Astronomy Today. Eighth edition. Eric Chaisson Steve McMillan

Transcription

1 Global edition Astronomy Today Eighth edition Eric Chaisson Steve McMillan

2 The Distance Scale ~1 Gpc Velocity Distance Hubble s law Supernovae ~200 Mpc Time Tully-Fisher ~25 Mpc ~10,000 pc Time Variable stars ~200 pc OBAFGKM Spectroscopic parallax Distance ~1 A.U. Stellar parallax Radar ranging Earth

3 SECTION 9.4 The Surface of Venus 247 These hopes for an Earth-like Venus were dashed in 1956, when radio observations of the planet were used to measure its thermal energy emission. Unlike visible light, radio waves easily penetrate the cloud layer and they gave the first indication of conditions on or near the surface: The radiation emitted by the planet has a blackbody spectrum characteristic of a temperature near 730 K! (Sec. 3.4) Almost overnight, the popular conception of Venus changed from that of a lush tropical jungle to an arid, uninhabitable desert. Radar observations of the surface of Venus are routinely carried out from Earth with the Arecibo radio telescope. (Sec. 5.5) With careful signal processing, this instrument can achieve a resolution of a few kilometers, but it can adequately cover only a fraction (roughly 25 percent) of the planet. The telescope s view of Venus is limited by the peculiar spin-orbit coincidence described in the previous section (which means that only one side of the planet can be studied) and also because radar reflections from regions near the edge of the planet are hard to obtain. However, the Arecibo data can usefully be combined with information received from probes orbiting Venus to build up a detailed picture of the planet s surface. Only with the arrival of the Magellan probe were more accurate data obtained. Process of SciencE Check Why did early studies of Venus lead astronomers to such an inaccurate picture of the planet s surface conditions? Figure 9.6 Venus, Up Close This image of Venus was made when the Pioneer spacecraft captured solar ultraviolet radiation reflected from the planet s upper clouds, which are probably composed mostly of sulfuric acid droplets, much like the corrosive acid in a car battery. 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 lower clouds. (NASA; ESA) cover Venus might be a habitable planet similar to our own. Indeed, in the 1930s, scientists had measured the temperature of the atmosphere spectroscopically at about 240 K, not much different from the temperature of our own upper atmosphere. (Secs. 4.5, 7.2) Calculations of the planet s surface temperature taking into account the cloud cover and Venus s proximity to the Sun, and assuming an atmosphere much like our own suggested that Venus should have a surface temperature only 10 or 20 degrees higher than Earth s. 9.4 The Surface of Venus Although the planet s clouds are thick and the terrain below them totally shrouded, we are by no means ignorant of Venus s surface. Detailed radar observations have been made both from Earth and from the Venera, Pioneer Venus, and Magellan spacecraft. (Sec. 6.6) Analysis of the radar echoes yields a map of the planet s surface. Except for the last two figures, all the views of Venus in this section are radargraphs (as opposed to photographs) created in this way. As Figure 9.7 illustrates, the early maps of Venus suffered from poor resolution; however, more recent probes especially Magellan have provided much sharper views. As in all the Magellan images, the light areas in Figure 9.7 represent regions where the surface is rough and efficiently scatters Magellan s sideways-looking radar beam back to the detector. Smooth areas tend to reflect the beam off into space instead and so appear dark. The strength of the returned signal as Magellan passed by thus results in a map of the planet s surface.

4 248 CHAPTER 9 Venus Figure 9.7 Venus Mosaics 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. A planetwide mosaic of Magellan images, colored in roughly the same way as part. 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 on page 242. (NASA) arge-scale Topography Figure 9.8 shows basically the same Pioneer Venus data of Venus as Figure 9.7, except that this figure has been flattened out into a more conventional map. The altitude of the surface relative to the average radius of the planet is indicated by the use of color, with white representing the highest elevations and blue the lowest. (Note that the blue has nothing to do with oceans, nor does white indicate snow-capped mountains!) Figure 9.8 shows a map of Earth to the same scale and at the same spatial resolution. Some of Venus s main features are labeled in Figure 9.8(c). The surface of Venus appears to be relatively smooth, resembling rolling plains with modest highlands and lowlands. Two continent-sized features, called Ishtar Terra and Aphrodite Terra (named after the Babylonian and Greek counterparts, respectively, of Venus, the Roman goddess of love), adorn the landscape and contain mountains comparable in height to those on Earth. The elevated continents occupy only 8 percent of Venus s total surface area. For comparison, continents on Earth make up about 25 percent of the surface. The remainder of Venus s surface is classified as lowlands (27 percent) or rolling plains (65 percent), although there is probably little geological difference between the two terrains. Note that, although Earth s tectonic plate boundaries are evident in Figure 9.8, no similar features can be seen in Figure 9.8. (Sec. 7.4) There simply appears to be no large-scale plate tectonics on Venus. Ishtar Terra ( and of Ishtar ) lies in the southern high latitudes (at the tops of Figures 9.7a and 9.8a recall our earlier discussion of Venus s retrograde rotation). The projection used in Figure 9.8 makes Ishtar Terra appear larger than it really is it is actually about the same size as Australia. This landmass is dominated by a great plateau known as akshmi Planum (Figure 9.9), some 1500 km across at its widest point and ringed by mountain ranges, including the Maxwell Montes range, which contains the highest peak on the planet, rising some 14 km above the level of Venus s deepest surface depressions. Again for comparison, the highest point on Earth (the summit of Mount Everest) lies about 20 km above the deepest section of Earth s ocean floor (Challenger Deep, at the bottom of the Marianas Trench on the eastern edge of the Philippines plate). Figure 9.9 shows a large-scale Venera image of akshmi Planum, at a resolution of about 2 km. The wrinkles are actually chains of mountains, hundreds of kilometers long and tens of kilometers apart. The red area immediately to the right of the plain is Maxwell Montes. On the western (right-hand) slope of the Maxwell range lies a great crater, called Cleopatra, about 100 km across. Figure 9.9 shows a Magellan image of Cleopatra, which was originally thought to be volcanic in origin. Close-up views of the crater s structure, however, have led planetary

5 SECTION 9.4 The Surface of Venus VENUS EARTH A TI T U DE Ishtar Terra Aphrodite Terra ONGITUDE E E V A TI O NS K M A 30 TI T 0 U DE ONGITUDE 240º 270º 300º 330º 0º 30º 60º 90º 120º 150º 180º 210º 240º 80º 80º 60º METIS U AKNA MONTES T V E S T A R U FREJIA MONTES ISHTAR AKSHMI Colette Secojaweo PANUM R U P P E E S S M A X W E MONTES TERRA TETHUS ATANTA PANITIA 60º G U I N E V E R E S E D N A E D A P A N I T I A 30º RHEA MONS THEIA ASTERIA MONS B E T A R E G I O Devana Chasma P A N I T I A P A N I T I A TEUS N I O B E P A N I T I A 30º Sappho 0º APHRODITE TERRA 0º -30º PHOEBE THEMIS AV I N A P A N I T I A APHA Eve A I N O P A N I T I A Arte m is C h a s m a Diana Chaima Dali Chaima -30º -60º H E E N P A N I T I A ise Meitrer -60º -70º -70º 240º 270º 300º 330º 0º 30º 60º 90º 120º 150º 180º 210º 240º (c) R I V U X G Figure 9.8 Venus Maps Radar map of the surface of Venus, based on Pioneer Venus data. Color represents elevation, with white the highest areas and blue the lowest. A similar map of Earth, at the same spatial resolution. (c) Another version of, 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) scientists to conclude that the crater is meteoritic in origin, although some volcanic activity was apparently associated with its formation when the colliding body temporarily breached the planet s crust. Notice the dark (smooth) lava flow emerging from within the inner ring and cutting across the outer rim at the upper right. It is now conventional to name features on Venus after famous women Aphrodite, Ishtar, Cleopatra, and so on. However, the early nonfemale names (e.g., Maxwell Montes, named after the Scottish physicist James Clerk Maxwell) predating this convention have stuck, and they are unlikely to change. Venus s other continent-sized formation, Aphrodite Terra, is located on the planet s equator and is comparable in size to Africa. Before Magellan s arrival, some researchers had speculated that Aphrodite Terra might have been the site of something akin to seafloor spreading at the Mid-Atlantic ridge on Earth a region where two lithospheric plates moved

6 250 CHAPTER 9 Venus Cleopatra akshmi Planum Maxwell Montes 1000 km 50 km Figure 9.9 Ishtar Terra A Venera orbiter image of a plateau known as akshmi Planum in Ishtar Terra. The Maxwell Montes mountain range (orange) 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. A Magellan image of Cleopatra showing a double-ringed structure that identifies the feature to geologists as an impact crater. (NASA) apart and molten rock rose to the surface in the gap between them, forming an extended ridge. (Sec. 7.4) With the low-resolution data then available, the issue could not be settled at the time. The Magellan images now seem to rule out even this small-scale tectonic activity, and the Aphrodite region gives no indication of spreading. Figure 9.10 shows a portion of Aphrodite Terra called Ovda Regio. The crust appears buckled and fractured, with ridges running in two distinct directions across the image, suggesting that large compressive forces are distorting the crust. There seem to have been repeated periods of extensive lava flows. The dark regions are probably solidified lava flows. Some narrow lava channels, akin to rilles on the Moon, also appear. (Sec. 8.5) Such lava channels appear to be quite common on Venus. Unlike lunar rilles, however, they can be extremely long hundreds or even thousands of kilometers. These lava rivers often have lava deltas at their mouths, where they deposited their contents into the surrounding plains. Figure 9.11 shows a series of angular cracks in the crust, thought to have formed when lava welled up from a deep fissure, flooded the surrounding area, and then retreated below the planet s surface. As the molten lava withdrew, the thin, new crust of solidified material collapsed under its own weight, forming the cracks we now see. Even taking into account the differences in temperature and composition between Venus s crust and Earth s, this terrain is not at all what we would expect at a spreading site similar to the Mid-Atlantic Ridge. (Sec. 7.4) Although there is no evidence for plate tectonics on Venus, it is likely that the stresses in the crust that led to the large mountain ranges were caused by convective motion within Venus s mantle the same basic process that drives Earth s plates. akshmi Planum, for example, is probably the result of a plume of upwelling mantle material that raised and buckled the planet s surface. 50 km Figure 9.10 Aphrodite Terra 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 (NASA)

7 SECTION 9.4 The Surface of Venus 251 Figure 9.11 ava 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) 20 km Volcanism and Cratering On Earth, the principal agent of long-term, planetwide surface change is plate tectonics, driven by convection in our planet s mantle. (Secs. 7.3, 7.4) Volcanic and seismic activity are predominantly (although not exclusively) associated with plate boundaries. On Venus, without global plate tectonics, large-scale recycling of the crust by plate motion is not a factor in changing the planet s surface. Nevertheless, many areas of Venus have extensive volcanic features. Most volcanoes on the planet are of the type known as shield volcanoes. Two large shield volcanoes, called Sif Mons and Gula Mons, are shown in Figure Shield volcanoes, such as the Hawaiian Islands on Earth, are not associated with plate boundaries. Instead, they form when lava wells up through a hot spot in the crust and are built up over long periods of time by successive eruptions and lava flows. A characteristic of shield volcanoes is the formation of a caldera, or crater, at the summit when the underlying lava withdraws and the surface collapses. The distribution of volcanoes over the surface of Venus appears random quite different from the distribution on Earth, where volcanic activity clearly traces out plate boundaries (see Figure 7.11) consistent with the view that plate tectonics is absent on Venus. More volcanic features are visible in Figure 9.13, which shows a series of seven pancake-shaped lava domes, each about 25 km across. They probably formed when lava oozed out of the surface, formed the dome, and then withdrew, Radius (km) 300 km 1 km 50 km Figure 9.12 Volcanism on Venus 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 as indicated by the scale at left. In Section 9.5, we will see how past volcanism has played a crucial role in determining current conditions in Venus s atmosphere and surface. This computer-generated view of Gula Mons, as seen from ground level (with colors based on data returned from Soviet landers) has a greatly exaggerated vertical scale (by about a factor of 40), so the mountain looks much taller relative to its width than it really is; Venus is actually a remarkably flat place. (NASA)