6.7 Key Terms
Anorthosites: silicate rocks that make up most of the Moon’s crust and are made of relatively low-density rock that solidified on the cooling Moon like slag floating on the top of a smelter. 6.3
Bar: a force of 100,000 Newtons acting on a surface area of 1 square meter; the average pressure of Earth’s atmosphere at sea level is 1.013 bars. 6.2
Caldera: volcanic crater on Venus. 6.5
Convection: the rising of hot materials. 6.5
Coronae: large circular or oval features on that are produced by bulges that form from the upwelling lava below the surface of a planet. 6.5
Deimos: one of Mars’ two small and odd-shaped moons that were most-likely captured asteroids or the result of a collision with Mars. 6.4
Differentiation is the process by which gravity helps separate a planet’s interior into layers of different compositions and densities.
Impact basins: huge depressions produced by collisions of large chunks of material with the Moon relatively early in its history. 6.3
Lunar highlands: the lighter, heavily cratered regions of the Moon, which are generally several kilometres higher than the maria. 6.3
Maria: areas of the Moon consisting mostly of flat plains with dark-coloured basalt (volcanic lava). 6.3
Outflow channels: a set of water-related features on Mars that were carved by huge volumes of running water, far too great to be produced by ordinary rainfall. 6.4
Ozone (O3): a heavy form of oxygen with three atoms per molecule instead of the usual two. 6.2
Permanent (residual) polar caps: polar caps that are always present near the poles on Mars. 6.4
Phobos: the larger of Mars’ two small and odd-shaped moons that were most-likely captured asteroids or the result of a collision with Mars. 6.4
Recurring slope lineae: dark streaks on Mars that elongate within a period of a few days, indicating that something is flowing downhill—either water or dark sediment. 6.4
Runoff channels: multitudes of small, sinuous (twisting) channels on Mars that look like what geologists would expect from the surface runoff of ancient rain storms. 6.4
Scarps: cliffs on Mercury that are more than a kilometer high and hundreds of kilometres long. 6.6
Seasonal polar caps: deposits of frozen CO2 (dry ice) on Mars that condense directly from the atmosphere when the surface temperature drops below about 150 K and develop during the cold martian winters. 6.4
Semimajor axis: a planet’s average distance from the Sun (or from its star) on its orbit. 6.6
Smaller gullies: a type of water feature on Mars that suggests intermittent outbreaks of liquid water even today as they have the remarkable property of changing regularly with the martian seasons. 6.4
Stratoshphere: the layer of Earth’s atmosphere above the troposphere and below the ionosphere. 6.2
Troposphere: a region of convection in a planet’s atmosphere. 6.5
Volatiles: elements and compounds that evaporate at relatively low temperatures. 6.3
