Geoscience Reference
In-Depth Information
Whether that resurfacing was a global cataclysm *500 Myr
ago, or was a more protracted process that yields this effective
age is still under debate, but there seems little doubt that the
Venusian surface is dominated by basalt. Without liquid
water on the surface, more evolved igneous rocks are not
produced (although perhaps outcrops of such, formed early in
Venus' history, may be present).
In the absence of fluvial processes (or glacial abrasion, or
freeze-thaw), it is hard to imagine how to make much sand.
Similarly, the high atmospheric pressure on Venus prevents
gas in magmas from expanding dramatically to produce
Strombolian eruptions that erupt a lot of fine ash—most of
the lava will have simply emerged from the ground and
flowed. Some chemical weathering processes may produce
some fines, and impact ejecta also produces fine particles
(dark parabolic haloes around some impact craters may be
radar-dark because they are made of smooth deposits of
fine-grained material—these appear around the apparently
youngest craters).
Regardless of how sand may be produced, there are
questions about how materials behave under unfamiliar
conditions (questions much like those which confront
Titan). Specifically, is basalt at 500 C hard enough to
behave as sand?
Marshall et al. (1991) studied the adhesion of basalt
particles at a range of temperatures, noting that rocks may
be appreciably softer at Venus surface temperatures. A
small jet of air, pulsed by a valve at roughly once a second,
was used to propel *3 mm angular particles of rock at a
polished target slab at known speeds inside a chamber.
They found that at temperatures below 440 K, abrasion
occurred as expected. Results were highly variable between
440 and 570 K: above 570 K particles almost always
accreted (i.e., welded themselves onto the target). This
adhesion threshold of about 500 K is roughly 40 % of the
melting temperature and is higher than temperatures even
on Venus' highest coolest spot (Maxwell Montes, *650 K)
and so grain adhesion may well be an important aspect of
aeolian processes on Venus.
Fig. 14.9
Wind tunnel data for Venus conditions (gravity is 'faked'
by using lower density rock, although the effect is in any case small)
showing the optimum size for saltation. The experiments suggest a
slightly higher threshold than theory would predict. This friction speed
is an order of magnitude lower than that required on Earth. Data from
Greeley and Iverson (1987)
slopes themselves are typically only *0.3. They also
argue that a Hadley-type flow may control many of the wind
directions inferred from streaks, although their maps of
streak orientation do not make this obvious. Most likely
local terrain may modify directions substantially.
There are sadly very few surface windspeed measure-
ments, only those by Venera 9 and 10 in 1976. These car-
ried cup anemometers able to operate at 500 C. The
Venera 9 wind record spans 49 min (Avduevsky et al.,
1977) and that of Venera 10 only 90 s, sampled at a 0.4 Hz
frequency with a resolution of the order of *0.2 m/s. The
Venera 9 and 10 records have been characterized as having
a means of 0.4 and 0.9 m/s respectively, with standard
deviations of 0.1 and 0.15 m/s (Golitsyn 1978). Surface
winds were also estimated from the intensity of sounds
recorded by the Groza microphone on Venera 13 and 14 to
be in a similar range (Ksanfomality et al. 1983). These data
do seem to show winds within a factor of 2 or so of the
saltation threshold, even for a small number of observa-
tions, so aeolian transport may be common. However, more
data are sorely needed.
14.8
Future Exploration of Venus
Post-Magellan exploration of Venus has been dominated by
atmospheric studies from two missions, Venus Express and
Akatsuki. Venus Express is a mission launched by the
European Space Agency in 2005, and has been operating at
Venus since 2006. However, its instrumentation (with the
exception of a handful of isolated radio science and infrared
observations) does not address the surface. Akatsuki (for-
merly 'Venus Climate Orbiter' or 'Planet-C') was launched
successfully in 2010, but suffered an engine failure during
14.7
Venus Sand
The expectation, given that Venus' relatively low population
of craters suggests it has been resurfaced—principally by
volcanism—is to give an effective surface age of *500 Myr.
Search WWH ::
Custom Search