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:

• Venus, Earth, and Mars have atmospheres, while there are only traces of an atmosphere on Mercury and the Moon.

• The Venusian and Martian atmospheres are predominantly carbon dioxide while the Earth's atmosphere is 78 % nitrogen and 21 % oxygen

• The Earth is the only planet whose atmosphere contains a significant amount of free oxygen; the abundant free oxygen on Earth is a product of life. There is oxygen in the Venusian and Martian atmoshperes, but it is tied up in the carbon dioxide.

• Venus essentially has no water, the Earth has abundant water, and Mars shows evidence of water.

• The surface temperatures of the planets vary wildly from T ~ 900 F for Venus to T ~ 60 F for the Earth.

• The atmospheric masses are in the rough ratio of 100:1:0.01 for Venus:Earth:Mars (based on their atmospheric pressures).

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:

• Because the planets formed from the accretion of solid rock particles, volatile elements were trapped inside of them. Later, as the interiors of the planetary bodies heated and melted, the volatiles were released througe volcanic eruptions, outgassing.

• The atmospheres were added to the planets after they were formed. This could occur either as a slow capture from the Solar Nebula directly (not likely), from material brought in by the intense Solar Wind from the young Sun, or by comets (recall the Clementine results for ice on the Moon).

It is not clear which of the above theories are 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.

For example, 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

If this rate is typical, it would have taken roughly 15 billion years to make the oceans via outgassing. If the rate were only 3 times higher in the past, then the oceans could be produced in 4.6 billion years (the age of the Solar System).

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.

At the current rate, deposition of water by comets would take tens of trillions of years. In order for the comet scenario to work, the cometary rate must have been significantly higher in the past or there must be a class of small comets (which are hard to detect), which completely dominates the more typical observed comets.

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:

• the mass of the planet is crucial because the escape velocity depends strongly on the mass of the planet (via the gravitational force); the more massive the planet the higher the escape speed

• the distance of the planet from the Sun because the temperature of the gas depends strongly upon how much energy it absorbs from the Sun; the closer to the Sun the hotter the planet's surface is likely to be, but there are tweaks to this idea.

EQUILIBRIUM SURFACE TEMPERATURES AND THE GREENHOUSE EFFECT

We now define the Equilibrium Temperature. Assume:

• the planet absorbs heat from the Sun at a certain rate and then re-radiates this energy at precisely the same rate (hence, the use of the word equilibrium).

• the planet radiates like an idealized creature which is defined to be a perfect emitter and absorber of radiation)--a blackbody radiator.

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 Earth.

Actual Atmospheic 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 albedo for each planet is 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.

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