The primary atmosphere for every terrestrial world was composed mostly of light gases that accreted during initial formation. These gases are similar to the primordial mixture of gases found in the Sun and Jupiter. That is 94.2% H, 5.7% He and everything else less that 0.1%.
However, this primary atmosphere was lost on the terrestrial planets. Why? a combination of surface temperature, mass of the atoms and escape velocity of the planet.
What determines if a particular atom is retained by a planet's gravitational field? if the atom is moving less than the escape velocity for the planet, it stays. If it moves faster than escape velocity, it escapes into outer space.
From the kinetic theory of gases, we know that the mean velocity of a bunch of atoms is set by the temperature of the planet's surface. Remember our microscopic description of macroscopic quantities such as pressure and temperature. Higher temperatures translate into higher velocities for the atoms.
Now consider of mix of elements in an atmosphere. Some atoms/molecules are low in mass (H, He) some are heavy (CO2, H2O, etc). The light elements are moving faster than the heavy elements and can reach escape velocity.
The second variable is the surface temperature of the planet. The inner worlds are closer to the Sun, therefore warmer. The opposite is true of the outer planets, farther from the Sun therefore cooler.
Combining the variables of escape velocity (mass, radius of planet) and surface temperature (distance from Sun plus effects of atmosphere heating) produces the following diagram. For key elements, lines are draw to show where the element escapes from the planet. If a planet is below that line, that element will escape.
Secondary Atmospheres:
For the warmer terrestrial worlds, the light, gaseous elements (H, He) are lost. The remaining elements are grouped into the rocky materials (iron, olivine, pyroxene) and the icy materials (H2O, CO2, CH4, NH3, SO2). The icy materials are more common in the outer Solar System, they are delivered to the inner Solar System in the form of comets (see later lecture).
The rocky and icy materials mix in the early crust and mantle. If the planet cools quickly, there is little to no tectonic activity and the icy materials are trapped in the mantle (see for example the Galilean moons). If the planet has a large mass (which means lots of trapped heat from formation), then there is a large amount of tectonic activity -> volcanos.
The icy materials are turned to gases in the warm mantle and returned to the planet surface in the form of outgassing to produce a secondary atmosphere. The atmospheres of Venus, Earth and Mars are secondary atmospheres.
The composition of outgassing is similar for Venus, Earth and Mars and is composed of 58% H2O, 23% CO2, 13% SO2, 5% N2 and traces of noble gases (Ne, Ar, Kr). The latter evolution of this outgassing is driven primarily by the surface temperature and chemistry of the planet.
On both Venus there was no liquid water (too hot) and, therefore, no place for the CO2 to dissolve. If the atmosphere is reducing in CO2 than lower ranking elements become important once the CO2 is gone. For the Earth, this meant that the atmosphere became primarily N2 based, with later additions of O2 from lifeforms. On Venus, CO2 was not reduced and stayed as the primary component to their atmospheres.
On Mars there was a period of liquid water very soon after formation. But there was insufficient temperature for this water to remain as a liquid, so it froze out leaving CO2 as the primary component in the atmosphere.
Also note how the noble gases are good traces of the amount of evolution an atmosphere undergoes. Noble gases do not react with other elements (they are inert). An atmosphere that is thin and undergoes sharp changes in mass has a high percentage of noble gases. In this case, Mars has had most of its atmosphere frozen out in the form of H2O and CO2 ice, leaving a high amount of noble gases. Thick atmospheres, such as Venus, have small percentages of noble gases since most of the outgassing material remains on the planet surface.
Greenhouse changes:
The greenhouse effect is controlled by the amount (by mass) of greenhouse gases in an atmosphere. These gases are primarily H2O, CO2, CH4, NH3. For secondary atmospheres on Venus, Earth and Mars, only CO2 has a major contribution to the greenhouse effect (although note that the amount of CH4 is increasing on the Earth due to the waste products of animals and agriculture).
The greenhouse effect currently raises the temperature of Venus, Earth and Mars by the following amounts:
Note that the greenhouse effect for the Earth is just enough to keep us out of a perpetual Ice Age (a little greenhouse effect is good for you). Whereas for Venus, a severe runaway greenhouse effect makes it the hottest place in the Solar System.
Also note that Mars probably had a stronger greenhouse effect in its distant past. But the large amounts of CO2 were converted to rocks in the early Mars oceans. The atmosphere thinned too fast stopping the greenhouse effect and the liquid H2O turned to ice (cold death).
The lesson to learn here is that Mars and Venus are exactly opposite in their evolution and the result of the greenhouse effect. The dynamics of planetary atmosphere's are unstable, and complex so that changes in Earth's atmosphere, even small, are a very serious matter for those of us who need a place to live.
Evolution of TP surfaces:
The evolution of a planetary surface is dominated by the following processes:
impact cratering
tectonic activity
erosion
Note that this list is also in temporal order since impact cratering occurs first, followed by tectonic activity and then erosion. Also note that all the planets receive the same amount of impacts from remnant debris in the early Solar System. But that the amount of tectonic activity and erosion varys from planet to planet.
Impact cratering:
After the formation of the planets some 4.5 billion years ago there was a tremendous amount of leftover material. This material was in the form of icy rocks that had various orbits out to the cometary Oort cloud. Often these orbits intersected with the forming planets and hence would impact on the newly formed surfaces with a great deal of kinetic energy.
While the surfaces were molten, these impacts would have just added more material to the planet (in fact, some of the H2 and CO2 in the mantle comes from early comet impacts). But as the planets cooled, the crust would have cooled and solidified first. Later impacts would have either 1) created craters or 2) burst through the crust to the mantle to release lava to form basins. Note that as time pasts and the planet cools, crust becomes thicker and impacts that form basins become rarer. Basins will be filled in, partially, with later cratering.
Planets with old surfaces have large amounts of impact cratering. Planets with young surfaces (young meaning later changes) have little evidence of the early epoch of cratering. Most impact basins were later destroyed due to more impacts (the smooth terrain was cratered) with the exception of the Moon, whose nearside was shielded by the Earth.
Tectonic Activity:
The amount of tectonic activity on a planet is controlled by the amount of heat stored in the planets interior after formation. The larger the amount of heat, the more energy stored that is transfered to the surface in the form of geological activity. Although the process of tectonic activity is still mostly unknown (see a Geology course), the connection between interior heat and activity is supported by the observations of the Galilean satellites where the inner moons, which are heated by tidal friction with Jupiter, are also the most geologically activity.
The amount of heat stored in a planet's interior comes from two sources:
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1) the energy of formation of the planet
2) heat generated by the decay of radioactive elements.
Formation energy or leftover heat is due to the fact that the debris and gas that the planet forms from coalesces into a ball. The potential energy from gravity of this infalling material is converted to kinetic energy (heat) as the debris falls together. Thus, the higher the mass of the planet, the greater the amount of energy deposited on it during formation, the greater the heat and, therefore, the greater the amount of tectonic activity.
The amount of heat from radioactive materials is also proportional to the mass of the planet. Again, more mass = more radioactive material = more heat from radioactive decay.
Tectonic activity displays itself in the following ways:
The more diverse the surface geography of a planet, the more involved is the tectonic activity. For example, the Earth is one of the most tectonically active planets in the Solar System and has extensive systems of plate boundarys, active volcanos, mountain ranges and canyons. Mars (small, low in mass) on the other hand has very few mountain ranges or active volcanos. The fact that the volcanos on Mars are large implies that Mars was once active, in its distant past but with limited plate motion.
Erosion:
Erosion can be cause by the following processes:
Depending on the mass of the atmosphere, this list is in order of strength. Atmospheric erosion has short timescales, on order of hundreds of thousands of years. Tectonic activity can take on order of millions of years. Gravity slumping is only visible on airless worlds with timescales of billions of years.
Note that large features, such as impact basins or extremely large impact craters can not be eroded away even after 100's of millions of years. Such large features on the Earth were eroded by tectonic activity, i.e. the crust was recycled by plate motion such that those ancient impact basins are gone.
Summary:
Moons:
Moons are `fossils" into a planet's past. The major, named moon systems are:
Earth: Luna (The Moon)
Mars: Deimos, Phobos
Jupiter: Adrastea, Amalthea, Ananke, Callisto, Carme, Elara, Europa, Ganymede, Himalia, Io, Leda, Lysithea, Metis, Pasiphae, Sinope, Thebe
Saturn: Atlas, Calypso, Dione, Enceladus, Epimetheus, Helene, Hyperion, Iapetus, Janus, Mimas, Pan, Pandora, Phoebe, Prometheus, Rhea, Telesto, Tethys, Titan
Uranus: Ariel, Belinda, Bianca, Cordelia, Cressida, Desdemona, Juliet, Miranda, Oberon, Ophelia, Portia, Puck, Rosalind, Titania, Umbriel
Neptune: Despina, Galatea, Larissa, Naiad, Nereid, Proteus, Thalassa, Triton
Pluto: Charon (note: Pluto/Charon form a binary planet, but Charon is the smaller so it is classed as the moon of Pluto)
New, smaller moons are being discovered all the time with recent space missions. The total count of moons (as of 12/18/2001) are:
Mercury - 0 moons Mars - 2 moons Uranus - 20 moons
Venus - 0 moons Jupiter - 28 moons Neptune - 8 moons
Earth - 1 moon Saturn - 30 moons Pluto - 1 moon
Moons range in shape from highly irregular to spheres. Their shape reflects
their formation history, irregular objects are ill-formed moons or
pieces of a larger moon, spherical objects were once molten
spheres, probably at the time of their formation.
Moons of Mars:
Deimos
Phobos
We speculate, from their irregular appearances and low mean densities, that Deimos and Phobos, are captured asteroids. Both Deimos and Phobos are saturated with craters. Deimos has a smoother appearance caused by partial filling of some of its craters.
Moons of Jupiter:
Adrastea is a typical small moon
Metis is the innermost known satellite of Jupiter
Amalthea is one of Jupiter's smaller, irregular moons, an example of moon collecting dust from another moon (Io)
Moons of Saturn:
Atlas the second of Saturn's known satellites, orbits near the outer edge of the A-ring
Enceladus is one of the innermost moons of Saturn. Enceladus reflects almost 100 percent of the sunlight that strikes it and has evidence of internal heating and recent resurfacing effects
Epimetheus and Janus are the fifth and sixth satellite of Saturn that share the same orbit, and have a possible origin as single moon that split
Hyperion is one of the smaller moons of Saturn. It has a pock-marked body and is the largest irregularly shaped satellite ever observed.
Iapetus is one of the stranger moons of Saturn, its leading side is dark with a slight reddish color while its trailing side is bright
Dione is the densest moon of Saturn other than Titan, and has several usual characteristics: 1) has a rocky core and ice crust, 2) is heavy cratering on trailing hemisphere, 3) has bright, wispy features
Rhea is the largest airless satellite of Saturn that has different regions with different crater sizes indicating that parts of the moon have undergone resurfacing since formation
Mimas is one of the innermost moons of Saturn with a very large impact crater that came close to fracturing the moon
Tethys is an icy body similar in nature to Dione and Rhea
Phoebe is the last of the known satellites of Saturn and orbits in a retrograde direction (opposite to the direction of the other satellites' orbits) in a plane much closer to the ecliptic than to Saturn's equatorial plane. Thus, Phoebe may be a captured asteroid with a composition unmodified since the time it was formed in the outer Solar System.
Moons of Uranus:
Ariel is a relatively small satellite and is the brightest moon of Uranus
Miranda with a jumbled surface unlike anything in the Solar System, indicates evidence of violent past with possible multiple shattering and reassembly
Titania is the largest moon of Uranus and is marked by a few large impact basins
Moons of Neptune:
Proteus is one of the darkest objects in the Solar System
Triton is the largest moon of Neptune and is colder than any other measured object in the Solar System with tidal heating after formation changed surface
