Geology Reference
In-Depth Information
Figure 6.11. This impact structure, named Lilian, is in the Guinevere
region and consists of four smaller crater-forms formed from the
breakup of a bolide as it passed through the dense Venus
atmosphere (NASA Magellan P-38290).
Figure 6.12. A Magellan image of a radar-dark
left by an
object that broke apart before reaching the surface; the area shown
is 290 km across (NASA Magellan C1 MIDR 45N350).
“
scar
”
indicative of basaltic compositions. Radar backscatter prop-
erties (the dark appearance on radar images) and views from
the landers
(Fig. 6.2)
suggest that some plains are mantled
with clastic materials, which could be windblown sediments,
ejecta deposits, basaltic materials from in situ weathering, or
some combination of these materials.
Most of the volcanic plains have no obvious vents, but a
few distinctive depressions with sinuous channels are
thought to be eruption sites
(Fig. 6.14)
. Plains units
show distinctive lobate
flows of volcanic origin and dis-
play differences in radar brightness
(Fig. 6.15)
that are
attributed to differences in surface textures, such as pahoe-
hoe, aa, or block
ows.
How were the vast lava plains emplaced? Although
hints of volcanic channels on Venus were seen on
Venera 15 and 16 images, the Magellan mission showed
these features in detail
(Figs. 6.14
and
6.16)
. More than
200 channels have been identi
ed, some of which are as
long as 6,800 km. Such a great length requires that the lava
remained molten for a substantial length of time. At
rst,
we might think that the high surface temperatures would
allow the
flows to retain heat, but, as discussed in the
analysis by Head and Wilson (
1986
), this effect would
be offset by the dense atmosphere that would conduct heat
ef
ciently away from the surface. In attempts to resolve
the issue, lavas other than basalt have been considered,
One consequence of the high surface temperature is the
formation of impact melt deposits as ejecta
(Fig. 6.13)
.
These have the appearance of lava
flows and are found in
association with many impact craters on Venus.
6.5.3 Volcanic features
The surface of Venus is dominated by volcanic and tec-
tonic features re
ecting an extensive history of internal
processes. Volcanic features include vast lava plains,
cones, domes, and huge shield structures (Head et al.,
1992
). Major volcanic provinces include Western Eistla,
Atla Regio, Beta Regio, and Bell Regio, all of which are
topographically high and have been modi
ed by exten-
sional tectonism. While most of the volcanic features are
probably basaltic, given their morphology and composi-
tional information, some volcanoes might include more
silica-rich lavas.
Many planetary geologists have studied and mapped the
venusian plains. Unfortunately, some of the terms and
descriptions are not used consistently, and the different cat-
egories overlap. For simplicity, the volcanic plains form
at,
smooth surfaces that are radar-dark and are found mostly in
the lowlands and rolling uplands. Ve n e r a 9 , 10 and 13 and
Ve g a 1 and 2 all landed on plains deposits and returned data
