Review Lecture 10

1) The solar system was formed from gravitational collapse of an enormous gas cloud, mostly hydrogen (74%) and helium (24%), but also some heavier elements present from burned out stars.

From radioactive decay dating, using very long-lived, naturally occurring, radioactive elements in the Earth’s crust 10 232 139. x 10 years 208 90Th→ 82 Pb one estimates the age of the Earth and the solar system to be 4.6 ± 1 billion years old. This can be compared to the estimated time for the gravitational accretion process to form the solar system of 100,000 years.

2) The temperature within the gaseous nebula surrounding the forming sun determined what elements coalesced out into planets. A) Refractory elements coalesced first to form Mercury, Venus, Earth, and Mars, the terrestrial planets. B) Hydrogen, helium, and methane formed the gas giants Jupiter, Saturn, Uranus, and Neptune further out . C) Pluto is a small ball of ice and rock. 1 3) The cooling rate of planets depends on the radiation emitted - . radius2 4) The primary atmosphere has been modified by gases escaping the gravitational attraction of the planets. The average velocity of an atom or molecule is given by Maxwell-Boltzmann 3kT v = . average m When this is on the order of the escape velocity from the planet the atom or molecule is lost to outer space. Thus, Earth no longer has free hydrogen in its atmosphere.

A picture of the gas cloud coalescing into a star.

a) A slowly rotating portion of a large nebula becomes a distinct globule and a mostly gaseous cloud collapses by gravitational attraction. This is called Helmholtz Contraction. b) The resultant distribution collapses to a disk, gravity flattening the cloud in one dimension, rotation of the cloud preventing complete collapse. The result is an equatorial disk with a dense central mass. c) Heat generated by the collapse heats the central mass. Radiation warms the inner part of the nebula, possibly vaporizing preexisting dust. Then, as the nebular cools, condensation produces solid grains that settle to the central plane of the nebula. d) The dusty nebula clears either by dust aggregation into larger particles (planets or planetesimals) or by ejection during a T-Tauri stage of the star’s evolution. A star energized by nuclear fusion and a system of cold bodies remains. Gravitational accretion of these small bodies eventually leads to the development of a small number of major planets. 5) Terrestrial Planet History

A) Undifferentiated core, molten magma surface, plastic asthenosphere in between.

B) Heat melts heavy metals - iron and nickel - which drop to form core. Inner core solid under heavy pressure Outer core molten

C) Rocky mantle covers the core. Crust and upper mantle ride on a plastic astheno- sphere. Crust broken into plates which butt together producing seismic activity.

D) Rifts allow new magma to replace old rock.

E) Heat radiates away as crust thickens, asthenosphere hardens and molten elements solidify.

F) The timing and relative important of these processes are unique to each planet and are determined by the planets composition, mass, and distance from the Sun.

Sources of planetary heating are shown below. The Inner Planets Mercury Venus Earth Mars Average Distance From Sun (106 57.91 108.2 149.60 227.93 km) Average Distance From Sun (AU) 0.3871 0.7233 1.0000 1.5236 Orbital Period (years) 0.2408 0.6152 1.0000 1.8808 Orbital Eccentricity 0.206 0.007 0.017 0.093 Inclination of Orbit to the Ecliptic 7.00o 3.39o 0.00o 1.85o Equatorial Diameter (km) 4880 12,104 12.756 6794 Equatorial Diameter (Earth = 1) 0.383 0.949 1.000 0.533 Mass (kg) 5.5 x 1023 4.9 x 1024 6.0 x 1024 6.4 x 1023 Mass (Earth = 1) 0.0553 0.8150 1.0000 0.1074 Average Density (kg/m3) 5430 5243 5515 3934 The Outer Planets Jupiter Saturn Neptune Uranus Pluto Average Distance From 778.30 1431.9 2877.4 4497.8 5914.7 Sun (106 km) Average Distance From 5.2026 9.5719 19.194 30.066 39.537 Sun (AU) Orbital Period (years) 11.856 29.369 84.099 164.86 248.60 Orbital Eccentricity 0.048 0.053 0.043 0.010 0.250 Inclination of Orbit to the 1.30o 2.48o 0.77o 1.77o 17.12o Ecliptic Equatorial Diameter (km) 142,984 120,536 51,118 49,528 2300 Equatorial Diameter 11.209 9.449 4.007 3.883 0.180 (Earth = 1) Mass (kg) 1.90 x 1027 5.69 x 1026 8.68 x 1025 1.02 x 1026 1.32 x 1022 Mass (Earth = 1) 317.8 95.16 14.53 17.15 0.002 Average Density (kg/m3) 1326 687 1318 1638 2000