ASTR 121 (O'Connell) Study Guide

A. UNIQUENESS

• Largest terrestrial planet

• Has largest satellite with respect to its own diameter (see images in Guide 13)

• Large atmospheric abundance of water is unique among terrestrials

• Open oceans: unique in solar system; water covers 2/3 of surface

• O₂-rich atmosphere (21% by volume)

• LIFE! Living organisms cover the Earth.
  There is no definite evidence yet for any other biospheres in the solar system. If these exist, the lifeforms are likely to be primitive. See Guide 23.

B. THE EARTH'S BIOSPHERE

• Consists of Earth's crust, oceans, lower atmosphere:
  
  • Thin!

  • For a scale model: Take a piece of paper. Fold once. Paste on a basketball. A thin smear of lifeforms on a huge sphere.

• Fragile!
  • We live in a delicate balance with nature

  • In cosmic time, our favorable ecosystem is transient and evolving rapidly.
    • For example: Earth's surface, as hard and unchanging as it may seem, is only temporary.

  • Although the rapidly growing human population (see Study Guides 9 and 19) is having deleterious effects on the biosphere, these are survivable (even if the costs could be catastrophic). The most serious, long-term threats to the ecosystem are extraterrestrial and beyond immediate human control: asteroid/comet impacts, solar evolution, supernovae and other stellar explosions, etc.

  • Astronomy is the ultimate ecology.

C. AGE

• Sedimentation rate/geological strata method
  Developed in 19th century
  The rock layers of, e.g., the Grand Canyon were once under water. They are hardened sediment that settled to the floors of ancient oceans, deposited over long periods of time by rivers that flowed into the oceans. Among other things, the sedimentary layers can contain fossils of ancient plants and animals; they can also harbor processed organic material in the form of oil and natural gas.
  Age-dating based on stratigraphy makes use of the relatively uniform rate of sediment deposition to estimate the age of different stata. For dating of the Grand Canyon, click on the picture. "Bio-stratigraphy" uses embedded fossils to refine ages.
  These methods are not very precise, but they were sufficient to prove to 19th century geologists that the Earth's surface had evolved over millions of years of time. They are useful in giving relative ages for rock formations.

• Radioisotope (or "radiometric") dating method
  • This is based on the well-determined decay rates of naturally occurring radioactive isotopes. e.g. uranium ==> lead. Much more precise than stratigraphy. Thoroughly understood on the basis of quantum mechanics.

  • "Half life" = time it takes for half of original sample of unstable isotope to decay to "daughter" isotope. See this Java Applet illustrating the decay process. Half lives for unstable isotopes range from microseconds to billions of years.

  • First, chemically separate parent from daughter isotopes; then estimate age from ratio of parent to daughter.
    "Carbon-14" dating is the best known type of radiometric dating. Because the half-life of ¹⁴C is only 5570 years, it is very important for studies of historical and pre-historical human artifacts; but it is not useful for geological time scales.

• Results
  • Oldest Earth surface rocks: 3.9 billion years (a few samples of zircon crystal date to 4.4 Byr)

  • Moon, meteorites: 4.5-4.6 billion years

  • Earth chronologies (click):
    Geological/Fossil Chronology
    Geological Chronology

Cross-section through the Earth (USGS).
Click for enlargement.

D. INTERIOR

• Earth's mass is determined from the orbit of the Moon or of artificial spacecraft by applying Kepler's 3rd law.
  • Reminder: Earth's mass can not be determined from its orbit around the Sun (see Study Guide 8).

• Mass/Volume = density, a clue to composition
  • Earth 5.5 grams/cc ==> heavier elements (like Si,O,Fe)

  • Jupiter 1.3 grams/cc ==> lighter elements (H,He)

  
• Probe interior with seismic waves from earthquakes
  • These show that the interior is differentiated: i.e. composition and density change with depth

  • 3 main zones: Core (innermost), mantle (body), crust (outermost).
    (See illustration above.)

  • Densities range from 12 grams/cc in the core to 3 grams/cc in the crust, implying that the core contains more heavy elements than the crust.

  • Temperature at the core is over 5000 K.

• The differentiation implies that Earth's interior was once molten, so that heavier materials could settle to the center.
  • Initial heat source: impacts of infalling planetesimals during formation stages. Further heat released during differentiation, as heavier materials sink.

  • Continuing (billions of years) interior heat source: radioactive decay of uranium and other materials: even though only a small fraction of the Earth's makeup, the heat generated by decay of these materials escapes only slowly, so the interior remains molten/plastic.

• Heat transfer/cooling
  • Interior heat escapes from planets through their surfaces. Heat is transferred by several processes:
    Conduction: transfer of energy by contact at the molecular level from hotter to cooler regions.
    Convection: transfer by actual motions of material. Hotter material in Earth's interior rises, producing "convection currents" that deposit heat at shallower levels.

  • As long as the interior retains significant heat, its transfer to the surface can drive various forms of "geological activity" (see next section). Convection currents are also responsible for producing a magnetic field extending into space.

E. "PLATE TECTONICS"

• A new (1950's - 60's) "paradigm" for the origin of geological structures. Click here for the history of plate tectonic interpretations.

• The outer layers of the Earth (the crust and the upper mantle, together called the lithosphere) are thin and cracked into pieces called "plates" (diagram at right). These float on the partially melted, plastic material (the asthenosphere) below them.

• The plates move in response to the slow convection currents in the mantle.
  Convection involves rising warm material and falling cool material; it is driven by the temperature gradient within the Earth.
  Typical motions are very small, about 1 cm per year. This sounds ridiculously tiny, but such motions can now be easily measured with technologies similar to those of the GPS system.
  On cosmic time scales such motions have drastic cumulative effects, as plates collide with each other or are exchanged with the mantle. Over 100 million years (a short time geologically), a motion of 1 cm per year adds up to 1000 km.

  
• Plate motion or "continental drift" is responsible for all the geological activity on the Earth's surface: mountain building, rifting, vulcanism (Mt. St. Helens at right), earthquakes, etc.

• Earthquakes and vulcanism are concentrated at the places where two plates are moving against each other (e.g. the "Pacific Ring of Fire", see the map above). Young mountain ranges are also associated with the edges of plates.

• The Earth's surface is being continuously recycled, as plates are pushed back into the interior.

• Illustrations:
  Continental drift animation (small)
  Continental drift animation (large)
  Building the Himalayas
  Maxi-Tsunami Simulation: tidal wave threat from "flank collapse" of Hawaii volcano. Note: the Hawaiian volcanoes are produced by a concentrated plume of hot magma rising from the interior rather than by plate collisions. This is the same kind of mechanism which produced the Tharsis volcanoes on Mars.

• Earth is the only terrestrial planet with continuous, large-scale tectonics

F. ATMOSPHERE

• The original atmosphere was "outgassed" from the interior soon after Earth formed.

• There has been strong evolution of the atmosphere over time
  The processes which drive atmospheric evolution for the terrestrial planets and their consequences will be discussed in Study Guide 19. A pictorial summary is given here.

• Now predominantly N₂, O₂. This is unlike Venus and Mars (mainly CO₂) and unlike the early Earth atmosphere.

• O₂ is produced mainly by photosynthesis in plants. Increased rapidly starting about 500 Myr ago. Since it is so reactive, free O₂ cannot persist in an atmosphere without life.

• Temperature at any height is determined by heating/cooling balance

• Circulation (winds): driven by heating (equator) vs. cooling (poles) and by effects of Earth rotation ("coriolis effect")

Atmosphere at sunset from Space Shuttle (300 mi altitude)

Bennett textbook: Secs. 8.5, 9.1, 9.6
Study Guide 12

Bennett textbook: Secs. 9.2, 9.3, 10.3
Study Guide 13

Introductory Course on Earth Systems (J. Schieber, Indiana U)
Tutorial on Remote Sensing and Earth Geology (Federation of American Scientists)
More Information on Age-Dating the Earth (Pogge, OSU)
Brandt's Evolutionary/Geological Timeline
The Geologic Time Scale (USGS)
Geochronology Methods (USGS)
Carbon-14 Dating
Dynamic Earth (Univ. of Leeds)
Background Information on Plate Tectonics (UC Berkeley)
More Background Information on Plate Tectonics (USGS)
The Electronic Volcano
Background Information on the Structure & Chemistry of the Atmosphere