Part 5C:

ATMOSPHERES OF THE TERRESTRIAL PLANETS

The Terrestrial planets are similar to each other in mass, diameter, and distance from the Sun. Because of this, it is expected that their atmospheres should share many similar qualities. Despite this, their atmospheres show significant differences.

Remarks:

There are other differences between the planets, but we consider the above as the key points for developing an understanding of the atmospheric evolution of the Terrrestrial planets.

ORIGIN OF THE ATMOSPHERES

Immediately after formation, Terrestrial planets essentially had no atmospheres (if they had captured some hydrogen and helium from the Solar Nebula, it was rapidly lost to space). Whatever atmosphere a Terrestrial planet has today was either captured or generated after the planet formed; the Terrestrial planets have secondary atmospheres.

There are two suggestions for the generation of secondary atmospheres:

Of the above, it is likely that the Outgassing Theory will eventually be shown to be correct.

Outgassing

Current studies of Terrestrial volcanoes show that they do emit large amounts of volatile materials such as water, carbon dioxide, nitrogen, and sulfur dioxide (at left is the Santa Maria volcano in Guatemala), however, it is not clear if enough volatile material can be trapped during the formation of the planets. Note that free oxygen is not among the original constituents of our secondary atmosphere.

As an example of outgassing, consider water. On the Earth, there is enough water to cover the planet to a depth of around 3.6 kilometers. The oceans thus contain a mass of water of

Mass ~ 1.5x1021 kilograms.

The current rate of outgassing of water from volcanoes is

Outgassing = 1011 kilograms per year

Comets

Comets are roughly half water ice and half rocky material. A 2 km comet with density 2 grams per cubic centimeter, thus has mass

M ~ 8x1012 kilograms.

So, roughly 4x108 comets are needed to explain the Earth's oceans.

There are many hundreds of billions of comets in the Solar System (in the Oort cloud) and so, there is an ample supply of comets but, is the rate of cometary impacts sufficiently large to warrant considering comets as a viable source for the Earth's oceans? Based on recent cratering history, the rate of crater formation by 1 km objects is roughly one every few tens of thousands of years.

Atmospheric Retention

Why does Venus have an atmosphere while Mercury does not?

There are two competing effects which determine whether a planet retains an atmosphere:

  • the strength of the gravitational field at the surface of the planet (as measured by the escape speed of the planet)

  • the speed with which the gas particles move around (as determined by the temperature and masses of the particles which make up the atmosphere).

There are therefore two important points:

EQUILIBRIUM SURFACE TEMPERATURES AND THE GREENHOUSE EFFECT

We now define the Equilibrium Temperature. Assume:

For a planet with an atmosphere, because of the presence of the atmosphere, not all of the solar radiation strikes the planet. Some of it is reflected by the cloud layer and returns to space. We measure this effect by defining the Albedo for the planet. The Albedo, A , is the fraction of the solar radiation which is reflected to space. This means that a fraction (1-A) of the radiation reaches the surface of the planet.

Actual Atmospheric and Equilibrium Temperatures

Venus

Earth

Mars

Actual Temp

>850 F

~60 F

-60 F --> -70 F

Eq. Temp

-20 F

-4 F

-70 F

The albedos for the planets are 0.65, 0.35, and 0.15 for Venus, Earth, and Mars, respectively. For Mars, the equilibrium and actual atmospheic temperatures are roughly the same while for Venus and Earth, the temperatures differ significantly. Why? Because both Venus and Earth have significant atmospheres and both exhibit the Greenhouse Effect.

In the Greenhouse Effect, some of the incoming Solar radiation penetrates the Earth's cloud layer and atmosphere and strikes the surface of the Earth. The Earth absorbs the energy and heats up. Because the surface temperature of the Sun is ~11,000 F and the surface the temperature of the Earth is 60 F, the Earth re-radiates much lower energy photons than those which it absorbed; the Earth re-radiates stronlgy in the Infrared (IR). The kicker is that the Greenhouse gases, carbon dioxide, water vapor, methane, CFCs, ... are much efficient at absorbing IR than they are at absorbing the incoming visible and UV light. This causes heat to build up in the vicinity of the Earth's surface raising the surface temperature of the Earth.


The Earth has a milder Greenhouse effect than does Venus. The mild Greenhouse effect is important because it is what makes the Earth comfortable.


In the past, the Sun was fainter than it is today, significantly fainter and yet the temperature of the Earth was more or less the same, The Faint Young Sun Paradox. The suggestion is that, in the past, there were more Greenhouse gases in the atmosphere (more carbon dioxide and/or more methane).