked up a cloud of dust. Since the
Venusian surface is hidden from view at optical wave-
lengths, there was no camera survey of Venus from orbit
comparable with the Mariner 9 and Viking orbiter surveys
of Mars which had revealed ample evidence of aeolian
activity on that world.
Wider reconnaissance of the Venusian surface was
accomplished with radar measurements using large radio
telescopes on Earth, and later by radar mapping of the
northern polar regions of Venus by the Venera 15 and 16
spacecraft in the early 1980s. However, these techniques
had spatial resolutions (1 * 2 km) too poor to resolve
dunes.
The next (and for the moment, latest) step was the near-
global radar mapping at *120 m resolution by the NASA
Magellan spacecraft in the early 1990s (Fig.
14.3
). Despite
the thick atmosphere which might a priori suggest aeolian
features might be widespread, only a couple of areas of
resolvable dunes were discovered on Venus in Magellan
radar imaging (e.g., Greeley et al. 1992; Weitz et al. 1994;
Since Venus has a thick atmosphere (Fig.
14.1
), it seems a
natural place to contemplate aeolian transport. Although
speculations about Venus' surface ranged from a global
ocean world (of water), to a tarry swamp, other speculations
were of a desert world. In many respects, this last per-
spective has been borne out, although Venus seems largely
to be a rocky rather than a sandy world, dominated by
volcanic and tectonic processes, rather than erosion and
deposition.
Authoritative research reviews of Venus science as a
whole are given in two topics in the University of Arizona
planetary science series, namely Venus (edited by Hunten
et al. 1983, written in the wake of Russian probes and
Pioneer Venus) and Venus II (Bougher et al. 1997, a mere
1362 pages compiled in the wake of Magellan) and an AGU
volume (Esposito et al. 2007). An excellent and readable
text is that by Marov and Grinspoon (1998). The best
comprehensive review of Venusian aeolian processes spe-
cifically is the chapter in Venus II by Greeley et al. (1997a).
14.2
History of Venus Exploration
Even before space probes revealed the details of surface
conditions on Venus, it was expected to be a windswept
place. Indeed, one early theory (Öpik 1961) even advanced
friction by wind-blown dust as being the reason that Venus'
surface might be hot, although this mechanism doesn't
stand up to thermodynamic scrutiny. As early spacecraft
data came in, it became obvious that the greenhouse effect
was responsible for elevating Venus' surface temperatures,
but until probes reached the surface in the 1970s, it was not
known how dense and hot the atmosphere really was, nor
which greenhouse gases might be responsible. One the-
ory—which formed the Ph.D. thesis of the later-prominent
climate scientist James Hansen—was that airborne dust
would provide greenhouse warming. In fact, we now know
1
Florensky et al. (1977) suggest on the basis of Venera 9 and 10
imaging that fines have been moved in the atmosphere, but there are no
obvious bedforms.
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