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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
90 Th
232
1 .39 x10 10 years
    → 82 Pb 208
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.
3) The cooling rate of planets depends on the radiation emitted -
1
.
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
v average =
3 kT
.
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 asthenosphere. 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
km)
57.91
108.2
149.60
227.93
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
5.5 x 1023
4.9 x 1024
6.0 x 1024
6.4 x 1023
0.0553
0.8150
1.0000
0.1074
5430
5243
5515
3934
Mass (kg)
Mass (Earth = 1)
Average Density (kg/m3)
The Outer Planets
Jupiter
Saturn
Neptune
Uranus
Pluto
Average Distance From
Sun (106 km)
778.30
1431.9
2877.4
4497.8
5914.7
Average Distance From
Sun (AU)
5.2026
9.5719
19.194
30.066
39.537
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
Ecliptic
1.30o
2.48o
0.77o
1.77o
17.12o
Equatorial Diameter (km)
142,984
120,536
51,118
49,528
2300
Equatorial Diameter
(Earth = 1)
11.209
9.449
4.007
3.883
0.180
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
Mass (kg)