Chemistry of Venus’ Recent Basalts as Clues to its Ancient Past

Perhaps the most important question
about Venus is whether it ever had a hydrosphere: liquid
water, oceans, and thus an environment suitable for life
as we know it. The D/H of Venus’ atmosphere
suggests extensive water loss, and climate models
may be consistent with global oceans. Rocks
dating from that epoch may be preserved in Venus’
tesserae (and especially Ishtar Terra) – inferred to of
felsic or silicic rock, which would suggest abundant
water. However, the exact elemental compositions
of tesserae (silicic or not) will be difficult to retrieve and
most of Venus’ surface is basalt flows and volcanic
constructs, inferred to be much younger than the oceanic
epoch. Remotely sensed data are (and will be)
ambiguous about the specific rock types that make up
tesserae, and lander spacecraft are almost certain
(in the near term) to avoid the rough and precipitous
topography of the tesserae and touch down instead on
flat, safe basaltic plains or volcanic rises.
‘Recent’ basalts at Venus’ surface could preserve
chemical tracers of an ancient aqueous past, if their
source regions (material that was re-melted) had been
affected by water. This scenario occurs on Earth in
Island Arc Basalts (IAB), where their compositions are
thought to reflect aqueous alteration of parental midocean
ridge basalts (MORB) and incorporation of
oceanic sediments. Ancient Venus might have
supported plate tectonic and thus have had IAB
equivalents; absent plate tectonics, basalt affected by
aqueous alteration could have been cycled into its
mantle by burial under thick sections of later basalt.
Our purpose is to suggest specific chemical clues in
current basalts that would permit recognition of those
with a history of aqueous interactions from those that
did not. We compare Earth’s Ocean Island Basalts
(OIB) that involved little water with IAB that show
chemical effects ascribable to water. It is not clear that
Venus’ basalts can be mapped into terrestrial tectonic
settings; however, it seems reasonable that aqueous
geochemical processes could have (had) similar effects
on both planets.