Download COMPARATIVE PLANETOLOGY

Comparative Planetology
A fundamental objective in studying the solar system is to understand how the
planets came to exist in their current state. Basic questions about planetary
evolution can be addressed through comparison of their global properties. This
approach, known as comparative planetology, is complementary to detailed
exploration of a specific planet or moon.
What is to be compared ?? Following is a listing of the various properties
which define the objects within our solar system. Within each class, a set of
specific attributes are given. The goal is to define and compare those attributes,
attempting to explain the origins and evolution of our planetary system. For the
topic of planetary atmospheres, an example of comparative properties is also
given. The units in which each quantity is normally expressed are given in
brackets, where appropriate.
Global properties:
 Mass: the amount of matter contained with an object [kilograms, Earth
masses]
 Size: dimensions of an object, usually described by its radius [kilometers,
Earth radii]
 Density: defined as mass per unit volume. The density of any object is a
strong clue to its composition [grams per cubic centimeter].
 Composition: Matter is composed of combinations of the 92 naturallyoccurring elements. An object’s composition is defined by the relative mix of
these different elements.
 Rotation rate: the period for an object to complete one rotation. Almost all
celestial objects spin around an axis, which remains fixed in space [hours,
days].
 Rotation axis orientation: An object’s rotation axis in principle can point in
any direction. For objects orbiting within the solar system, the angle between
the rotation axis and the ecliptic is of prime importance: that angle defines the
seasons on any particular object [degrees].
 Surface temperature: the heat content of the physical or visible “surface” of
an object. Many celestial objects, like Earth, will possess a range of surface
temperatures [degrees Kelvin]
Orbital Properties:
 Distance from the Sun: the mean distance of an object from the Sun, during
the course of its orbit [kilometers, astronomical units]
 Orbital period: the time required to complete one revolution around the Sun
[days, years]
 Eccentricity: the degree to which an orbit deviates from a circle [0 to 1,
circular to extremely elliptical]
Atmospheres:
Composition: The constituents of a planetary atmosphere, as a function of
altitude.
Pressure & Temperature: Profiles with height above the surface
Weather: Motions of the atmosphere, precipitation, clouds
An example of comparative planetology:
 Mercury: essentially no atmosphere
 Venus: dense, hot, dry, 96% CO2, light winds, clouds of sulfuric acid
 Earth: 71% nitrogen, 28% oxygen, precipitable H2O, trace CO2, dynamic
weather, clouds of H2O
 Moon: none
 Mars: dry, extremely thin, cold, 95% CO2, winds, dust storms, H2O & CO2
clouds
 Jovian planets: hydrogen + helium composition, thick, dense, turbulent,
clouds of ammonia compounds
Surface features: What structures are observed on the object’s surface ?
 Overall texture: presence of mountain ranges, basins, volcanic features, ice
caps
 The number and size of impact craters
 Chemical composition across the surface
Surface processes: How is/was the surface modified ?
 Global geological activity, tectonic motions, earthquakes
 Volcanic activity; the ejection of liquid or gaseous material from the interior
 Effects of the atmosphere on the surface; erosion, winds, flowing liquids
Interiors: What’s inside ?
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Density, pressure & temperature, as a function of radius.
Composition change with radius
The nature of the core; rocky, metallic, solid, molten ?
Global magnetic field: Does a magnetic field exist ?
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Strength
Orientation