Venus (planet)

Article Outline

Introduction; Observation from Earth; Orbit and Rotation; Venus’s Past; Exploration

Surface Features

Because Venus and Earth are so similar in size, they are sometimes called twin planets. Venus is only slightly smaller and less dense than Earth. Its radius is 6,052 km (3,760 mi) and its average density is 5.2 g/cm³. The planet’s surface gravity is nine-tenths as strong as surface gravity on Earth; an object that weighs 10 kg on Earth would weigh 9 kg on Venus.

It is clear from data collected by space probes, however, that the geological processes that shaped the surfaces of both planets are different. Blocked by thick clouds, the surface features on Venus can only be studied by radar or by special infrared detectors. The first maps of Venus were made by radar on Earth. Curiously, the periods of Venus’s orbit and rotation cause the same side of the planet to always face Earth when the two planets are closest. At such times, the side facing Earth can be viewed and mapped by Earth-based radar.

In contrast to the very large antenna needed for Earth-based radar mapping of Venus, a modest instrument on Pioneer Venus 1 was able to conduct a nearly global survey. Combined with data from the Soviet probes, the Magellan orbiter, and Earth-based radar, the survey shows that the surface of Venus is primarily a rolling plain interrupted by two continent-sized highland areas, which have been named Ishtar Terra and Aphrodite Terra after two manifestations of the goddess Venus. Aphrodite Terra, although not as high as Ishtar Terra, extends nearly halfway around the equatorial region; it occupies the planet’s far side as viewed from Earth at closest approach.

The more powerful radar aboard the Magellan spacecraft has revealed huge volcanoes, large solidified lava flows, and a large array of meteorite craters. The largest impact crater is almost 300 km (190 mi) across—the smallest about 5 km (3 mi). Although the probe’s radar could resolve even smaller craters, if any were present, Venus’s dense atmosphere apparently prevents smaller meteorites from impacting the surface of the planet. It is believed that all craters older than about 500 million years have been obliterated, while the more recent ones show little sign of modification.

Venus’s Past

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Because the size and density of Venus and Earth are so similar, scientists think the two planets originated in the same way. Like Earth, Venus formed about 4.6 billion years ago out of the spinning disk of dust and debris that surrounded the newborn Sun. The materials accreted (clumped together) to form larger and larger objects called planetesimals, resulting in bodies with sizes between those of the Moon and Mars. A number of these bodies with similar or intersecting orbits eventually collided and merged to form both Venus and Earth at difference distances from the Sun.

The earliest history of Venus was probably very similar to that of Earth. However, impacts from leftover Moon or Mars-size bodies in the inner solar system could have given Venus its odd, extremely slow, backward (clockwise) rotation. One or more of these impacts may have created a moon or moons for Venus, much the same way that our Moon is thought to have formed from a giant impact with Earth. Some theories suggest that Venus’s ancient moon or moons eventually crashed back into the planet, reversing its original faster counterclockwise rotation.

Scientists are not certain if Venus had large amounts water like Earth after it formed or if it has always been dry. In the wet scenario, Venus may have had oceans and an atmosphere similar to the early Earth for millions of years. Because it received more sunlight, Venus began to heat up, increasing the water vapor in the atmosphere. Water vapor is an extremely efficient greenhouse gas. The planet eventually suffered a runaway greenhouse effect that raised its surface temperature to a point where the oceans boiled away entirely. Ultraviolet light from the Sun then broke down the water vapor in the atmosphere into hydrogen and oxygen. The light hydrogen atoms escaped into space, carried off by the solar wind, while the oxygen atoms reacted with minerals in the crust. With most of the water lost, carbon dioxide could not combine with water to form carbonate rocks as it had on Earth. The massive amount of carbon dioxide in the atmosphere added to the greenhouse effect begun by the nearly vanished water vapor.

Studies of the impact craters on the surface have led some scientists to propose that the entire surface of the Venus melted and reformed in a planet-wide eruption around 500 million years ago—no older impact craters have been identified. Such a global event may have happened a number of times during the history of Venus. Unlike the constant tectonic plate movement and volcanic activity that gradually reshapes Earth’s crust, Venus may undergo rare but catastrophic tectonic processes that resurface the entire crust all at once.

Exploration

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Venus’s complete cloud cover and deep atmosphere make it difficult to study from Earth. Most knowledge of the planet has been obtained through the use of space vehicles, particularly those carrying probes that descend through the atmosphere. The first flyby was that of Mariner 2, launched by the United States in 1962, followed by Mariner 5 in 1967 and Mariner 10 in 1974. The former Union of Soviet Socialist Republics developed several entry probes, some combined with flybys or orbiters: Venera 4 and 5 (1967), 6 (1969), 7 (1970), 8 (1972), 9 and 10 (1975), 11 and 12 (1978), 13 and 14 (1981), and 15 and 16 (1983); Vega 1 and 2, sent toward Halley’s comet in 1984, also flew by Venus and released descent capsules. Several of these probes successfully reached the planet’s surface. The United States sent two Pioneer Venus missions in 1978. Pioneer Venus 2 sent four probes to the surface, while the remaining craft explored the upper atmosphere. Pioneer Venus 1, an orbiter, measured the upper atmosphere for 14 years. The Magellan probe, launched toward Venus in 1989, transmitted radar images of the planet from 1990 to 1994.

Other spacecraft have made flybys of Venus to use its gravity to change their orbits around the Sun. The Galileo probe flew by Venus in 1990 to gain energy to reach Jupiter. The Cassini probe made flybys of Venus in 1998 and 1999 for gravitational boosts to reach Saturn. NASA’s MESSENGER probe, however, is using three passes by Venus to lose energy so the craft can enter orbit around the inner planet Mercury in 2011. All of these probes have also gathered data about Venus on their flybys.

In 2005 the European Space Agency (ESA) launched the Venus Express spacecraft on a mission to Venus. The spacecraft began orbiting Venus in 2006. It is equipped with instruments designed to study the structure, chemistry, and dynamics of the planet’s atmosphere, particularly its hurricane-force winds and its cloud system. The Venus Express also carried the first infrared instrument designed to study the planet’s surface at infrared wavelengths, making it possible to detect active volcanoes if they exist.

The extreme conditions on Venus make it highly unlikely that humans will ever set foot on the planet—there are no current plans for manned exploration. If humans ever reached Venus, they would weigh about the same as on Earth but would otherwise find a totally alien world requiring heavy protection from heat and pressure. The stifling atmosphere is so dense that even a slow breeze would feel like a tremendous gust. Although nighttime would last for over 58 days, it would not be dark—the entire rocky landscape would glow a dull red like the burner on an electric stove. During the daytime, the Sun would be only faintly visible through the dense clouds that give the entire surface a dull, yellow-orange cast. At higher elevations, particles of heavy metals might fall like snow. Venus may be the closest thing in the solar system to the way humans have imagined hell.

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Surface Features

Venus’s Past

Exploration