chapter 7.2 composition and structure of planets & 7.3 dating planetary surfaces

giant planets; jupiter and saturn

• jupiter and saturn have similar chemical compositions

  • 75% hydrogen 25% helium

  • so large that the hydrogen becomes a liquid due to compression

• jupiter and saturn have cores of heavier rock, metal, ice

• biggest

• nearly all oxygen present is combined with hydrogen to form water

• hydrogen-dominated composition is referred to as reduced

giant planets; uranus and neptune

• smaller than jupiter and saturn

• core of rock, metal, ice

• less efficient at attracting hydrogen and helium gas (smaller atmospheres in proportion to their cores compared to jupiter and saturn)

• dominated by hydrogen

• nearly all oxygen present is combined with hydrogen to form water

• hydrogen-dominated composition is referred to as reduced

terrestrial planets

• composed primarily of rocks and metals

• most abundant rocks (silicates); made of silicon and oxygen

• most common metal found is iron

• mercury; greatest proportion of metals (most dense)

• moon; lowest proportion of metals (least dense)

• earth, venus, mars; 1/3 iron-nickel or iron-sulfur combinations, 2/3 silicates

• largely composed of oxygen compounds (such as the silicate minerals of their crusts), their chemistry is said to be oxidized

• densest material in the core, lighter silicates near surface (heavier elements sink due to gravity) (differentiation)

differentiation

the process by which gravity helps separate a planet’s interior into layers of different compositions and densities

• heavier metals sink to form a core, while the lightest minerals float to the surface to form a crust

• later, when planets cool, this layered structure is preserved

• in order for a rocky planet to differentiate, it must be heated to the melting point of rocks, which is typically more than 1300K

moons, asteroids, and comets

• Earth’s Moon; chemically and structurally like the terrestrial planets

• moons of outer solar system; compositions similar to the cores of the giant planets around which they orbit

• The three largest moons—Ganymede and Callisto in the jovian system, and Titan in the saturnian system—are composed half of frozen water, and half of rocks and metals

• Most of these moons differentiated during formation, and today they have cores of rock and metal, with upper layers and crusts of very cold and—thus very hard—ice

• some larger asteroids (e.g. Vesta) appear to be differentiated

• asteroids and comets retain their original composition, thus they represent relatively unmodified material dating back to the time of the formation of the solar system.

temperatures within the solar system

• farther a planet or moon from Sun, the cooler the surface

• radiant energy of the Sun gets weaker with the square of the distance

• mercury: 280–430 °C on its sunlit side,

• pluto: –220 °C,

• temperature of a planet can be influenced by its atmosphere

counting the craters

• estimate the age of a surface by counting the number of impact craters works because the rate at which impacts have occurred has been roughly constant for several billion years

• number of craters is proportional to the length of time the surface has been exposed

radioactivity

used to measure the age of rocks

• Around the beginning of the twentieth century, physicists began to understand that some atomic nuclei are not stable but can split apart (decay) spontaneously into smaller nuclei.

• The process of radioactive decay involves the emission of particles such as electrons, or of radiation in the form of gamma rays

• We should also note that the decay of radioactive nuclei generally releases energy in the form of heat

half-time

specific time period where if there is a large number of radioactive atoms of one type during which the chances are fifty-fifty that decay will occur for any of the nuclei