Venus - Introduction to Planetary Geomorphology - page 122

Figure 6.26. Complex-ridged terrain in Ovda
Regio, showing the square blocks of tesserae
(arrows) that are highly fractured. The dark band
cutting through the terrain is a younger graben
partly filled with smooth-surfaced lava; the area
shown is 225 km by 150 km (NASA Magellan
P-37788).

fractured terrain, clastic materials produced from pyroclas-
tic eruptions and tectonic “ grinding ” of crustal rocks also
generate smaller fragments.
As clastic materials are generated and reduced in size,
additional weathering of materials would occur in the
presence of the high temperatures and sulfuric acid envi-
ronment. Coupled with in situ weathering of bedrock,
the resulting fine-grained debris could account for the
generally low radar backscatter signatures seen in many
areas of Venus.
Once produced, clastic materials generated either by
primary processes or from weathering are subject to
agents of transportation through wind and gravity. Mass
movement in the form of landslides is seen in several
areas where steep slopes occur (Fig. 6.27) . These occur
on a wide range of scales and involve mostly rock ava-
lanches, some of which are surrounded by radar-dark
surfaces interpreted to be fine-grained materials generated
from the mass movement.
In the present surface environment, wind appears to
be the primary means for redistributing surface materials
on Venus. Surface winds are very sluggish, due in part to
the high density of the atmosphere; they were measured by
the Soviet Veg a landers and NASA Pioneer Venus probes
to be about 0.5

Figure 6.27. This fan-shaped landslide mass (arrow) is about 25 km
long and originated from a volcano. The radar-bright mass indicates
rugged, blocky topography (NASA Magellan F-MIDR 75N327).

2m/s. But, because the atmosphere is so
dense, winds need not be moving very fast to entrain sand
and dust ( Fig. 3.36) , and the measured winds are well
within the range predicted for particle transport by the wind.
Fields of sand dunes were identi ed in Magellan radar
images in two areas. The Aglaonice field covers 1,300 km 2
and is associated with ejecta deposits from the impact crater

-

and the formation of gradation features. Physical weath-
ering to produce loose rocks and small grains (Fig. 6.2)
results from a variety of processes, including impacts
( Fig. 4.7) . For example, NASA scientist Jim Garvin esti-
mates that the amount of impact-generated debris on Venus
would produce a layer 0.3 - 1m thick if spread evenly over
the entire planet. Given the extensive volcanic features and