General Astronomy, 29:61
Fall, 2005
December 12, 2005
Glossary: Mars and the Outer Planets (Material of Last Four Weeks)

Temperatures of Planets

• Equilibrium Temperature: The temperature at which the power radiated to space by a planet equals the power absorbed in the form of sunlight.
• Albedo: The reflectivity of a celestial object. Precisely, the fraction of radiation power which is reflected into space rather than absorbed on the planetary surface. An albedo of 0 indicates all sunlight is absorbed. An albedo of 1 indicates all power is reflected back to space.
• Greenhouse Effect: The blocking of the flow of blackbody radiation to space by absorption at infrared wavelengths in the atmosphere of a planet. The Greenhouse Effect results in an increase in the surface temperature of a planet.
• Blackbody Radiation: A physics term referring to the properties of electromagnetic radiation emitted by objects at finite temperatures. Blackbody radiation laws predict the amount of power radiated per unit surface area (Stefan-Boltzmann Law) and the wavelength at which a blackbody radiator is brightest (Wien's Law).

Mars

• Valley Networks: River-size dry river channels which are found in the oldest Martian terrains.
• Outflow Channels: Huge dry water channels which apparently occurred over a longer geological period than the Valley Networks.
• Martian Geological Eras: (1) Noachian, 4.5 - 3.5 Gyr ago, (2) Hesperian, 3.5 - 2.9 Gyr ago, (3) Amazonian, 2.9 Gyr - present.
• ``Warm Wet Mars'': A theory about the remote past of Mars which holds that 3.5 - 4.5 Gyr ago Mars had a much denser atmosphere and warmer temperatures than now, and that liquid water was stable on the surface, allowing lakes and seas to exist.
• ``Cold Dry Mars'': The opposite point of view, which contends that the surface of Mars has been pretty much as it presently is for the whole history of the solar system.
• Hematite: An iron oxide (Fe$_2$O$_3$) which, on Earth, forms in the presence of water. Hematite is found in abundance in the Meridiani Planum region of Mars where the Opportunity spacecraft landed.

The Jovian Planets

• Liquid Metallic Hydrogen: An extreme form of matter in which hydrogen becomes a dense, electrical current-conducting liquid. It is almost beyond our capabilities to produce in terrestrial physics laboratories, but forms most of the mass of Jupiter and Saturn.
• Magnetosphere: A region of space around a planet which is controlled by its magnetic field. The magnetospheres of the planets with the strongest magnetic fields, Jupiter, Earth, and Saturn, occupy volumes much larger than the planets themselves and deflect the solar wind. Within the magnetosphere are regions of trapped energetic particles, which on Earth comprise the Van Allen belts.
• Cassini's Division: A gap in the ring of Saturn which is produced by a 2:1 resonance with the moon Mimas.
• Roche Limit: The mimimum distance that a small object, held together by its own gravity, can get to a more massive object before it is pulled apart by the tidal stresses caused by the massive object. It is approximately 2.5 times the radius of the more massive object. The ring of Saturn lies within the Roche limit for Saturn.
• Resonant Perturbation: A situation in celestial mechanics when a perturbation of a celestial object is periodic, and has a period which is commensurate with the period of the object being perturbed, i.e. twice the period of the perturber equals three times the period of the object being perturbed. Resonant perturbations can strongly and quickly alter the orbital elements of a celestial object.
• Kirkwood's Gaps: Gaps in the distribution of semimajors axes of asteroids, due to resonant perturbations from Jupiter.