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Physics LibreTexts

6. 34 Gravity and Atmospheres

Schematic of volcano injection of aerosols and gases into the atmosphere. Click here for original source URL.

Image of the ash cloud from erupting Mount Cleveland in Alaska taken by the International Space Station crew. Click here for original source URL.

Several "pancake dome" volcanoes on the surface of Venus imaged by the Magellan spacecraft. Click here for original source URL.

Olympus Mons, the largest volcano/mountain in the solar system, located on Mars. Olympus Mons is a shield volcano, similar to the large volcanoes that make up the Hawaiian Islands. Click here for original source URL.

Pit-floor craters on Mercury. The result of the collapse of subsurface magma chambers, indicating volcanic processes on Mercury. Click here for original source URL.

As Isaac Newton discovered in the 1600s, all planets have gravity, and the force of gravity increases as the mass of the planet increases. According to the kinetic theory of matter, the gases in a planet's atmosphere are composed of atoms and molecules that are constantly in motion. If the atoms or molecules near the top of the atmosphere find themselves moving upward faster than the escape velocity of the planet, they will escape into space, never to return. That is true even if the average speed of the atoms or molecules is less than the escape velocity. Because the velocity distribution has a long tail out to high values, there are always some speedy particles leaving the system. Thus, planet atmospheres continually and slowly leak away into space.

Planets with stronger gravity have larger escape velocities, so gas molecules have to move faster in order to exceed escape velocity. For example, a molecule moving upward at 4 kilometers per second would easily escape from the Moon, but the Earth’s stronger gravity would retain it. Therefore, a large planet will hold onto its atmosphere longer than a small planet (all other things being equal).

The kinetic energy of a gas molecule is proportional to its mass and the square of its velocity. So in a mixture of gases at the same temperature or energy, light molecules will be moving faster than heavy molecules. Therefore, a light gas like hydrogen or helium is more likely to escape into space than a heavy gas like oxygen. There was plenty of hydrogen and helium is the Earth's atmosphere early on but it quickly escaped, as did hydrogen being pushed out from the surface due to geological activity. Terrestrial planets are only massive enough to hold onto heavy molecules like oxygen, nitrogen, and carbon dioxide.

So if we order the terrestrial worlds by size, we can see if these generalizations are true: Do small worlds really have thinner atmospheres? In the following list, satellites and planets are listed in order of increasing size, and the atmospheric pressure at the planet's surface is indicated in units of bars. One bar is defined to be equal to the pressure at sea level on Earth. Notice the sequence of properties:

• Deimos - No atmosphere.

• Phobos - No atmosphere.

• Moon - Negligible atmosphere.

• Mercury - Essentially no atmosphere - slight concentration of some gases including oxygen, sodium, and potassium.

• Mars - Thin CO2 atmosphere (0.006 bars).

• Venus - Massive CO2 atmosphere (90 bars).

• Earth - Moderate N2 plus O2 atmosphere (1 bar).

Among atmospheres in the inner solar system, everything checks — more massive planetary bodies have atmospheres, and less massive planetary bodies do not. The atmospheres are composed of heavy molecules, indicating most lighter atoms were lost to space. The only exception is Venus, which is slightly less massive than Earth but has a much thicker atmosphere. What accounts for this anomaly? The explanation is that all of Venus's carbon dioxide is still in the atmosphere, while the Earth's carbon dioxide has dissolved in the oceans and bound into rocks