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the
incidence
angle
of
the
radar
observation
was
22-25.
Small dunes, however, might be much more widespread
than the fields of resolved dunes above. In many areas
where no resolvable dunes were detected, it was noted that
the radar echo had a substantially different strength when
the area was observed from one side versus another. One
explanation of this radar asymmetry is that unresolved
'microdunes' may be present (Weitz et al. 1994; Kreslavsky
and Vdovichenko 1998), wherein the asymmetry is due to
shallow stoss slopes being prominent from one direction
against the steeper slip faces seen from the other (Fig.
14.6
).
A possible additional factor in shaping the radar reflectivity
of microdunes is that wind tunnel experiments (see later)
show that in a sand of mixed composition, dense (and
therefore radar-reflective) minerals such as pyrite or chro-
mite become concentrated on the windward (stoss) side of
the microdune (Greeley et al. 1991a).
A significant reason for the lack of observed dunes may
be a planetwide paucity of sediment. The Venusian surface
may have been completely resurfaced by lava flows about
500 Myr ago, and the kinetics of breakdown of basalt by the
Venusian atmosphere is uncertain (and of course, other
sand-generating processes such as freeze-thaw, glacial
action or fluvial erosion do not occur on present-day
Venus). Indeed, the dominant source of sand-sized sediment
may be the ejecta from impact craters. Garvin (1990) cal-
culated that impacts could produce enough fine-grained
materials (\1 mm) to form at most a globally-averaged
layer 1 m thick. Note that the Fortuna and Algaonice
dunefields likely account for only about 1000 km
3
Fig. 14.1
Venus in ultraviolet light, observed in 1972 by Mariner 10.
At visible wavelengths (like Titan) Venus appears nearly featureless
because of its thick, cloudy atmosphere. Credit NASA
Greeley et al. 1995), although wind streaks and downwind
dispersal of impact ejecta were observed in many locations,
and some possible yardangs were also identified.
14.3
Venus Dunes
of sedi-
ment in total.
The two prominent dune fields identified on Venus are
Algaonice (Fig.
14.4
) and Fortuna-Meshkenet (Greeley
et al. 1992) (Fig.
14.5
). The Algaonice dunes at 25S,
340E cover some 1300 km
2
(about the same size as the
Lencois Maranhenses dunefield in Brazil) at the end of the
ejecta outflow channel from the impact crater of that name
(the dunes themselves were subsequently formally named
Menat Undae, although the Algaonice name seems to have
been more widely used). The dunes are 0.5-5 km in length
and are quite bright, likely because there are slip faces
oriented towards the radar illumination, which was at an
incidence of 35.
The more northern dune field, Fortuna-Meshkenet lies at
67N, 91E in a valley between Ishtar Terra and Meshkenet
Tessera (the dunes are formally named Al-Uzza Undae).
The dunes are 0.5-10 km long, 0.2-0.5 km wide and spaced
by an average of 0.5 km. They appear to be transverse
dunes, in that there are several bright wind streaks visible in
the region, which seem generally orthogonal to the dunes.
Glints are not observed strongly on these dunes, although
14.4
Aeolian Transport Under Venus
Conditions: Experiments
The conditions on the surface of Venus are far from com-
mon terrestrial experience (although somewhat similar
conditions are generated in certain industrial processes).
The three major factors (surface gravitational acceleration
at 8.79 ms
-2
being not too different from the terrestrial
9.81 ms
-2
) are the 90 bar pressure, the 750 K temperature,
and the principal gas being CO
2
. One reason these features
were interpreted to be microdunes rather than ripples is that
(Figs.
14.7
and
14.8
).
Just above the saltation threshold, at 0.63 m/s, *18 cm
long dunes with slip faces developed. The wavelength
became shorter as windspeed was increased, reaching 8 cm
for a windspeed of 1.07 m/s. The dunes became more
degraded
at
this
point,
with
the
slip
faces
tending
to
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