Terrestrial, Telluric or Rocky planet:
A terrestrial planet is a term used to describe the four planets in the solar system that are closest to the Sun, Mercury, Venus, Earth, and Mars. These four planets are composed primarily of rock and have solid surfaces. The word terrestrial is derived from the Latin word terra, meaning ground or soil.
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.
So note that for the outer Jovian worlds, all the primary, initial atmosphere is held. But for the inner worlds, most of the original H and He has been lost. These inner worlds then will form a secondary atmosphere composed of the outgassing from tectonic activity.
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.
Note that H2O is the key catalyst for the evolution of a secondary atmosphere. On the Earth, the temperature was just right for the formation of liquid water = oceans. The CO2 released by outgassing was dissolved in the liquid water to produce carbonate rocks. Thus, the Earth had a reducing atmosphere.
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.
Note that most of the O2 released by outgassing is locked up in liquid H2O. Since O2 is highly reactive, it must constantly be replenished. Some is released by photodisintegration with H2O vapor in the upper atmosphere.
But most of the O2 in today's atmosphere is from the photosynthesis process associated with lifeforms. This occurred about 1 billion years after the Earth formed. The original secondary atmosphere of the Earth was lacking large amounts of O2 and was rich in N2 and CO2.
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:
- Mars -> +5 degrees
- Earth -> +35 degrees
- Venus -> +500 degrees
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.
One of the thickest atmosphere's in the Solar System (2nd only to Venus) is Titan. Titan's current atmosphere is 90% N2 and 7% CH4 (methane). Since Titan formed in the outer Solar System where it is much cooler, and contains more icy materials such as NH3 (ammonia) and CH4. NH3 is easily separated into N2 and H2 by sunlight. The N2 is retained by Titan's gravity (see the chart above), but H2 escapes. Thus, over time, Titan has built up a N2 atmosphere like the Earth's from an original secondary atmosphere that was rich in NH3.
Note that the interaction of sunlight and CH4 induces chemical reactions that build hydrocarbons such as ethane, acetylene, propane; all of which have been detected in Titan's atmosphere. Hydrocarbons can join together to form long molecular chains called polymers. Droplets of polymers can remain suspended in an atmosphere to form an aerosol (heavy smog) whereas others will sink to form a thick layer of tar on the surface.
Composition of a Secondary Atmosphere:
In summary, the composition of an atmosphere on a terrestrial planet will be determined by the following:
- Distance from Sun (surface temperature of planet)
- Mass and radius of planet = surface gravity = escape velocity
- chemical reactions = different molecules are created and destroyed in various environments, higher temperatures mean faster reactions
- geological activity = amount of outgassing, more activity = more outgassing = thicker atmosphere
- living organisms = change the composition through their waste products
Why are Terrestrial Planets small and rocky? The primary atmosphere has boiled off leaving the rocky core. Why did this happen? Distance from the Sun for the primary worlds, distance from warm Jupiter or Saturn for the terrestrial moons.
Evolution of TP surfaces:
The evolution of a planetary surface is dominated by the following processes:
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.
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.
Typically the surface of airless worlds will display rock fragments which have been violently broken and recemented. They are highly irregular and angular and are called breccia. Due to the fact that impacts continue well beyond the initial era of a planetarys evolution when it was warm and molten, the surface of most airless worlds are covered a layer of rubble and fine dust called regolith.
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:
- the energy of formation of the planet
- 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:
- plate motion
- mountain/terrain formation
- crustal fractures
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 can be cause by the following processes:
- atmospheric erosion (wind, weather)
- tectonic activity (crustal movement/recycling, volcanos)
- gravity (slumping)
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.
- impact cratering is a measure of age for a planet's surface. Old surfaces are heavily cratered. Young surfaces are dominated by tectonic and erosion effects.
- the mass of a planet determines the amount of heat available for tectonic activity -> more mass = more activity. Tectonic activity determines the global surface features of a planet. Even if the planet is inactive today, surface features may reflect an era of early activity when the planet's interior was still hot (i.e. Mars). Note also that the amount of tectonic activity also determines the amount of outgassing, again see Mars = small planet, thin atmosphere.
- Erosion to varying degrees dominates the later stages of surface feature evolution. Airless worlds have little to none (i.e. Mercury). Planets with thick atmospheres have few features left over from the early eras (i.e. Venus and Mars).
Can not examine the interiors of planets directly (even our own). Thus, we build computer models which contain the following parameters:
- density as a function of radius
- rotation rate
- magnetic field
- temperature as function of radius
- chemical composition
The boundary conditions are what we can measure:
- total mass
- mean density
- surface temperature
- strength of magnetic field at surface
- thickness of crust
- seismic activity
Our understanding of the origin to magnetic fields in planets is very poor. We know that the Earth's magnetic field is not due to the presence of a giant permanent magnet, such as iron, deep in the Earth's core because 1) the core temperature is above the Cure temperature and 2) the field is too variable. It must be a generated field and we know that a conducting fluid in motion generates a dipole magnetic field. The nature of this field and its evolution is governed by the field known as magnetohydrodynamics. The liquid outer core of the Earth is the conducting fluid, free electrons being released from metals, such as Fe (iron) and Ni (nickel), by friction and heat. Variations in the global magnetic field represent changes in fluid flow in the core.
The field of paleomagnetism examines the behavior of the Earth's magnetic field with time. Whenever a rock crystallizes from molten lave, the magnetic elements in the rock act like tiny compass needles and are frozen into position aligned with the direction of Earth's field. Fossil evidence for field reversals on timescales of 100,000 years indicates that the process of magnetic field generation is unstable.
For the planets it is key to know that a magnetic field indicates that:
- the planet has a large, liquid core
- the planet has a core rich in metals (source of free electrons)
- the planet has a high rotation rate
The strength of the magnetic field is telling you something about the combination of the above factors. For example, Mercury has a weak magnetic field. But, since it has a very low rotation rate we conclude that it has a large liquid core. Mars has a high rotation rate (similar to Earth's), but a magnetic field that is 1/800th the strength of the Earth's. Therefore, we conclude that Mars has a very small core.
The most important process early in the formation of a planet that influences its structure of its interior is gravity. Gravity causes heavier elements to sink to the core of a planet, this is called chemical fractionation.
Since this is a slow process, the planet may solidify before chemical fractionation can fully develop. Thus, large, massive planets, like the Earth and Venus, are molten long enough for a Fe and Ni core to form. Whereas, smaller planets, like Mars, cool faster and solidify before the heavier elements sink to the core. Thus, elements like Fe are over abundant in the soil, giving Mars its red color.
The thickness of a planet's crust is directly proportional to the rate at which the planet cooled in the distant past. A fast cooling rate (i.e. a small planet) will result in a thick crust. For the major terrestrial worlds, the crust thickness is proportion to the diameter of the planet is:
Note that the cooling rate is proportional to the total mass of the planet. Large worlds cool slower, have thinner crusts. High cooling rates also determine the interior structure. Slow cooling rates imply planets that still have warm interiors now. Warmer interiors imply more diversified structure (inner core, outer core, semi-solid mantle, etc.)
Note also that a thicker crust means less tectonic activity.
Summary of Terrestrial Planet Interiors:
The make-up of planet interiors is dominated by the physics of materials under high temperatures and pressures. Starting with cold, low pressure regions, rocky materials are straight solids. As one goes deeper into a planet the temperature and pressures go up. Solids become semi-solid, plastic-like materials. With higher temperatures and pressures, semi-solids become liquids. With even higher temperatures and pressures liquid or molten rocky materials undergo a phase change and become solids again. That is why the very inner cores of the Earth and Venus are solid, surrounded by liquid outer cores.
Just from examination of mean densities, the primary terrestrial worlds fall into two classes. High mean density worlds, Earth, Venus and Mars, with values around 5 gm/cc. And low mean density worlds, Mercury and the Moon, with values around 3 gm/cc. The high density worlds have Fe/Ni cores and clearly defined interior structure. Low density worlds have weaker structure, i.e. no strong cores. Cross sections of the major terrestrial worlds are found below. Areas marked in red are in a liquid or semi-solid state.
Several trends should be noted:
- The two largest worlds have the largest liquid outer cores and, therefore, the highest amounts of current tectonic activity.
- Magnetic fields are generated by planets with molten inner or outer cores. The strength is determined by the size of the core and rotation rate of the planet. For example, the Earth has a strong magnetic field because it has a large core and high rotation rate. Mercury has a weak field since it has a large core, but very slow rotation rate (56 days). Venus has almost no magnetic field because even though it has a large core, its rotation rate is extremely low (243 days).
- Tectonic activity is caused by heat in the core. But, visible effects (volcanos) is also determined by thickness of crust. Mercury has a large core, but a very thick crust and shows no tectonic activity.
Galilean Satellites Interiors:
The primary difference in the formation of the Galilean satellites is the much higher concentration of icy materials in the outer solar system compared to the inner terrestrial worlds. Due to this, the composition of the crusts is dominated by H2O and CO2 ice. Other points to note:
- The distance to Jupiter determines the amount of tidal friction. The amount of tidal friction determines the amount of heat in the moons core and, therefore, the level of geological activity.
- Large amounts of outgassing have drained the inner moons, Io and Europa of their icy materials making them rich in rocky materials.
- The lack of tectonic activity for the outer worlds has left them with warm interiors and large, liquid ice mantles.