1. Concepts 2. Solar System Origin 3. Planetary Processes 4. Earth Processes 5. Meteorites
6. Our Moon 7. Remote Sensing 8. Mercury 9. Mars 10. Venus, Our Twin
11. Jupiter & Jovian Moons 12. Saturn, Rings & Moons 13. Uranus 14. Neptune 15. Pluto, Charon & Comets

Planetary Processes

Review Chapter 2, read Chapters 6, 12, 13 in The New Solar System.

by Kari Hetcher and Scott Hughes

IMPACTING

Impact events, like the ones that formed Meteor Crater about 50,000 years ago in Arizona and the Manicouagan impact structure about 210 million years ago in Quebec, represent the dominant process of planetary accretion (growth) and surface restructuring. Planets without significant tectonic reworking, weathering or erosion of their surfaces have old surfaces that reflect numerous impacts during their early growth stages. Although the rate of impacting has diminished over the past 4.5 billion years, these events still happen periodically, occasionally with enough energy to cause massive destruction. We will cover more of this topic when we discuss lunar geology and mass extinctions on the Earth.

Manicouagan Crater

Meteor Crater

Impact Crater Chains on Callisto:

Gipul Catena is the longest of 12 or so such chains on Callisto, one of Jupiter's 4 planet-sized satellites. It is 620 kilometers long and the largest.  Visit NASA's Callisto page for more information

 

Visit the Terrestrial Impact Craters Slide Show

Compiled by Christian Koeberl and Virgil L. Sharpton

http://www.lpi.usra.edu/publications/slidesets/craters/

Other sites to visit are the following:

http://seds.lpl.arizona.edu/nineplanets/nineplanets/callisto.html

http://photojournal.jpl.nasa.gov/catalog/PIA03379

http://www.lpl.arizona.edu/SIC/impact_cratering/intro/

http://observe.arc.nasa.gov/nasa/exhibits/craters/impact_home.html

Comet Shoemaker-Levy Collision with Jupiter: http://www.jpl.nasa.gov/sl9/sl9.html

Near-Earth Objects (Resources at the NASA HQ Library): http://www.hq.nasa.gov/office/hqlibrary/pathfinders/aster.htm

Asteroids, Comets, Meteors, and Near-Earth Objects

: http://impact.arc.nasa.gov/index.html

 

Eugene and Carolyn Shoemaker wrote chapter 6 of our textbook. As a team these authors have contributed immensely to our knowledge of impacting and the prospects of devastating collisions in the future. The Shoemaker-Levy comet that struck Jupiter in 1996 was discovered by the Shoemakers and their colleague David Levy. (Tragically, Gene Shoemaker was killed and Carolyn Shoemaker was injured in an auto collision while working on impact structures in Australia during the summer of 1997.)

PLANETARY DIFFERENTIATION

In terms of planetary evolution and geologic processes, differentiation means to make a homogenous body heterogeneous. This often reflects changes in the relative proportions of chemical and mineralogical constituents from one place to another. Planetary differentiation, therefore, refers to the processes that cause an essentially homogeneous accreted body that is made up of primordial solar material to become separated into layers having different chemical and/or physical properties. If a planetary body is large enough it will develop a core, mantle and crust each of which may be further subdivided. Each layer in the Earth has its own set of subdivisions, for example: upper, middle and lower crust.

NOTE: The Earth’s lithosphere is comprised of the entire crustal layer plus the uppermost part of the mantle. The mantle immediately beneath the lithosphere is the asthenospheric mantle, which is chemically and mineralogically similar to the lithospheric mantle, but is partially melted to provide a plastic layer over which the lithospheric plates move. The lithosphere-asthenosphere transition is a consequence of processes beyond those that caused differentiation into layers. See module 4 – Earth.

Planetary differentiation is primarily heat-related, i.e. it is the manifestation of internal heating, melting, and segregation of components. Denser components sink to the center to form the Fe-metal rich core while less dense material rises to form the silicate crust. Pressure and temperature increase with depth in a planetary body, so minerals that are stable at one depth might not be stable at another depth.

Planets begin to heat up in their early stages of evolution and the energy budget of the planet involves several processes:

<>Impact Heating results when a bolide (comet, asteroid, meteor) strikes a body in space. Kinetic energy of the bolide goes into pulverizing and vaporizing both the impactor and part of the planetary surface. Some of it is transformed to shock waves that propagate through the planet and the remainder is transformed into heat. Rock is instantaneously melted during some large impacts. Evidence for impact melting includes tektites found on Earth and glass beads found in lunar soils (they are chemically distinguished from volcanic glasses also found in lunar soil).
Tidal Heat is generated by the slight internal deformation against frictional resistance as one planetary body revolves around another. Fluctuations in gravity result from variation in the relative positions of the two bodies. For example, the tides on Earth are a direct response to the positions of the Moon and Sun. Gravitational perturbations also result from an irregular orbit, such that the distance separating the planetary bodies is not constant. The best example of tidal heating in the Solar System is in the Jupiter system, where the small inner moon Io exhibits active volcanism due to intense internal heating. 
Solar Heat is responsible for surficial weathering and erosional processes on planets that have atmosphere, but also causes surface warming on planets with thin or no atmosphere. The amount of solar energy that actually reaches the surface depends on several factors, such as the density and composition of clouds. The surface of Venus reaches temperatures of around 700 degrees Celsius due to solar heating. This suggests that the thermal gradient beneath the surface is also quite high due to the elevated starting surface temperature. Thus, the amount of internal heat lost to space may be greatly affected by the surface temperature.
Radiogenic Heat is produced during the decay of radioactive isotopes. We know from Module 1 that nucleosynthesis produces a wide variety of nuclides that make up a solar nebula. As planets form, they incorporate naturally-occurring radioactive elements such as 235U and 40K that have half-lives measured in billions of years. These long-lived nuclides are still present in the Earth and other planetary bodies, although in lower abundance than when the Solar System formed. This allows for sustained long-term heating during planetary evolution. Radionuclides with relatively short half-lives measured in thousands to millions of years, such as 26Al, decayed out early in planetary evolution and were responsible for initial internal heating.
Internal heating can also be caused by core formation during which process the potential energy of sinking dense matter is transformed into heat as the material descends toward a deeper level. This is not considered a significant factor in bringing a planetary interior close to melting because the material involved would have to become partially melted for segregation to occur in the first place.

The most important heat-generating process involved in planetary differentiation is radiogenic decay. Rocks are insulating materials, so heat is transferred by conduction very slowly to the surface where it is transferred by radiation off to space. Because of this slow rate of heat transfer, various parts of a planet’s interior will become heated to the point of partial melting. When a magma is formed and injected into other regions of the planetary body (usually upward into overlying layers) heat is transferred by convection due to the mobility of the molten material. Volatile elements and compounds, such as water, carbon dioxide, sulfur, etc., enhance the transfer of heat by convection.

 

Visit W. M. White's online geochemistry textbook for a thorough discussion of the evolution of the Earth.  This textbook is also a wonderful resource for any questions related to geochemistry.

 

ROCK FORMING MINERALS

 

Rocks are made of minerals, most of which are silicates formed by the combination of certain cations (Mg, Fe, Ca, Na, K, etc.) with SiO2 (silicon dioxide). Other minerals include simple oxides (e.g. magnetite, chromite), halides (salt = halite, sylvite), sulfides (pyrite, galena), sulfates (gypsum), carbonates (calcite, dolomite), minerals composed of a single element (diamond, graphite), and so on. By far most of the rock-forming minerals are silicates which are present on (or in) every planetary body.

Mineralogy Information Source: http://www.mindat.org/

SiO2 is an oxide, but when Si combines with O in a tetrahedral arrangement, there are four O atoms for each Si atom. This is the silicate tetrahedron, which acts as a complex anion due to a charge imbalance with the two extra oxygen atoms. Oxygen is ionized to O2- and silicon is ionized to Si4+, thus the combination of Si + 4O leaves a charge imbalance of –4.

NOTE: Imagine if each O atom is shared by two Si atoms such that a three-dimensional network of SiO4 tetrahedra exists with all of them interconnected like a framework. In this case, there would be no charge imbalance and the formula would be SiO2, quartz.

Each side of an SiO4 tetrahedron is identical, so it can be drawn as a geometric tetrahedron in order to simplify the structures of various silicate mineral types.

 

Note the structure of the SiO4 molecule in each of the following

Olivine, a silicate solid solution mineral, has the formula (Mg, Fe)2SiO4 which means that Mg and Fe cations substitute for each other in the crystal lattice. The actual composition of olivine varies from one end-member composition (forsterite = Mg2SiO4) to the other (fayalite = Fe2SiO4). The chemical balance of cations (Mg, Fe) and anions (SiO4) causes the structure of olivine to be made of independent SiO4 tetrahedra surrounded by Mg and Fe. Olivine is called a ferromagnesian mineral (Fe and Mg) and has a high melting temperature.

Olivine phenocrysts (green crystals) in basaltic lava flow. Olivine crystals probably formed before the lava was erupted. Note vesicles due to gas exsolution and expansion as the lava cooled. Image is approximately 1x2 cm.

Pyroxene is made up of Mg, Fe and sometimes Ca (along with other substitute cations like Ti, Na, Al, etc.) that fit around single chains of SiO4 tetrahedra. Pyroxene is found with many different compositions, and has various names like augite, enstatite, hypersthene, pigeonite, etc. depending on the relative proportions of Ca, Mg, and Fe. The silicate chains, called polymers, are produced when two of the four O atoms in each SiO4 tetrahedron are shared with another tetrahedron. Note that every other tetrahedron in the chain is reversed ("upside down").

Amphibole is an even more complex mineral. It is formed by an arrangement of SiO4 chains that are attached side-by-side to make double chain silicates. The most common example of amphibole is hornblende, often-called a "garbage can" mineral because much substitution of cations is allowed in the crystal lattice. It is a ferromagnesian mineral like olivine and pyroxene, but often contains abundant Ca, Na, and Al, and it contains structurally bound water in the form of the hydroxyl (OH-) ion. Also, Al substitutes for Si in some of the tetrahedral sites, creating a charge imbalance that is offset by changes in the relative proportions of Na, Ca, etc.

Hornblende crystal is approximately 6 cm long, an unusually large size for a common rock-forming mineral. It probably formed in a pegmatite or other fluid-rich late magmatic or high-grade metamorphic system (see below). The dark color is typical of this mineral.

Another type of hydrous mineral is mica, formed by book-like layers of two-dimensional sheets of SiO4 tetrahedra. These minerals have one direction of perfect cleavage, like a deck of cards, which allows them to be split into very thin wafers. Common examples are biotite (which often occurs in rocks that contain amphibole), muscovite, chlorite and phlogopite. Like amphibole and some pyroxenes, mica compositions can be quite variable, especially biotite.

Feldspars are framework silicates found in nearly every igneous rock, and in many sedimentary and metamorphic rocks. They are alumino-silicates of Ca, Na, and K, and occur in various states of atomic order. Ca and Na feldspars comprise a solid solution series called plagioclase ranging in composition from anorthite (CaAl2Si2O8) to albite (NaAlSi3O8). The importance of this mineral will become apparent in the Moon module. K feldspars are classified according to how well the crystal lattice is ordered. Sanidine is the least-ordered form of KAlSi3O8 found in siliceous volcanic rocks, whereas increasing degrees of ordering are found in plutonic K-feldspars orthoclase and microcline.

Plagioclase crystal is approximately 10 cm high and, like the hornblende crystal shown above, probably grew in a hydrothermal or pegmatite system. Plagioclase occurs in nearly every type of igneous rock, so it is ubiquitous in the Earth as well as other terrestrial planets. The light colored regions of the Moon, called the Highlands, are mostly anorthosite, a rock made up of mostly Ca-rich plagioclase. Look at the full moon and try to outline the arrangement of Highlands and Maria.

All Silicate Minerals:Note the decrease in ratio of SiO2 to cations, from independent tetrahedra silicates to framework silicates, indicating an increase in the relative molecular proportion of SiO2 in the mineral. As noted above, the simplest framework silicate is quartz. Found in many rocks, the presents of quartz indicates the availability of free SiO2 molecules in a magma, meaning that cations like Mg, Fe, Ca, Na, etc. have been used up in the formation of other minerals. Quartz and olivine generally are not found together in nature because pyroxene has a composition intermediate between the two.

Consider the following balanced reaction between chemical compounds: Mg2SiO4 + SiO2 <=> 2MgSiO3 In mineralogical terms this equation is: Olivine + Quartz <=> 2 Pyroxenes.

Assignment -- Part 1:

Answer the following study questions and e-mail your answers to the instructor.

1.  Define and understand these terms:

-thermal energy

-kinetic energy

-gravitational potential energy

-chemical potential energy

-refractory element

-core, mantle, crust

-lithosphere, aesthenosphere

-planetismal

-impact heating

2.  What are the five major stages of planetary formation?

3.   What is the definition of a mineral?  What is a rock?  What is the difference between rocks and minerals?

4.  Why are the terrestrial planets found closer to the sun that the Jovian planets?>

5.  What does the presence of hydrous minerals like amphibole mean in terms of environment of rock formation?

On to Module 3