Venus

Venus is the second planet from the Sun and is in some superficial geological features like Earth . In many important geochemical features, however, it is quite different. Next to the Sun and Moon , Venus can be the brightest object in the sky. At its most brilliant, Venus is sixteen times brighter than Sirius (? Canis Majoris), the brightest star in the night sky. The extreme brilliance of Venus is partly due to its occasional closeness to Earth, and partly due to it having a highly reflective atmosphere.

Basic properties

Venus orbits the Sun on a near circular orbit at mean distance of 0.723 Astronomical Units (AU). At aphelion the planet is at a maximum distance of 0.728 AU from the Sun, while at perihelion Venus it is 0.718 AU away. Coming as near as 0.272 AU, no other planet can approach the earth as close as Venus does.

Since it circles the Sun in an orbit smaller than that of Earth's, Venus is never very far away from the Sun for terrestrial viewers. The greatest angular distance between the Sun and Venus, as seen from Earth, is 47 degrees. This means that even under the most favorable of conditions Venus will set at most three hours after the Sun, or rise no earlier than three hours before the Sun. Observed since the most ancient of times, Venus is often called the "morning star," if it rises before dawn or the "evening star" if it sets after the Sun. The Greek philosopher Homer referred to Venus in his Illiad as "the most beautiful star set in the sky."

The time required for Venus to complete one orbit about the Sun, its sidereal period, is 224.701 days, whereas the time for Venus to repeat alignments with respect to the Sun and Earth is 584 days. The best times for viewing Venus are when it is near greatest eastern, or greatest western elongation. Greatest western elongation follows about five months after greatest eastern elongation, and greatest eastern elongation's repeat about every 19 months.

As it circles the Sun, Venus shows phases just like the Moon, and the planet Mercury. The phase changes of Venus were first recorded by Galileo Galilei in 1610. Galileo's observations of Venusian phase changes were important since they strengthened his belief in the heliocentric model of the solar system which had been proposed by Copernicus in his now famous book, De Revolutionibus, published in 1543. Galileo reasoned that if Venus circled Earth, as the then accepted geocentric model of the Universe decreed, it would not show the full range of phases that were observed. Likewise, the correlation between angular size and phase would also be different to that observed if Venus orbited Earth.

Transits of Venus across the disk of the Sun, as seen from Earth, are not common. If a transit is to occur, however, it will take place in either June or December, when Earth is at the line along which Venus's orbit cuts the ecliptic—the line of nodes for Venus. A very precise geometrical alignment of Earth, Venus and Sun is required for a Venusian transit to take place. The last Venusian transit occurred on December 6, 1882. The next transit will take place on June 8, 2004. In general, Venusian transits are seen in pairs. Each of the transits in a pair are eight years apart, and the pair-cycle repeats on an alternating cycle of 121.5 and 105.5 years.

The rotation rate of Venus

The Venusian atmosphere is both optically thick and highly reflective. The upper cloud deck has an albedo of 0.76, meaning that it reflects 76% of the sunlight that falls on it. In addition, the low-lying Venusian clouds are so dense that they completely obscure any optical view of the planet's surface. Not being able to monitor variations in surface detail has meant that astronomers have only recently discovered the true rotation rate of Venus.

While the Venusian atmosphere presents an impenetrable shroud against optical observations, it is transparent to radio and microwave radiation . Using the giant radio telescope at the Arecibo Observatory in Puerto Rico, astronomers were able to bounce microwave signals off the surface of Venus. By analyzing the Doppler shift in the returned signals the astronomers were then able to determine the planet's rotation rate. The results were a complete surprise.

The microwave measurement of Venus's rotation rate showed that the planet was spinning on its axis in the opposite sense to which it orbits the Sun. If one could stand on the surface of Venus, sunrise would be in the west and sunset would be in the east, the exact opposite to that seen on Earth. When viewed from above, all the planets in the solar system orbit the Sun in a counter clockwise direction. Most of the planets also rotate about their spin axies in a counter clockwise sense. The only exceptions to this rule are the planets Venus, Uranus , and Pluto . When a planet spins on its axis in the opposite sense to its orbital motion , the rotation is said to be retrograde.

Venus takes 243.01 days to spin once on its axis. With this slow rotation rate it actually takes 18.3 days longer for Venus to spin once on its axis than it takes for the planet to orbit the Sun. The Venusian day, that is the time from one noon to the next, is 116.8 terrestrial days long. A curious relationship exists between the length of the Venusian day and the planet's syndic period. The syndic period of Venus, that is, the time for the planet to repeat the same alignment with respect to Earth and Sun, is 584 days, and this is five times the Venusian day (584 = 5 × 116.8). It is not known if this result is just a coincidence, or the action of some subtle orbital interaction. The practical consequence of the relationship is that, should a terrestrial observer make two observations of Venus that are 584 days apart, then they will "see" the same side of the planet turned towards Earth.

Venusian surface detail

At is closest approach the planet Venus can have an angular diameter just slightly larger than 1/60th of a degree. This angular size translates to a physical diameter of 7,520 mi (12,104 km), making the planet about 95% the size of Earth. Given the near similar sizes of Earth and Venus, it might be expected that the surface of Venus has been shaped by the same geological processes that operate on Earth. This expectation is only partly true.

While radar maps of the surface of Venus were constructed during the 1970s, the most detailed topographic maps obtained to date are those from the Magellan spacecraft mission. The Magellan spacecraft was placed into Venusian orbit by NASA in 1990, and a powerful on-board imaging radar system was used to map the entire Venusian surface to a resolution of a few hundred meters.

The Magellan radar data showed that Venus is remarkably flat, and that some 80% of the planet's surface is covered by smooth volcanic plains, the result of many lava out-flows. The altitude map constructed from Magellan data has revealed the existence of two large continent-like features on Venus. These features are known as Ishtar Terra (named after the Babylonian Goddess of love), and Aphrodite Terra (named after the Greek Goddess of love). Ishtar terra, which measures some 621 mi (1,000 km) by 931 mi (1,500 km), lies in Venus's northern hemisphere, and has the form of a high plateau ringed with mountains . The largest mountain in the region, Maxwell Montes, rises to a height of 7 mi (11 km). Aphrodite Terra is situated just to the south of the Venusian equator and is some 9,936 mi (16,000 km) long by 1,242 mi (2,000 km) wide. It is a region dominated by mountainous highlands and several large volcanoes.

Venus has three major terrains, based mainly upon elevation (above mean planet radius). The lowlands are rolling volcanic plains (~ 60% of the planet) with under 1,600 ft (500 m) of relief. The uplands are intermediate in elevation and represent a transition between lowlands and highlands. Elevations range from 0 to 1.2 mi (0 to 2 km) in the uplands. The highlands have up to 3 mi (5 km) relief and are quite mountainous in some places. The highlands have compression ridges and fractured rocks and comprise about 15% of the planet's surface.

Given the similarity in size of Venus and Earth, geologists had speculated on the possibility that plate tectonics , which is the primary agent for re-shaping Earth's surface, might operate on Venus. The Magellan probe found no evidence, however, for large scale tectonic activity on Venus. The reasons underlying the absence of any large-scale tectonic activity on Venus are presently unclear, but it may be indicative that the planet has a thinner and weaker lithosphere than Earth. The ridges and folds that cover many of the plain regions of Venus is indicative, however, of a certain amount of local tectonic activity on the planet.

The Magellan maps revealed many large craters on the Venusian surface. There is an apparent cut-off for craters with diameters less than a few kilometers. This is a selection effect imposed by the dense Venusian atmosphere. The atmosphere is in fact such a good filter of incoming meteoroids, that only those objects larger than a few kilometers in diameter survive their passage through the atmosphere, with sufficient mass , to produce a crater at the Venusian surface.

The relative age of different regions on a planet's surface can be gauged by counting the number of craters that appear per unit area. This method of crater counting has proved very useful for dating the various regions on our Moon. The essential idea being exploited in crater count dating is that, assuming the cratering rate is the same over the whole planet, if one region has fewer craters per unit area than another, it implies that some resurfacing, e.g., by a lava flow, has taken place in the region with fewer craters. The re-surfacing has in effect erased the older craters and re-set the cratering clock.

The number of craters in the Venusian plains is typically about 15% of the crater counts for the lunar maria. This observation indicates that the Venusian plains have an age roughly equivalent to 15% the age of the lunar maria. From the lunar rock samples that were returned from the Apollo Moon landing missions we know that the lunar maria are about 3.2 billion years old, and consequently the likely age of the Venusian plains is about 500 million years.

The main agent for re-surfacing the Venusian plains is believed to be aperiodic but widespread volcanism. Certainly, many (apparently) extinct volcano's were mapped by Magellan during its five-year survey. Some lava flow regions observed by Magellan are believed to be no more than ten millions years old, and they may be much younger.

Some of the more remarkable surface features discovered by Magellan were the pancake-shaped volcanoes. These flat-topped, circular volcanoes are unique to Venus, and it is thought that they are probably formed through the surface extrusion of a very thick and viscous lava.

In recognition of Venus being named in honor of the Greek Goddess Aphrodites, whom the Romans called Venus, the International Astronomical Union assigns only female names to the planet's surface features. Craters, for example, can be named after any famous women; linear features are named after Goddesses of War, while plains are named in honor of mythological heroines.

Venusian surface processes

Processes affecting Venus include impact cratering, lava flows, solid-state creep (viscous flow of rocks at the surface due to high temperatures and pressures), and eolian (wind ) effects. The latter include deflation (blowing away fine particles), wind erosion (forming yardangs), and wind deposition (formation of streaks, transverse dunes, and wind-shadow dunes). Weathering on the surface is due to interaction between carbon dioxide and sulfur dioxide at high temperature with silicate rocks. At elevations above about 2.2 mi (3.5 km) above mean planet radius, weathering seems to favor formation of iron-sulfur compounds and below that level, iron-oxide and calcium-sulfate compounds, tend to form. The latter was inferred from the nature of the reflected radar signal off surficial materials.

Venusian internal structure

Venus has no natural satellites and consequently its mass has only been determined through the gravitational effect that the planet has on passing space probes. A mass equivalent to 82% that of Earth's, or 4.9x1024 kg, has been found for Venus. The bulk density of Venus is 5240 kg/m3, slightly smaller than that of Earth's.

The similarity between the mass, radius, and bulk density of Venus and Earth suggests that the two planets have similar internal structure. Venus most probably has, therefore, a thin rocky crust, a large iron- and magnesium-silicate mantle, and an inner nickel-iron alloy core (~ 25% of the planet's mass).

One Venusian anomaly that defies present-day theory relates to the planet's magnetic field, or more correctly to the complete lack of any detectable magnetic field. It is believed that the earth's magnetic field is created by a dynamo effect that operates in its hot, liquid nickel-iron alloy core. If, as has been previously argued, Venus has an internal structure similar to that of Earth, why does it not have a similar magnetic field? The answer may lie with the slow Venusian rotation rate. One of the key ingredients of the dynamo theory is that the conducting, liquid core is rotating. Since Venus rotates much more slowly than Earth, by a multiplicative factor of 1/243, it may be that the dynamo effect cannot operate in the planet's core.

The Venusian atmosphere

While Venus is often referred to as Earth's twin on the basis that the two planets have similar physical characteristics (radius, mass, density, composition, etc), it is far from being Earth's twin when atmospheric characteristics are compared.

The many spacecraft that have flown past, or landed on, the Venusian surface have found that the uppermost cloud tops, which obscure Earth-based observers' view of the planet, are about 40 mi (65 km) above the surface. For comparison, on Earth, the highest clouds are about 10 mi (16 km high.) Observations taken of ultraviolet wavelengths reveal that the upper Venusian clouds follow a jet stream-like pattern and circle the planet once every four days, or so. The circulation velocity of the upper cloud deck is much greater than the rotation rate of the planet, and it is believed that this is the result of extensive atmospheric convection driven by solar heating.

The upper cloud deck is about 3 mi (5 km) thick. At about 31 mi (50 km) altitude there is a second much more dense cloud deck. Below about 18 mi (30 km) in altitude the Venusian atmosphere is clear of clouds. The upper cloud deck has been found to contain substantial amounts of sulfur , which give the clouds their dark yellow to yellow-orange color . The lower cloud deck has been found to contain large concentrations of sulfur dioxide, hydrogen sulfide compounds, and droplets of sulfuric acid . It has been suggested that the presence of atmospheric sulfides is indicative of very recent volcanic activity on the planet's surface.

The greenhouse effect

Astronomers began to suspect that the surface of Venus was a decidedly inhospitable place when radio telescope measurements, made in the 1950s, indicated surface temperatures as high as 750K (891°F; 477°C). It is believed that a greenhouse effect is responsible for maintaining the high surface temperature on Venus.

A greenhouse effect occurs whenever incoming sunlight warms the planetary surface, but the atmospheric gases do not allow the infrared radiation emitted by the heated surface to escape back into space. The net result of the atmospheric trapping of infrared radiation is that the atmosphere heats up, and the surface temperature continues to rise.

That a strong greenhouse effect can operate at Venus is not surprising given that its primary atmospheric component is carbon dioxide. This gas has long been recognized as a problematic "greenhouse" gas on Earth.

Building a spacecraft to land on the Venusian surface has proved to be a major engineering challenge. Not only must a lander be able to operate at temperatures that exceed the melting point of lead , but it must also withstand an atmospheric pressure some 90 times greater than that experienced at sea level on Earth. The pressure exerted by the Venusian atmosphere, at the planet's surface, is equivalent to that exerted by a 0.6 mi (1 km) column of water in Earth's oceans. The first of only four spacecraft to soft-land, and successfully transmit images of the surface of Venus back to Earth, was the former Soviet Union-built spacecraft Venera 7. The lander, which set-down on August 17, 1970, managed to transmit data for 23 minutes.

Impact craters on Venus

Venus' atmosphere protects the surface from smaller objects that would otherwise impact the surface if the atmosphere were thinner. Smaller objects, especially those under 0.6 mi (1 km) in diameter are largely broken up in the atmosphere and do not directly impact the surface. For this reason and due to Venus' high volcanic resurfacing rate, rather few impact craters on Venus are known (less than 1000). Impact craters on Venus are frequently attended by flows of impact ejecta that look much like lava flows. This is due to the effect of the thick atmosphere on ejecta behavior during impact. Meade Patera is the largest impact crater basin on Venus at 174 mi (280 km) in diameter. There are only six known multi-ring impact basins on Venus, of which Meade is one. There are also large "splotches" (radar bright spots) about 6–44 mi (10–70 km) in diameter thought to be due to the effect of near-surface atmospheric detonation of incoming objects.

Venus geologic history

Venus has two alternative histories, depending upon how one views the age of the extensive resurfacing volcanic lavas. In the "catastrophic" model of Venus history, there was a huge resurfacing event in Venus' history about 200 to 700 million years ago, probably due to rather sudden solidification of the interior of the planet. Vast volcanic features of about the same age favor this interpretation. In the "gradualistic" model of Venus history, global resurfacing has occurred gradually over the whole of Venus' history and the rate of this activity has been rather high so no older surface areas still exist. The mechanism for this is random and continuous volcanic activity. This model does not explain the lack of magnetic field (explained in the other model by the internal solidification event). Future studies of Venus will hopefully help us understand which of these (or perhaps some other) model is correct about Venus' past.

See also Planetary atmospheres.

Resources

books

Beatty, J. Kelly, Carolyn Collins Petersen, and Andrew L. Chaikin. The New Solar System. Cambridge: Cambridge University Press, 1999.
de Pater, Imke, and Jack J. Lissauer. Planetary Sciences. Cambridge, UK: Cambridge University Press, 2001.

Morrison, D., and Tobias Owen. The Planetary System. 3rd ed. Addison-Wesley Publishing, 2002.

Taylor, F.W. The Cambridge Photographic Guide to the Planets. Cambridge University Press, 2002.

periodicals

Goldman, Stuart. "Venus Unveiled." Sky and Telescope 83 (March 1992).

Luhmann, Janet, J. Pollack, and C. Lawrence. "The Pioneer Mission to Venus." Scientific American (April 1994). Paul, Jeffrey. "Venus in 3-D." Sky & Telescope 86 (August 1993). Saunders, Stephen. "The Surface of Venus." Scientific American (December 1990).

other

Arnett, B. SEDS, University of Arizona. "The Nine Planets, A Multimedia Tour of the Solar System." November 6, 2002 [cited February 8, 2003] <http://seds.lpl.arizona.edu/nineplanets/nineplanets/nineplanets.html>.

Martin Beech

David T. King, Jr.

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Albedo

—The fraction of sunlight that a surface reflects. An albedo of zero indicates complete absorption, while an albedo of unity indicates total reflection.

Doppler effect

—The apparent change in the wavelength of a signal due to the relative motion of the source and the observer.

Dynamo effect

—A model for the generation of planetary magnetic fields: the circulation of conducting fluids within a planet's hot, liquid inner-core results in the generation of a magnetic field.

Greenhouse effect

—The phenomenon that occurs when gases in a planet's atmosphere capture radiant energy radiated from a planet's surface thereby raising the temperature of the atmosphere and the planet it surrounds.

Lithosphere

—The solid outer layer, or crust, of a planet's mantle.

Mantle

—The major portion of a terrestrial planet's interior, made of plastic rock.

Retrograde rotation

—Axial spin that is directed in the opposite sense to that of the orbital motion.

Tectonic activity

—The theory of crustal motion.