1. Chapter 9 Venus
Chapter 9 opener. Often called Earth’s sister planet, Venus is nothing like Earth. When it comes to surface temperature, it’s hot enough there (730 K) to melt lead. We now know that Venus’s climate, like Earth’s, has varied over time—largely the result of geological activity and atmospheric change. What we do not know well is why Venus became so very much hotter than Earth—or if Earth could someday heat up similarly. Here, this global view of the surface of Venus was created when the Magellan spacecraft’s radar data were mapped onto a computer-simulated globe. ( JPL)

2. Orbital Properties
Venus is much brighter than Mercury, and can be farther from the Sun
Called morning or evening star, as it is still “tied” to Sun
Brightest object in the sky, after Sun and Moon
Figure 9-1. Venus at Sunset The Moon and Venus in the western sky just after sunset. Venus clearly outshines even the brightest stars in the sky. ( J. Schad/Photo Researchers, Inc.)

3. Orbital Properties
Apparent brightness of Venus varies, due to changes in phase and distance from Earth
Figure 9-2. Venus’s Brightness Venus appears full when it is at its greatest distance from Earth, on the opposite side of the Sun from us (superior conjunction). As its distance decreases, less and less of its sunlit side becomes visible. When closest to Earth, it lies between us and the Sun (inferior conjunction), so we cannot see the sunlit side of the planet at all. Venus appears brightest when it is about 39° from the Sun. (Compare Figure 2.12.) ( Insets: UC/Lick Observatory)

4. Physical Properties Radius: 6000 km Mass: 4.9 x 1024 kg
Density: 5200 kg/m3
Rotation period: 243 days, retrograde
Figure 9-3. Terrestrial Planets’ Spins The inner planets of the solar system—Mercury, Venus, Earth, and Mars—display widely differing rotational properties. Although all orbit the Sun in the same direction and in nearly the same plane, Mercury’s rotation is slow and prograde (in the same sense as its orbital motion), that of Venus is slow and retrograde, and those of Earth and Mars are fast and prograde. Venus rotates clockwise as seen from above the plane of the ecliptic, but Mercury, Earth, and Mars all spin counterclockwise. This is a perspective view, roughly halfway between a flat edge-on view and a direct overhead view.

5. Physical Properties Slow, retrograde rotation
1 Solar day = 117 Earth days
Figure 9-4. Venus’s Solar Day Venus’s orbit and retrograde rotation combine to produce a solar day on Venus equal to 117 Earth days, or slightly more than half a Venus year. The red arrows represent a fixed location, or an observer standing, on the planet’s surface. The numbers in the figure mark time in Earth days.

6. Long-Distance Observations of Venus
Dense atmosphere and thick clouds make surface impossible to see
Surface temperature is about 730 K—hotter than Mercury!
Figure 9-5. Venus This photograph, taken from Earth, shows Venus with its creamy yellow mask of clouds. No surface detail can be seen, because the clouds completely obscure our view of whatever lies beneath them. (AURA)

7. Long-Distance Observations of Venus
Even probes flying near Venus, using ultraviolet or infrared, can see only a little deeper into the clouds
Figure 9-6. Venus, Up Close (a) Venus as photographed by the Pioneer spacecraft’s cameras 200,000 km away from the planet. This image was made by capturing solar ultraviolet radiation reflected from the planet’s clouds, which are probably composed mostly of sulfuric acid droplets, much like the highly corrosive acid in a car battery. (b) Venus in the infrared, as seen by Venus Express on approach to the planet. The longer infrared wavelength allows us to “see” deeper into Venus’s clouds. ( NASA; ESA)

8. The Surface of Venus Surface is relatively smooth
Two continent-like features: Ishtar Terra and Aphrodite Terra
No plate tectonics
Mountains, a few craters, many volcanoes and large lava flows

9. The Surface of Venus Surface mosaics of Venus:
Figure 9-7. Venus Mosaics (a) This image of the surface of Venus was made by a radar transmitter and receiver on board the Pioneer spacecraft, which is still in orbit about the planet, but is now inoperative. The two continent-sized landmasses are named Ishtar Terra (upper left) and Aphrodite (lower right). Colors represent altitude: blue is lowest, red highest. The spatial resolution is about 25 km. (b) A planetwide mosaic of Magellan images, colored in roughly the same way as part (a). The largest “continent” on Venus, Aphrodite Terra, is the yellow dragon-shaped area across the center of this image. See also the full-page, chapter-opening photo. (NASA)

10. The Surface of Venus Surface maps of Venus, with Earth comparison:
Figure 9-8. Venus Maps (a) Radar map of the surface of Venus, based on Pioneer Venus data. Color represents elevation, with white the highest areas and blue the lowest. (b) A similar map of Earth, at the same spatial resolution. (c) Another version of (a), with major surface features labeled. Compare with Figure 9.7, and notice how the projection exaggerates the size of surface features near the poles. (NASA)

11. The Surface of Venus
Ishtar Terra is one of two continent-sized features on the surface of Venus
Figure 9-9. Ishtar Terra (a) A Venera orbiter image of a plateau known as Lakshmi Planum in Ishtar Terra. The Maxwell Montes mountain range (red) lies on the western margin of the plain, near the right-hand edge of the image. A meteor crater named Cleopatra is visible on the western slope of the Maxwell range. Note the two larger craters in the center of the plain itself. (b) A Magellan image of Cleopatra showing a double-ringed structure that identifies the feature to geologists as an impact crater. (NASA)

12. The Surface of Venus The other is Aphrodite Terra
Figure Aphrodite Terra (a) A Magellan image of Ovda Regio, part of Aphrodite Terra. The intersecting ridges indicate repeated compression and buckling of the surface. The dark areas represent regions that have been flooded by lava upwelling from cracks like those shown in Figure (b) This lava channel in Venus’s south polar region, known as Lada Terra, extends for nearly 200 km. (NASA)

13. The Surface of Venus Lava has flowed from cracks on the surface
Figure Lava Flows These cracks in Venus’s surface, detected by Magellan in another part of Aphrodite Terra, have allowed lava to reach the surface and flood the surrounding terrain. The dark regions are smooth lava flows. The network of fissures visible here is about 50 km long. (NASA)

14. The Surface of Venus Volcanoes on Venus; most are shield volcanoes:
Figure Volcanism on Venus (a) Two larger volcanoes, known as Sif Mons (left) and Gula Mons, appear in this Magellan image. Color indicates height above a nominal planetary radius of 6052 km and ranges from purple (1 km, the level of the surrounding plain) to orange (corresponding to an altitude of about 4 km). The two volcanic calderas at the summits are about 100 km across. (b) A computer-generated view of Sif Mons, as seen from ground level. (c) Gula Mons, as seen from ground level. In (b) and (c), the colors are based on data returned from Soviet landers, and the vertical scales have been greatly exaggerated (by about a factor of 40), so these mountains look much taller relative to their widths than they actually are; Venus is actually a remarkably flat place. (NASA)

15. The Surface of Venus
Other volcanic features include lava domes and coronas
Figure Lava Dome (a) These dome-shaped structures resulted when viscous molten rock bulged out of the ground and then retreated, leaving behind a thin, solid crust that subsequently cracked and subsided. Magellan found features like these in several locations on Venus. (b) A three-dimensional representation of four of the domes. This computer-generated view is looking toward the right from near the center of the image in part (a). Colors in (b) are based on data returned by Soviet Venera landers. (NASA)
Figure Venus Corona This corona, called Aine, lies in the plains south of Aphrodite Terra and is about 300 km across. Coronae probably result from upwelling mantle material, causing the surface to bulge outward. Note the pancake-shaped lava domes at top, the many fractures in the crust around the corona, and the large impact craters with their surrounding white (rough) ejecta blankets that stud the region. (NASA)

16. The Surface of Venus
Impact craters on Venus, the largest named after Margaret Mead:
Figure Impact Craters on Venus (a) A Magellan image of an apparent multiple-impact crater in Venus’s southern hemisphere. The irregular shape of the light-colored ejecta seems to be the result of a meteoroid that fragmented just prior to impact. The dark regions in the crater may be pools of solidified lava associated with individual fragments. (b) Venus’s largest crater, named Mead after anthropologist Margaret Mead, is about 280 km across. Bright (rough) regions clearly show its double-ringed structure. (NASA)

17. The Surface of Venus
Photographs of the surface, from the Venera landers:
Figure Venus In Situ (a) The first direct view of the surface of Venus, radioed back to Earth from the Soviet Venera 9 spacecraft, which made a soft landing on the planet in The amount of sunlight penetrating Venus’s cloud cover is about the same as that reaching Earth’s surface on a heavily overcast day. (b) Another view of Venus, in true color, from Venera 14. Flat rocks like those visible in part (a) are seen among many smaller rocks and even fine soil on the surface. This landing site is not far from the Venera 9 site shown in (a). The peculiar filtering effects of whatever light does penetrate the clouds make Venus’s air and ground appear peach colored—in reality, they are most likely gray, like rocks on Earth. (Russian Space Agency)

18. The Atmosphere of Venus
Venus’s atmosphere is very dense
Solid cloud bank 50–70 km above surface
Atmosphere is mostly carbon dioxide; clouds are sulfuric acid
Figure Venus’s Atmosphere The structure of the atmosphere of Venus, as determined by U.S. and Soviet probes. (One bar is the atmospheric pressure at sea level on Earth.)

19. The Atmosphere of Venus
Upper atmosphere of Venus has high winds, but atmosphere near surface is almost calm
Figure Atmospheric Circulation Three ultraviolet views of Venus, taken by the Pioneer Venus orbiter, showing the changing cloud patterns in the planet’s upper atmosphere. The wind flow is from right to left (or clockwise from above), in the direction opposite the sideways “V” in the clouds. Notice the motion of the dark region marked by the blue arrow. Venus’s retrograde rotation means that north is at the bottom of these images and west to the right. The time difference between the left and right photographs is about 20 hours. (NASA)

20. The Atmosphere of Venus
There are also permanent vortices at the poles; the origin of the double-lobed structure is a mystery
Figure Venus Polar Vortex The left side of this composite image shows Venus during the day when sunlight reflects from its cloud tops. By contrast, the false-color image at right is a nighttime view of radiation arising from deeper layers within, emphasizing the dynamic swirls and vortices of its lower-atmospheric cloud structures. (ESA)

21. The Atmosphere of Venus
Venus is the victim of a runaway greenhouse effect—just kept getting hotter and hotter as infrared radiation is reabsorbed
Figure Greenhouse Effect on Earth and Venus Because Venus’s atmosphere is much deeper and denser than Earth’s, a much smaller fraction of the infrared radiation leaving the planet’s surface escapes into space. The result is a much stronger greenhouse effect than on Earth and a correspondingly hotter planet. The outgoing infrared radiation is not absorbed at a single point in the atmosphere; instead, absorption occurs at all atmospheric levels. (The arrows indicate only that absorption occurs, not that it occurs at one specific level; the arrow thickness is proportional to the amount of radiation moving in and out.)

22. Venus’s Magnetic Field and Internal Structure
No magnetic field, probably because rotation is so slow
No evidence for plate tectonics
Venus resembles a young Earth (1 billion years)—no asthenosphere, thin crust