Terrestrial Surfaces

Solid, inorganic substances that make up
A list of the most important:
 Feldspars (K,Na,Ca)AlSi3O8
 Density: 2600-2800 kg/m3.
 Most common mineral in terrestrial surface rocks;
even more common on the moon.
 Tend to float in magmas and form surface crust
 Quartz (silica: SiO2):
 Density 2600 kg/m3.
 Even less dense than many feldspars
 One of the first to solidify from magma, after the
 Pyroxenes (Mg, Fe, Ca, Na, Al and Ti silicates):
 high density, 2800-3700 kg/m3.
 Common in meteorites and igneous rocks, but only
10% of Earth’s crust.
 Ampiboles and Micas:
 Like pyroxenes, but different crystal structure
 Slightly less dense
Beyond the asteroid belt, ice becomes a major “mineral”
constituent of terrestrial bodies.
Water ice
 Density 917 kg/m3 (at standard temperature and
 Dominates the surfaces of many moons and ring
 Main constituent of most comets
 Also present in the polar caps of inner planets (Mars
and Earth)
Carbon dioxide ice:
 Density 1560 kg/m3.
 Begins to sublime at about 10 AU from the Sun.
Ammonia ice:
 Density 817 kg/m3.
 Low melting point (195 K), so could be fluid or gas in
some slightly heated bodies
Methane ice:
 Low density, 415 kg/m3.
 Very volatile, low melting temperature. The first to
sublime in comets approaching the Sun.
 Only condenses on most distant bodies.
 Present on Pluto, Triton and Titan.
Other minerals
 Olivine:
 High density, 3300-4400 kg/m3.
 Sinks to the lowest level in a magma
 Probably a major component of all terrestrial
 Iron oxides:
 Related to ordinary rust
 Both magnetic and non-magnetic forms
 Responsible for the red soil on Mars
 Troilite (FeS):
 High density, 4600 kg/m3.
 Makes up 5-10% of chondrites
 Graphite (C):
 Low density, 2200 kg/m3.
 Responsible for the low albedo of asteroids and
comet nuclei in the outer solar system
 Clay minerals:
 Hydrous aluminum silicates
 Main component of the fine dust on Mars
Solids made of more than one mineral
mix of minerals in rocks varies from one part of the SS
to another and well as within a given body.
The types of rocks and minerals we find, then, are clues
to their history and the history of the SS.
Igneous rocks:
 once heated and subsequently cooled
 The rate of cooling and the temperature and
pressure conditions under which they cooled are
detectable in their crystalline structure.
 basalts have been found widely on the surfaces of
SS bodies.
 granitic rocks have a higher % of SiO3 and have
undergone several cycles of repeated heating and
sedimentary rocks:
 formed by deposits of fine grains due to water or
 Common on Earth where both water and atmosphere
are important. The only other place where they
have probably been found to date is the surface of
metamorphic rocks:
 altered slowly under conditions of high T and P
 Examples are marble (formerly limestone), slate
(shale) and gneiss (granite).
 So far identified only on Earth.
Crater formation
 The energy of impact is so great that it obliterates the
impactor and generates a high energy explosion.
 The formation of an impact crater involves an almost
instantaneous transfer of energy, in three stages:
 impact and compression: In such a hypervelocity
impact the energy is transferred to the surface
through a shock wave which spreads out in a
spherical pattern away from the point of impact.
During this very brief (few seconds or less) period
the rock behaves almost like a fluid.
 ejection and excavation: After the shock wave has
passed the rock relaxes and a rarefaction wave
(essentially a reflection of the original shock wave)
travels upward carrying rock with it. The rock is
ejected, forming the crater floor and rim.
Depending on the energies involved, some of the
ejecta will carry well beyond the crater in “rays”
which we see especially around young craters.
Material ejected directly upward will often fall back,
creating a central peak.
 collapse and modification: As time passes the crater
walls may slump forming terraces, the floor may
experience additional rebounds from secondary
impacts, and the crust generally re-establishes
itself over time.
 Typically the diameter of a crater is D  E 1 / 2 and this
translates into crater diameter ~10x the impactor’s
Craters and surface age
 An impact remnant will be produced only if the surface
and underlying layers are rigid enough to transmit the
shock wave and then rebound.
 The longer the surface has been rigid, the greater the
number of impact craters we will see. So, potentially, we
can use crater numbers to tell us surface age
 For the moon, we see evidence for periods of heavy
bombardment at 4.3Gyr and 3.9Gyr ago, after which the
rate drops substantially.
 This makes sense in the context of our model of SS
formation in which we expect there to be large
numbers of planetesimals left over after the
creation of the major planets and satellites.
 dynamical models suggest that collisions are a major
reason we do not see them today.
 Since we expect all SS bodies to have suffered the
same bombardment during the early stages of the
SS, we can use this lunar profile to estimate surface
ages for other SS bodies. This will provide us with
at least relative ages which give some idea of when a
SS member cooled enough to have a crust rigid
enough to retain impact evidence.
Measuring surface ages
Atmospheric effects and surface evolution will destroy
craters of different sizes.
Atmospheres can also have an effect by preferentially
destroying small meteoroids and limiting the impact
evidence to larger craters.
This can be taken into account by plotting cumulative
crater density vs crater size. For the Moon, such a plot
shows near saturation numbers of craters at all
diameters. By contrast, Venus has almost no craters
smaller than a few km and the numbers of large craters
are well below saturation level, indicative of both a
younger surface age than the Moon and greater surface
We also expect that the number of impacts will be
different for different bodies depending on their mass
and closeness to other massive bodies – i.e gravitational
focussing. This effect is modeled and taken into account
when using crater density data to estimate surface age.