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The Sun and Planets
Lecture Notes 3.
Spring Semester 2017
Prof Dr Ravit Helled
Lecture 3
Planetary Geology
Questions
1. What are the physical processes that shape terrestrial planets?
2. What causes the differences between the terrestrial planets?
Planetary Geology
Planetary geology is the discipline concerned with the geology of planetary objects. The
geological features that we see on the surface of planet’s are actually related to the processes taking place in the planet’s deep interior.
The Geology of Earth
Inside the Earth, pressure and temperature increases as you go deeper, eventually reaching
3.2 million bar and 5000 K in the inner core. The most detailed information about the
interior of the Earth comes from seismic waves. These seismic waves are vibrations that
travel both through the interior and along the surface after earthquakes.
Layering by density: All terrestrial worlds have layered interiors. The layered structure
of the terrestrial worlds tells us that, in the past, these worlds must have been hot enough
in their interior for rock and metal to melt and separate by density. The three density
layers in the Earth are:
Crust
The highest density material, which consists primarily of metals (nickel, iron)
resides in the central core.
Mantle Rocky material of moderate density, made mostly of minerals that contain
silicon, oxygen, and other elements, that forms the thick mantle that
surrounds the core.
Core
The lowest density rock, such as granite and basalt, that forms the thin crust
covering the Earth.
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Relative to solar abundances, the Earth is depleted in hydrogen and helium.
Layer
Crust
Mantle
Core
Material
rocky
rocky
metallic
Composition
basalt, granite
peridotite, olivine
Fe-Ni and some lighter elements
Figure 1: The interior structure of Earth.
Heat Sources in Planets
A hot interior contains thermal energy. Where does this energy come from? This energy
does not come from sunlight—sunlight heats the planetary surface.
1. Accretion Accretion deposits energy originated by the colliding planetesimals. An
incoming planetesimal has a lot of gravitational potential energy when it is far away.
As it approaches a forming planet its gravitational potential energy is converted to
kinetic energy, causing it to accelerate. After impact, much of the kinetic energy is
converted to heat.
2. Differentiation The process by which gravity separates materials by density and
causes the distinct layering of the terrestrial worlds. The process of differentiation
adds mass to the planet’s core and reduces the mass in the outer layers (denser
material sinks while the lighter elements rise), which means that mass effectively
moves inward and loses gravitational potential energy.
3. Radioactive decay Radioactive decay affects the planet’s heat budget because the
rock and metal planetesimals that built the planet contain radioactive isotopes of
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the elements (uranium, potassium, etc.). When radioactive nuclei decay, subatomic
particles fly off at high speeds, colliding with neighboring particles, which heats
them. This transfers some of the mass-energy (E = mc2 ) of the radioactive nucleus
to thermal energy of the planetary interior.
While accretion and differentiation provide heat only when the planets are young, radioactive decay provides an ongoing source of the heat. The rate of radioactive decay
however, declines with time, so it is more significant at early times.
Heat Transport in Planets
1. Convection is the transfer of heat from one place to another by the movement of
a fluid (liquid or gas). The fluid motion is caused by buoyancy forces that result
from temperature variations in the fluid. Convection is heat transfer via fluids.
2. Conduction is the transfer of heat (internal energy) by microscopic collisions of
particles and movement of electrons within a body. Conduction is heat transfer via
solids.
3. Radiation is the transfer of energy as electromagnetic waves or as moving subatomic particles.
Figure 2: The three mechanisms of heat transport.
Magnetic Fields
The presence or absence of a magnetic field is an interior property of the planet. By
examining how a planetary interior generates a magnetic field, we’ll know why the rest of
the terrestrial planets do not have them.
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Earth’s magnetic field is generated by a process similar to that of an electromagnet, in
which the magnetic field arises as a battery forces charged particles (electrons) to move
along a coiled wire. In Earth, charged particles move with the molten metals in its liquid
outer core. Internal heat causes the liquid metals to rise and fall (convection), while
Earth’s rotation twists and distorts the convection pattern of these molten metals. The
result is that electrons in the molten metals move within the Earth’s outer core in a same
way they move in an electromagnet, generating the Earth’s magnetic field.
Figure 3: The magnetic field of Earth.
Requirements for a magnetic field
The three basic requirements for a global magnetic field are:
1. An interior region of electrically conducting fluid (liquid/gas), such as molten metal,
2. Convection in that layer of fluid,
3. At least moderately rapid rotation.
Earth is the only terrestrial planet which meets all these requirements. The Moon has no
magnetic field either because it has no metallic core or because its core cooled and ceased
convection. Mars has no magnetic field today, probably because of core solidification.
Venus is likely to have a molten core like Earth, but either its convection or rotation
period is too slow to generate a magnetic field. Mercury has a magnetic field despite its
small size and slow rotation. Mercury has a huge metal core that may still be molten and
convecting. All of the outer planets have strong magnetic fields.
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Geologic Processes
1. Impact cratering The blasting of bowl-shaped impact craters by asteroids or
comets striking a planet’s surface.
2. Volcanism The eruption of molten rock, or lava, from a planet’s interior onto its
surface.
3. Tectonics The disruption of a planet’s surface by internal stresses.
4. Erosion The wearing down or building up of geological features by wind, water,
ice, and other phenomena of planetary weather.
Impact Cratering
An impact crater forms when a body slams into a solid planetary surface. Impacting
objects hit the surface at high speeds. At such high speeds, the impact releases enough
energy to vaporize solid rock and form a crater. Debris shoots high enough above the
surface and rains down over a large area. If the impact is large enough, some of the ejected
material can completely escape from the planet. Impact craters generally end up being
circular because an impact blasts our material in all directions. Laboratory experiments
show that craters are generally about 10 times as wide as the object that created them
and about 10–20% as deep as they are wide.
The scarred faces of the Moon and Mercury reveal the large number of impacts by leftover planetesimals (i.e., asteroids and comets). Small craters far outnumber big craters,
confirming that many more small asteroids and comets orbit the Sun than large ones.
Volcanism
Any eruption of molten lava onto the surface. Volcanism occurs when underground molten
rock, typically called magma, finds a path to the surface (where it is then called lava). Lava
that is “runny” can flow a longer distance before cooling and solidifying, while “thick”
lava tend to collect in one place. Lava plains and volcanoes are made of basalt, a mixture
of many different minerals. All terrestrial worlds—as well some Jovian moons—show
evidence of basalt.
Plate Tectonics
The term tectonics comes from the Greek word tekton, meaning “builder”. In geology,
tectonics refers to any surface reshaping resulting from stretching, compression, or other
forces acting on the lithosphere. Most tectonic activity is a direct or indirect result of of
mantle convection.
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Erosion
Erosion is the action of surface processes (such as water flow, rain, or wind) that remove
soil, rock, or dissolved material from one location on the Earth’s crust, then transport it
away to another location.
Planetary properties and geology
Both volcanism and tectonics require internal heat, which means they depend on planetary
size: larger planets have more internal heat and hence more volcanic and tectonic activity.
Erosion is linked to three fundamental planetary properties:
1. Size
2. Distance from the Sun
3. Rotation rate
Planetary size is important because erosion requires an atmosphere. Distance from the
Sun is important because it controls the temperature (e.g., erosion is most efficient with
liquid water). Rotation is important since it’s the primary driver of winds and other
weather—faster rotation means stronger winds and storms.
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