Venus

Interior structure and geologic evolution

Much less is known about the interior of Venus than about its surface and atmosphere. Nevertheless, because the planet is much like Earth in overall size and density and because it presumably accreted from similar materials (see solar system: Origin of the solar system), scientists expect that it evolved at least a crudely similar internal state. Therefore, it probably has a core of metal, a mantle of dense rock, and a crust of less-dense rock. The core, like that of Earth, is probably composed primarily of iron and nickel, although Venus's somewhat lower density may indicate that its core also contains some other, less-dense material such as sulfur. Because no intrinsic magnetic field has been detected for Venus, there is no direct evidence for a metallic core, as there is for Earth. Calculations of Venus's internal structure suggest that the outer boundary of the core lies a little more than 3,000 km (1,860 miles) from the centre of the planet.

Above the core and below the crust lies Venus's mantle, making up the bulk of the planet's volume. Despite the high surface temperatures, temperatures within the mantle are likely similar to those in Earth's mantle. Even though a planetary mantle is composed of solid rock, the material there can slowly creep or flow, just as glacial ice does, allowing sweeping convective motions to take place. Convection is a great equalizer of the temperatures of planetary interiors. Similar to heat production within Earth, heat within Venus is thought to be generated by the decay of natural radioactive materials. This heat is transported to the surface by convection. If temperatures deep within Venus were substantially higher than those within Earth, the viscosity of the rocks in the mantle would drop sharply, speeding convection and removing the heat more rapidly. Therefore, the deep interiors of Venus and Earth are not expected to differ dramatically in temperature.

As noted above, the composition of the Venusian crust is believed to be dominated by basalt. Gravity data suggest that the thickness of the crust is fairly uniform over much of the planet, with typical values of perhaps 20–50 km (12–30 miles). Possible exceptions are the tessera highlands, where the crust may be significantly thicker.

Convective motions in a planet's mantle can cause materials near the surface to experience stress, and motions in the Venusian mantle may be largely responsible for the tectonic deformation observed in radar images. On Venus the gravity field is found to correlate more strongly with topography over broad regional scales than it is on Earth—i.e., large regions where the topography is higher than the mean elevation on Venus also tend to be regions where the measured gravity is higher than average. This implies that much of the increased mass associated with the elevated topography is not offset by a compensating deficit of mass in the underlying crust that supports it (so-called low-density roots), as it is on Earth (see isostasy). Instead, some of the broad-scale relief on Venus may owe its origin directly to present-day convective motions in the mantle. Raised topography, such as Beta Regio, could lie above regions of mantle upwelling, whereas lowered topography, such as Lavinia Planitia, could lie above regions of mantle downwelling.

Despite the many overall similarities between Venus and Earth, the geologic evolution of the two planets has been strikingly different. Evidence suggests that the process of plate tectonics does not now operate on Venus. Although deformation of the lithosphere does indeed seem to be driven by mantle motions, lithospheric plates do not move mainly horizontally relative to each other, as they do on Earth. Instead, motions are mostly vertical, with the lithosphere warping up and down in response to the underlying convective motions. Volcanism, coronae, and rifts tend to be concentrated in regions of upwelling, while plains deformation belts are concentrated in regions of downwelling. The formation of rugged uplands such as Aphrodite and Ishtar is not as well understood, but the mechanism probably involves some kind of local crustal thickening in response to mantle motions.

The lack of plate tectonics on Venus may be due in part to the planet's high surface temperature, which makes the upper rigid layer of the planet—the lithosphere—more buoyant and hence more resistant to subduction than Earth's lithosphere, other factors being equal. Interestingly, there is evidence that the Venusian lithosphere may be thicker than Earth's and that it has thickened with time. A gradual, long-term thickening of Venus's lithosphere in fact could be related to the curious conclusion drawn from Venus's cratering record (see above the section Impact craters)—that most of the planet underwent a brief but intense period of geologic resurfacing less than a billion years ago. One possible explanation is that Venus may experience episodic global overturns of its mantle, in which an initially thin lithosphere slowly thickens until it founders on a near-global scale, triggering a brief, massive geologic resurfacing event. How many times this may have occurred during the planet's history and when it may happen again are unknown.

Observations from Earth

Since Galileo's discovery of Venus's phases, the planet has been studied in detail, using Earth-based telescopes, radar, and other instruments. Over the centuries telescopic observers, including Gian Domenico Cassini of France and William Herschel of England, have reported a variety of faint markings on its disk. Some of these markings may have corresponded to the cloud features observed in modern times in ultraviolet light, while others may have been illusory.

Important early telescopic observations of Venus were conducted in the 1700s during the planet's solar transits (see eclipse: Transits of Mercury and Venus). In a solar transit an object passes directly between the Sun and Earth and is silhouetted briefly against the Sun's disk. Transits of Venus are rare events, occurring in pairs eight years apart with more than a century between pairs. They were extremely important events to 18th-century astronomy, since they provided the most accurate method known at that time for determining the distance between Earth and the Sun. (This distance, known as the astronomical unit, is one of the fundamental units of astronomy.) Observations of the 1761 transit were only partially successful but did result in the first suggestion, by the Russian scientist Mikhail V. Lomonosov, that Venus has an atmosphere. The second transit of the pair, in 1769, was observed with somewhat greater success. Transits must be viewed from many points on Earth to yield accurate distances, and the transits of 1761 and, particularly, 1769 prompted the launching of many scientific expeditions to remote parts of the globe. Among these was the first of the three voyages of exploration by the British naval officer James Cook, who, with scientists from the Royal Society, observed the 1769 transit from Tahiti. The transit observations of the 1700s not only gave an improved value for the astronomical unit but also provided the impetus for many unrelated but important discoveries concerning Earth's geography. By the time the subsequent pair of transits occurred, in 1874 and 1882, the nascent field of celestial photography had advanced enough to allow scientists to record on glass plates what they saw through their telescopes. No transits took place in the 20th century; the first of the next pair was widely observed and imaged in 2004.

In the modern era Venus has also been observed at wavelengths outside the visible spectrum. The cloud features were discovered with certainty in 1927–28 in ultraviolet photographs. The first studies of the infrared spectrum of Venus, in 1932, showed that its atmosphere is composed primarily of carbon dioxide. Subsequent infrared observations revealed further details about the composition of both the atmosphere and the clouds. Observations in the microwave portion of the spectrum, beginning in earnest in the late 1950s and early '60s, provided the first evidence of the extremely high surface temperatures on the planet and prompted the study of the greenhouse effect as a means of producing these temperatures.

After finding that Venus is completely enshrouded by clouds, astronomers turned to other techniques to study its surface. Foremost among these has been radar (see radio and radar astronomy). If equipped with an appropriate transmitter, a large radio telescope can be used as a radar system to bounce a radio signal off a planet and detect its return. Because radio wavelengths penetrate the thick Venusian atmosphere, the technique is an effective means of probing the planet's surface.

Earth-based radar observations have been conducted primarily from Arecibo Observatory in the mountains of Puerto Rico, the Goldstone tracking station complex in the desert of southern California, and Haystack Observatory in Massachusetts. The first successful radar observations of Venus took place at Goldstone and Haystack in 1961 and revealed the planet's slow rotation. Subsequent observations determined the rotation properties more precisely and began to unveil some of the major features on the planet's surface. The first features to be observed were dubbed Alpha, Beta, and Maxwell, the last after James Clerk Maxwell, the Scottish physicist who first derived some of the basic equations that describe the propagation of electromagnetic radiation. These three features are among the brightest on the planet in radar images, and their names have been preserved to the present as Alpha Regio, Beta Regio, and Maxwell Montes.

By the mid-1980s Earth-based radar technology had advanced such that images from Arecibo were revealing surface features as small as a few kilometres in size. Nevertheless, because Venus always presents nearly the same face toward Earth when the planets are at their closest, much of the surface went virtually unobserved from Earth.

Spacecraft exploration

The greatest advances in the study of Venus were achieved through the use of robotic spacecraft. The first spacecraft to reach the vicinity of another planet and return data was the U.S. Mariner 2 in its flyby of Venus in 1962. Since then, Venus has been the target of more than 20 spacecraft missions.

Successful early Venus missions undertaken by the United States involved Mariner 2, Mariner 5 (1967), and Mariner 10 (1974). Each spacecraft made a single close flyby, providing successively improved scientific data in accord with concurrent advances in spacecraft and instrument technology. After visiting Venus, Mariner 10 went on to a successful series of flybys of Mercury. In 1978 the United States launched the Pioneer Venus mission, comprising two complementary spacecraft. The Orbiter went into orbit around the planet, while the Multiprobe released four entry probes—one large probe and three smaller ones—that were targeted to widely separated points in the Venusian atmosphere to collect data on atmospheric structure and composition. The Orbiter carried 17 scientific instruments, most of them focused on study of the planet's atmosphere, ionosphere, and interaction with the solar wind. Its radar altimeter provided the first high-quality map of Venus's surface topography. Pioneer Venus Orbiter was one of the longest-lived planetary spacecraft, returning data for more than 14 years.

Venus was also a major target of the Soviet Union's planetary exploration program during the 1960s, '70s, and '80s, which achieved several spectacular successes. After an early sequence of failed missions, in 1967 Soviet scientists launched Venera 4, comprising a flyby spacecraft as well as a probe that entered the planet's atmosphere. Highlights of subsequent missions included the first successful soft landing on another planet (Venera 7 in 1970), the first images returned from the surface of another planet (Venera 9 and 10 landers in 1975), and the first spacecraft placed in orbit around Venus (Venera 9 and 10 orbiters).

In terms of the advances they provided in the global understanding of Venus, the most important Soviet missions were Veneras 15 and 16 in 1983. The twin orbiters carried the first radar systems flown to another planet that were capable of producing high-quality images of the surface. They produced a map of the northern quarter of Venus with a resolution of 1–2 km (0.6–1.2 miles), and many types of geologic features now known to exist on the planet were either discovered or first observed in detail in the Venera 15 and 16 data. Late the following year the Soviet Union launched two more spacecraft to Venus, Vegas 1 and 2. These delivered Venera-style landers and dropped off two balloons in the Venusian atmosphere, each of which survived for about two days and transmitted data from their float altitudes in the middle cloud layer. The Vega spacecraft themselves continued past Venus to conduct successful flybys of Halley's Comet in 1986.

In 1990, on its way to Jupiter, the U.S. Galileo spacecraft flew by Venus. Among its more notable observations were images at near-infrared wavelengths that viewed deep into the atmosphere and showed the highly variable opacity of the main cloud deck.

The most ambitious mission yet to Venus, the U.S. Magellan spacecraft, was launched in 1989 and the next year entered orbit around the planet, where it conducted observations until late 1994. Magellan carried a radar system capable of producing images with a resolution better than 100 metres (330 feet). Because the orbit was nearly polar, the spacecraft was able to view essentially all latitudes on the planet. On each orbit the radar system obtained an image strip about 20 km (12 miles) wide and typically more than 16,000 km (almost 10,000 miles) long, extending nearly from pole to pole. The image strips were assembled into mosaics, and high-quality radar images of about 98 percent of the planet were ultimately produced. Magellan also carried a radar altimeter system that measured the planet's surface topography as well as some properties of its surface materials. After the main radar objectives of the mission were completed, the spacecraft's orbit was modified slightly so that it passed repeatedly through the upper fringes of the Venusian atmosphere. The resulting drag on the spacecraft gradually removed energy from its orbit, turning an initially elliptical orbit into a low, circular one. This procedure, known as aerobraking, has since been used on other planetary missions to conserve large amounts of fuel by reducing the use of thrusters for orbital reshaping. From its new circular orbit, the Magellan spacecraft was able to make the first detailed map of Venus's gravitational field.

The U.S. Cassini-Huygens spacecraft flew by Venus twice, in 1998 and 1999, on the way to its primary target, Saturn. During its brief passages near Venus, Cassini failed to corroborate signs of the existence of lightning in the planet's atmosphere that had been observed by previous spacecraft. This suggested to some scientists that lightning on Venus is either rare or different from the lightning that occurs on Earth.

The European Space Agency's Venus Express, which was launched in 2005, entered into orbit around Venus the following year, becoming the first European spacecraft to visit the planet. Venus Express carried a camera, a visible-light and infrared imaging spectrometer, and other instruments to study Venus's magnetic field, plasma environment, atmosphere, and surface for a planned mission of more than two Venusian years. Among its early accomplishments was the return of the first images of cloud structures over the planet's south pole.

Steven W. Squyres

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