1.5 The Atmospheres of the Planet: Gas Giants (thick) vs Terrestrial Planets (thin)

The Jovian planets have very thick atmospheres of hydrogen, helium, methane, and ammonia. By contrast, the terrestrial planets, including Earth, have meager atmospheres at best. There are two reasons for this difference.

The first reason is the location of each planet within the solar nebula during its formation. The outer planets formed where the temperature was low enough to allow water vapor, ammonia, and methane to condense into ices. Hence, the Jovian planets contain large amounts of these volatiles. However, in the inner regions of the developing solar system the environment was too hot for ices to survive. Consequently, one of the long-standing questions for the nebular hypothesis was how did Earth acquire water and other volatile gases. The answer seems to be that during its protoplanet stage, Earth was bombarded with icy fragments, planetesimals, that originated beyond the orbit of Mars.

But why do Mercury and our moon lack an atmosphere? Like the inner planets, they surely would have been bombarded by icy bodies. That brings us to the second reason that the inner planets, and the moon, lack substantial atmospheres. A planet’s ability to retain an atmosphere depends on its mass and its temperature. Simply stated, more massive planets have a better chance of retaining their atmospheres because atoms and molecules need a higher speed to escape. On the moon the escape velocity is only 2.4 km per second compared with over 11 km per second on Earth.

Because of their strong gravitational fields, the Jovian planets have Escape velocities that are much higher than that of Earth, which is the largest terrestrial planet. Consequently, it is much more difficult for gases to escape from the outer planets. Also, because the molecular motion of a gas is temperature dependent, at the low temperatures of the Jovian Planets even the lightest gases, hydrogen and helium, are unlikely to acquire the speed needed to escape.

By contrast, a comparatively warm body with a small surface gravity, such as Mercury and our Moon, is unable to hold even heavy gases such as carbon dioxide and radon. Mercury does have trace amounts of gas present. The slightly larger terrestrial planets of Earth, Venus, and Mars retained some heavy gases such as water vapor, nitrogen, and carbon dioxide, but even their atmospheres make up only a very small portion of their total mass.

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