The Solar System

We begin with a tour (in natural color or black and white, unless otherwise specified) of some of the well-photographed bodies in our Solar System. We've seen many of these already, but it is instructive to see them in the context of their brethren. Both the planets and their moons are ordered with increasing orbital distance; asteroids and comets appear alphabetically. As you take the tour, pay attention to atmospheric features such as density, coloration and movement:

	Mercury, the innermost planet.	source
	Venus, second from the Sun.	source
	A radar map of Venus.	source
	Earth; hurricane Andrew is about to make landfall.	source
	A panorama of the meteor crater near Winslow, AZ.
	An aurora on Earth.	source
	Earth's Moon (Luna).	source
	The far side of the Moon.	source
	Mars, closest of the outer planets.	source
	A radar map of Mars.	source
	Impact ejecta on Mars.	source
	Volcanic rilles (trenches) on Ascraeus Mons, on Mars.	source
	Lava flows from Olympus Mons on Mars (not true color).	source
	Hecate Thola lava dome on Mars.	source
	Phobos, moon of Mars.	source
	Deimos, moon of Mars.	source
	Ceres, a dwarf planet, largest object in the asteroid belt.	source
	Eros, an asteroid.	source
	Gaspra, an asteroid.	source
	Ida, an asteroid.	source
	Lutetia, an asteroid.	source
	Mathilde, an asteroid.	source
	Vesta, an asteroid (south pole).	source
	Jupiter, the largest planet.	source
	X-ray auroras on Jupiter.	source
	Io, one of the Galilean moons of Jupiter.	source
	Tvashtar, a volcano on Io (color enhanced).	source
	Europa, one of the Galilean moons of Jupiter.	source
	Ganymede, one of the Galilean moons of Jupiter.	source
	Callisto, one of the Galilean moons of Jupiter.	source
	Saturn, sixth planet from the Sun.	source
	Ultraviolet auroras on Saturn.	source
	Mimas, moon of Saturn.	source
	Enceladus, moon of Saturn.	source
	Geysers of Enceladus.	source
	Tethys, moon of Saturn.	source
	Dione, moon of Saturn.	source
	Rhea, moon of Saturn.	source
	Hyperion, moon of Saturn.	source
	Titan, moon of Saturn.	source
	Radar image of methane seas on Titan.	source
	A radar map of Titan.	source
	Iapetus, moon of Saturn.	source
	Uranus, the penultimate planet.	source
	Miranda, moon of Uranus.	source
	Ariel, moon of Uranus.	source
	Titania, moon of Uranus.	source
	Neptune, furthest of the true planets.	source
	Triton, moon of Neptune.	source
	Comet Hartley 2.	source
	Pluto, dwarf planet.	source
	Charon, moon of Pluto.	source
	Comet Tempel 1.	source
	Comet Wild 2.	source
	The planets to scale.	source
	The moons to scale.	source
	The orbits of the outer planets.	source
	The orbits of over 1400 potentially hazardous asteroids known as of early 2013.	source
	Some of the smallest constituents of the Solar System are dust particles.	source

Yellow and orange coloration are largely due to sulfur compounds, although in Titan's case its coloration comes from a heavy hydrocarbon haze. Jupiter's ammonia content also affects its color, and can be seen in the greenish tints reflected from the night sides of the Galilean moons. Methane absorbs light in the red and yellow wavelengths, leading to Uranus' (with less methane) and Neptune's (with more methane) coloration.

Also look closely for surface features which might give clues to past tectonic, volcanic, geyser or meteor activity. Large scale mountain ranges are indicative of tectonic plate collision (ie., the Rocky or Himalayan ranges). Distinguish between volcanic and meteor craters by looking for impact debris and ejecta (some craters could be calderas). Look for volcanic rilles (trenches), lava flows and domes, as well as ridges and scarps (cliffs) formed by cooling and shrinking. Note the presence of liquids, dust or ice, and evidence of major collisions. What do these tell you about the surface history?

To get a feel for how difficult this business is, read this analysis of Europa.

(View Cosmos DVD 4, episode 6, on Voyager at Europa.)
(View Cosmos DVD 3, episode 4, comet simulations, on the Tunguska Event.)

This image shows zodiacal light scattered by solar system dust. It also shows the Milky Way (home of our solar system), and that the plane of our solar system is not parallel to the plane of our galaxy.

A meteoroid is a small (< 100 m) piece of interplanetary debris, often found along the orbits of comets. A meteor is the bright streak that meteoroids make as they burn up in the atmosphere. A meteorite is a meteoroid that survives the atmosphere to hit the surface. Here is a movie of the Peekskill fireball (.95 Mb) (source).

The Minor Planet Center has produced animations of small objects in the Solar System. The TNOs (Trans-Neptunian Objects) in the outer Solar System animation are also known as Kuiper Belt objects.

Portfolio Exercise: Categorize each body in the list above as to the distinguishing feature(s) of its landscape: tectonic, volcanic, geyser or meteoric. You may have some bodies in which multiple features are present.
For any characterization other than meteoric, find NASA images which support your categories; include them (with source references) in your portfolio. Annotate the pictures, circling and identifying the features which support your categorization.

Planetary magnetic fields are believed to be the product of rotation and liquid conducting fluids in their cores. We have been able to measure the magnetic fields of some of these bodies using space probes:

Body	Magnetic Field (Earth)
Sun	2
Mercury	0.011
Venus	0.001
Earth	1
Mars	0.001
Jupiter	13.89
Saturn	0.67
Uranus	0.74
Neptune	0.43

The designation "(Earth)", here and below, indicates that the number is a multiple of Earth's value.

When charged particles from the Solar wind encounter Earth's magnetic fields, the fields are distorted, and energy is transferred into the atmosphere, causing the electrons in Oxygen and Nitrogen atoms to transition to a higher energy level. When they relax, photons are emitted, which we see as aurora. Molecular Oxygen emits photons in the green part of the optical spectrum, while molecular Nitrogen emits in the blue. Atomic Oxygen exists at high altitudes; it emits red photons.

There is a good movie of terrestrial auroras (2.43 Mb) from the POLAR mission (source), and another taken from the International Space Station (reported at SpaceWeather.com on September 22, 2011). Note that more intense magnetic fields result in higher energy photons; hence Saturn's UV auroras, and Jupiter's x-ray auroras.

Temperature and Atmosphere

By measuring the wavelength of maximum power output, and using Wien's Law:

wavelength of maximum thermal output_Angstroms = 2.9 * 10⁷ Angstroms / temperature_K

we can measure the effective (black body) temperature of many of these bodies. Using Stefan's Law:

power emitted or absorbed per unit area = 5.67 * 10^-8 W/K⁴ * temperature⁴

and the fact that intensity decreases with the square of the distance, we can compare the intensity of the radiation they receive to that which Earth receives (1365 W/m²), and the ratio of their output to input intensity I_out/I_in (for moons, we include the input both from the Sun and from their host planet).

In addition, for some of these bodies we have been able to measure the surface temperature using space probes. Using these temperatures, we can learn something about possible atmospheres. If the average molecular speed of a gas:

average molecular speed = 157 * (temperature / molecular weight_{H = 1})^1/2

is less than 1/6 of the planet's escape velocity at the surface:

escape velocity = (2 * 6.674 * 10^-11m³/(kg s²) * mass / radius)^1/2

there is a high probability that the gas still exists in an atmosphere near the surface. We can use these two equations to find the minimum molecular weight of any atmosphere for these bodies (the majority atmospheric component has been provided for comparison purposes, when known):

Body	Teff_K	I_Sun (Earth)	I_out/I_in	Max Tsurface	Min Mol. Wt.	Major Atmos. Comp.	Surface Atmos. Pressure (Earth)
Sun	5777				0	H
Mercury	533	6.674	1.005	700	34		2 * 10^-12
Venus	227	1.911	0.2308	735	6	C O₂	90
Earth	255	1	0.7025	331	2	N₂	1
The Moon	387	1	3.727	396	62		1 * 10^-12
Mars	217	0.4308	0.8553	268	9	C O₂	.006
Jupiter	125	0.03694	1.098		0	H₂
Io	128	0.03694	1.198		17
Europa	128	0.03694	1.204		27		10^-7
Ganymede	128	0.03694	1.206		15
Callisto	128	0.03694	1.207		19
Saturn	95	0.0101	1.231		0	H₂
Titan	85	0.0101	0.7883	94	11	N₂	1.6
Uranus	57	0.002715	0.646		0	H₂
Miranda	86	0.002715	3.327		2044
Ariel	84	0.002715	3.038		239
Neptune	59	0.001106	1.82		0	H₂
Triton	38	0.001106	0.3126		15	N₂	1.4 * 10^-5

"I" denotes intensity. The closer I_out/I_in is to 1, the nearer the body is to being in thermal equilibrium; values less than one indicate the body is warming (within the accuracy of the data, and Wien's and Stefan's laws, which are both approximations).

Some possible atmospheric components of interest are:

Atom or Molecule	Molecular Weight
H₂	2
He	4
C H₄ (methane)	16
N H₃ (ammonia)	17
H₂ O	18
C O	28
N₂	28
C₂ H₆ (ethane)	30
O₂	32
Ar	40
C O₂	44
C₃ H₈ (propane)	44

Weather

Weather is a natural characteristic of bodies with atmospheres, and wherever there is a large amount of energy available, the storms can get very large indeed. Here is a GOES movie (28.25 Mb) of hurricane Isabel (2003). (source)

Saturn displays a hexagonal jet stream about its north pole. (source)

(View Cosmos DVD 4, episode 6, on Jupiter, the Red Spot and Neptune.)

After viewing these large-scale weather patterns on various planets, it is interesting to observe something much smaller on Earth and on Mars:

Dust devils on Earth.

Martian Dust Devils as photographed by Spirit. (source)

Two dust storms, one on Earth off the northwest coast of Africa, and one on Mars in the north polar regions. (source)

Dust is responsible for heat transport on Mars much as water is here on Earth.

Gravity Waves are seldom noticed in action because of their long periods; this movie (4 Mb) is a time lapse over 40 minutes. (source)

Solar System Formation

Temperatures in the protoplanetary disc are much higher in the central regions where the star is forming, due to the increased density and pressure. Further away the temperatures cool quickly, with the outer regions cooling faster. The general makeup of planets as a function of distance from the protostar can be understood by the various melting points of the materials involved:

Material	Melting Point (K)
Iron	1808
Silicon	1684
Ice	273
Ammonia	195
Methane	91

So we have gas giants beyond the so-called snow line (beyond which water freezes), and rocky planets inside.

AU Microscopii is a red dwarf star approximately 12 million years old with a debris disc:

AU Microscopii; the radius of the cleared central region is approximately 7 AU. (source)

HD 107146 is a yellow star much like our Sun but between 30 and 250 million years old; the radius of the cleared central region is approximately 40 AU. (source)

In comparison, the Kuiper Belt (home to short period comets) extends to about 50 AU from the Sun. The Oort Cloud (home to long period comets) has a radius of approximately 100000 AU. Eta Corvi has been observed to possess a region similar to the Kuiper Belt.

The mass of Comet Halley is about 10¹⁴ kg. It is about 40% water and about 20% organic compounds. Assuming that it is an average comet, and that comets were responsible for the presence of water on the Earth, we can compute the frequency of cometary impact in the Earth's early history. These impacts occurred mainly during a period of about 200 million years, approximately 4 billion years ago. If the total amount of water on the Earth is 1.65 * 10²¹ kg, it would have taken 41.25 million cometary impacts, or an average of about 5 impacts a year. These impacts would have deposited a further 8.25 * 10²⁰ kg of organic material.
The ratio of Deuterium (²H) to Hydrogen (¹H) in the Earth's oceans has been shown to be substantially the same as that in at least one Kuiper Belt comet (103P/Hartley 2).
More recently, the Rosetta spacecraft has shown that another Kuiper Belt comet (67P/Churyumov-Gerasimenko) has a very different ²H/¹H ratio.

Here is a Hubble animation revealing a protoplanetary disc (.62 Mb) in the Orion Nebula (source). Note that during formation, accretion and fragmentation are both occurring, as well as frequent collisions.

The standard model of planetary system formation suggests that a huge number of asteroid-size planetesimals accrete from dust particles, first through collisions and then by gravitational attraction, in the first hundred thousand years of solar system formation. The growth of planets then takes place primarily through gravitationally-induced collisions, with the mass increasing roughly as
1 - e^-t
on time scales of tens of millions of years:

(source)

Here is an animation of Solar System Formation (3.84 Mb) (source). Here is another animation of Solar System Formation (1.94 Mb) (source).

Please send comments or suggestions to the author.