A Positron Named Priscilla Scientific Discovery at the Frontier (1994) / Chapter Skim
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5 Magellan's Venus: A World Revealed
5 Magellan's Venus: A World Revealed
Pages 124-153
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From page 124... ...
They had orbited Mars and photographed volcanoes nearly three times as high as Mt. Everest, a canyon that would stretch the length of the United States, and dry river valleys that spoke of an ancient watery epoch on this desert world.
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From page 125... ...
The planet's surface features had been glimpsed in the preceding two decades, first using Earth-based radar and then from orbiting Soviet and American probes. But Magellan's pictures showed features as small as a few hundred meters across, far more detail than any previous views of Venus in fact, better resolution than that available for vast areas of Earth, namely the ocean basins.
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From page 126... ...
Roughly 2 kilometers wide and some 6800 kilometers long, more than the distance from Havana, Cuba, to Anchorage, Alaska, the River Styx is a puzzle: What carved it? With no water available, geologists speculated on a variety of other fluids, including liquid sulfur; today molten lava of an unusual composition is considered most likely.
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From page 127... ...
Images made from the radar data showed just enough to tantalize: circular features that seemed to be impact craters, bright spots thought to be volcanoes, and banded terrain that might be the result of crustal folding. Clearly, Venus' surface had been shaped by a variety of geologic processes, but no one would be able to say more without getting a better look.
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From page 128... ...
Smaller planets, whose ratio of surface area to volume is higher, deplete their heat supplies sooner than large ones. A survey of the rocky worlds that populate the inner solar system, the so-called terrestrial planets,2 illustrates a direct relationship between size and degree of geologic activity.
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From page 129... ...
It is possible that Mars's geologic activity has continued to the present but at very low levels. The simple pattern that emerges is that the larger terrestrial planets held on to their heat longer and thus had longer geologic lifetimes.
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From page 130... ...
than the height of Mount Everest above sea level. Ishtar bears a tantalizing resemblance to the Tibetan plateau, where due to plate tectonics India is colliding with Asia, pushing up the Himalayas in the process.
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From page 131... ...
This has important implications for Venus' surface composition. On Earth, composition is closely linked to elevation, since continental crust is rich in granite and oceanic crust is basaltic; since granite is less dense than basalt, the continents tend to be more buoyant than the ocean basins.
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From page 132... ...
Schaber, along with the University of Arizona's Robert Strom and several colleagues proposed that Venus suffered an episode of volcanic activity so pervasive and so intense that nearly the entire planet was covered in fresh lava. In recent years, geologists have theorized that massive volcanic outbursts resurfaced parts of Earth, including a basalt "mega-eruption" 250 million years ago that covered much of what is now Siberia.
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From page 133... ...
Since the resurfacing event, Schaber proposed volcanic and tectonic activity could continue at reduced levels, especially along the fracture belts that connect the broad low rises in the equatorial highlands. The main attraction of Schaber's so-called global resurfacing model was that it neatly explained the observed crater distribution.
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From page 134... ...
But Magellan has required geophysicists like Phillips to cross over into geology, to try to make sense of Magellan's strange findings. By the fall of 1991 Phillips and some of his colleagues had spent time studying the Magellan images, in particular looking for craters that had been cracked by faults or partially invaded by lava flows.
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From page 135... ...
"It's important to realize," Grimm says, "that what we see on the surface isn't the whole story." As a geophysicist, Grimm finds the global resurfacing model difficult to accept. "Here's Venus going like gangbusters, and suddenly it comes grinding, screeching, choking to a halt," he says, explaining that the current level of geologic activity is only a tiny fraction of what would have been required during the resurfacing event.
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From page 136... ...
In effect, it decouples them somewhat from the convective action of the mantle, thus moderating the intensity of surface tectonic activity. On Venus, geophysicists have had only one probe of the interior: a map of the planet's gravity field compiled during the Pioneer Venus mission.
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From page 137... ...
The surprising result from PVO's gravity data is that the apparent depths of compensation for many features on Venus range from 100 to 300 kilometers, much greater than on Earth. This suggests that these areas are dynamically compensated simply because the high temperatures found at great depths would cause a crustal root to weaken and flow away over geologic time.
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From page 138... ...
For that reason, Turcotte believes that in most places the lithosphere of Venus is several hundred kilometers thick; however, he says, it's only temporary. While Turcotte believes Venus today is far less geologically "alive" than Earth, he does not believe the planet is dead or even dying; rather, it is in a state of quiescence or, as Sue Smrekar terms it, "hibernation." Turcotte believes a hibernating Venus is the explanation for the global resurfacing model.
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From page 139... ...
Plumes of hot mantle material rise to meet the lithosphere, thinning and buoying it, causing topographic highs. In these places some geologic activity may resume.
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From page 140... ...
VENUS IS ALIVE One of the recurring themes of Magellan's Venus is that the same data may draw widely differing interpretations from different scientists. In the case of the ADC values indicated by the gravity data, Bob Grimm and Sue Smrekar take the opposite view from Donald Turcotte.
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From page 141... ...
Hot-spot volcanism, for example, is thought to have formed the Hawaiian islands; over time the motion of the Pacific plate over the hot mantle plume resulted in a series of volcanoes arrayed along a line. At least five of the Venusian uplands studied by Smrekar and Phillips occupy broad, dome-like uplifts and seem to be places where volcanism and crustal rifting have occurred.
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From page 142... ...
This makes sense if we consider how the Earth's asthenosphere is formed: When oceanic crust subducts into the mantle, it carries seawater down with it, which drastically reduces the melting temperature of the surrounding mantle rocks. As a result, the rocks of the uppermost mantle the asthenosphere-exist almost at their melting temperature and probably contain pockets of molten rock.
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From page 143... ...
This suggests that volcanism has not played a major role in erasing Venus' craters in recent geologic history. If Roger Phillips is right if something is eating craters on Venus-scientists must look for another explanation.
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From page 144... ...
Malin asks, "How do you resurface a planet? " With volcanism apparently not a viable mechanism, he suspects tectonic activity.
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From page 145... ...
The remaining 60 percent of the craters are found on the 40 percent of the planet that hasn't been tectonically altered. A close look at the tortured regions known as tesserae shows that they have fewer craters than the surrounding plains; the enormous fracture belts that girdle the planet's equator have even fewer craters.
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From page 146... ...
But Smrekar believes that even on a very dry Venus "slopes of 30 degrees are still not going to hang around for 500 million years." She estimates the age of Maxwell Montes at 10 million years or less. But as if to emphasize the ambiguity with which Venus confronts planetary scientists, the crater Cleopatra, 100 kilometers across, sits on the high slopes of Maxwell Montes (see Figure 5.5~.
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From page 147... ...
When a hot plume of mantle rock rises toward the surface, Smrekar says, it encounters steadily decreasing pressure; this allows it to succumb to the high temperatures. It does not melt completely, however.
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From page 148... ...
Because of this layer, a hot plume of mantle material would not be able to reach the lithosphere, thus reducing the amount of volcanic activity. However, the presence of the residuum layer beneath a relatively thin lithosphere would not inhibit tectonic activity.
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From page 149... ...
For now, however, Phillips believes this assortment of terrains, with sharply different crater ages, shows that Venus' history cannot be explained in terms of one major resurfacing event. Phillips's finding seems to challenge the premise that is the foundation of the global resurfacing model, that Venus' craters are simply a result of half a billion years in the cosmic shooting gallery.
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From page 150... ...
There may be tectonic activity or volcanism at the margins of these blocks, but the blocks themselves are too tightly packed to undergo significant lateral motion. Nevertheless, Grimm says, such a configuration would not limit the extent of Venus' geologic activity.
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From page 151... ...
For example, the lithosphere under the eastern United States reaches a thickness of 175 kilometers, while in the western portion of the continent the lithosphere is less than 30 kilometers thick and consists only of crustal rocks; consequently, the western United States is a site of volcanic and tectonic activity. On Venus, Turcotte says, the thickest lithosphere could be 300 or even 400 kilometers thick; the thinnest, at the center of some active highland regions, could be only 12 kilometers thick.
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From page 152... ...
And scientists are eager to see NASA, the National Aeronautics and Space Administration, approve a special seismic lander to touch down on the Venusian surface to probe the planet's interior. For now, they are hopeful that Magellan will provide a critical key to the puzzle in the form of new, high-resolution data on the gravity field.
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From page 153... ...
1984. The Geology of the Terrestrial Planets.
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