The Insignificant Magnetic Fields and Atmosphere

The Moon has a magnetic field less than 0.001 that of the Earth.This field is not an active one being produced inside the planet, but simply the record of a magnetic field from the past. Some lunar samples preserve old magnetic fields as remanent magnetization. A rock obtains a magnetic field by crystallizing in a larger field, for example, the field of the planet it is on.When the rock cools fully, its iron-based minerals (such as magnetite) hold the record of the magnetic field frozen for as long as the rock remains cool. If the rock is heated above a certain temperature called the Curie temperature, the minerals lose their record of the magnetic field. These frozen remanent fields can thus remain in rocks after the planet's own magnetic field has ceased. This is probably what happened on the Moon: It may have had a planetary magnetic field early in its history, and this field remains recorded in certain crustal rocks even though the Moon has only the tiniest magnetic field now (for more on planetary magnetic fields, see the section "Magnetic Field" on page 46).

The fact that the Moon had any magnetic field in the past was a surprise to some scientists, since until recently many scientists thought the Moon entirely lacked an iron core, and an iron core was thought necessary for the production of a magnetic field. Now it appears that the Moon likely has a small iron core, no larger than 10 or 20 percent of the Moon's radius. Any magnetic field created on the Moon occurred early in its evolution and lasted only a brief time. Today there is no magnetic field being created by the body.

The atmospheres ofVenus, the Earth, and Mars, as well as those of all the gas giant planets, are protected by their magnetic fields. The planet's magnetic field creates a magnetosphere, a region around the planet in space from which the solar wind is blocked. When energetic particles from the Sun strike the magnetic field, they are carried along the field lines rather than passing through to the planet. If the energetic solar wind were able to strike the planet's atmosphere, the gases in the atmosphere would be knocked off into space and depleted over a short period of time. Without a magnetic field and atmosphere the planet's surface would also be bombarded by high-energy radiation from the Sun.

The Moon has almost no atmosphere now. There is a small region around the Moon that has a few tenuous gases, made from radon and a few other elements degassing from the Moon's interior, and from the solar wind itself. The solar wind contributes mainly hydrogen and helium to the planet's surface.The Apollo 17 specifically measured the lunar atmosphere, and found that the three primary gases in the lunar atmosphere are neon, helium, and hydrogen, in approximately equal amounts. Methane, carbon dioxide, ammonia, and water are also present in smaller amounts. Argon was also detected, and its abundance appears to coincide with quakes on the Moon. Moonquakes may cause new fractures in the lunar crust, allowing 40Ar to escape from the lunar interior, which it had previously been produced by decay of potassium-40 (40K). Each atom in the lunar atmosphere has a lifetime of only a few months on average before it escapes into space. Some atoms or molecules, however, get caught in "cold traps," that is, cold areas in permanent or semipermanent shadow in which molecules freeze and where they are unlikely to be struck by energetic particles or heated by radiation. These molecules can remain stuck in the cold traps indefinitely.

Unlike Mars, which apparently started with a significant atmosphere, the Moon probably never had one.The tiny gravity field of the Moon was almost certainly incapable of retaining any atmosphere after the giant impact that created the Moon, especially because at the time of the impact all the matter was highly energized and liable to be lost to space or to the Earth's greater gravity field.While the side of the Moon facing the Sun obtains a few atoms from the solar wind, on the far side of the Moon from the Sun is a region called the plasma umbra that may be the most complete vacuum in the solar system. The Moon itself shields this region of space from corpuscular radiation, and in the absence of solar heating and radiation the Moon itself contributes almost no particles to this region.

The exceptional heat of formation of the Moon is thought to have driven off some fraction of the planet's oxygen. With no oxygen atmosphere and with an oxygen-depleted interior, the Moon is highly reduced, meaning that fewer atoms are bonded to oxygens than they are on Earth.Where on Earth iron would commonly be bonded with oxygen as Fe O or as FeO, on the Moon iron is often so reduced that it is simply metallic iron. On Earth metallic iron on the surface is rapidly oxidized into rust.

Without an atmosphere and a magnetosphere, the temperature on the lunar surface is controlled by the cycles of heating by the Sun and cooling into space at night. In the day, the temperature of the Moon averages 225°F (107°C), although it rises as high as 253°F (123°C).At night the surface cools to an average of —243°F (—153°C), and an extreme of —387°F (—233°C) in the permanently shaded South Polar basin. A typical minimum temperature at the nonpolar Apollo 15 landing site was —294°F (—181°C).These extreme temperature swings of 200 to 300 degrees between night and day make the lunar surface an inhospitable place and highly stressful on materials.

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