Planetary studies

Mercury is too hot to retain an atmosphere, but Venus’s brilliant white appearance is the result of its being completely enveloped in thick clouds of carbon dioxide, impenetrable at visible wavelengths. Below the upper clouds, Venus has a hostile atmosphere containing clouds of sulfuric acid droplets. The cloud cover shields the planet’s surface from direct sunlight, but the energy that does filter through warms the surface, which then radiates at infrared wavelengths. The long-wavelength infrared radiation is trapped by the dense clouds such that an efficient greenhouse effect keeps the surface temperature near 465 °C (870 °F, 740 K). Radar, which can penetrate the thick Venusian clouds, has been used to map the planet’s surface. In contrast, the atmosphere of Mars is very thin and is composed mostly of carbon dioxide (95 percent), with very little water vapour; the planet’s surface pressure is only about 0.006 that of Earth. The outer planets have atmospheres composed largely of light gases, mainly hydrogen and helium.

Each planet rotates on its axis, and nearly all of them rotate in the same direction—counterclockwise as viewed from above the ecliptic. The two exceptions are Venus, which rotates in the clockwise direction beneath its cloud cover, and Uranus, which has its rotation axis very nearly in the plane of the ecliptic.

Some of the planets have magnetic fields. Earth’s field extends outward until it is disturbed by the solar wind—an outward flow of protons and electrons from the Sun—which carries a magnetic field along with it. Through processes not yet fully understood, particles from the solar wind and galactic cosmic rays (high-speed particles from outside the solar system) populate two doughnut-shaped regions called the Van Allen radiation belts. The inner belt extends from about 1,000 to 5,000 km (600 to 3,000 miles) above Earth’s surface, and the outer from roughly 15,000 to 25,000 km (9,300 to 15,500 miles). In these belts, trapped particles spiral along paths that take them around Earth while bouncing back and forth between the Northern and Southern hemispheres, with their orbits controlled by Earth’s magnetic field. During periods of increased solar activity, these regions of trapped particles are disturbed, and some of the particles move down into Earth’s atmosphere, where they collide with atoms and molecules to produce auroras.

Jupiter has a magnetic field far stronger than Earth’s and many more trapped electrons, whose synchrotron radiation (electromagnetic radiation emitted by high-speed charged particles that are forced to move in curved paths, as under the influence of a magnetic field) is detectable from Earth. Bursts of increased radio emission are correlated with the position of Io, the innermost of the four Galilean moons of Jupiter. Saturn has a magnetic field that is much weaker than Jupiter’s, but it too has a region of trapped particles. Mercury has a weak magnetic field that is only about 1 percent as strong as Earth’s and shows no evidence of trapped particles. Uranus and Neptune have fields that are less than one-tenth the strength of Saturn’s and appear much more complex than that of Earth. No field has been detected around Venus or Mars.