MAGNETOSPHERES OF THE TERRESTRIAL PLANETS
It has been known for many years that the earth has a magnetic field that extends far beyond the body of the earth to form a magnetic envelope called a magnetosphere. The earth's magnetic field is attributed to electrical currents flowing in the outer core; so the earth is somewhat like a giant electromagnet. If the other terrestrial planets also have electrical currents flowing in their deep interiors, then they too will have magnetospheres. Or we can turn the argument around:
The spacecraft detection of a magnetic field tells us something about the interior of a planet; that is, it is sufficiently fluid for matter to flow in the deep interior to generate electrical currents.
Mariner 10 found evidence for Mercury's having a magnetic field, but it is much too weak to hold radiation belts such as the earth's Van Allen belts.
Before the advent of planetary exploration with spacecraft, astronomers had suspected that the earth's sister planet, Venus, might have an internally generated magnetic field comparable to that of the earth. However, with the first few Mariner and Venera spacecraft it became evident that the magnetic field for Venus is much smaller than that of the earth. Venus has a well-developed bow shock formed by solar wind particles impinging on the planet's magnetic field; but that weak field, like that of Mercury, is too feeble to trap solar wind particles in a radiation belt. The weakness of Venus's magnetic field is probably due to the very slow rotation of the planet.
In 1965 Mariner 4 accomplished the first measurements of Mars's magnetic field. The intensity of the magnetic field is much less than that of the earth's field. There is a feeble bow shock formed between the onrushing solar wind particles and the field, but no radiation belts appear to exist.
In summary the earth is the only one of the terrestrial planets with a field intense enough to maintain radiation belts. This suggests that the earth is the only one possessing in its core a circulation pattern capable of generating a strong magnetic field.
