The interior orbit of Venus means that it never strays far from the Sun in the sky. Because of its highly reflective cloud cover, Venus is brighter than any star in the sky, as seen from Earth. It is so bright that it can be seen even in the daytime.
Venus's rotation is slow and retrograde, most likely because of a collision between Venus and some other solar system body during the late stages of the planet's formation.
The extremely thick atmosphere of Venus is nearly opaque to visible radiation, making the planet's surface invisible from the outside. Spectroscopic examination of sunlight reflected from the planet's cloud tops shows the presence of large amounts of carbon dioxide. Venus's atmosphere is nearly 100 times denser than Earth's. The temperature of the upper atmosphere is much like that of Earth's upper atmosphere, but the surface temperature is 730 K.
Venus's surface cannot be seen in visible light from Earth, but it has been thoroughly mapped by radar from Earth-based radio telescopes and orbiting satellites. The most recent and most thorough survey has been carried out by the U.S. Magellan satellite.
Venus's surface is mostly smooth, resembling rolling plains with modest highlands and lowlands. Two elevated continent-sized regions are called Ishtar Terra and Aphrodite Terra. There is no evidence for plate tectonic activity as on Earth.
Many lava domes and shield volcanoes have been found by Magellan on Venus's surface, but none of the volcanoes has yet been proven to be currently active. The planet's surface shows no sign of plate tectonics. Features called coronae are thought to have been caused by an upwelling of mantle material that, for unknown reasons, never developed into full convective motion. The surface of the planet appears to be relatively young, resurfaced by volcanism every few hundred million years. Some craters on Venus are due to meteoritic impact, but the majority are volcanic in origin. The evidence for currently active volcanoes on Venus includes surface features resembling those produced in earthly volcanism, fluctuating levels of sulfur dioxide in Venus's atmosphere, and bursts of radio energy similar to those produced by lightning discharges that often occur in the plumes of erupting volcanoes on Earth. However, no actual eruptions have been seen.
Soviet spacecraft that landed on Venus photographed surface rocks with sharp edges and a slablike character. Some rocks on Venus appear predominantly basaltic in nature, implying a volcanic past. Other rocks resemble terrestrial granite and are probably part of the planet's ancient crust.
Venus is comparable in both mass and radius to Earth, suggesting that the two planets started off with fairly similar surface conditions. However, the atmospheres of Earth and Venus are now very different. The total mass of Venus's atmosphere is about 90 times greater than Earth's. The greenhouse effect stemming from the large amount of carbon dioxide in Venus's atmosphere is the basic cause of the planet's current high temperatures. Almost all the water vapor and carbon dioxide initially present in Earth's early atmosphere quickly became part of the oceans or surface rocks. Because Venus orbits closer to the Sun than does Earth, surface temperatures were initially higher, and the planet's greenhouse gases never left the atmosphere. On Venus, the runaway greenhouse effect caused all the planet's greenhouse gases—carbon dioxide and water vapor—to end up in the atmosphere, leading to the extreme conditions we observe today.
Venus has no detectable magnetic field, almost certainly because the planet's rotation is too slow for any appreciable dynamo effect to occur. To some planetary geologists, Venus's interior structure suggests that of the young Earth, before convection became established in the mantle.
1. Venus appears brighter in the sky than any star. HINT
2. Venus has the least eccentric orbit of any of the planets in the solar system.
3. Venus is brightest when it is in its full phase. HINT
4. Although Venus is slightly smaller than Earth, its atmosphere is significantly more extensive. HINT
5. The average surface temperature of Venus is about 260 K. HINT
6. Numerous large surface features on Venus can be seen from Earth-based observations made in the ultraviolet part of the spectrum. HINT
7. The top cloud layers of Venus are composed of sulfuric acid. HINT
8. Other than lacking oxygen, Venus's atmosphere is quite similar to that of Earth. HINT
9. Venus has roughly the same temperature at its equator as at its poles. HINT
10. The surface of Venus is relatively rough, compared with the surface of Earth, with higher highs and lower lows. HINT
11. Images from Magellan show signs of tectonic activity on Venus. HINT
12. Lava flows are common on the surface of Venus. HINT
13. There is strong circumstantial evidence that active volcanism continues on Venus. HINT
14. Venus has a magnetic field similar to that of Earth. HINT
15. Soviet spacecraft are still sending back data from the surface of Venus. HINT
1. Venus's mass has now been well determined through the use of _____ orbiting it. HINT
2. Because Venus has a mass and average _____ only slightly lower than Earth's, we might expect that its internal structure and evolution should be Earthlike. HINT
3. Venus's rotation is unusual because it is _____. HINT
4. The most abundant gas in the atmosphere of Venus is _____. HINT
5. Water vapor is found in _____ amounts in the atmosphere of Venus. HINT
6. The process that makes Venus so hot is known as the _____. HINT
7. The runaway greenhouse effect on Venus was a result of the planet's being _____ to the Sun than is Earth. HINT
8. The surface of Venus has been mapped using _____. HINT
9. Ishtar Terra and Aphrodite Terra are two _____ on the surface of Venus. HINT
10. Most craters on the surface of Venus are the result of _____. HINT
11. Huge, roughly circular regions on the surface of Venus are known as ____. HINT
12. A meteoroid less than about 1 km in diameter will most likely _____ as it passes through Venus's atmosphere. HINT
13. The scarcity of large impact craters on the surface of Venus indicates that its surface is quite _____. HINT
14. The surface of Venus appears to be resurfaced by _____ every few hundred million years. HINT
15. The main difficulties in using landers to study Venus's surface are the planet's extremely high _____ and _____. HINT
1. Why does Venus appear so bright to the eye? Upon what factors does the brightness of Venus depend? HINT
2. Explain why Venus is always found in the same region of the sky as the Sun. HINT
3. If you were standing on Venus, how would Earth look? HINT
4. How did radio observations of Venus made in the 1950s change our conception of the planet? HINT
5. What did ultraviolet images returned by Pioneer Venus show about the planet's high-level clouds? HINT
6. Name three ways in which the atmosphere of Venus differs from that of Earth. HINT
7. What are the main constituents of Venus's atmosphere? What are clouds in the upper atmosphere made of? HINT
8. What component of Venus's atmosphere causes Venus to be so hot? Explain why there is so much of this gas in the atmosphere of Venus, compared with Earth's. What happened to all the water that Venus must have had when formed? HINT
9. Earth and Venus are nearly alike in size and density. What primary fact caused one planet to evolve as an oasis for life, while the other became a dry and inhospitable inferno? HINT
10. If Venus had formed at Earth's distance from the Sun, what do you imagine its climate would be like today? Why do you think this? HINT
11. How do the "continents" of Venus differ from earthly continents? HINT
12. How are the impact craters of Venus different from those found on other bodies? HINT
13. What evidence exists that volcanism of various types has changed the surface of Venus? What is the evidence for active volcanoes? HINT
14. Given that Venus, like Earth, probably has a partially molten iron-rich core, why doesn't it also have a magnetic field? HINT
15. Do you think there might be life on Venus? Explain your answer. HINT
1. Using the data given in the text, calculate Venus's angular diameter, as seen by an observer on Earth, when the planet is (a) at its brightest, (b) at greatest elongation, and (c) at the most distant point in its orbit.
2. How long does a radar signal take to travel from Earth to Venus and back when Venus is brightest (see problem 1)? Compare this with the round-trip time when Venus is at its closest point to Earth.
3. Using the data presented in the Venus Data box, calculate the length of a solar day on Venus (that is, the time from one sunrise to the next). How would your answer change if Venus's rotation were prograde instead of retrograde?
4. Compare the magnitude of the tidal gravitational force on Venus due to Earth at closest approach with that due to the Sun. Assume circular orbits for both planets.
5. What is the size of the smallest feature that can be distinguished on the surface of Venus by the Arecibo radio telescope, at an angular resolution of 1'? HINT
6. Pioneer Venus observed high-level clouds moving around Venus's equator in 4 days. What was their speed in km/h? In mph? HINT
7. Venus's atmosphere is 90 times more massive than Earth's atmosphere. Approximating Venus's atmosphere as a layer of gas of uniform density 70 kg/m3, calculate its effective thickness, in kilometers. (Compare Chapter 7, problem 3.)
8. According to Stefan's law (see 
Sec. 3.4), how much more radiation—per square meter, say—is emitted by Venus's surface at 750 K than is emitted by Earth's surface at 300 K?
9. In the absence of any greenhouse effect, Venus's average surface temperature, like Earth's, would be about 250 K. In fact, it is about 750 K. Use this information and Stefan's law to estimate the fraction of infrared radiation leaving Venus's surface that is absorbed by carbon dioxide in the planet's atmosphere.
10. Calculate the orbital period of the Magellan spacecraft, moving around Venus on an elliptical orbit with a minimum altitude of 294 km and a maximum altitude of 8543 km above the planet's surface. In 1993 the spacecraft's orbit was changed to have minimum and maximum altitudes of 180 km and 541 km, respectively. What was the new period?
1. Is Venus in the morning or evening sky right now? Look for it every few days, over the course of several weeks. Draw a picture of the planet with respect to foreground trees or buildings. If you always observe at the same time every day, you may begin to notice that the planet is getting higher or lower in the sky.
2. Consult an almanac to determine the next time Venus will pass between Earth and Sun. How many days before and after this event can you glimpse the planet with the eye alone?
3. Consult the almanac again to find out the next time Venus will pass on the far side of the Sun from Earth. How many days before and after this event can you see the planet with the naked eye?
4. When Venus ornaments the predawn sky, try keeping track of the planet with your eye alone until it appears in a blue sky, after sunrise. As always, be careful not to look at the Sun!
5. Using a powerful pair of binoculars or a small telescope, examine Venus as it goes through its phases. Note the phase and the relative size of it. (You can compare its size to the field of view in a telescope; always use the same eyepiece for this.) Look at it every few days or once a week. Make a table of the shape of the phase, the size, and the relative brightness to the naked eye. After you have observed it through a significant change in phase, can you see the correlations between these three properties first recognized by Galileo?
