Life Beyond Earth - The Habitable Zone
Discovery:
Because Venus is so bright the brightest light in Earth's skyexcept for the Sun and our Moon, it was likely recognizable in earlyhistory as a unique feature in the sky. In 1610, Galileo Galilei was thefirst to note that Venus had a visible disk and phases like the Moon.
Size:
Venus is the second planet from the Sun and the sixth largest in our solar system.
Orbit:
Venus orbits the Sun at an average distance of 108,200,000 kilometers fromthe Sun. A year on Venus is approximately 225 Earth days.
Comparison to Earth:
Venus and Earth have roughly the same size and mass. At its equator Venusis 12,104 kilometers in diameter (or 0.95 times that of the Earth), andits mass is 0.81 times that of the Earth. Both planets also have fewcraters, indicating relatively young surfaces. The planets also havesimilar density and composition as well.
Comparisons of the two planets' rotations, however, illustrate a keydifference. Venus rotates on its axis in a direction opposite that of theEarth and the other planets of our solar system and takes 243 Earth-daysfor a complete rotation. One revolution around the Sun for Venus takes 225Earth days. Because Venus takes longer to rotate on its own axis than tocomplete a journey around the Sun, one day on Venus is slightly longerthan one year there!
Terrain:
Most of Venus' surface consists of gently rolling plains with littlerelief. There are also several broad depressions: Atalanta Planitia,Guinevere Planitia, Lavinia Planitia. There two large highland areas:Ishtar Terra in the northern hemisphere (about the size of Australia) andAphrodite Terra along the equator (about the size of South America). Thehighest point on the planet, lying in the Maxwell mountains, rises about11 kilometers above average surface elevation. The Maxwell range, namedafter physicist James Clerk Maxwell, is the only Venus feature named aftera man. All other features on the planet are named after women, followingplanetary nomenclature rules established by the International Astronomical Union.
Data collected by Magellan's imaging radar shows that much of Venus iscovered by lava flows. There are several large shield volcanoes (similarto Hawaii) such as Sif Mons. Recently announced findings indicate thatVenus is still volcanically active, but only in a few hot spots. For themost part, it has been geologically quiet for the past few hundred millionyears.
Water:
At one time, Venus may have had large amounts of water much likeEarth, but now it is a dry, barren landscape. In fact, when it "rains" onVenus (its rain consists primarily of sulfur and carbon dioxide), anywater evaporates before it ever hits the ground.
Exploration:
Numerous U.S. space missions have visited Venus, beginning with a flightpast the planet by the U.S. Mariner 2 in 1962. Mariner 5 flew past Venusin 1967 and Mariner 10 in 1974. In 1978 two U.S. spacecraft were sentthere. Pioneer Venus 1 orbited the planet, collecting data on surfaceheights and returning radar images, while Pioneer Venus 2 released threeprobes to the surface to collect atmospheric data. The most recent U.S.mission to Venus, Magellan, launched from the Space Shuttle Atlantis onMay 4, 1989 and entered Venus' orbit on August 10, 1990. The Magellanspacecraft remained in Venus' orbit collecting data for four years, andmapped 99 percent of the planet's surface utilizing a state-of-the-artradar. On October 12, 1994 the Magellan spacecraft entered Venus' denseatmosphere and disintegrated.
The former Soviet Union has had an active Venus exploration program withits sixteen Venera missions conducted between 1961 and 1983. Theseexplorations included orbiters, landers, and balloon studies of theplanet's atmosphere. In 1985, the Soviet Union missions Vega 1 and Vega 2also conducted flybys of Venus.
The Interior:
The interior of Venus is probably very similar to that of Earth: an ironcore about 3000 km in radius, a molten rocky mantle comprising themajority of the planet. Recent results from the Magellan gravity dataindicate that Venus' crust is stronger and thicker than had previouslybeen assumed. Like Earth, convection in the mantle produces stress on thesurface, which is relieved in many relatively small regions instead ofbeing concentrated at plate boundaries, as is the case on Earth.
Atmosphere:
There are several layers of clouds many kilometers thick composed ofsulfuric acid. These clouds completely obscure our view surface. Theatmosphere is composed mostly of carbon dioxide. There are strong winds up to 350 kilometers per hour at the cloud tops, but windsat the surface are very slow, no more few kilometers per hour. Two Veneraspacecraft carrying lightning detectors recorded massive lightning strikesin the Venus atmosphere, occuring at an average rate of more than 10 timesper second.
Probes from the Pioneer Venus mission discovered that, below the clouds,the atmosphere contains about 0.1 to 0.4 percent water vapor and 60 partsper million of free oxygen. These components indicate that Venus may havehad abundant water at one point early in its history, water that has sincebeen lost.
Temperature:
Temperatures on the planet's surface average 462 degrees Celcius (864 degrees Fahrenheit).
Magnetic fields:
Venus lacks any magnetic field, perhaps because of its excruciatingly slow rotation.
Satellites:
Venus has no satellites (moons).
More Information:
Learn more about Venus and its relationship to Earth in an interview withcomparative planetology expert David Grinspoon.
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