Venus Fast Facts about a Planet in Orbit

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Venus, Earth's closest neighbor, sometimes shines brightly enough to cast shadows on Earth. The planet shines from the west after sunset and from the east after sunrise. The clear shining of the planet both in the morning and evening caused the ancient Greeks to believe for some time that the brilliance was caused by two separate heavenly objects, which they called Vesper (evening) and Phosphor (morning). The planet's brightness made it an obvious target for observations starting in the 17th century with the invention of the telescope, but its seemingly blank surface posed a conundrum for astronomers. Early astronomers were able to discern dark patches. Several, including Francesco Fontana, a Roman Italian architect and theorist who lived from 1668 to 1708, and Giovanni Cassini, the Genoan astronomer who lived from 1625 to 1712, created maps of Venus based on these dark and bright patches, which they labeled seas and continents.

Venus is covered with dense clouds that make direct photographic imaging in visible light, such as can be done of the Moon, impossible. Not until 1761 did anyone postulate that the planet had a thick atmosphere. At that time, Mikhail Lomonosov, a scientist at the St. Petersburg Observatory, saw an unusual refraction of sunlight from the planet. The figure on page 116 shows four views of Venus as seen by Galileo at distances of 1.4—2 million miles (2.24—3.2 million km). The top two images were taken four and five days after Galileo's closest approach to the planet. The bottom two images were taken on day six, two hours apart.These violet light images show the dynamic cloud patterns and movements that always dominate the atmosphere of the planet. These clouds never part and are exceptionally dense, so the surface of the planet is never visible to the eye.

As shown in the upper color insert on page C-8, to see the surface of the planet a space mission needs to use radar imaging. The wavelengths of radar can pass through Venus's clouds and bounce off its surface. Characteristics of the surface change the waves when they bounce, so when they return to the spacecraft, its instruments can use the changes in the waves to make images of the surface (see the sidebar "Remote Sensing," on page 36).A bright radar image is created by a higher percentage of the radar signal being bounced back to the detector, which can be caused by several things. The surface being examined may be at a higher altitude, so there is less atmosphere for the signal to pass through (though variations in altitude are slight on Venus, a difference of even a few kilometers can make a difference in reflection of up to 25 percent). The roughness of the surface also influences its reflectivity, as does the composition of the surface.

Venus is Earth's closest twin in terms of size, composition, and distance from the Sun (see the sidebar "Fundamental Information about

Clouds on Venus taken by Galileo in violet light over four days show the dynamic atmospheric movements on the planet. (NASA/GaMeo/JPL)

The complex surface of Venus, created by radar imaging, was created by a wide variety of volcanic and tectonic processes, as well as by impact cratering. (NASA/ Magellan /Pioneer/JPL)

Venus," on page 118). Despite their similar beginnings, the planets have evolved into very different states, as already evidenced by Venus's thick atmosphere and strange surface features.

The figure above is a complete radar image of one side of Venus, showing its widely varied surface, which carries chasms, faults, craters, volcanoes, and enigmatic channels that may have been formed by swiftly flowing magma. The light colors in the radar image occur where the radar waves are reflected intensely, generally implying rough terrain. Smooth terrain remains dark. These tones, therefore, correspond to terrain types, and not to composition or anything relating to surface color as it would appear to human eyes.

Each planet and some other bodies in the solar system (the Sun and certain asteroids) have been given a symbol as a shorthand in scientific writing.The symbol for Venus is shown on page 119.

Fundamental Information about Venus ^/enus's radius is almost identical to the Earth's, being only about 5 percent shorter (see the table below. The slight discrepancy in radius means that its volume is about 12 percent less than the Earth's. Because Venus's density is comparable to the Earth's, but its volume is slightly less, its gravitational field is also slightly weaker. In all these respects, though, Venus is still the most similar to Earth of all the planets. Measuring a planet's mass is most easily done when the planet has a moon orbiting it. The speed and radius of the moon's orbit depends upon the mass of the planet it is orbiting. Venus has no moons and as a result its mass was not known with great accuracy until the Mariner 10 mission craft flew past it and experienced the planet's gravitational pull.

Though Venus closely resembles the Earth in its bulk physical characteristics, its rotational and orbital characteristics are entirely different, as are its surface conditions. Venus's exceptionally slow rotation also means that it experiences only weak forces from spinning and so has no discernible equatorial bulge. Unlike faster-spinning planets, Venus is virtually spherical. In addition to being virtually spherical as an overall shape, Venus also has little surface relief (differences in elevation, especially hills or mountains). Nearly 90 percent of the planet lies within six-tenths of a mile (1 km) of the average level.

Fundamental Facts about Venus

equatorial radius 3,760.4 miles (6,051.8 km), or 0.949 of the Earth's

ellipticity ([equatorial radius—

0, meaning the planet is almost a perfect sphere,

polar radius]/polar radius)

outside of surface features

volume

2.23 X 1011 cubic miles (9.284 X 1011 km3), 0.88 times

Earth's volume

mass

1.07 X 1025 pounds (4.87 X 1024 kg), or 0.814 times Earth

average density

330 pounds per cubic feet (5,240 kg/m3), comparable to

Earth

acceleration of gravity on

29 feet per second squared (8.87 m/s2), 0.9 times Earth

the surface at the equator

magnetic field strength at

about 2 X 10-9 Tesla, or 5 X 10-5 times the Earth's field

the surface

rings

0

moons

0

Symbol Woman

Many solar system objects have simple symbols; this is the symbol for Venus.

Because Venus orbits between the Earth and the Sun, it displays phases when viewed from the Earth, similar to those of the Moon. When Venus is between the Earth and Sun it shows its unlit face to the Earth but only during the Earth's daytime, and so it is seldom seen. When Venus is opposite the Sun from the Earth, it is in full light when viewed from the Earth; between these stages Venus shows partially lit faces like the Moon's phases. Because Venus is always within 46° of the Sun when viewed from the Earth, it can only be seen in the morning and the evening.

Venus rotates on its axis in a retrograde sense, that is, in the opposite direction from the Earth's rotation.Venus's day length is therefore listed as a negative number in the table "Venus's Orbit," on page 120. On Venus, the Sun rises in the west and sets in the east. Its rotation, though, is unusually slow, and the planet completes a revolution around the Sun (a Venusian year) before it completes a rotation on its axis (a Venusian day).The simplest way to measure the rotational period of a planet is to watch surface features move across the disk of the planet. For three centuries astronomers attempted to measure Venus's rotational period this way. Confused by both thick clouds and retrograde rotation, attempts resulted in estimates from 23 hours to 225 days. Only the advent of radar observations in the late 1950s and 1960s made the measurement possible.

There are three main theories for Venus's retrograde rotation.The first two theories stand upon the idea that the planet's retrograde

Many solar system objects have simple symbols; this is the symbol for Venus.

VENUS'S ORBIT

rotation on its axis ("day")

-243 Earth days

rotation speed at equator

0.29 miles per second (0.47 km/s)

rotation direction

retrograde (clockwise when viewed from above the North Pole; opposite direction to Earth's spin)

sidereal period ("year")

224.7 days ("day" longer than "year")

orbital velocity (average)

21.75 miles per second (35.02 km/s)

sunlight travel time (average)

6 minutes and 1 second to reach Venus

average distance from the Sun

67,239,750 miles (108,208,930 km,) or 0.723 AU

perihelion

66,952,000 miles (107,476,000 km), or 0.718 AU from the Sun

aphelion

67,695,000 miles (108,942,000 km), or 0.728 AU from the Sun

orbital eccentricity

0.006773°

orbital inclination to the ecliptic

3.39°

obliquity (inclination of equator

to orbit)

177.3° (because the rotation axis is within 3° of the vertical, Venus has no discernible seasons)

rotation is likely the result of its rotation axis being tipped almost 180° from the vertical.Venus almost certainly began rotating normally as a result of forming from the rotating planetary nebula, but because the planet has been almost completely tipped over, the planet appears to be rotating in the opposite direction. Some astronomers believe that high internal frication and turbulence in its atmosphere caused the planet to flip over; others believe that some catastrophic event such as a giant meteorite impact knocked the planet over.

A third theory has been proposed by French astronomers Alexandre Correia and Jacques Laskar of the CNRS Institute of Celestial Mechanics, who argue that chaotic effects could have reversed the planet's spin while its rotation axis remained stationary. These scientists created large computer models of planets with different atmospheric tides, gravitational forces, internal friction, and obliquities. For the rotation axis of Venus to flip, Correia and Laskar calculated that the planet's equator must once have had a high obliquity. Although this widely accepted idea is still possible, Correia and Laskar calculated that chaotic behavior in the atmosphere of Venus could have slowed and then reversed the rotation of Venus without a high initial obliquity.Venus therefore may have flipped its rotation axis to produce its retrograde rotation, or its rotation may have slowed and reversed as a result of atmospheric turbulence.Together,Venus's slow retrograde rotation and its intense surface conditions cause the planet to differ significantly from the Earth despite their similar sizes, compositions, and distances from the Sun.

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