The Moon Has No Atmosphere
Unlike the Earth, the Moon has essentially no atmosphere.
The Moon might have had an atmosphere when it was formed, or outgassed significant atmosphere since then,
but it has since lost any previous atmosphere.
Thus, it has no protective blanket to
moderate its temperatures (see the right panel) or to shield it from meteors.
Retention of Atmospheres
To understand why the Earth has an atmosphere but the Moon does not, we need to consider
four basic factors governing whether a planet or a moon
retains an atmosphere: (1) the mass of the
planet; (2) the mass of the gas molecules in
the atmosphere; (3) the temperature of the surface and atmosphere; (4) the strength of the
The first factor governs how strongly the planet attracts the gas in its atmosphere, the second and
third govern how rapidly the
molecules move in the atmosphere, and the fourth determines whether there is a magnetosphere to shield the
atmosphere from the solar wind.
Maxwell Velocity Distribution
The atoms and molecules in a planetary atmosphere have a distribution of velocities
called a Maxwell distribution (see adjacent figure). This
graph plots the number of atoms or molecules with a given velocity versus the velocity.
The higher the temperature, the higher the average velocity (indicated by the dotted vertical lines)
and the higher the velocity of the fastest molecules. In addition, the higher the mass of a molecule at a
given temperature, the lower the average velocity in the atmosphere. For example, at a fixed temperature,
light hydrogen molecules will on average be moving faster than heavy oxygen molecules.
The critical question is whether the fastest
molecules attain escape velocity, which is determined by the mass of the planet
(here is an escape velocity
calculator for planets in the Solar System). High
temperatures, light gases, and weak gravitational fields favor the loss of the atmosphere into space.
The situation in the Solar System is illustrated in the adjacent diagram.
dots indicate the escape
velocities of various objects in the Solar System, plotted versus
the temperature at the tops of their atmospheres (or their surfaces if they have no atmosphere). The
curves indicate the typical highest velocities for different gas molecules as a function of temperature.
Generally, if the point representing the escape velocity for an object is well above the curve for a given
atom or molecule, it is likely that the object can retain the atom or molecule in its gravitational field.
The rough "rule of thumb" is that the escape velocity needs to be larger by a factor of about 6 for
the gas to have been retained by the planet or moon.
Magnetospheres and the Solar Wind
The solar wind of charged particles
from the Sun can interact with atmospheres and strip ions from the upper part if
there is no magnetic field to deflect the wind. The Moon has no magnetic field, but this is not important
because it has lost any atmosphere that it had anyway. However, Venus and Mars also lack a strong magnetic field. Scientists
believe that their inability to shield themselves from the solar wind
has had a significant influence on
the evolution of their atmospheres.
Atmospheres in the Solar System
So, for example, the gas giant planets have sufficiently high escape velocities that they can retain with
high probability all the gases listed. Since the majority of the original solar nebula was hydrogen and
helium, they have large concentrations of these gases. At the other extreme, low-mass objects like the
Moon or the asteroid Ceres cannot retain any of the gases listed.
Planets like the Earth are intermediate
cases: the Earth easily retains heavy gases like oxygen or water vapor, but has lost most of its free
hydrogen and helium (there is significant hydrogen on Earth in water, but water is a heavy enough molecule
to be retained by the gravitational field).
The Moon Has Insufficient Gravity to Retain an Atmosphere
We conclude that the Moon has no atmosphere because its gravitational field is too weak to have retained
any atmosphere that it had
over times comparable to the age of the Solar System. Notice that retention of an atmosphere
on the temperature.
The Saturnian moon Titan has an escape velocity only about twice that of the
Moon, but it has retained an atmosphere even thicker than that of the Earth because it is far from the Sun
and therefore much colder than the Moon.