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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 ? 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 ? Strength Orientation