* As temperature increases, so does the average speed of the atoms or molecules in a gas. Average speed declines with increasing atomic mass. A small fraction of the atoms in a gas move much faster than the average speed.
* Atoms can't escape from the lower parts of the atmosphere of
a planet because they collide with other particles after traveling
a short distance. In the thin outer atmosphere, however, a rapidly
moving atom is unlikely to collide and be reflected back by another
atom before escaping. If the average atomic speed there is greater
than about 1/6 escape velocity, all of that kind of atom will
escape relatively quickly.
* Density is the mass of gas per unit volume. Gas pressure is
the force per unit area exerted by the gas. Gas pressure is proportional
to the product of density and temperature.
* Hydrostatic equilibrium occurs when the decrease of gas pressure
with height balances the downward pull of gravity. Pressure falls
gradually with height until the atmosphere blends into interplanetary
space.
- A blackbody is one that absorbs all of the electromagnetic
radiation that falls on it. A heated blackbody emits electromagnetic
radiation at the maximum rate possible for a body of a given temperature
and size.
* A blackbody emits electromagnetic radiation with a characteristic
spectrum that depends only on the temperature of the blackbody.
The wavelength at which the spectrum is brightest grows shorter
with increasing temperature. The total rate at which a blackbody
emits radiation varies with the fourth power of its temperature.
* If the temperature of a body reaches the point where the body
emits radiation at the same rate at which it absorbs radiant energy
falling on it, the body is said to be in thermal equilibrium.
The decline of solar radiation with increasing distance from the
Sun leads to temperatures that decline with increasing distance.
Highly reflecting bodies are cooler at a given distance than dark
bodies, which absorb sunlight well.
* Nearly all of the mass of an atom is contained in its nucleus,
which is composed of neutrons and protons. The number of protons
in a nucleus determines what element it is. Nuclei that are of
the same element and differ only in numbers of neutrons are called
isotopes.
* In some nuclear reactions, matter is transformed into energy.
This happens when the particles that react are more massive than
the products of the reaction. The two types of nuclear reactions
are fission, in which a massive nucleus splits to produce less-massive
nuclei, and fusion, in which light nuclei combine to form a heavier
one.
- Radioactive decay occurs when an unstable isotope decays
to form another nucleus. The rate of radioactive decay is described
by its half-life, the time needed for half of the nuclei to decay.
Radioactive decay is useful for determining the ages of material
containing unstable isotopes because the decay occurs at a predictable
rate.
- For most solar system bodies, radioactive decays are the
major source of internal heat. As the amount of radioactive material
in the solar system has declined, so has the rate of radioactive
heating. Solar system bodies also were heated by the release of
kinetic energy as they accreted. However, accretional heating
was significant primarily for larger bodies.
- Conduction, convection, and radiative transfer are the
processes that carry energy from place to place. Within a body,
the rate at which heat flows is determined by the decrease of
temperature from center to surface. Because they have large areas
relative to their volumes, small bodies cool more rapidly than
large ones.
- Planetology is the comparative study of the Earth and other
planets. Planetology emphasizes trying to understand an individual
planet by accounting for the ways in which it is similar to and
different from other planets. Different planets show how the same
processes work in bodies of different mass, diameter, composition,
and distance from the Sun.
accretion accretional, heating, albedo,
atomic number, blackbody, blackbody radiation, conduction, convection
density, deuterium, electron, fission, fusion ,half-life, hydrostatic
equilibrium, isotope, mass number, neutral gas neutron, nuclei,
nucleus ,number density, planetology, plasma ,pressure, proton,
radiative transfer radioactivity, solar constant, Stefan-Boltzmann
law, thermal equilibrium, Wien's law.
1. An astronomical unit is: the distance between the sun and Earth
2. Which of the following is a characteristic of a terrestrial
planet? mean density of about 5000 kg per cubic meter
3. Suppose there are 100 grams of an isotope which has a half-life
of 10 minutes. How much of the isotope will be left after 30 minutes?
12.5 grams
4. If an object emits light by thermal, or blackbody radiation,
there is a unique correspondence between the temperature of the
object and the wavelength at which it is brightest
5. Suppose a blackbody is brightest at a wavelength of one micron.
If the temperature of the blackbody is doubled, at what wavelength
would it be brightest? 0.5 microns
6. Blackbodies A and B have the same size. A is three times hotter
than B. What is the ratio of the rate of energy emitted by A to
the rate of energy emitted by B? 81
7. Pluto is now 30 times farther from the sun than is the Earth.
How much fainter would the sun appear from Pluto than from Earth?
900 times
8. The energy transport process in which heated material rises
and cooled material sinks is convection
9. A planet has an albedo of 0.4. What percentage of the sunlight
which strikes the planet is absorbed by the planet? 60%
10. Which of the following planets is most likely to retain a
thick atmosphere? a planet with a cool atmosphere, high gravity
4. If an object emits light by thermal, or blackbody radiation, there is a unique correspondence between the temperature of the object and the wavelength at which it is brightest
5. Suppose a blackbody is brightest at a wavelength of one micron.
If the temperature of the blackbody is doubled, at what wavelength
would it be brightest? 0.5 microns
6. Blackbodies A and B have the same size. A is three times hotter
than B. What is the ratio of the rate of energy emitted by A to
the rate of energy emitted by B? 81
7. Pluto is now 30 times farther from the sun than is the Earth.
How much fainter would the sun appear from Pluto than from Earth?
900 times
8. The energy transport process in which heated material rises
and cooled material sinks is convection
9. A planet has an albedo of 0.4. What percentage of the sunlight
which strikes the planet is absorbed by the planet? 60%
10. Which of the following planets is most likely to retain a
thick atmosphere? a planet with a cool atmosphere, high gravity