ASTR 1210 (O'Connell) Study Guide
12. The Earth
A. Earth as a Planet: 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
- Open oceans: unique in the solar system; water covers 2/3 of
Earth's surface and produces its signature blue color
- 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!. A total thickness of about 25 km (15 miles),
compared to Earth's 8000 mile diameter. For a sense of the thinness
of the atmosphere, see this
- For a scale model: Take a piece of paper. Fold once. Paste on
a basketball. A thin smear of lifeforms on a huge sphere.
- We live in a delicate balance with nature
- In cosmic time, our favorable ecosystem is transient and
- 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 enormous). 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. The Age of the Earth
Our experience of everyday time is of little help in visualizing the
age of the Earth. Hundreds or thousands of years represent the
longest time scales we can easily comprehend, yet the age of the Earth
is millions of times longer. The model time scale described
in Study Guide 1, based on the pages in our
textbook, is probably the best way of attempting to retrain your
thinking about cosmic time. Another useful illustration is in the
form of a "cosmic clock." An example is shown at the right. At this
scale, you can't even see the current era of human dominance. Click
for a full-sized version.
Throughout the vast majority of its history, the Earth's atmosphere,
surface features, and especially its lifeforms, were dramatically
different from what we experience today.
There are two primary methods for dating the Earth's surface:
- Sedimentation rate/geological strata method
- Developed in the 19th century (after road and railroad
construction exposed deep rock layers)
- The rock layers of the Grand Canyon, for example, were once
under water. They are hardened sediment that settled to the
floors of ancient seas, deposited over long periods of time by
rivers that flowed into the oceans. Among other things, the
sedimentary layers can
of ancient plants and animals. They can also harbor processed
organic material in the form of oil and natural
- Age-dating based on stratigraphy makes use of
the relatively uniform rate of sediment deposition to estimate the
age of different stata. Deeper layers are older, and thicker layers
were exposed to deposition for a longer period of time. For an
example of dating of the Grand Canyon, click on the picture below.
"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 at least millions of years of time.
This recognition has been called "the discovery of Deep Time."
Stratigraphy is still very important in determining the relative
ages for different rock formations, which can be traced across
areas. Here is a
pertinent example, showing the distribution of the recently
exploited gas-rich Marcellus shale formation across the eastern
Sedimentary layers in the walls of
the Grand Canyon (Reiner Stenzel). Click for cross-sectional ages.
Cross-section through the Earth (USGS)
D. Earth's Interior
- Earth's mass can be determined from the orbit of the Moon or of
artificial spacecraft by applying Kepler's 3rd
- Reminder: the Earth's mass can not be determined from its
orbit around the Sun (see Study
- The density of a planet, defined to be its mass
divided by its volume, is an essential clue to its composition.
Despite is much smaller diameter, Earth has a higher density
than does Jupiter:
- Earth 5.5 grams/cc ==> heavier elements (like Si,O,Fe)
- Jupiter 1.3 grams/cc ==> lighter elements (H,He)
- We are able to probe the interior
waves from earthquakes
- These show that the interior is differentiated: i.e.
the 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
- Initial heat source: impacts of infalling planetesimals during
formation stages. Further heat released during differentiation, as heavier
- A persistent (lasting billions of years) interior heat source:
radioactive decay of uranium and other materials. Even though
they constitute 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 of energy by actual
rising/falling motions of material. These "convection currents"
systematically deposit interior heat at shallower depths.
- As long as the interior retains significant heat, its transfer to
the surface drives 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
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 above). These float on
the partially melted, plastic material (the
asthenosphere) below them.
- The plates move in response to the
currents in the mantle.
Convection circulation includes 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 geological 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, earthquakes, vulcanism (as at right), etc.
- Earthquakes and vulcanism are concentrated primarily at
the periphery of plates where two plates are moving against
each other (e.g. the "Pacific Ring of Fire", see
the map above). Young mountain
ranges (e.g. the Andes and Himalayas) are also found at the
edges of plates.
- The Earth's surface is being continuously recycled.
Older plates are pushed back into the interior and melted
while newer materials are added to the surface in upwelling zones.
Melting of plates gassifies some of their materials and cycles
those back into the atmosphere.
- Earth is the only terrestrial planet with continuous,
Earth's Atmosphere From Orbit
- The original atmosphere was "outgassed" from the interior soon after Earth formed.
- There has been strong evolution of the constituents of the
atmosphere over time
Many different physical processes drive atmospheric evolution for the
terrestrial planets. A pictorial summary is
These and their consequences for Earth, Venus, and Mars will be
discussed further in Study Guide 19.
- Earth's atmosphere is now predominantly N2, and
O2. This is unlike the atmospheres of Venus and
Mars (which are mainly CO2) and unlike the early Earth
- O2 is directly linked the the presence of life on Earth.
It is produced mainly by photosynthesis in plants and
started rapidly increasing in the atmosphere about 500 million
years ago. Since it is so reactive, free O2 cannot
persist in an atmosphere without life.
- The temperature at any height is determined by the balance of heating
- Circulation (wind) is driven by heating (near equator)
vs. cooling (near poles) and by effects of Earth's rotation (the "coriolis
Reading for this lecture:
Bennett textbook: Secs. 8.5, 9.1, 9.6
Study Guide 12
Reading for next lecture:
Bennett textbook: Secs. 9.2, 9.3, 10.3
Study Guide 13
April 2018 by rwo
Drawing of seismic waves from ASTR 161 University of Tennessee.
Text copyright © 1998-2018 Robert W. O'Connell. All rights
reserved. These notes are intended for the private, noncommercial use
of students enrolled in Astronomy 1210 at the University of Virginia.