Download Planetary Physics and Chemistry 2012 1. Introduction and Overview

Planetary Physics and Chemistry 2012
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1. Introduction and Overview
1.1 Why study planetary science?
There are many reasons to study other planets, such as:
• Studying processes on other planets helps us understand our own planet. Often,
similar processes take place but in different environments or different ways. Thus we
can gain a more complete understanding of processes.
• Helps us understand the origin and evolution of the solar system hence the origin of
our planet. In order to understand the origin of our own planet, we have to consider
the solar system as a whole.
• Risk. Asteroids and comets have struck Earth before causing much destruction, for
example causing the extinction of the dinosaurs (according to the most widelybelieved hypothesis) and will do so in the future.
• Possible valuable resources on other bodies. Other planets/moons/asteroids might
contain valuable materials that could be mined.
• Colonisation of other worlds (not enough room/resources on Earth?)
• It’s interesting: There is much popular interest in space exploration, as exemplified by
various science fiction works (Star Wars, Star Trek etc.)
• To look for life elsewhere – both in own solar system and around other stars. In our
own solar system, Mars and Europa are places that scientists are looking for evidence
of life. Around other stars, about 270 “extra-solar” planets have so far been
discovered. These are almost all gas giants but 4 terrestrial “super-Earths” have been
found and more Earth-like planets are expected to be discovered within the next 10
years.
1.2 How did our knowledge of the solar system develop?
1.2.1 Basic observations
The ancient people spent a lot of time studying the skies and noted several basic
observations. A successful model of our solar system must explain these observations:
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Stars are fixed with respect to each other, but appear to rotate through the sky on a
daily basis. The path changes with the season.
The Sun and Moon move through the sky on a restricted path through the stars. This
path is called the Zodiac. Groups of stars known as constallations (12 of them) were
identified along this path. For the Sun, the exact path is called the ecliptic.
Planets (wanderers) also move through the stars on the same path (Zodiac), but more
slowly.
In general, all objects move in the same direction through the Zodiac, but sometimes
planets temporarily move backwards, which is known as retrograde motion.
Venus and Mercury always stay close to horizon (i.e., they are visible soon after
sunset or before sunrise), whereas other planets move through the entire sky.
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1.2.2 Geocentric and Heliocentric models
Expanations of these started off as either mystical/supernatural, or based on mathematical
ideas, the latter particularly with the ancient Greeks. The two major models are:
• Geocentric: The Earth is at the center of the solar system/universe
• Heliocentric: The Sun is at the center of the solar system
Early thinkers favoured the geocentric model. Major versions of this were
• Anaximander (610-546 BC): In his model, the planets and the stars are embedded in
concentric spherical shells surrounding the Earth.
• Ptolemy (100 AD): In his model the planets move in orbits around the Earth that are
described as perfect circles + epicycles. Epicycles are small circles superimposed on
the main circles and are useful to explain retrogrades. The Ptolomaic system fit the
major observations and it wasn’t until much more accurate observations were made
that it was superceded.
The heliocentric model didn’t get much support until ~1500 years later. The major
developments are:
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Copernicus (1473-1543) proposed the heliocentric model, to much controversy. The
circular orbits he proposed naturally explain retrogrades, but epicycles are still needed
to fit observations of planetary motions (because orbits are not perfect circles)
Tyco Brahe (1546-1601). Made very detailed observations of the movements of
various celestial objects (but didn’t try to explain them). Without using a telescope.
Johannes Kepler (~1610). Found a mathematical fit to Brahe’s observations: orbits
are elliptical, not perfectly circular. He came up with 3 laws regarding the orbits, as
discussed later.
Galileo Galilei (~1610). Was the first person to use a telescope to observe the sky. He
made various observations that support the heliocentric view of the solar system,
particularly:
o 4 moons orbiting Jupiter (Europa, Io, Ganymede, Callisto). If some objects
orbit another planet, then obviously everything does not orbit the Earth,
contradicting the geocentric model.
o Phases of Venus. Like the moon, we generally see only the part of Venus that
is lit by the Sun, but unlike the Moon the size of Venus changes greatly
because its distance changes. The observed phases can only be explained if
both Venus and Earth are orbiting the Sun, and not in the Ptolemaic system.
(see figures)
o The Moon is “imperfect”, with a heavily cratered and varied surface,
contradicting Ptolemy’s ideas that celestial bodies are perfect spheres.
In summary, the heliocentric model explains all of the basic observations listed earlier:
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The pattern of stars shifts with the season because the Earth is orbiting the Sun so the
side that is experiencing night (i.e., dark) faces different directions.
The Sun, Moon and planets move on the same path because everything is moving in
(almost) the same plane.
Retrogrades can be explained by the Earth ‘overtaking’ other planets in their orbits
(see diagram)
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Venus and Mercury stay close to the Sun (from Earth’s perpective) because they orbit
closer to the Sun than Earth does.
1.2.3 Fundamental physics of planetary motions
So far, our knowledge was only descriptive: Kepler’s Laws say HOW the planets move, but
not WHY. A physical understanding came thanks to Isaac Newton (1687). Using his law of
gravity and three laws of motion, Kepler’s laws can be derived from fundamental principals.
This explained almost everything about planetary motions, but not some detailed things,
particularly the precession of Mercury. This is because Newton’s laws are not perfect but are
an approximation valid at low velocity relative to the speed of light. Albert Einstein (18791955) discovered the full description of gravitational interactions with his General Theory of
Relativity, in which gravity can be explained as a distorsion of the space-time continuum.
With this, the precession of Mercury can be explained.
1.2.4 The modern era
Our knowledge of the solar system is still expanding, due mainly to telescope observations
and to spacecraft exploration.
Telescopes observe the skies at many different wavelengths, from radio waves to X-rays, and
come in a range of sizes. Radio telescopes are huge dishes tens of meters across, whereas the
largest optical telescopes are 8-10 meters diameter. Telescopes can observe many things,
including the surfaces, atmospheres, orbits, shapes and rotation rates of solar system objects.
They record reflected radiation (i.e., sunlight reflected by the object) or radiation emitted by
the object. Using these various observations information can be deduced about composition
(surface & atmosphere), mass, internal mass distribution, temperature, and so on.
Bodies can look different at different wavelengths. For example:
- In the class presentation we see the Sun in visible light, ultraviolet, far UV, and X-rays. As
higher frequency corresponds to higher temperatures, these views bring out regions of higher
and higher temperature.
- Venus looks like an opaque ball in visible light. In the ultraviolet clouds become visible,
whereas radar can see straight to the surface.
Spacecraft exploration of planets. This started in 1959 with the USSR Luna 1 mission to
the moon.
• Unmanned spacecraft have visited every planet except Pluto, whereas
• Humans have been only to the Moon (the Apollo missions of the USA in the late
1960s and early 1970s).
• Unmanned spacecraft have landed on the Moon, Venus (Venera; USSR), Mars
(several: Viking, Pathfinder, Rovers), and most recently, Titan, the major moon of
Saturn (Cassini-Huygens 2005). Additionally,
• a probe was dropped into Jupiter by the Galileo spacecraft.
Some important missions for this course are:
• Voyager 1&2 (Jupiter, Saturn, Uranus, Neptune) 1970s-1990
• Magellan (Venus) early 1990s
• Galileo (Jupiter system) late 1990s
• Mars Global Surveyor late 1990s
• Mars Odyssey NOW
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Mars Exploration Rovers NOW
Cassini/Huygens (Saturn system) NOW
1.3 Tour of the solar system
1.3.1 Overview
Planet
Distance
(AU)
Radius
(km)
Mass
(Earth=1)
Mercury
0.3871
2440
0.0553
Venus
0.7233
6052
0.8150
Earth
1.0
6378
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Mars
1.5237
3396
0.1074
Jupiter
5.2026
71492
317.710
Saturn
9.5549
60268
95.162
Uranus
19.2184
25559
14.535
Neptune
30.1100
24766
17.141
Pluto
39.5447
1150
0.002
1 AU = the Sun-Earth distance = 1.496x1011 m
Mass of Earth=5.974x1024 kg
Sideal day = rotation period relative to stars
Mean
Density
(g/cm3)
5.43
5.20
5.52
3.91
1.33
0.69
1.318
1.638
~2.0
Sidereal
day
58.64d
243.02d
23.9345h
24.6230h
9.925h
10.6562 h
17.24 h
16.11 h
6.3872 d
Orbital
period
(days)
87.97
224.69
365.24
686.93
4330.60
10,746.94
30,588.7
59,799.9
90,589
The Sun contains 99.9% of the mass of the solar system and is far larger than Jupiter, the
largest planet, which in turn is ~10 times larger than Earth. See lecture slides for some
comparisons.
The planets can be grouped into the
• rocky inner terrestrial planets (Mercury, Venus, Earth, Mars) and the
• large outer gas or ice giants (Jupiter, Saturn, Uranus, Neptune).
• Pluto is the odd one out and is no longer considered a full planet but rather a “dwarf
planet”, together with many other small icy solar system objects.
The main features of the planets are as follows:
1.3.2 Planets
Mercury has an old, heavily cratered surface, similar to the Moon. Until the recent
Messenger mission, only half of Mercury had been photographed by spacecraft. One unusual
feature is scarps (cliffs) that cross its surface, which may have been caused by the planet
shrinking as it cooled.
Venus is similar size to Earth but quite different in many respects. It has a dense atmosphere
(about 90 times denser than Earth’s) containing clouds. It has a varied surface with an
average age of about 600 million years containing tectonic features and volcanic features,
with no plate tectonics.
Earth is the only planet with current plate tectonics. Most earthquakes and volcanoes occur
at or near plate boundaries.
Mars is smaller than Earth but is the most similar in its surface geological features. It has a
thin atmosphere with (water-ice) clouds. Its surface is quite old, with many craters, but has
interesting features including rift valleys, large volcanoes and sand dunes. Its crust has a
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dichotomy: smooth northern lowlands and more cratered southern highlands. Mars has two
small moons that look like asteroids.
Jupiter is the largest planet (~11 times Earth) and is made mostly of gas. It doesn’t have a
solid surface but instead the gas gets denser going inwards. It has colourful clouds that reveal
a banded, time-dependent atmospheric structure, with the atmospheric wind going in bands of
alternating directions. A prominent feature is the “Great Red Spot”. It has many moons.
Saturn is almost as large as Jupiter (~9 times Earth) and also made mostly of gas, but is
much less massive. It’s atmosphere is less colourful than Jupiter’s but has similar dynamics.
It’s impressive rings have a lot of small-scale structure including bands, gaps and spokes.
Uranus & Neptune are large (~4 times Earth) but mostly “ice” with an outer layer of gas.
Neptune has banded atmospheric features similar to Jupiter’s but these are not visible on
Uranus.
Pluto is a small, icy body with a (relatively large) moon Charon. The discovery of several
other icy bodies beyond Pluto has led to the demotion of Pluto from a full planet to a “dwarf
planet”.
1.3.3 Small objects
Asteroids: Mostly found between Mars and Jupiter, these are irregularly-shaped rocky or
metallic bodies ranging in size from ~500 km to < 1 km. They are thought to be ancient
bodies left over from solar system formation. Meteorites (rocks that fall to Earth from space)
mostly come from asteroids.
Comets are icy bodies beyond the orbit of Neptune, often on highly elliptical orbits that bring
them through the inner solar system. In the inner solar system the ice sublimates (evaporates)
taking dust with it and creating two tails: a dust tail and an ion tail.
1.3.4 Moons
Terrestrial planets:
• Earth’s Moon is the only large moon of a terrestrial planet. It has a very old surface
with highly cratered, light-coloured highlands and less cratered, dark-coloured
lowland ‘maria’, which are probably due to volcanic lava infilling low-lying areas.
• Mars has two small moons, Phobos and Diemos, that look like asteroids
Outer planets have many (10s of) moons. Some of them larger than Earth’s moon, and one
of them (Ganymede) is larger than Mercury, so these might be considered planets if they
were orbiting the Sun. These moons have a range of sizes down to very small (~few km),
more of which are still being discovered. Most outer planet moons have icy outer layers +
rocky interiors
• Jupiter has four large (“Galilean”) moons, which trend in composition going
outwards: Io has a rocky surface. It is the most volcanically active body in the solar
system, due to heat generated by tidal flexing by Jupiter. Europa has a young, icy
surface which seems to be undergoing some sort of tectonics. Ganymede and
Callisto have older, heavily cratered icy surfaces.
• Saturn has one big one moon – Titan – the only moon with a major atmosphere,
several intermediate sized moons and many smaller ones. Some of the smaller ones
are inside the rings, where the other ones are all outside the rings.
• Uranus has 5 intermediate-sized moons plus several smaller ones.
• Neptune has 1 intermediate-sized moon (Triton) plus several smaller ones.
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1.4 Constraints on composition
1.4.1 Density
The most important constraint on the composition of a solar system body is its density. This
can be compared to the densities of commonly-available solar system materials to estimate its
composition. The composition of its outermost layer is constrained by observations of its
surface (visual and spectra).
How can density be measured? As density = mass / volume, this requires knowing the mass
and the volume.
• The volume is easily calculated from its diameter, which is easily observed and
measured.
• Mass can be determined by a body’s gravitational influence on surrounding objects.
This is straightforward if it has a moon: a moon’s orbital period is related to the mass
of the planet and the orbital distance, through Kepler’s 3rd law. If there is no moon,
then its influence on more distant bodies must be studied, for example the planets
have a small but measurable effect on other planets’ orbits.
Typical densities are:
• Terrestrial planets 4-5.5 g/cm3.
• Gas and ice giant planets 0.7-1.6 g/cm3.
• Icy moons ~2 g/cm3.
NOTE: g/cm3 is not an SI unit – convert densities to kg/m3 (multiply by 1000) before doing
any calculations using them.
For bodies larger than Mercury and the Moon it is important to take into account selfcompression, i.e., the effect of interior pressure on density. Planets have high pressures in
their interiors, and pressure increases density. Without this effect, Earth would have a mean
density of 4.1 g/cm3 instead of its actual value of 5.5 g/cm3. It is important to correct for this
effect before comparing densities, i.e., compare the ‘uncompressed’ density.
1.4.2 Available materials
What materials are available to make a planet? The abundances of elements in our solar
system are well-known and are given by the cosmic abundances, i.e., abundances in the Sun,
which are similar to the abundances of (non-volatile) elements in primitive meteorites.
From these cosmic abundances can be derived the compounds available to make planets. The
most abundant elements are H and He, which are dominant in Jupiter and Saturn, but not in
other planets. Two important points are:
1. Elements join together to make compounds (water, carbon dioxide, quartz, calcite, …)
2. Solid planets are made of substances that were solid at the (HOT) temperature that the
planets formed, so gases and ‘volatiles’ are missing, and ‘icy’ substances only found in outer
solar system
The compounds that form depend on the relative abundances of H and O, which are two of
the most abundant and most reactive elements and are involved in most common substances,
e.g., CH4, CO2, H2O, NH3, O2.
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If O is more abundant=> ‘oxidizing’ environment, lots of O compounds form. The inner,
terrestrial planets tend to be oxidized.
If H is more abundant=> ‘reducing’ environment, lots of H compounds form. The outer
planets tend to be reduced
In general, there are four types of planet-forming matter
Gas: mostly H and He
Ice (‘volatiles’): e.g., H2O, CO2, CO, NH3, CH4. These are solid at low temperatures, as exist
in the outer solar system but at inner solar system temperatures these are ostly gas. When
solid they have an uncompressed density of around 1 g/cm3.
Rock: Rocks are made of silicon oxide, i.e., silicate, plus some amounts of Mg, Fe, Al, etc..
They have an uncompressed density ~3 g/cm3.
Metal: This means mostly Fe, but with some Ni and perhaps lighter elements too. The
uncompressed density is ~ 8 g/cm3.
Often ‘rock’ and ‘metal’ are lumped together into one ‘rock’ category, as they are in
primitive meteorites.
1.4.3 Planetary compositions
The four terrestrial planets have rocky surfaces, which means that they are made of a
mixture of rock and metal.
• Because they formed hot, the metal and rock have gravitationally separated into a
metal core and a rocky mantle+crust.
• There is a trend of decreasing uncompressed density with increasing distance from the
Sun (5.3, 4.0. 4.1, 3.7 g/cm3). This means that Mercury has a very large core, while
Mars has a relatively small core, and Earth and Venus are similar.
• An anomaly is Earth’s moon, which has a density close to that of pure rock, indicating
a very small core. Thus, while the Moon and Mercury have similar-looking surfaces,
they are quite different in their average composition and their core size!
Jupiter and Saturn (1.3 & 0.7 g/cm3) are made mostly of gas, i.e., H with some He.
• The high pressures greatly increase the density of this gas! Even so, Saturn has a
density lower than than of water.
• Their densities cannot be matched by pure H and He- some heavier material is
necessary in addition. Thus, it is thought that they have dense ‘cores’ of about 10
Earth masses, probably made of rock (and metal) and ice.
Uranus and Neptune (1.3 & 1.6 g/cm3). Because they are small than Jupiter and Saturn, they
have much less compression, so plenty of heavier material is necessary to match their
densities. They are thought to be made mostly of “ice”, with an outer layer of “gas” and some
“rock” in their centers.
Moons of outer planets typically have a density of around 2 g/cm3, implying a mixture of
“ice” and “rock”.
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