Mercury 131
significantly less than many of the surfaces of the Moon and
other terrestrial planets. Earth-based microwave and mid-
infrared observations also indicate that Mercury’s surface
has less FeO plus TiO 2 , and at least as muchfeldsparas
the lunar highlands. This has been interpreted as indicat-
ing that Mercury’s surface is largely devoid of basalt, but it
could also mean that the basalts only have a low iron content
or are fluid sodium-rich basalts. It has been suggested that
eruption of highly differentiated basaltic magma may have
produced alkaline lavas. On Earth there are low viscosity
alkali basalts that could produce the type of volcanic mor-
phology represented by Mercury’s plains. Mercury could
be the only body in the inner solar system that has not
experienced substantial high-iron basaltic volcanism and,
therefore, may have undergone a crustal petrologic evolu-
tion different from other terrestrial planets.
In summary, both Earth-based spectroscopic observa-
tions and calibratedMariner 10images indicate that the
surface composition of Mercury has a varied composition
with a wide range of SiO 2 content. The FeO content appears
to be between 1 and 3%. This is abnormally low compared
to other terrestrial planets and the Moon. There is spec-
trographic evidence for the Mg-rich mineral pyroxene. The
spectroscopic data are consistent with compositions rang-
ing from low-iron basalts toanorthosites.We will have
to await theMESSENGERmission data to discover the
detailed composition of Mercury and its variation across
the surface.
6.3 Tectonic Framework
No other planet or satellite in the solar system has a tec-
tonic framework like Mercury’s. It consists of a system of
contractional thrust faults called lobate scarps (Figs. 16 and
17). Individual scarps vary in length from∼20 to>500 km
and have heights from a few 100 m to about 3 km. They have
a random spatial and azimuthal distribution over the imaged
half of the planet and presumably occur on a global scale.
Thus, at least in its latest history, Mercury was subjected
to global contractional stresses. The only occurrences of
features indicative of extensional stresses are localized frac-
tures associated with the floor of the Caloris Basin and at
its antipode, both of which are the direct or indirect result
of the Caloris impact. No lobate scarps have been embayed
by intercrater plains on the region viewed byMariner 10,
and they transect fresh as well as degraded craters. Few
craters are superimposed on the scarps. Therefore, the sys-
tem of thrust faults appears to postdate the formation of
intercrater plains and to have been formed relatively late in
Mercurian history. This tectonic framework was probably
caused by crustal shortening resulting from a decrease in the
planet radius due to cooling of the planet. The amount of ra-
dius decrease is estimated to have been anywhere between
0.5 and 2 km.
FIGURE 16 Photomosaic of Discovery scarp. This lobate scarp
is a thrust fault about 1 km high and 500 km long. It cuts across
two craters 55 and 35 km in diameter. (Courtesy NASA.)Also there is apparently a system of structural lineaments
consisting of ridges, troughs, and linear crater rims that
have at least three preferred orientations trending in north-
east, northwest, and north–south directions. The Moon also
shows a similar lineament system. The Mercurian system
has been attributed to modifications of ancient linear crustal
joints formed in response to stresses induced by tidal spin
down.6.4 Thermal History
All thermal history models of planets depend on compo-
sitional assumptions, such as the abundance of uranium,
thorium, and potassium in the planet. Since our knowledge
of the composition of Mercury is so poor, these models can
only provide a general idea of the thermal history for cer-
tain starting assumptions. Nevertheless, they are useful in
providing insights into possible modes and consequences
of thermal evolution. Starting from initially molten condi-
tions for Mercury, thermal history models with from 0.2 to
5% sulfur in the core indicate that the total amount of plan-
etary radius decrease due to cooling is from∼6to10km
depending on the amount of sulfur (Fig. 18). About 6 km of
this contraction is solely due to mantle cooling during about
the first 700 million years before the start of inner core for-
mation. The amount of radius decrease due to inner core
formation alone is about 1 km for 5% sulfur and about 4 km
for 0.2% sulfur.
Thermal models suggest that inner core formation may
have begun about 3 billion years ago, and, therefore, after