Later Development

Planetary scientists continue to puzzle over the ages of the major geologic and geophysical events that took place on Mercury after its formation. On the one hand, it is tempting to model the planet's history after that of the Moon, whose chronology has been accurately dated from the rocks returned by the U.S. Apollo manned landings and Soviet Luna robotic missions. By analogy, Mercury would have had a similar history, but one in which the planet cooled off and became geologically inactive shortly after the Caloris impact rather than experiencing persistent volcanism for hundreds of millions of years, as did the Moon. On the presumption that Mercury's craters were produced by the same populations of remnant planetary building blocks (plan-etesimals), asteroids, and comets that struck the Moon, most of the craters would have formed before and during an especially intense period of bombardment in the inner solar system, which on the Moon is well documented to have ended about 3.8 billion years ago. Caloris presumably would have formed about that time, representing the final chapter in Mercury's geologic history, apart from occasional cratering.

On the other hand, there are many indications that Mercury is very much geologically alive even today. Its dipolar field seems to require a core that is still at least partially molten in order to sustain the magnetohydrodynamic dynamo. Indeed, recent radar measurements of Mercury's spin state have been interpreted as proving that at least the outer core is still molten. In addition, as suggested above, Mercury's scarps show evidence that the planet may not have completed its cooling and shrinking.

There are several approaches to resolving this apparent contradiction between a planet that died geologically before the Moon did and one that is still alive. One hypothesis is that most of Mercury's craters are younger than those on the Moon, having been formed by impacts from so-called vulcanoids—the name bestowed on a hypothetical remnant population of asteroid-sized objects orbiting the Sun inside Mercury's orbit— that would have cratered Mercury over the planet's age. In this case Caloris, the lobate scarps, and other features would be much younger than 3.8 billion years, and Mercury could be viewed as a planet whose surface has only recently become inactive and whose warm interior is still cooling down. No vulcanoids have yet been discovered, however, despite a number of searches for them. Moreover, objects orbiting the Sun so closely and having such high relative velocities could well have been broken up in catastrophic collisions with each other long ago.

A more likely solution to Mercury's thermal conundrum is that the outer shell of Mercury's iron core remains molten because of contamination, for instance, with a small proportion of sulfur, which would lower the melting point of the metal, and of radioactive potassium, which would augment production of heat. Also, the planet's interior may have cooled more slowly than previously calculated as a result of restricted heat transfer. Perhaps the contraction of the planet's crust, so evident about the time of formation of Caloris, pinched off the volcanic vents that had yielded such prolific volcanism earlier in Mercury's history. In this scenario, despite present-day Mercury's lingering internal warmth and churnings, surface activity ceased long ago, with the possible exception of a few thrust faults as the planet continues slowly to contract.

Venus

No planet approaches closer to Earth than Venus, the second planet from the Sun. At its nearest it is the closest large body to Earth other than the Moon. Because Venus's orbit is nearer the Sun than Earth's, the planet is always roughly in the same direction in the sky as the Sun and can be seen only in the hours near sunrise or sunset. When it is visible, it is the most brilliant planet in the sky. Venus is designated by the symbol Q. Venus is the sixth largest planet in the solar system in size and mass.

Venus was one of the five planets—along with Mercury, Mars, Jupiter, and Saturn—known in ancient times, and its motions were observed and studied for centuries prior to the invention of advanced astronomical instruments. Its appearances were recorded by the Babylonians, who equated it with the goddess Ishtar, about 3000 BCE, and it also is mentioned prominently in the astronomical records of other ancient civilizations, including those of China, Central America, Egypt, and Greece. Like the planet Mercury, Venus was known in ancient Greece by two different names—Phosphorus when it appeared as a morning star and Hesperus when it appeared as an evening star. Its modern name comes from the Roman goddess of love and beauty (the Greek equivalent being Aphrodite), perhaps because of the planet's luminous jewellike appearance.

Venus has been called Earth's twin because of the similarities in their masses, sizes, and densities and their similar relative locations in the solar system. Because they presumably formed in the solar nebula from the same kind of rocky planetary building blocks, they also likely have similar overall chemical compositions. Early telescopic observations of the planet revealed a perpetual veil of clouds, suggestive of a substantial atmosphere and leading to popular speculation that Venus was a warm, wet world, perhaps similar to Earth during its prehistoric age of swampy carboniferous forests and abundant life. Scientists now know, however, that Venus and Earth have evolved surface conditions that could hardly be more different. Venus is extremely hot, dry, and in other ways so forbidding that it is improbable that life as it is understood on Earth could have developed there. One of scientists' major goals in studying Venus is to understand how its harsh conditions came about, which may hold important lessons about the causes of environmental change on Earth.

Venus photographed in ultraviolet light by the Pioneer Venus Orbiter (Pioneer 12) spacecraft, Feb. 26,1979. Although Venus's cloud cover is nearly featureless in visible light, ultraviolet imaging reveals distinctive structure and pattern, including global-scale V-shaped bands that open toward the west (left). NASA/JPL

BASIC

_ASTRONOMICAL DATA_

Viewed through a telescope, Venus presents a brilliant yellow-white, essentially featureless face to the observer. Its obscured appearance results from the surface of the planet being hidden from sight by a continuous and permanent

Global image of the topography of Venus below its obscuring clouds, based on radar data from the Magellan spacecraft with supplemental data from Venera and Pioneer Venus missions and Earth-based radar studies. NASA/JPL/California Institute of Technology cover of clouds. Features in the clouds are difficult to see in visible light. When observed at ultraviolet wavelengths, the clouds exhibit distinctive dark markings, with complex swirling patterns near the equator and global-scale bright and dark bands that are V-shaped and open toward the west. Because of the all-enveloping clouds, little was known about Venus's surface, atmosphere, and evolution before the early 1960s, when the first radar observations were undertaken and spacecraft made the first flybys of the planet.

Venus orbits the Sun at a mean distance of 108 million km (67 million miles), which is about 0.7 times Earth's distance from the Sun. It has the least eccentric orbit of any planet, with a deviation from a perfect circle of only about 1 part in 150. Consequently, its distances at perihelion and aphelion (i.e., when it is nearest and farthest from the Sun, respectively) vary little from the mean distance. The period of its orbit—that is, the length of the Venusian year—is 224.7 Earth days. As Venus and Earth revolve around the Sun, the distance between them varies from a minimum of about 42 million km (26 million miles) to a maximum of about 257 million km (160 million miles). Because Venus's orbit lies within Earth's, the planet exhibits phases like those of the Moon when viewed from Earth. In fact, the discovery of these phases by the Italian scientist Galileo in 1610 was one of the most important in the history of astronomy. In Galileo's day the prevailing model of the universe was based on the assertion by the Greek astronomer Ptolemy almost 15 centuries earlier that all celestial objects revolve around Earth. Observation of the phases of Venus was inconsistent with this view but was consistent with the Polish astronomer Nicolaus Copernicus's idea that the solar system is centred on the Sun. Galileo's observation of the phases of Venus provided the first direct observational evidence for Copernican theory.

The rotation of Venus on its axis is unusual in both its direction and its speed. The Sun and most of the planets in the solar system rotate in a counterclockwise direction when viewed from above their north poles; this direction is called direct, or prograde. Venus, however, rotates in the opposite, or retrograde, direction. Were it not for the planet's clouds, an observer on Venus's surface would see the Sun rise in the west and set in the east. Venus spins very slowly, taking about 243 Earth days to complete one rotation with respect to the stars—the length of its sidereal day. Venus's spin and orbital periods are very nearly synchronized with Earth's orbit such that, when the two planets are at their closest, Venus presents almost the same face toward Earth. The reasons for this are complex and have to do with the gravitational interactions of Venus, Earth, and the Sun, as well as the effects of Venus's massive rotating atmosphere. Because Venus's spin axis is tilted only about 3° toward the plane of its orbit, the planet does not have appreciable seasons. Astronomers as yet have no satisfactory explanation for Venus's peculiar rotational characteristics. The idea cited most often is that, when Venus was forming from the accretion of planetary building blocks, or planetesimals, one of the largest of these bodies collided with the proto-Venus in such a way

PLANETARY DATA FOR VENUS

mean distance from Sun

108,200,000 km (0.72 AU)

eccentricity of orbit

0.007

inclination of orbit to ecliptic

3.4°

Venusian year (sidereal period of revolution)

224.7 Earth days

maximum visual magnitude

-4.6

mean synodic period*

584 Earth days

mean orbital velocity

35 km/s

radius (equatorial and polar)

6,051.8 km

surface area

4.6 x 108 km2

mass

4.87 x 1024 kg

mean density

5.25 g/cm3

mean surface gravity

860 cm/s2

escape velocity

10.4 km/s

rotation period (Venusian sidereal day)

243 Earth days (retrograde)

Venusian mean solar day

116.8 Earth days

inclination of equator to orbit

177°

atmospheric composition

carbon dioxide, 96%; molecular nitrogen, 3.5%; water, 0.02%; trace quantities of carbon monoxide, molecular oxygen, sulfur dioxide, hydrogen chloride, and other gases

mean surface temperature

737 K (867 °F, 464 °C)

surface pressure at mean radius

95 bars

mean visible cloud temperature

about 230 K (-46 °F, -43 °C)

number of known moons

none

*Time required for the planet to return to the same position in the sky relative to the Sun as seen from Earth.

as to tip it over and possibly slow its spin as well.

Venus's mean radius is 6,051.8 km (3,760.4 miles), or about 95 percent of Earth's at the equator, while its mass is 4.87 x 1024 kg, or 81.5 percent that of Earth. The similarities to Earth in size and mass produce a similarity in density—5.25 g/cm3 (3.04 oz/in3) for Venus, compared with 5.52 for Earth. They also result in a comparable surface gravity; humans standing on Venus would possess nearly 90 percent of their weight on Earth. Venus is more nearly spherical than most planets. A planet's rotation generally causes a bulging at the equator and a slight flattening at the poles, but Venus's very slow spin allows it to maintain its highly spherical shape.

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