Venus

Venus is the second planet from the Sun, and often been described as Earth's "sister planet" because it bears a striking similarity to Earth in size, density, and composition. But its similarities end there: the surface of Venus is one of the most inhospitable places in the solar system.

Orbit and Observation

Venus orbits the Sun with a period 225 days, at an average distance of 68 million miles (108 million km) or 0.72 AU. Its nearly-circular orbit is only inclined about 3.4 degrees to the plane of the Ecliptic. Venus orbits inside the orbit of Earth; so like Mercury, Venus is only seen in the morning or evening sky. At maximum elongation, Venus is about 47 degrees from the Sun. With an apparent magnitude ranging from -3.8 to -4.4, Venus is brighter than any star. Venus is often called the "Morning Star" or "Evening Star" because of its prominence in the sky.

In a small telescope, Venus shows phases, similar to the phases of the Moon. Its appearance changes from a gibbous shape, when it is on the far side of the Sun, to a thin crescent when it is closer to Earth. The first person to observe the phases of Venus was Galileo, in 1610. From this observation, Galileo deduced that Venus must orbit the Sun - this disproving the Ptolemaic model of the solar system.

Like Mercury, Venus occasionally transits (passes across) the Sun, and such transits can be observed telescopically. Transits of Venus are much less frequent than those of Mercury. They occur in pairs 8 year apart, separated by gaps of 120 years; the last transit of Venus occurred in 2004, and the next one will happen in 2012.

Atmosphere and Climate

Almost no surface detail is visible on Venus with even the largest telescopes. Astronomers realized early on that Venus's surface is completely covered by clouds. Early spectroscopic observations revealed the presence of carbon dioxide in the atmosphere, but little else. Since Venus's surface was not visible, its nature was a subject of intense speculation. Authors and scientists postulated warm, wet, swamps with nearly-constant rainfall; or hot, dry deserts with billowing dust clouds; or a global ocean of carbonated water.

Even Venus's rotational period was unknown, until astronomers were able to determine it accurately using radar in 1963. These observations showed that Venus turns very slowly, taking 243 Earth days to complete one rotation, and in a retrograde direction - in other words, opposite from the direction of Venus's orbital motion around the Sun.

Further observations from Earth and by spacecraft revealed that the Venusian atmosphere is very dense, and made up mostly of carbon dioxide. At the surface of Venus, the atmospheric pressure is 92 times that on Earth - similar to the pressure a depth of 1000 meters below the surface of our oceans. Venus's atmosphere is also very hot, with a surface temperature of about 870°F (470°C). That is hotter than any household oven, and hot enough to melt lead. The thick atmosphere spreads heat evenly across the planet, with only a few degrees of temperature difference between the day and night sides, or from equator to pole. Venus's clouds are not composed of water droplets, like Earth's; but rather of sulfuric acid. The hot, dense, and poisonous atmosphere makes Venus completely uninhabitable to life as we know it.

The carbon dioxide in Venus's atmosphere is transparent to visible light, but opaque to infrared radiation. It traps heat, like the glass of a greenhouse. On Earth, the oceans dissolve carbon dioxide and store it in the crust as carbonate rocks (like limestone), and green plants convert carbon dioxide into oxygen through photosynthesis. On Venus, any oceans that were present when the planet formed have long since boiled away, crippling Venus's ability to remove carbon dioxide from its atmosphere. Today, the amount of carbon present in the Earth's oceans and crust is about the same as the amount present in the atmosphere of Venus. Venus has suffered a "runaway greenhouse effect".

Geology and Exploration

Venus's thick atmosphere prevents us from seeing its surface directly. However, Venus has been mapped in great detail by radar observations from spacecraft like NASA's 1991 Magellan mission. Radar mapping shows Venus to have a mountainous, rugged topography with active volcanoes, and very few impact craters.

Although Venus must have a molten core, like Earth, it does not appear to have any plate tectonic activity. This may be due to the lack of water, which serves to "lubricate" the Earth's crustal plates. Venus also has essentially no magnetic field. The reasons for this are that Venus rotates very slowly, and that there is an absence of convection in the liquid core (as evidenced by the lack of plate tectonics).

Only a few spacecraft have landed on the surface of Venus, and none of them have survived for more than an hour or so. They have returned pictures of a barren surface covered with slabs of rock. The Soviet Venera missions have lofted balloons into the atmosphere of Venus, and have detected powerful thunderstorm-like convection cells and lightning.

Other interplanetary spacecraft, such as NASA's Galileo, Cassini, and MESSENGER missions, have used Venus's gravity to assist them in "slingshot" maneuvers to help them reach their destinations.

Venus, like Mercury, has no known moons.