Planetary Atmospheres

Evolution of Terrestrial Planets After the condensation and accretion phases of planet formation, terrestrial bodies can go through 4 different stages of evolution. (The rates of evolution can vary greatly.)  Differentiation – in a molten planet, heavy materials sink  Cratering – left over bodies impact the planet’s surface  Flooding – water, lava, and gases trapped inside the planetcome to the surface and cover the terrain.  Erosion – surface features are destroyed due to running water, atmosphere, plate tectonics, and geologic motion. What has the Most Craters?

a) Mercury Mercury VenusEarthMars

Why is the Earth’s Core Hot? c) Heat from radioactivity Accretion Contraction Radioactivity

What is the Density of Kuiper Belt Objects? b) similar to water.

Which Heating Source Doesn’t Decline? c) Heating by the Sun AccretionSolarContraction Radioactivity

At What Wavelength Can You Observe Planet Formation? d) in the infrared part of the spectrum

The Atmospheres of the Solar System PlanetAtmosphere Mercurynone Venusthick CO 2 EarthN 2, O 2, [CO 2 ] Marsthin CO 2 TitanN 2, CH 4, NH 3 Jupiter, Saturn, Uranus, NeptuneH 2, He, CH 4, NH 3

The Atmosphere of Jovian Planets Jovian planets are similar to the Sun. Due to their smaller mass, their central pressures and temperatures are not great enough to fuse hydrogen. They are thus cooler, so hydrogen can react with other atoms to form molecules. H + H  H 2 4H + C  CH 4 3H + N  NH 3 Since the inside of Jupiter is hot (due to the pressure), while the cloud tops are cool, the composition of the atmosphere changes with depth. The fast rotation rate of the Jovian planets also drives strong currents and storms, similar to the trade winds and hurricanes on Earth.

The Structure of Jupiter’s Atmosphere The inside of Jupiter is extremely hot and, in fact, Jupiter shines (in the infrared) by gravitational contraction.

Jupiter’s Trade Winds Jupiter’s equator is moving faster than the poles (it has farther to go in a day). This drives a network of very strong winds and storms.

Formation of an Atmosphere When a terrestrial planet enters its flooding stage, gases trapped inside the during formation (or created as a result of radioactive decay) will be outgassed. These gases include  H 2, He, H 2 O, N 2, CO 2, and probably CH 4 and NH 3 As the planet cools, water vapor condenses out of the atmosphere and falls as rain. Oceans form. But will the planet be able to keep this atmosphere?

Temperature versus Gravity Escape velocity:the speed a particle must have to escape the gravity of a body and not come back Temperature:the average kinetic energy of an atom or molecule The kinetic energy of an atom or molecule depends both on its speed, and its mass. Light particles move quickly; heavy particles move slowly. It’s easier for a body to hold onto heavy gases than light gases.

The Masses of Gases Atom# protons# neutronsTotal weight H101 He224 C6612 N7714 O8816 MoleculeWeightMoleculeWeight H2H2 2He4 CH 4 16NH 3 17 N2N2 28O2O2 32 CO 2 44

Mercury versus Titan Mercury and Titan are both low-mass bodies. But …  Mercury is close to the Sun, so it is hot. Its gravity is not strong enough to keep its gases from escaping into space.  Titan is in the outer solar system and is cold. The molecules are moving slowly, so the moon can hang onto its atmosphere (except for the lightest gases of H 2 and He).

Titan’s Lakes The atmospheric pressure at the surface of Titan is about twice that on Earth. In fact, Titan’s surface is not all solid -- it has lakes (of liquid methane and ammonia).

The Atmosphere of Mars The composition of Mars’ atmosphere is determined by  The mass of the planet. Since Mars is only about 0.1 M , it does not have the gravity to hold onto H 2 and He. It can barely hold onto N 2.  Proximity to the Sun. Gases such as CH 4 and NH 3 are destroyed by ultraviolet light. Mars’ atmosphere is not thick enough to shield itself from ultraviolet photons.  Chemistry. Oxygen (O 2 ) reacts with almost anything (i.e., minerals in rocks), so it cannot stay free. Consequently, Mars’ atmosphere is primarily CO 2 with a little bit of N 2.

Carbon-Dioxide and Mars Mars’ pole is tipped 24° from the ecliptic. It therefore undergoes seasons, just like the Earth. In winter at the pole, CO 2 freezes out and becomes dry ice. In summer, this ice evaporates and becomes part of the atmosphere. This cycle produces strong winds and dust storms.

Carbon-Dioxide and Mars Mars’ pole is tipped 24° from the ecliptic. It therefore undergoes seasons, just like the Earth. In winter at the pole, CO 2 freezes out and becomes dry ice. In summer, this ice evaporates and becomes part of the atmosphere. This cycle produces strong winds and dust storms.

Venus and Earth The similarities:  The planets have similar masses (0.82 M  versus 1.0 M  )  The planets have similar compositions (density 4.2 vs. 5.5)  The planets’ distances from the Sun are similar (0.72 A.U. versus 1.0 A.U.)  Neither planet can hold onto light gases (H 2 and He)  Neither planet can keep large amounts of CH 4 and NH 3 in its atmosphere (due to ultraviolet light from the Sun) The main difference:  The Earth’s temperature is between  50° C and +50° C, while Venus’ temperature is +470° C

Properties of Carbon-Dioxide CO 2 has two interesting properties:  CO 2 dissolves into liquid water (H 2 O) to create H 2 CO 3 (carbonic acid). Carbonic acid (i.e., the fizz in soda) then reacts with any number of minerals. For instance H 2 CO 3 + Ca  H 2 + CaCO 3 (limestone) The result is that, if liquid water is around, CO 2 will be removed from the air, and locked up in rocks.  CO 2 is a greenhouse gas. It is transparent to optical light, but it absorbs infrared light. So sunlight can make it through CO 2, but the heat it brings cannot get out.

Runaway Greenhouse Effect Venus and Earth both started out with similar atmospheres. But because Venus is slightly closer to the Sun …  Venus was a bit warmer, and had a bit less liquid water  With less liquid water, less CO 2 dissolved away  With more CO 2 in the atmosphere, the greenhouse effect was more effective  The warmer temperature caused more water to evaporate  With even less liquid water, even less CO 2 dissolved away  As all the water evaporated and the temperature increased, outgassing of greenhouse gases (CO 2 and CH 4 ) became easier. CO 2 was “baked out” of the rocks  Ultraviolet light destroyed the CH 4, NH 3, and H 2 O in the atmosphere, leaving a thick atmosphere of CO 2

The Atmosphere of Earth Venus and Earth both started out with similar atmospheres. But because the Earth is slightly farther away from the Sun …  Earth was a bit cooler, and had a bit more liquid water  With more liquid water, more CO 2 dissolved away  With less CO 2 in the atmosphere, the greenhouse effect was less effective  With more liquid water and a comfortable environment, photosynthetic life developed  Photosynthesis removed even more CO 2 from the atmosphere, replacing it with O 2. (When dinosaurs lived, there was 5 times more CO 2 in the air!)  Lightning plus atmospheric O 2 created ozone, which shielded the Earth from ultraviolet light. Water molecules in the atmosphere survived longer (along with life)

Today on Earth and Venus A small change in the conditions now can lead to large changes later on!

There is a Tide …

The Earth-Moon System Tides are the difference in gravity from one side of a body to the other. On Earth, the tides draw the water out towards/away from the Moon. But the Earth is constantly rotating, pulling the tidal bulge out of alignment. As a result, the water is continually moving in the opposite direction of the Earth’s rotation.

Tidal Friction The movement of the water on Earth has two effects:  It slows down the Earth’s rotation. When dinosaurs roamed the Earth, a day was 22 hours long.  It pulls the Moon along a bit faster, slinging it out further from the Earth.

Tides on the Moon The movement of the water will eventually stop the rotation of the Earth. But what about the Earth’s tidal force on the Moon? Since the Earth is about 80 times more massive than the Moon, its tidal force is 80 times greater. Tidal friction of flowing rocks (lava) has long since locked the Moon to the Earth.

Jupiter’s Moons Jupiter is much more massive than the Earth, so the tidal effect on its moons is much greater. Recall the 4 Galilean satellites … Moon Mass (lunar) Density (water = 1) Distance (1000 km) Period (days) Io Europa Ganymede Callisto

Io Io’s density is that of rock. It has no impact craters no visible water, and is entirely molten, except for a thin crust that is constantly being resurfaced by volcanism.

Europa Europa’s icy crust is thin. Below the crust is (probably) liquid water. There are few (if any) impact craters on Europa.

Ganymede Ganymede has many craters, but also a network of grooves that lie on top of the craters. These are mostly likely caused by expansion and contraction of ice layers.

Callisto Callisto has an extremely old, icy surface covered with impact craters. It is essentially unchanged since the time it was formed.

Jupiter and the Sun There is very little difference between Jupiter and a star.  The composition of Jupiter is similar to that of a star.  Jupiter formed in a mini-nebula, just like the solar nebula.  During formation, Jupiter shined by gravitational contraction, just like a star.  Jupiter’s luminosity prevented light elements from condensing on its inner moons, just like the Sun. The only difference between Jupiter and a star is that Jupiter hasn’t been able to fuse hydrogen.

Jupiter’s Moons The four Galilean moons of Jupiter show a range of properties:  Io is entirely molten, except for a thin crust. Volcanos are erupting all the time, covering the surface with lava.  Europa is warm enough under its surface to have liquid water.  Ganymede has rills and grooves on its surface, as if ice has been warmed and cooled.  Callisto is an old, cold moon, with no sign of evolution since it was formed. Why the difference?

Jupiter and Tides The tidal force of Jupiter on its moons is much stronger than the tides of the Earth-Moon system. These objects should be tidally locked to Jupiter. But …  Io, Europa, and Ganymede orbit in a 1:2:4 resonance. Io is constantly being perturbed by its neighbors.  Io’s orbit is elliptical – its speed changes during its orbit. Io can’t become tidally locked!

Heat and the Moons of Jupiter As a result of Jupiter’s tides …  Io is continually stressed by the tides of Jupiter. Its interior is kept entirely molten.  Europa feels some tidal stress as well. However, since it is further away, the stress is less. Europa’s interior is probably warm enough to melt ice into liquid water.  Ganymede has been thermally stressed in the past, either by heat from Jupiter’s gravitational contraction, or by tides. The grooves in its surface are probably due to ice expansion and contraction. It is now tidally locked.  Callisto is far enough away from Jupiter to be thermally unaffected. It is a cold body.