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A Public Outreach Module:
Solar Wind, Genesis, and the Planets

Do other solar system planets have protective magnetospheres?

In the past, Mars and Venus may have lost former oceans and Mars may have lost much of its atmosphere to space as a direct result of solar wind. Mercury and Venus are between the sun and the Earth, where the solar wind is more dense and has a greater magnetic field strength when it passes these planets than when it impacts the Earth's magnetosphere. Do they have magnetospheres, and, if so, are they strong enough to protect them from a Solar Max?

Mercury has a weak magnetic fieldMercury has a magnetic field whose strength is about 1% that of the Earth's, but it is strong enough that a bow shock forms where the field interacts with the solar wind. Even though Mercury lacks an appreciable atmosphere and ionosphere, its magnetic field is apparently too weak to maintain a belt of charged particles but strong enough to trap some atoms from the solar wind.

Venus' very weak magnetosphere is no more than 0.09% the strength of the Earth's and no radiation belts surround the planet. There is a well-developed bow shock in Venus' Venus's weak magnetic fieldouter atmosphere, but there is no evidence of charged particles being trapped. The solar wind approaches Venus much more closely than it does the Earth. When the solar wind encounters a nonmagnetic planet with an atmosphere, the planet's ionosphere creates forces that slow and divert the flow. Venus' dense atmosphere and the large-scale currents induced in its conducting ionosphere prevent the solar wind from reaching its surface.

The remaining planets are farther from the sun than the Earth. Mars is located at 1.5 AU, Jupiter at 5.2 Au, Saturn at 9.5 AU, Uranus at 19 AU, Neptune at 30 AU, and Pluto at almost 40 AU. The solar wind's density and speed decrease with distance from the sun, so these planets would not need magnetospheres as strong as the Earth's to protect them.

The global magnetic field of Mars is too weak (about 0.2% that of the Earth's) and the atmosphere too thin to protect the planet's surface from solar flares. In such a weak magnetic field, no radiation belts have formed. The solar wind apparently interacts with the planet's conducting ionosphere, creating a weak bow shock. There is some indication that the solar wind may remove as much as 30,000 tons of atmospheric gases from Mars each year.

Jupiter's magnetosphere is about 100 times Saturn's magnetospherelarger and 20,000 times greater than that of the Earth's. Extremely high-energy electrons have been found in an enormous disk-shaped region of its radiation belts. The density of Jupiter's charged particles is several orders of magnitude greater than that of Earth's., However, Jupiter's solar wind pressure is only 4% that of Earth's since it is 5.2 AUs from the sun.

Saturn's magnetosphere is populated with charged particles making it 500 to 1000 times stronger than the Earth's. Because of this intense magnetosphere and the low solar wind pressures at 9.5 AUs from the sun, the bow shock is found 1.44 million km from the center of Saturn on its sun-ward side. Data from Voyager 1 indicates that Titan, Saturn's largest satellite, is the source of substantial quantities of nitrogen ion plasma for Saturn's inner magnetosphere. Other sources may be the planet's inner satellites, the planet's rings and its hydrogen-dominated upper atmosphere. The intensity of all trapped particles decreases dramatically at the outer edge of ring A.

There have been aurora-like emissions in Uranus' upper atmosphere, leading us to think that this planet has a magnetosphere. Voyage 2 found that Uranus has a magnetic field that is about 0.1 that of Saturn and that it is larger than the sun. It appears to be comprised of plasma ad a large population of energetic particles. Uranus' upper ionosphere seems to be the primary source of energized particles, but helium and heavier nuclei that characterize the solar wind are conspicuously absent.

Pioneer 10 established that the solar wind extends beyond Neptune. Voyager 2 found that Neptune's magnetic field is tilted from the planet's rotational axis and is offset from the center of the planet. This causes marked changes in the magnetic field as the planet rotates in the solar wind. Its field strength varies from 0.1 gauss in the northern hemisphere to more than 1.0 gauss in the southern hemisphere. Voyager 2 also detected auroras similar to those on Earth, but Neptune's registered 50 million watts, compared to Earth's 100 billion watts and occurred over wide regions of the planet's surface.

For classroom materials that focus on the similarities and differences of the
Solar System Planets, see the Genesis education module
Cosmic Chemistry: Planetary Diversity. button
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Curator: Aimee Meyer
Updated: November 2009

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