Venus: Surface and Interior 159
Variations in elemental abundance do suggest that some
real differences exist. The bulk composition of Venus can
be extrapolated from these measures. Within the uncertain-
ties, the composition is similar to that of Earth. Similarly
available data on Fe/Mg and Fe/Mn suggest that the core
composition is similar to Earth’s. Some variation may occur
after the rock forms. For example, the amount of Al, Ti, Ca,
or Si may change through chemical weathering or meta-
morphism when the rock experiences changes in pressure
and/or temperature.
The initial chemical measurements of the surface have
provided invaluable constraints on the surface composition.
However, the overall number and geographic diversity of
sites remains limited. The precision of the measurements
that were possible with instrumentation built in the 1970s is
very low compared with measurements possible today. The
uncertainties in the measurements mean that numerous
questions such as the size of the core (which is constrained
by the ratios of Fe/Mn/Mg) and the amount of crustal re-
cycling cannot be addressed. In fact, the uncertainties in
the Venusian measurements are so large that they encom-
pass the entire range of composition for basalts on Earth,
Mars, the Moon, and meteorites. In contrast, basalts from
the Moon and Mars (as represented in meteorites) have a
distinct chemical signature from Earth. [SeeMeteorites.]
These variations represent key differences in the formation
and evolution of these bodies, such as the formation of a
magma ocean on the Moon.
In addition to direct measurements of the composi-
tion, morphology can be used as a very crude indication
of composition. For example, lavas with a basaltic compo-
sition tend to be very fluid, forming long, narrow flows and
broad, low volcanoes. As the silica content increases, the
viscosity of the lava increases. The thicknesses of flows in-
crease, their lengths decrease, and the slopes of volcanoes
formed increases. Terrestrial examples are Mauna Loa in
Hawaii (basaltic) and Mt. St. Helens in Washington (more
silica-rich). On Venus, the morphology of flows is generally
consistent with low-viscosity, basaltic compositions. There
are some features that appear to represent much thicker,
shorter flows (see description of “pancakes” in Section 6).
However, these morphologies cannot be considered diag-
nostic of composition as factors such as the volume and
rate of material erupting, the atmospheric pressure during
eruption, and the amount of gas in the lava also shape the
morphology of the flow.
5.2 Surface Weathering
Although weathering of the surface by wind is relatively
mild on Venus as compared to Earth, the environment for
chemical weathering is extremely harsh. In addition to the
searing temperature and high pressure, the atmosphere
contains highly corrosive and chemically active gases such
as SO 2 (sulfuric acid), CO, OCS, HCl (hydrochloric acid)
and CO 2. A variety of minerals form in laboratory experi-
ments that simulate Venus conditions, such as wollastonite,
anhydrite, and hematite, but no landers have measured ac-
tual minerals. Measurement of the specific minerals present
and their abundances is highly desirable as they provide in-
sight into the nature of the chemical interaction between
the surface and the atmosphere. This information is a criti-
cal piece of understanding the larger problem of how Venus
arrived at the hellish climate that now exists.
One of the key questions is how much CO 2 is trapped as
carbonates on the surface of Venus. Most of the CO 2 found
on Earth is trapped as carbonates via biological processes,
specifically the formation and accumulation of seashells.
This process is an important element of the overall balance
that makes Earth habitable. Available information from sur-
face composition and laboratory experiments suggests that
significant amounts of carbonates could be present on the
surface of Venus, perhaps up to 10%. If so, this would mean
that CO 2 in surface rocks is an important part of determin-
ing the atmospheric pressure and composition. Another key
question is how atmospheric SO 2 interacts with the sur-
face. On Earth, most of the SO 2 is dissolved in the oceans.
Rates of chemical reactions involving SO 2 are known for
the conditions in the atmosphere of Venus and predict that
the SO 2 in the present-day sulfuric acid clouds on Venus
should react with other chemicals and disappear over time.
This analysis indicates that SO 2 should disappear from the
atmosphere within 2 Ma. The fact that sulfuric acid clouds
are present today implies that new sulfur gases have been
added to the atmosphere with this time by volcanic erup-
tions. Other important measurements for understanding
the surface–atmosphere interactions are the oxidation state
of iron minerals and minerals that react with hydrogen chlo-
ride (HCl) or hydrogen fluoride.6. Volcanism
With the exception of Jupiter’s moon Io, Venus is the most
volcanic world in the solar system. Volcanic features of a
broad range in morphology cover the surface, from sheet-
like expanses of lava flows to volcanoes shaped like pancakes
and ticks, as illustrated later. The high surface tempera-
ture and pressure on Venus make explosive volcanism less
likely, though some possible deposits produced by explo-
sive volcanism have been mapped. The extreme conditions
on Venus also result in volcanoes that tend to be taller and
broader than those on Earth or Mars.Magellandata il-
lustrated that volcanic features do not occur in chains or
specific patterns, indicating the lack of plate tectonics on
Venus.
The plains or low-lying regions on Venus are covered by
sheet and digitate deposits that are interpreted to be vol-
canic in origin (Fig. 8). These extensive deposits are likely
to be flood basalts, formed in similar ways to the Columbia