Introduction to Planetary Science

mars: the little planet that could 213
several large impact basins and volcanic centers.
The difference in the crater density indicates that
the surface of the southern hemisphere of Mars is
older than the surface of the northern hemisphere.
The technical terms used to describe the
topographic features on Mars are similar to the
terms used on Venus. For example, a planitia is a
low-lying plain, whereas a planum is an elevated
plain, mountains are called mons (singular) or
montes (plural), valleys are referred to as vallis
(singular) or valles (plural). A fossa on Mars
is a rift valley (i.e., a graben) and a chasma is
a canyon. In addition, the topography of some
areas on Mars is described by Latin phrases
such as Vastitas Borealis (northern vastness) and
Noctis Labyrinthus (labyrinth of the night).
Mars is a much smaller planet than the Earth
because its radius is only 3398 km compared to
6378 km for the radius of the Earth, and the
mass of Mars 642 × 10 23 kg is about one tenth
the mass of the Earth 5978 × 10 24 kg. Nevertheless,
the former presence of liquid water on
the surface of Mars means that it could have
harbored life in the past and that it could be
colonized in the future by humans from the Earth.
In this sense, Mars is the little planet that “could.”
The on-going exploration of the surface of Mars
has transformed it from a point of light studied by
astronomers into a world to be explored by geologists.
The geology and history of Mars have been
described in several excellent books including
those by Moore (1977), Greeley (1985), Kieffer
et al. (1992), Carr (1996), Sheehan (1996),
Sheehan and O’Meara (2001), Morton (2001),
Hartmann (2003), Kargel (2004), Hanlon (2004),
De Goursac (2004), and Tokano (2005). Each
of these books contains references to additional
publications about our favorite planet as well as
numerous well-chosen images of the landscape.
12.1 Origin and Properties
Mars formed at the same time, from the same
chemical elements, and by the same process as
all of the planets and their satellites of the solar
system. Although the details may vary depending
on the distance from the Sun and the amount of
mass of a particular body, the theory of the origin
of the solar system provides enough guidance to
allow us to reconstruct the sequence of events
that led to the accumulation and internal differentiation
of each of the planets, including Mars.
12.1.1 Origin by Accretion
of Planetesimals
Mars accreted from planetesimals within the
protoplanetary disk at a distance of 1.52 AU from
the Sun. Consequently, it may have contained
a greater abundance of volatile elements and
compounds than Mercury, Venus, and Earth.
Like the other terrestrial planets of the solar
system, Mars heated up because of the energy
that was released by the impacts of planetesimals
and by decay of unstable atoms of uranium,
thorium, potassium, and others. As a result, Mars
was initially molten and differentiated by the
gravitational segregation of immiscible liquids
into a core composed primarily of metallic iron,
a magma ocean consisting of molten silicates,
and a sulfide liquid that may have been trapped
within the core of liquid iron and within the
magma ocean. The volatile compounds (H 2O,
CO 2,CH 4,NH 3, and N 2) were released during
the impacts of the planetesimals and formed a
dense primordial atmosphere.
As the magma ocean crystallized, an anorthositic
crust may have formed by plagioclase
crystals that floated in the residual magma.
However, this primitive crust has not yet been
recognized on Mars, perhaps because it was
deeply buried by ejecta during the period of
intense bombardment between 4.6 and 3.8 Ga.
The present crust of Mars is composed of lava
flows interbedded with sedimentary rocks and
ejecta deposits derived from large impact basins.
The slow cooling of the magma ocean also
caused the crystallization of olivine, pyroxene,
and other refractory minerals which sank in the
residual magma and thus formed the mantle
composed of ultramafic rocks (e.g., peridotite).
The present mantle of Mars is solid and is
divisible into an asthenosphere and a lithosphere
on the basis of the temperature-dependent
mechanical properties of the ultramafic rocks.
The presence of large shield volcanoes in the
northern as well as the southern hemisphere of
Mars suggests that its asthenosphere convected
by means of plumes that rose from depth until
they impinged on the underside of the lithosphere.
The basalt magmas that formed by