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Union University Department of Physics

The Science Guys

Science Guys > September 2004

September 2004

Do other planets have magnetic fields like our Earth?

It is great that our Earth possesses a magnetic field because it protects us from harmful particles in the solar wind. The Earth’s magnetic field curves outward near the South Pole, turns upward, and reenters the Earth near the North Pole. The magnetic field does not exactly line up with the geographic poles. The difference in these two positions is called the magnetic declination.

Since moving electric charges create magnetic fields, scientists think that the motion of molten material in the outer core of our Earth is responsible for the magnetic field. The molten material possesses electric charges and their motion is believed to produce our magnetic field.

High-energy cosmic rays (principally protons) constantly stream toward our Earth from the Sun. This "solar wind" flows from the sun at speeds close to 400 km/s (895,000 mph.). The Earth’s magnetic field causes these charged particles to be deflected away from Earth, however a few collect at our poles, where they impact the upper atmosphere, dissipate their energy, producing beautiful auroras - the "Northern Lights" (or aurora borealis) and "Southern Lights" (aurora australis.)

Space probes, such as the Voyager probes, have measured the magnetic fields of the planets and even auroras have been photographed on other planets. The spacecraft Mariner 10 flew by Mercury in 1974 and surprised the science community. Mercury was thought to be cold and dead inside, thus having no magnetic field. However, Mariner measured a weak magnetic field, meaning Mercury must have some internal activity. Probes found that Mars and Venus do not have a significant magnetic field.

Jupiter, Saturn, Uranus, and Neptune all have magnetic fields much stronger than that of the Earth. Jupiter is the champion- having the largest magnetic field. The mechanism that causes their magnetic fields is not fully understood. It is believed that in the case of Saturn and Jupiter that their magnetic fields may be caused by hydrogen conducting electricity deep within the planet. Hydrogen near the planets core may be compressed so densely by all the planetary layers above that it becomes an electrical conductor.

The planet Uranus has an interesting magnetic field. Uranus’ poles lie almost in the plane of its orbit around the Sun. The magnetic poles are fully 60 degrees away from the geographic poles, which results in a wild rotation of Uranus’ magnetic field as the planet rotates. On the other hand, Saturn’s magnetic field and rotation axes seem to be pretty much the same, making Saturn somewhat magnetically unique.

Our Moon lacks a magnetic field, which implies its interior is cold and inactive. However, rocks from the Moon show permanent magnetism, suggesting that at one time the Moon had a magnetic field. The physics of planetary magnetic fields still contains many mysteries for scientists.

A wonderful summary of these facts may be found on Adlerplanetarium.org