Planetary Interiors & Volcanism
Basic Properties
Some of the most basic information on planets tells us a lot about what is going on inside them:
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Planet |
Semi-major Axis (AU) |
Mass (Mearth) |
Radius (km) |
Density (g/cm3) |
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Mercury |
0.39 |
0.06 |
2,439 |
5.43 |
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Venus |
0.72 |
0.81 |
6,051 |
5.25 |
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Earth |
1.00 |
1.00 |
6,378 |
5.52 |
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Mars |
1.52 |
0.11 |
3,397 |
3.93 |
The density of “average” rocks is about 3.3 g/cm3, while that of heavy metals is
higher for iron it is about 9 g/cm3. So we can immediately see that
Mercury, Venus, and Earth must be made of rock with a substantial amount of
metals, while Mars is rocky, but more deficient in metals. By contrast, the
most abundant material potentially available, water (liquid or ice) has a
density of about 1 g/cm3, and
must be comparatively rare.
GENERAL REGIONS OF THE TERRESTRIAL PLANETS
Core - densest innermost region, consisting primarily of metal Mantle - intermediate, lower density region, composed of rock Crust Hydrosphere - thin layer of water at the surface Cryosphere Atmosphere Magnetosphere - the general region influenced by the planet's magnetic field
For now we will look at the first 3….. |
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Not all of the TPs have all 7 regions. Mercury lacks an atmosphere in the usual sense. Only Earth has a true hydrosphere.
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A comparison of the interiors of the TPs and the Moon. Note the similarities between the Earth and Venus. One would think that these 2 planets would have similar surfaces, but they are very different. Venus has no oceans, and a surface temperature that make rock behave somewhat differently than on the Earth.
Mars has a much thicker lithosphere that Earth, which will govern how its surface has evolved with time, compared to the Earth.
A greater fraction of the mass of Mercury is in its core than any other TP.
The Moon has the smallest core, and only recently ahs evidence accumulated that it may be partly molten.
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We will begin with a detailed description of the Earth and the processes that have shaped the evolution of its interior and surface. This will be used as a jumping-off point for a comparative study of the rest of the TPs, and leads us naturally into the material we will cover for Mars. Stuff on the atmosphere and magnetosphere will be postponed until another chapter, unless needed here.
THE MATERIALS OF THE EARTH AND TERRESTRIAL PLANETS
Most of the Earth’s solid surface is rock. What is rock? In essence, it is a collection of atoms that may be tightly bound by atomic bonds into specific well-ordered crystals, as in the case or pure minerals. Or it may consist of a looser assemblage of materials that may be chemically inhomogeneous (what we usually mean by ‘rock’ as opposed to “mineral”).
Rocks & Minerals
Minerals are a naturally occurring solid composed of a single element, or a specific combination of elements. These often will take the form of a crystalline structure, as in quartz.
Some of the most important and common types of minerals that we will encounter over and over again…) are:
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Silica
(as in quartz) |
Olivines - (Fe,Mg)2SiO4 |
Pyroxenes
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Plagioclase
Feldspars |
Calcite |
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Rocks are collections of different minerals combined into a single mass. These are usually classified into one of 3 broad types, depending on how they were formed:
Igneous - created by the solidification of molten minerals. Examples are granite and basalt.
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Basalt is an extrusive igneous rock: it forms by cooling on the surface after being extruded from the interior. Because it cools quickly, the atoms in the crystal only have enough time to line up with their nearest neighbors before solidification. Because of this, the crystals are typically smaller than in rocks that solidified more slowly. The process whereby large-scale order is achieved through very slow cooling is called annealing. Basalts are composed of feldspars, olivines, and pyroxenes. Basalt is the principle rock underlying the oceans and makes up most of the large lava flows on the surface. The Hawaiian islands are almost entirely basalt.
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Gabbro solidifies underground, where it can cool more slowly. Due to more effective annealing, mineral crystals it contains are larger than those in basalt. Again, fedspars, olivines, and pyroxenes are major mineral groups of this rock. They also contain hornblende, a relative of the pyroxenes. When composed primarily of a variety of plagioclase feldpspar called labradorite, this rock is termed anorthosite. As we will see, anorthosite is an important rock found on the Moon.
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Granite
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Sedimentary - rocks often made in layers due to erosion by wind, water, and ice (or biological action), and the subsequent deposition and cementing of the fragments. Examples are limestone & sandstone.
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Limestone |
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Sandstone
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Metamorphic - Igneous or sedimentary rocks that have undergone further transformation into a new rock by high temperature, pressure, or the addition of other chemicals.
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Marble is one example, produced by transforming limestone under the effects of temperature, pressure, and water.
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Quartzite is a processed form of sandstone. Because sandstone is largely a bunch of quartz crystals, that is why this rock has the name it does.
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Changes of State - solid/liquid melting
In the interior of a planet, the temperature and pressure may become high enough for a mineral to melt or have its internal solid structure re-arranged. When the temperature exceeds the melting point for that material at that pressure, the material will be liquid. This is what we would call a phase change.
Differentiation separates material into different
chemical & physical properties.
Example - Magma - different minerals crystallize out at different T's, and denser minerals sink while lighter minerals rise.
Example When the force of gravity separated out
molten materials by having the densest materials sink and the less dense
materials rise. This is why the TPs have metallic cores and rocky outer
regions.
FORMATION OF THE TERRESTRIAL PLANETS
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The current state of the TPs is, in part, rooted in the formation of the solar system. As the proto-solar system began to collapse from an initially large cloud of gas and dust, conservation of angular momentum forced it to take on a flattened configuration, from which the precursors of the planets would eventually form.
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Exactly what sort of planet, terrestrial or jovian, would form depended on the distance fro the Sun. Closer to the Sun, only refractory materials (those that can withstand high temperatures and remain solid) could condense, while further out volatiles (those that vaporize more easily) could also condense.
So close to the Sun, planetary embryos and protoplanets would be dominated by rock and metal, while beyond a certain distance (variously called the “snow line” or “frost line” ices could also go into forming planets. Because H and O ate the two most abundant chemically active elements in the universe, water ice is potentially the most abundant solid possible. |
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The precise set of minerals to form depends on the condensation sequence.
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There is still some debate as to the precise sequence of events leading to the formation of these planetary embryos.
It is possible that these objects formed from a collection of relatively uniform (chemically) objects, which because of the internal heat generated by both collisions and radioactive decay partially melted and differentiated.
It is also possible that the embryos were formed while the condensation sequence was still “operating”. Here the most refractory materials were able to form the beginnings of a core, while less refractory materials came later. Further internal heating would then finish the differentiation.
Those small objects that never partook of this process are still with us today. The asteroids (also called minor planets) contain the original material that formed the TPs, while comets and Kuiper Belt Objects (KBOs) contain volatiles (ices) as well. If we want to sample the least processed materials in these regions, we need to get samples of them.
These come to us in the form of some meteorites (from
asteroids) and captured comet grains (which make up a significant fraction of
interplanetary dust particles
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Chondrites are meteorites that contain small spheroidal chondrules that formed very close to the Sun. You can also easily see flecks of metal (FeNi usually) in them. So these are samples from an undifferentiated body. |
Iron meteorites (actually a mix of iron & nickel) are from a body that underwent differentiation, solidification, and was later broken apart (probably by collisions). |
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In some cases, we have samples of the core-mantle boundary!! This is an example of a stony-iron meteorite called a pallasite. The crystal material here is olivine. |
The most “primitive” (least processed) meteorites are the carbonaceous chondrites, which as their name implies, contain C, but also water, amino acids, and a host of other interesting stuff. |
Because the bulk of the composition of the Earth was basically that of chondrites, we often say that is composition was chondritic.
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The comets leave trails of debris for every passage they make into the inner solar system. Many survive entry into the atmosphere (like micro-meteorites) and have been collected by high-flying airplanes. These can be studied in the laboratory.
In January of 2006, NASA’s Stardust mission also brought back samples from Comet Wild 2 for analysis. We can also study comets spectroscopically. We can also examine the spectra debris disks surrounding other stars! What do these tell us? |
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A “typical” IDP of cometary origin |
The IR spectrum of the debris disk surrounding HD 100546, compared to that of Hale-Bopp, and crystalline forsterite. |
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An IDP containing forsterite |
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Crystalline forsterite grain from Comet Wild 2, returned to Earth by Stardust |
Despite the comment in your book, we really can say a lot about the mineralogy of other planetary systems!!
Finally, in the last phase of planet formation, it is apparent that these planetary embryos collided with one another, forming what became the familiar planets we have today. It is thought that the proto-Earth suffered a collision with a Mars-mass embryo. Much of the material coalesced to form the Earth, while some of the debris generated by the impact collected to form our Moon. The process of collisions is still going on, albeit at a much reduced rate from what it was 4.5 Byr ago!
DIFFERENTIATION
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Regardless of which initial condensation sequence is correct (the truth may lie somewhere between the two models outlined), if the internal temperature was high enough for silicates to melt, then the heavier iron-nickel particles would slowly settle to the core. If the temperature was high enough to melt these metals, then they would “rain out” of the molten material. The net migration of denser materials to the core would also release gravitational potential energy, keeping the interior hot for a long time. The phase of major differentiation probably continued through the ‘bombardment” phase of planetary formation.
Currently the Interior of the Earth consists of a solid inner core of metal (mostly iron & nickel), a molted outer core also of metal, a rocky mantle, and a rocky crust.
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The mantle is made mostly of minerals such as olivine & pyroxene, while the crust contains significantly greater amounts of feldspars (see Table 2.1 in McBride & Gilmore).
The uppermost layer of the mantle is solid, and along with the solid crust, forms the lithosphere of the Earth. The lower portion of the mantle is “soft” enough to flow, and forms the aesthenosphere.
Note that the boundary between the crust and mantle is a chemical one, while that between the lithosphere and aesthenosphere is a physical one (phase change).
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Because the lower mantle flows due to convective motions, the solid lithosphere “scum” floating on top of it is carried around like raft on a river. The lithosphere is divided into a number of large and small “plates”, each which is carried about by the fluid mantle below. The physics and chemistry of this process is referred to as plate tectonics, and is the source for the older term “continental drift”.
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However, not only are continental plates in motion, but oceanic ones as well.
The two types of lithospheric plates also differ form one another in terms of overall composition and density.
Where plates collide, one often gets plate subduction, where one plate slides below another.
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The heating that takes place as plates collide and subduct is responsible for the volcanic activity found along plate boundaries. The most famous of these is the “ring of fire” surrounding the Pacific Ocean. Collisions between two continental plates is responsible for mountain-building.
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Sometimes, a piece of the subducted lithosphere is carried deep within the mantle.
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One group of geologists have recently reported a ribbon settling to the core, “like taffy”!
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In a few instances, an oceanic plate can drift across a relatively stationary “hot spot”, causing a string of volcanoes to be built in succession. This seems to be the origin of the Hawaiian Island chain. Hot spot volcanism is also believed to be responsible for the huge volcanic mountains found on Mars. |
Plate Non-Tectonics on a Hot Waterless World: Venus
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Venus will not have plate tectonics most of the time, for 2 related reasons.
First, the very hot surface makes the crust more soft and “pliable” compared to that on Earth, so plate collisions tend to make “pile-ups” rather than having one plate get subducted below another. On Earth it also helps that the oceanic plates (the ones that are usually getting shoved down) are thinner than the continental plates.
The lack of liquid water also prevents hydration-induced melting, a process we will get to a little later.