Chapter 9 Venus O"en called Earth s sister planet because of their comparable sizes, Venus is actually nothing like our own world. Surface condi?ons on Venus have changed radically over?me due to geological ac?vity and environmental change, and today the planet s surface temperature is hot enough (730 K) to melt lead, while the atmosphere rains sulfuric acid. This global view of the surface of Venus was created when radar data from the Magellan spacecra" were mapped onto a computer- generated globe. The color here is probably close to reality. (JPL)
Units of Chapter 9 9.1 Orbital Properties 9.2 Physical Properties 9.3 Long-Distance Observations of Venus 9.4 The Surface of Venus 9.5 The Atmosphere of Venus 9.6 Venus s Magnetic Field and Internal Structure
9.1 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 a;er sunset. Venus clearly outshines even the brightest stars in the sky. ( J. Schad/Photo Researchers, Inc.)
9.1 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 conjuncgon). 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 conjuncgon), 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)
9.2 Physical Properties Radius: 6000 km Mass: 4.9 x 10 24 kg Density: 5200 kg/m 3 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 rotagonal properges. Although all orbit the Sun in the same direcgon and in nearly the same plane, Mercury s rotagon is slow and prograde (in the same sense as its orbital mogon), 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 eclipgc, but Mercury, Earth, and Mars all spin counterclockwise. This is a perspecgve view, roughly halfway between a flat edge- on view and a direct overhead view.
9.2 Physical Properties Slow, retrograde rotation of Venus results in large difference between solar day (117 Earth days) and sidereal day (243 Earth days); note that the solar day is a large fraction of the year, and the sidereal day is even longer than the year. Figure 9-4. Venus s Solar Day Venus s orbit and retrograde rotagon 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 locagon, or an observer standing, on the planet s surface. The numbers in the figure mark Gme in Earth days.
9.3 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)
9.3 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) This image of Venus was made when the Pioneer spacecra; captured solar ultraviolet radiagon reflected from the planet s clouds, which are probably composed mostly of sulfuric acid droplets, much like the highly corrosive acid in a car ba[ery. (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)
9.4 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
9.4 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 transmi[er and receiver on board the Pioneer spacecra;, which is sgll in orbit about the planet, but is now inoperagve. The two congnent- sized landmasses are named Ishtar Terra (upper le;) and Aphrodite (lower right). Colors represent algtude: blue is lowest, red highest. The spagal resolugon is about 25 km. (b) A planetwide mosaic of Magellan images, colored in roughly the same way as part (a). The largest congnent 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)
9.4 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 elevagon, with white the highest areas and blue the lowest. (b) A similar map of Earth, at the same spagal resolugon. (c) Another version of (a), with major surface features labeled. Compare with Figure 9.7, and nogce how the projecgon exaggerates the size of surface features near the poles. (NASA)
9.4 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 idengfies the feature to geologists as an impact crater. (NASA)
9.4 The Surface of Venus The other is Aphrodite Terra Figure 9-10. Aphrodite Terra A Magellan image of Ovda Regio, part of Aphrodite Terra. The intersecgng 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 9.11. (NASA)
9.4 The Surface of Venus Lava has flowed from cracks on the surface Figure 9-11. 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)
9.4 The Surface of Venus Volcanoes on Venus; most are shield volcanoes Figure 9-12. Volcanism on Venus (a) Two larger volcanoes, known as Sif Mons (le;) 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 algtude of about 4 km). The two volcanic calderas at the summits are about 100 km across. (b) A computer- generated view of Gula Mons, as seen from ground level. The colors here are based on data returned from Soviet landers, and the vergcal scales have been greatly exaggerated (by about a factor of 40), so the mountain looks much taller relagve to its width than it really is. Venus is actually a remarkably flat place. (NASA)
9.4 The Surface of Venus Other volcanic features include lava domes and coronas Figure 9-13. 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 locagons on Venus. (b) A three- dimensional representagon 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 9-14. 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)
9.4 The Surface of Venus Impact craters on Venus, the largest named after Margaret Mead Figure 9-15. Impact Craters on Venus (a) A Magellan image of an apparent mulgple- 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 a;er anthropologist Margaret Mead, is about 280 km across. Bright (rough) regions clearly show its double- ringed structure. (NASA)
9.4 The Surface of Venus Photographs of the surface, from the Venera landers Figure 9-16. Venus In Situ (a) The first direct view of the surface of Venus, radioed back to Earth from the Soviet Venera 9 spacecra;, which made a so; landing on the planet in 1975. The amount of sunlight penetragng 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 the planet s air and ground appear peach colored in reality, they are most likely gray, like rocks on Earth. (Russian Space Agency)
9.5 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 9-17. 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.)
9.5 The Atmosphere of Venus Upper atmosphere of Venus has high winds, but atmosphere near surface is almost calm Figure 9-18. Atmospheric CirculaCon Three ultraviolet views of Venus, taken by the Pioneer Venus orbiter, showing the changing cloud pa[erns in the planet s upper atmosphere. The wind flow is from right to le; (or clockwise from above), in the direcgon opposite the sideways V in the clouds. NoGce the mogon of the dark region marked by the blue arrow. Venus s retrograde rotagon means that north is at the bo[om of these images and west to the right. The Gme difference between the le; and right photographs is about 20 hours. (NASA)
9.5 The Atmosphere of Venus There are also permanent vortices at the poles; the origin of the double-lobed structure is a mystery Figure 9-19. Venus Polar Vortex The le; side of this composite image shows Venus during the day when sunlight reflects from its cloud tops. By contrast, the false- color insets at right are nighfme views of radiagon arising from deeper layers within, emphasizing the dynamic swirls and vorgces of its lower- atmospheric cloud structures. These images of the southern vortex were taken a few hours apart by Europe s Explorer spacecra;. (ESA)
9.5 The Atmosphere of Venus Venus is the victim of a runaway greenhouse effect just kept getting hotter and hotter as infrared radiation was reabsorbed Figure 9-20. Greenhouse Effect on Earth and Venus Because Venus s atmosphere is much deeper and denser than Earth s, a much smaller fracgon of the infrared radiagon leaving the planet s surface escapes into space. The result is a much stronger greenhouse effect than on Earth and a correspondingly ho[er planet. The outgoing infrared radiagon is not absorbed at a single point in the atmosphere; instead, absorpgon occurs at all atmospheric levels. (The arrows indicate only that absorpgon occurs, not that it occurs at one specific level; the arrow thickness is proporgonal to the amount of radiagon moving in and out.)
9.6 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
Summary of Chapter 9 Venus is never too far from Sun and is the brightest object in the sky (after the Sun and Moon) Atmosphere very dense, mostly carbon dioxide Surface hidden by cloud cover Surface temperature 730 K Rotation slow and retrograde
Summary of Chapter 9 (cont.) Many lava domes and shield volcanoes Venus is comparable to Earth in mass and radius Large amount of carbon dioxide in atmosphere, and closeness to Sun, led to runaway greenhouse effect and very hot surface