Abstract
Venusian canali, outflow channels, and associated volcanic deposits
resemble fluvial landforms more than they resemble volcanic features
on Earth and Mars. Some canali have meandering habits and features
indicative of channel migration that are very similar to meandering
river channels and flood plains on Earth, venusian outflow channels
closely resemble water-carved outflow channels on Mars and the Channeled
Scabland in Washington, collapsed terrains at the sources of some
venusian channels resemble chaotic terrains at the sources of martian
outflow channels, venusian lava deltas are similar to bird's-foot
deltas such as the Mississippi delta, and venusian valley networks
indicate sapping. The depositional fluvial-type features (deltas,
braided bars, and channeled plains) are generally among the smoothest
terrains at the Magellan radar wavelength (12.6 cm) on Venus. These
features suggest the involvement of an unusual lava, unexpected processes,
and/or extraordinary eruption conditions. Possibly the lava was an
ordinary silicate lava such as basalt or a less common type of silicate
lava, and conditions unique to Venus or to those particular eruptions
may have caused an unusual volcanological behavior. We have developed
the alternative possibility that the lava had a water-like theology
and a melting point slightly greater than Venus' surface temperature,
thus accounting for the unusual behavior of the lava. Unlike silicate
lavas, some carbonatites (including carbonate-sulfate-rich liquids)
have these properties; thus they can flow great distances while retaining
a high fluidity, significant mechanical erosiveness, and substantial
capacity to transport and deposit sediment. Venusian geochemistry
and petrology are consistent with extensive eruptions of carbonatite
lavas, which could have crustal and/or mantle origins. Venus' atmosphere
(especially CO2, HCl, and HF abundances) and rocks may be in local
chemical equilibrium, which suggests that the upper crust contains
large amounts of calcite, anhydrite, and other salts. Chemical analyses
indicate, according to some models, that Venusian rocks may contain
4-19% calcite and anhydrite. Mixtures of crustal salts could melt
at temperatures a few tens to a few hundred Kelvins higher than Venus'
surface temperature; hence, melting may be induced by modest endogenetic
or impact heating. Salts may have many of the same geologic roles
on Venus as water and ice have on Mars. A molten salt (carbonatite)
''aquifer'' may exist beneath a few hundred meters to several kilometers
of solidified salt-rich ''permafrost.'' Many geologic features can
be explained by carbonatite magmatism: (1) impact melting of crustal
salts can explain crater outflows, (2) small, sustained eruptions
from molten salt aquifers can explain sapping valleys, (3) large,
sustained eruptions may explain canali and their flood plans, and
(4) catastrophic outbursts may have formed outflow channels and chaotic
terrain. Landforms created by carbonate-rich lavas would be thermally
stable on Venus' surface, though some minerals may weather to other
solid substances. (C) 1994 Academic Press, Inc.
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