Encyclopedia of the Solar System 2nd ed

158 Encyclopedia of the Solar System

In contrast, Mars, Mercury, and the Moon have surfaces
that preserve the large impact basins from early bombard-
ment and reflect a gradual loss of heat and decline in geo-
logic activity. Some models have proposed that resurfacing
on Venus occurs episodically. In one scenario, the litho-
sphere thickens and becomes denser due to both cooling
and chemical phase transitions. The lithosphere is predicted
to founder, or get mixed into the mantle, when it becomes
gravitationally unstable. However, how the lithosphere
actually breaks and initiates this process is unclear. In an-
other scenario, the stagnant lid heat insulates the mantle,
causing it to heat up to the point that widespread melting
occurs, eventually erupting on the surface. Other models
show that volcanism that is globally distributed and resur-
faces small regions in each event can produce the observed
distribution. High mantle temperatures could facilitate this
kind of widespread volcanism.

5. Composition

5.1 Global Implications

The similarity between Venus and Earth in terms of size and
location in the solar system indicates that their bulk compo-
sitions should be comparable. The exact composition of the
crust is related to the composition and temperature in the
interior of the planet when the rock melts, as well how much
of the original rock is melted. The typical rock type that
forms on Earth when the interior melts and erupts is basalt.
Thus, it is not surprising that geochemical measurements
on the surface of Venus have a gross composition similar
to terrestrial basalts, with some variation. On Earth, basalts
make up the majority of the oceanic crust and are found
in volcanic regions of continents. When processes such as
subduction remelt basalts, the resulting rocks are enriched
in silica (SiO 2 ). Continental rocks are a result of billions of
years of remelting of a basaltic crust driven by convective
and plate tectonic processes. They are lower density than
basalt due to the enrichment of silica relative to iron and
magnesium. The presence of at least small amounts of water
may be essential to the formation of such silica-rich rocks.
Continents stand higher than the oceanic crust due to both
their lower density and the greater thickness of continen-
tal crust. As we will discuss, there is limited evidence for
silica-rich rock on Venus.
The abundances of primary mineral-forming and radio-
genic elements were measured by spectrometers onVenera
landers.Venera 8, 9 , and 10 andVega 1and 2 landers mea-
sured the amounts of uranium (U), thorium (Th), and potas-
sium (K) using a gamma ray spectrometer (Table 1). The
Venera 13and 14 landers measured these elements as well
the major–element forming minerals (see Table 2). Due to
the orbital dynamics of delivering probes to the surface of
Venus in any given time period,Venera 8–14landers are

TABLE 1 Abundances of Primary

Mineral-Forming and Radiogenic

Elementsa

Lander U (ppm) Th (ppm) K (weight percent)

Venera 8 2.2±0.7 6.5±0.2 4.0±1.2

Venera 9 0.6±0.2 3.6±0.4 0.5±0.1

Venera 10 0.5±0.3 0.7±0.3 0.3±0.2

Vega 1 0.68±0.47 1.5±1.2 0.5±0.3

Vega 2 0.68±0.38 2.0±1.0 0.4±0.2

aThe abundances for each element represent an interpretation of the

most likely minerals on the surface of Venus, as is standard practice in

geochemical analysis.

all located in a relatively small region on Venus within 270–

330 ◦E and 15◦S–30◦N. This area includes the eastern flank

of Beta Regio, a major hot spot, and the plains to the east

of Beta and Pheobe Regiones. TheVega 1and 2 landers,

sent at an earlier time, are located near 170◦E, 10◦N and

180 ◦E, 10◦S to the west of Atla Regio.

The silica content and the relative abundances of iron

and magnesium for rocks at theVeneralander sites (Ta-

ble 1) are characteristic of basalt. Although some variations

in composition do exist, when the overall abundance of el-

ements is considered in the context of minerals that occur

stably together, all the rock compositions are consistent with

a basaltic composition. Early analysis of the relatively high

value of U, Th, and K atVenera 8suggested that this loca-

tion was composed of a more silica-rich rock, possibly even

granite. However, subsequent analyses have discounted

this idea and concluded that theVenera 8site is mostly

basaltic, although it has more silica than other lander sites.

TABLE 2 Elements and Major–Element Forming

Minerals Measured by theVenera 13and

14 Landersa

Constituent Venera 13 Venera 14 Vega 2

SiO 2 45.1±3.0 48.7±3.6 45.6±3.2

TiO 2 1.59±0.45 1.25±0.41 0.2±10.1

Al 203 15.8±3.0 17.9±2.6 16.0±1.8

FeO 9.3±2.2 8.8±1.8 7.74±1.1

MnO 0.2±0.1 0.16±0.08 0.14±0.12

MgO 11.4±6.2 8.1±3.3 11.5±3.7

CaO 7.1±0.96 10.3±1.2 7/5±0.7

K 2 O 4.0±0.63 0.2±0.07 0.1±0.08

S 0.65±0.4 0.35±0.31 1.9±0.6

Cl <0.3 <0.4 <0.3

aValues are in weight percent. The raw data is converted from

measurements of elemental abundance into likely chemical

combinations.