Venus (planet)

I

Introduction

Venus (planet), second planet in distance from the Sun, but the hottest planet in the solar system (hotter than Mercury). Its hellish surface has broiling temperatures that make rocks glow red under a crushing atmosphere that shrouds the planet in thick layers of clouds. Venus is nearly the same size as Earth, but takes 243 days to rotate on its axis in the opposite direction. It also lacks a magnetic field and a moon. Why conditions on Venus and Earth are so different remains a major puzzle for planetary scientists. Venus circles the Sun at a distance of 108 million km (67 million mi) in a little over seven months (about 225 days). The planet was named for Venus, the Roman goddess of beauty.

Except for the Sun and the Moon, Venus is the brightest object in the sky. It is often called the morning star when it appears in the east at sunrise, and the evening star when it is in the west at sunset. In ancient times the evening star was called Hesperus and the morning star Phosphorus, Eosphoros, or Lucifer.

II

Observation from Earth

When viewed through a telescope, Venus exhibits phases like the Moon. This effect occurs because Venus orbits closer to the Sun than Earth does. An observer on Earth can see Venus illuminated from the side and from behind as well as face-on—planets further away from the Sun than Earth only show a nearly fully lit, face-on view toward our planet. The phases of Venus were one of the proofs that the 17th-century astronomer Galileo used to show that Earth itself orbits the Sun. Maximum brilliance (a stellar magnitude of -4.4, 15 times as bright as the brightest star) is seen in the crescent phase when the planet is closer to Earth. Venus’s full phase appears smaller and dimmer because it occurs when the planet is on the far side of the Sun from Earth. The phases and positions of Venus in the sky repeat every 1.6 years (see Time; Year).

Transits of Venus (when the planet moves across the face of the Sun as seen from Earth) are rare, occurring in pairs at intervals of a little more than a century. The most recent transit occurred in 2004. The second transit of the pair will be in 2012. The next pair of transits will occur in 2117 and 2125.

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The thick cover of clouds around Venus meant that earlier generations of astronomers using telescopes had little information about conditions on the surface. Some researchers speculated that Venus might be a lush tropical world or an ocean planet drenched by thick rain clouds. Other scientists predicted a dry desert swept by dust storms, or with petroleum seas. The first clues that conditions on Venus might be extremely hot came from microwave observations in 1956. Earth-based radar studies in the 1960s discovered the planet’s slow retrograde rotation. It would take space probes to provide much more detailed information.

III

Orbit and Rotation

Venus orbits the Sun at an average distance of about 108 million km (67 million mi), or 0.7233 astronomical units (AU). An AU is equal to the average distance between Earth and the Sun, or about 150 million km (93 million mi). Venus is the nearest planet to Earth in distance at about 41 million km (25.4 million mi) away at its closest approach.

Venus circles the Sun once every 224.7 days in a counterclockwise direction, the same direction as the other planets in the solar system. Its axis is nearly vertical and its orbit is nearly circular so Venus does not experience seasons the way Earth and Mars do because of their more tilted axes and more elliptical orbits. Venus rotates very slowly, once every 243 Earth days. Venus’s rotation is retrograde, which means that the planet turns clockwise (from east to west) as seen looking down on its north pole. Earth and most other planets turn counterclockwise (from west to east).

Venus’s counterclockwise orbit and slow, clockwise rotation combine to make the periods of day and night on the planet very different from its 243-day period of rotation (called a sidereal day). A full solar day on Venus—the time when the Sun next passes the noon point in the sky—is 116.8 days long. Viewed from a spot on the equator of Venus, the Sun rises in the west and takes 58.4 days to cross the sky until it sets in the east. Night also lasts 58.4 days. Venus’s sidereal day (one complete rotation on its axis) is longer than its year (224.7 days), but the planet experiences almost two complete solar days per orbit around the Sun. On Earth, the solar day (24 hr) is four minutes longer than the sidereal day (23 hr 56 min) to add our planet’s extra counterclockwise orbital motion around the Sun to its counterclockwise rotation. As a result Earth counts 365 solar days and 366.2 sidereal days in a year.

A

Atmosphere

Although Venus is 30 percent closer to the Sun and receives about twice the amount of sunlight that Earth does, its thick clouds reflect back about 76 percent of the Sun’s energy. Only 10 percent of the Sun’s energy penetrates the clouds to reach the surface. Nonetheless, Venus has the hottest surface of any planet in the solar system because of its atmosphere.

The atmosphere of the planet consists of 97 percent carbon dioxide (CO₂) and is so thick that the surface pressure is 96 bars (compared with 1 bar on Earth). The surface temperature on Venus varies little from place to place and is extremely hot, about 462°C (736 K/864°F), or hot enough to melt lead. (The average surface temperature of Earth is about 15°C (288 K/59°F).) The high surface temperature is explained by an intense greenhouse effect. Even though only a small percentage of the solar energy that falls on Venus reaches the surface, the planet stays hot because the thick CO₂ atmosphere prevents the energy from escaping. Carbon dioxide is a very efficient “greenhouse” gas that has been linked to global warming on Earth, where the gas makes up only about 0.035 percent of the atmosphere.

That nearly all of Venus’s atmosphere is CO₂ is not as strange as it might seem; in fact, the crust of Earth contains almost as much CO₂ chemically bound in the form of limestone, a mineral that forms in the presence of water. About 3 percent of the Venusian atmosphere is nitrogen gas (N₂). By contrast, 78 percent of Earth’s atmosphere is nitrogen. The actual amount of nitrogen molecules in the atmospheres of both planets is virtually the same, however. Water and water vapor are extremely rare on Venus. Many scientists argue that Venus, being closer to the Sun, was subjected to a so-called runaway greenhouse effect, which caused any oceans to evaporate into the atmosphere. The hydrogen atoms of the water molecules could have been lost to space and the oxygen atoms to the crust. Another possibility is that Venus had very little water to begin with.

Cloud particles on Venus mostly consist of concentrated sulfuric acid. Earth’s atmosphere also contains a very thin haze of sulfuric acid particles in the stratosphere. On Earth, however, sulfuric acid does not build up because rain carries it down to react with surface materials. On Venus the acid evaporates at the cloud base, which lies about 50 km (31 mi) above the surface, and so remains in the atmosphere. The upper parts of the clouds, visible from Earth and from Pioneer Venus 1, extend as haze 70 to 80 km (44 to 50 mi) above the surface. The clouds contain a pale yellow impurity, better detected at near-ultraviolet wavelengths. Variations in the sulfur dioxide content of the atmosphere may indicate active volcanism on the planet. Some evidence suggests that lightning may occur in the atmosphere. A possible source might be electrically charged sulfuric acid droplets, but most researchers are not in favor of this idea.

Certain cloud patterns and weather features that can be discerned in the cloud tops give some information about complex wind motion in the atmosphere. The upper-level winds circle the planet at 360 km/h (225 mph), making a complete rotation in only four days. These winds are said to super-rotate because they travel much faster than the rotation of the planet itself. These high-speed winds cover the planet completely, blowing toward the west at virtually every latitude from equator to pole. The motions of descending probes, however, have shown that the bulk of Venus’s tremendously dense atmosphere, closer to the planet’s surface, is almost stagnant. From the surface up to 10 km (6 mi) altitude, wind speeds are only about 3 to 18 km/h (2 to 11 mph). The high-speed winds probably result from the transfer of momentum from Venus’s slow-moving, massive lower atmosphere to higher altitudes where the atmosphere is less massive, so that the same momentum results in a much higher velocity.

Data from the Venus Express and other space probes have revealed that a double vortex forms at both poles of Venus—two huge hurricane-like structures that rotate very slowly side-by-side. Sinking air at the poles may create the double spinning systems, which have a complex vertical structure that scientists are only beginning to understand.

The upper atmosphere and ionosphere were studied in great detail by Pioneer Venus 1, which passed through them once each day. On Earth this region is very hot; on Venus it is not, even though Venus is closer to the Sun. Surprisingly, on the night side of Venus the upper atmosphere is extremely cold. (Day-side temperatures are 40°C/104°F, compared to night-side temperatures of -170°C/-274°F.) Scientists believe that strong winds blow from the day side toward the near vacuum that is caused by the low temperatures on the night side. Such winds would carry along light gases, such as hydrogen and helium, which are concentrated in a night-side “bulge.”

On Earth the ionosphere is isolated from the solar wind (a flow of charged particles from the Sun) by the magnetosphere, the magnetic envelope created by the dynamo effect of Earth’s rotating core. Earth’s magnetosphere deflects charged particles from the Sun, although some particles can strike the atmosphere at the poles to create auroras. Venus lacks a magnetic field of its own, but the solar wind seems to generate an induced magnetosphere around the planet by interacting with the ionosphere. This magnetosphere is similar to ones created around comets.