Compared to regulations in other countries, those of the United States tends to be narrower in scope, with an emphasis on manufacturing processes and specific categories of pollution, and little or no attention to the many other factors that affect environmental quality. An example is the focus on controlling pollution rather than influencing decisions about processes, raw materials, or products that determine environmental impacts. Regulation in the United States tends to isolate specific aspects of production processes and attempts to control them stringently, which means that some aspects of business are regulated tightly, although sometimes not cost-effectively, while others are ignored. Other countries and several American states have recently made more progress in preventing pollution at its source and considering such issues as product life cycles, packaging waste, and industrial energy efficiency.
Environmental regulation in the United States is also more prescriptive than elsewhere, in the sense of requiring specific actions, with little discretion left to the regulated firm. There also is a great reliance on action-forcing laws and technology standards.
These contrasts are illustrated nicely in a 1974 book that used a hare and tortoise analogy to compare air quality regulation in the United States and Sweden. While the United States (the hare) codified ambitious goals in statutes that drove industry to adopt new technologies under the threat of sanctions, Sweden (the tortoise) used a more collaborative process that stressed results but worked with industry in deciding how to achieve them. In the end air quality results were about the same. Similar results have been found in other comparative analyses of environmental regulation. For example, one study of a multinational firm with operations in the United States and Japan found that pollution levels in both countries were similar, despite generally higher pollution abatement expenditures in the United States. The higher costs observed in the United States thus were due in large part, not to more stringent standards, but to the higher regulatory transaction costs. Because agencies in different countries share information about technologies, best practices, and other issues, the pollution levels found acceptable in different countries tend to be quite similar.
Compared to regulations in other countries, those of the United States tends to be narrower in scope, with an emphasis on manufacturing processes and specific categories of pollution, and little or no attention to the many other factors that affect environmental quality. An example is the focus on controlling pollution rather than influencing decisions about processes, raw materials, or products that determine environmental impacts. Regulation in the United States tends to isolate specific aspects of production processes and attempts to control them stringently, which means that some aspects of business are regulated tightly, although sometimes not cost-effectively, while others are ignored. Other countries and several American states have recently made more progress in preventing pollution at its source and considering such issues as product life cycles, packaging waste, and industrial energy efficiency.
Environmental regulation in the United States is also more prescriptive than elsewhere, in the sense of requiring specific actions, with little discretion left to the regulated firm. There also is a great reliance on action-forcing laws and technology standards.
These contrasts are illustrated nicely in a 1974 book that used a hare and tortoise analogy to compare air quality regulation in the United States and Sweden. While the United States (the hare) codified ambitious goals in statutes that drove industry to adopt new technologies under the threat of sanctions, Sweden (the tortoise) used a more collaborative process that stressed results but worked with industry in deciding how to achieve them. In the end air quality results were about the same. Similar results have been found in other comparative analyses of environmental regulation. For example, one study of a multinational firm with operations in the United States and Japan found that pollution levels in both countries were similar, despite generally higher pollution abatement expenditures in the United States. The higher costs observed in the United States thus were due in large part, not to more stringent standards, but to the higher regulatory transaction costs. Because agencies in different countries share information about technologies, best practices, and other issues, the pollution levels found acceptable in different countries tend to be quite similar.
Compared to regulations in other countries, those of the United States tends to be narrower in scope, with an emphasis on manufacturing processes and specific categories of pollution, and little or no attention to the many other factors that affect environmental quality. An example is the focus on controlling pollution rather than influencing decisions about processes, raw materials, or products that determine environmental impacts. Regulation in the United States tends to isolate specific aspects of production processes and attempts to control them stringently, which means that some aspects of business are regulated tightly, although sometimes not cost-effectively, while others are ignored. Other countries and several American states have recently made more progress in preventing pollution at its source and considering such issues as product life cycles, packaging waste, and industrial energy efficiency.
Environmental regulation in the United States is also more prescriptive than elsewhere, in the sense of requiring specific actions, with little discretion left to the regulated firm. There also is a great reliance on action-forcing laws and technology standards.
These contrasts are illustrated nicely in a 1974 book that used a hare and tortoise analogy to compare air quality regulation in the United States and Sweden. While the United States (the hare) codified ambitious goals in statutes that drove industry to adopt new technologies under the threat of sanctions, Sweden (the tortoise) used a more collaborative process that stressed results but worked with industry in deciding how to achieve them. In the end air quality results were about the same. Similar results have been found in other comparative analyses of environmental regulation. For example, one study of a multinational firm with operations in the United States and Japan found that pollution levels in both countries were similar, despite generally higher pollution abatement expenditures in the United States. The higher costs observed in the United States thus were due in large part, not to more stringent standards, but to the higher regulatory transaction costs. Because agencies in different countries share information about technologies, best practices, and other issues, the pollution levels found acceptable in different countries tend to be quite similar.
Compared to regulations in other countries, those of the United States tends to be narrower in scope, with an emphasis on manufacturing processes and specific categories of pollution, and little or no attention to the many other factors that affect environmental quality. An example is the focus on controlling pollution rather than influencing decisions about processes, raw materials, or products that determine environmental impacts. Regulation in the United States tends to isolate specific aspects of production processes and attempts to control them stringently, which means that some aspects of business are regulated tightly, although sometimes not cost-effectively, while others are ignored. Other countries and several American states have recently made more progress in preventing pollution at its source and considering such issues as product life cycles, packaging waste, and industrial energy efficiency.
Environmental regulation in the United States is also more prescriptive than elsewhere, in the sense of requiring specific actions, with little discretion left to the regulated firm. There also is a great reliance on action-forcing laws and technology standards.
These contrasts are illustrated nicely in a 1974 book that used a hare and tortoise analogy to compare air quality regulation in the United States and Sweden. While the United States (the hare) codified ambitious goals in statutes that drove industry to adopt new technologies under the threat of sanctions, Sweden (the tortoise) used a more collaborative process that stressed results but worked with industry in deciding how to achieve them. In the end air quality results were about the same. Similar results have been found in other comparative analyses of environmental regulation. For example, one study of a multinational firm with operations in the United States and Japan found that pollution levels in both countries were similar, despite generally higher pollution abatement expenditures in the United States. The higher costs observed in the United States thus were due in large part, not to more stringent standards, but to the higher regulatory transaction costs. Because agencies in different countries share information about technologies, best practices, and other issues, the pollution levels found acceptable in different countries tend to be quite similar.
At the peak of tulip mania in Holland, in March 1637, some single tulip bulbs sold for more than 10 times the annual income of a skilled craftsman. It is generally considered the first recorded speculative bubble. The term "tulip mania" is now often used metaphorically to refer to any large economic bubble (when asset prices deviate from intrinsic values).
The event was popularized in 1841 by British journalist Charles Mackay. According to Mackay, at one point 12 acres of land were offered for a Semper Augustus bulb. Mackay claims that many such investors were ruined by the fall in prices, and Dutch commerce suffered a severe shock. Some modern scholars, however, feel that the mania was not quite as extraordinary as Mackay described. Some even argue that not enough price data remain, historically, to represent an all out tulip bulb bubble.
In her 2007 scholarly analysis Tulipmania, Anne Goldgar states that the phenomenon was limited to "a fairly small group," and that most accounts from the period are based on a few contemporary pieces of propaganda. While Mackay's account held that a wide array of society was involved in the tulip trade, Goldgar's study of archived contracts found that even at its peak the trade in tulips was conducted almost exclusively by merchants and skilled craftsmen who were wealthy, but not by members of the nobility. Thus, any economic fallout from the bubble was very limited. Goldgar, who identified many prominent buyers and sellers in the market, found fewer than half a dozen who experienced financial troubles in the time period, and even of these cases it is not clear that tulips were to blame. This is not altogether surprising. Although prices had risen, money had not exchanged hands between buyers and sellers. Thus profits were never realized for sellers; unless sellers had made other purchases on credit in expectation of the profits, the collapse in prices did not cause anyone to lose money.
There is no dispute that prices for tulip bulb contracts rose and then fell in 1636–37, but even a dramatic rise and fall in prices does not necessarily mean that an economic or speculative bubble developed and then burst. For tulip mania to have qualified as an economic bubble, the price of tulip bulbs would need to have become unhinged from the intrinsic value of the bulbs. Modern economists have advanced several possible reasons for why the rise and fall in prices may not have constituted a bubble. For one, the increases of the 1630s corresponded with a lull in the Thirty Years' War, which occurred between 1618 and 1648. Hence market prices were responding rationally to a rise in demand. However, the fall in prices was faster and more dramatic than the rise, and did not result from a sudden resurgence in the war.
At the peak of tulip mania in Holland, in March 1637, some single tulip bulbs sold for more than 10 times the annual income of a skilled craftsman. It is generally considered the first recorded speculative bubble. The term "tulip mania" is now often used metaphorically to refer to any large economic bubble (when asset prices deviate from intrinsic values).
The event was popularized in 1841 by British journalist Charles Mackay. According to Mackay, at one point 12 acres of land were offered for a Semper Augustus bulb. Mackay claims that many such investors were ruined by the fall in prices, and Dutch commerce suffered a severe shock. Some modern scholars, however, feel that the mania was not quite as extraordinary as Mackay described. Some even argue that not enough price data remain, historically, to represent an all out tulip bulb bubble.
In her 2007 scholarly analysis Tulipmania, Anne Goldgar states that the phenomenon was limited to "a fairly small group," and that most accounts from the period are based on a few contemporary pieces of propaganda. While Mackay's account held that a wide array of society was involved in the tulip trade, Goldgar's study of archived contracts found that even at its peak the trade in tulips was conducted almost exclusively by merchants and skilled craftsmen who were wealthy, but not by members of the nobility. Thus, any economic fallout from the bubble was very limited. Goldgar, who identified many prominent buyers and sellers in the market, found fewer than half a dozen who experienced financial troubles in the time period, and even of these cases it is not clear that tulips were to blame. This is not altogether surprising. Although prices had risen, money had not exchanged hands between buyers and sellers. Thus profits were never realized for sellers; unless sellers had made other purchases on credit in expectation of the profits, the collapse in prices did not cause anyone to lose money.
There is no dispute that prices for tulip bulb contracts rose and then fell in 1636–37, but even a dramatic rise and fall in prices does not necessarily mean that an economic or speculative bubble developed and then burst. For tulip mania to have qualified as an economic bubble, the price of tulip bulbs would need to have become unhinged from the intrinsic value of the bulbs. Modern economists have advanced several possible reasons for why the rise and fall in prices may not have constituted a bubble. For one, the increases of the 1630s corresponded with a lull in the Thirty Years' War, which occurred between 1618 and 1648. Hence market prices were responding rationally to a rise in demand. However, the fall in prices was faster and more dramatic than the rise, and did not result from a sudden resurgence in the war.
At the peak of tulip mania in Holland, in March 1637, some single tulip bulbs sold for more than 10 times the annual income of a skilled craftsman. It is generally considered the first recorded speculative bubble. The term "tulip mania" is now often used metaphorically to refer to any large economic bubble (when asset prices deviate from intrinsic values).
The event was popularized in 1841 by British journalist Charles Mackay. According to Mackay, at one point 12 acres of land were offered for a Semper Augustus bulb. Mackay claims that many such investors were ruined by the fall in prices, and Dutch commerce suffered a severe shock. Some modern scholars, however, feel that the mania was not quite as extraordinary as Mackay described. Some even argue that not enough price data remain, historically, to represent an all out tulip bulb bubble.
In her 2007 scholarly analysis Tulipmania, Anne Goldgar states that the phenomenon was limited to "a fairly small group," and that most accounts from the period are based on a few contemporary pieces of propaganda. While Mackay's account held that a wide array of society was involved in the tulip trade, Goldgar's study of archived contracts found that even at its peak the trade in tulips was conducted almost exclusively by merchants and skilled craftsmen who were wealthy, but not by members of the nobility. Thus, any economic fallout from the bubble was very limited. Goldgar, who identified many prominent buyers and sellers in the market, found fewer than half a dozen who experienced financial troubles in the time period, and even of these cases it is not clear that tulips were to blame. This is not altogether surprising. Although prices had risen, money had not exchanged hands between buyers and sellers. Thus profits were never realized for sellers; unless sellers had made other purchases on credit in expectation of the profits, the collapse in prices did not cause anyone to lose money.
There is no dispute that prices for tulip bulb contracts rose and then fell in 1636–37, but even a dramatic rise and fall in prices does not necessarily mean that an economic or speculative bubble developed and then burst. For tulip mania to have qualified as an economic bubble, the price of tulip bulbs would need to have become unhinged from the intrinsic value of the bulbs. Modern economists have advanced several possible reasons for why the rise and fall in prices may not have constituted a bubble. For one, the increases of the 1630s corresponded with a lull in the Thirty Years' War, which occurred between 1618 and 1648. Hence market prices were responding rationally to a rise in demand. However, the fall in prices was faster and more dramatic than the rise, and did not result from a sudden resurgence in the war.
At the peak of tulip mania in Holland, in March 1637, some single tulip bulbs sold for more than 10 times the annual income of a skilled craftsman. It is generally considered the first recorded speculative bubble. The term "tulip mania" is now often used metaphorically to refer to any large economic bubble (when asset prices deviate from intrinsic values).
The event was popularized in 1841 by British journalist Charles Mackay. According to Mackay, at one point 12 acres of land were offered for a Semper Augustus bulb. Mackay claims that many such investors were ruined by the fall in prices, and Dutch commerce suffered a severe shock. Some modern scholars, however, feel that the mania was not quite as extraordinary as Mackay described. Some even argue that not enough price data remain, historically, to represent an all out tulip bulb bubble.
In her 2007 scholarly analysis Tulipmania, Anne Goldgar states that the phenomenon was limited to "a fairly small group," and that most accounts from the period are based on a few contemporary pieces of propaganda. While Mackay's account held that a wide array of society was involved in the tulip trade, Goldgar's study of archived contracts found that even at its peak the trade in tulips was conducted almost exclusively by merchants and skilled craftsmen who were wealthy, but not by members of the nobility. Thus, any economic fallout from the bubble was very limited. Goldgar, who identified many prominent buyers and sellers in the market, found fewer than half a dozen who experienced financial troubles in the time period, and even of these cases it is not clear that tulips were to blame. This is not altogether surprising. Although prices had risen, money had not exchanged hands between buyers and sellers. Thus profits were never realized for sellers; unless sellers had made other purchases on credit in expectation of the profits, the collapse in prices did not cause anyone to lose money.
There is no dispute that prices for tulip bulb contracts rose and then fell in 1636–37, but even a dramatic rise and fall in prices does not necessarily mean that an economic or speculative bubble developed and then burst. For tulip mania to have qualified as an economic bubble, the price of tulip bulbs would need to have become unhinged from the intrinsic value of the bulbs. Modern economists have advanced several possible reasons for why the rise and fall in prices may not have constituted a bubble. For one, the increases of the 1630s corresponded with a lull in the Thirty Years' War, which occurred between 1618 and 1648. Hence market prices were responding rationally to a rise in demand. However, the fall in prices was faster and more dramatic than the rise, and did not result from a sudden resurgence in the war.
Even though physiological and behavioral processes are maximized within relatively narrow ranges of temperatures in amphibians and reptiles, individuals may not maintain activity at the optimum temperatures for performance because of the costs associated with doing so. Alternatively, activity can occur at suboptimal temperatures even when the costs are great. Theoretically, costs of activity at suboptimal temperatures must be balanced by gains of being active. For instance, the leatherback sea turtle will hunt during the time of day in which krill are abundant, even though the water is cooler and thus the turtle's body temperature requires greater metabolic activity. In general, however, the cost of keeping a suboptimal body temperature, for reptiles and amphibians, is varied and not well understood; they include risk of predation, reduced performance, and reduced foraging success.
One reptile that scientists understand better is the desert lizard, which is active during the morning at relatively low body temperatures (usually 33.0 C), inactive during midday when external temperatures are extreme, and active in the evening at body temperatures of 37.0 C. Although the lizards engage in similar behavior (e.g., in morning and afternoon, social displays, movements, and feeding), metabolic rates and water loss are great and sprint speed is lower in the evening when body temperatures are high. Thus, the highest metabolic and performance costs of activity occur in the evening when lizards have high body temperatures. However, males that are active late in the day apparently have a higher mating success resulting from their prolonged social encounters. The costs of activity at temperatures beyond those optimal for performance are offset by the advantages gained by maximizing social interactions that ultimately impact individual fitness.
Even though physiological and behavioral processes are maximized within relatively narrow ranges of temperatures in amphibians and reptiles, individuals may not maintain activity at the optimum temperatures for performance because of the costs associated with doing so. Alternatively, activity can occur at suboptimal temperatures even when the costs are great. Theoretically, costs of activity at suboptimal temperatures must be balanced by gains of being active. For instance, the leatherback sea turtle will hunt during the time of day in which krill are abundant, even though the water is cooler and thus the turtle's body temperature requires greater metabolic activity. In general, however, the cost of keeping a suboptimal body temperature, for reptiles and amphibians, is varied and not well understood; they include risk of predation, reduced performance, and reduced foraging success.
One reptile that scientists understand better is the desert lizard, which is active during the morning at relatively low body temperatures (usually 33.0 C), inactive during midday when external temperatures are extreme, and active in the evening at body temperatures of 37.0 C. Although the lizards engage in similar behavior (e.g., in morning and afternoon, social displays, movements, and feeding), metabolic rates and water loss are great and sprint speed is lower in the evening when body temperatures are high. Thus, the highest metabolic and performance costs of activity occur in the evening when lizards have high body temperatures. However, males that are active late in the day apparently have a higher mating success resulting from their prolonged social encounters. The costs of activity at temperatures beyond those optimal for performance are offset by the advantages gained by maximizing social interactions that ultimately impact individual fitness.
Even though physiological and behavioral processes are maximized within relatively narrow ranges of temperatures in amphibians and reptiles, individuals may not maintain activity at the optimum temperatures for performance because of the costs associated with doing so. Alternatively, activity can occur at suboptimal temperatures even when the costs are great. Theoretically, costs of activity at suboptimal temperatures must be balanced by gains of being active. For instance, the leatherback sea turtle will hunt during the time of day in which krill are abundant, even though the water is cooler and thus the turtle's body temperature requires greater metabolic activity. In general, however, the cost of keeping a suboptimal body temperature, for reptiles and amphibians, is varied and not well understood; they include risk of predation, reduced performance, and reduced foraging success.
One reptile that scientists understand better is the desert lizard, which is active during the morning at relatively low body temperatures (usually 33.0 C), inactive during midday when external temperatures are extreme, and active in the evening at body temperatures of 37.0 C. Although the lizards engage in similar behavior (e.g., in morning and afternoon, social displays, movements, and feeding), metabolic rates and water loss are great and sprint speed is lower in the evening when body temperatures are high. Thus, the highest metabolic and performance costs of activity occur in the evening when lizards have high body temperatures. However, males that are active late in the day apparently have a higher mating success resulting from their prolonged social encounters. The costs of activity at temperatures beyond those optimal for performance are offset by the advantages gained by maximizing social interactions that ultimately impact individual fitness.
Originally, scientists predicted small asteroids to be hard and rocky, as any loose surface material (called regolith) generated by impacts was expected to escape their weak gravity. Aggregate small bodies were not thought to exist, because the slightest sustained relative motion would cause them to separate. But observations and computer modeling are proving otherwise. Most asteroids larger than a kilometer are now believed to be composites of smaller pieces. Those imaged at high-resolution show evidence for copious regolith despite the weak gravity. Most of them have one or more extraordinarily large craters, some of which are wider than the mean radius of the whole body. Such colossal impacts would not just gouge out a crater—they would break any monolithic body into pieces. In short, asteroids larger than a kilometer across may look like nuggets of hard rock but are more likely to be aggregate assemblages—or even piles of loose rubble so pervasively fragmented that no solid bedrock is left.
The rubble hypothesis, proposed decades ago by scientists, lacked evidence, until the planetologist Schumaker realized that the huge craters on the asteroid Mathilde and its very low density could only make sense together: a porous body such as a rubble pile can withstand a battering much better than an integral object. It will absorb and dissipate a large fraction of the energy of an impact; the far side might hardly feel a thing. At first, the rubble hypothesis may appear conceptually troublesome. The material strength of an asteroid is nearly zero, and the gravity is so low one is tempted to neglect that too. The truth is neither strength nor gravity can be ignored. Paltry though it may be, gravity binds a rubble pile together. And anybody who builds sandcastles knows that even loose debris can cohere. Oft-ignored details of motion begin to matter: sliding friction, chemical bonding, damping of kinetic energy, etc. We are just beginning to fathom the subtle interplay of these minuscule forces.
The size of an asteroid should determine which force dominates. One indication is the observed pattern of asteroidal rotation rates. Some collisions cause an asteroid to spin faster; others slow it down. If asteroids are monolithic rocks undergoing random collisions, a graph of their rotation rates should show a bell-shaped distribution with a statistical “tail” of very fast rotators. If nearly all asteroids are rubble piles, however, this tail would be missing, because any rubble pile spinning faster than once every two or three hours fly apart. Recently, several astronomers discovered that all but five observed asteroids obey a strict rotation limit. The exceptions are all smaller than about 150 meters in diameter, with an abrupt cutoff for asteroids larger than 200 meters. The evident conclusion—that asteroids larger than 200 meters across are rubble piles—agrees with recent computer modeling of collisions. A collision can blast a large asteroid to bits, but those bits will usually be moving slower than their mutual escape velocity (the lowest velocity that a body must have in order to escape the orbit of a planet). Over several hours, gravity will reassemble all but the fastest pieces into a rubble pile.
Originally, scientists predicted small asteroids to be hard and rocky, as any loose surface material (called regolith) generated by impacts was expected to escape their weak gravity. Aggregate small bodies were not thought to exist, because the slightest sustained relative motion would cause them to separate. But observations and computer modeling are proving otherwise. Most asteroids larger than a kilometer are now believed to be composites of smaller pieces. Those imaged at high-resolution show evidence for copious regolith despite the weak gravity. Most of them have one or more extraordinarily large craters, some of which are wider than the mean radius of the whole body. Such colossal impacts would not just gouge out a crater—they would break any monolithic body into pieces. In short, asteroids larger than a kilometer across may look like nuggets of hard rock but are more likely to be aggregate assemblages—or even piles of loose rubble so pervasively fragmented that no solid bedrock is left.
The rubble hypothesis, proposed decades ago by scientists, lacked evidence, until the planetologist Schumaker realized that the huge craters on the asteroid Mathilde and its very low density could only make sense together: a porous body such as a rubble pile can withstand a battering much better than an integral object. It will absorb and dissipate a large fraction of the energy of an impact; the far side might hardly feel a thing. At first, the rubble hypothesis may appear conceptually troublesome. The material strength of an asteroid is nearly zero, and the gravity is so low one is tempted to neglect that too. The truth is neither strength nor gravity can be ignored. Paltry though it may be, gravity binds a rubble pile together. And anybody who builds sandcastles knows that even loose debris can cohere. Oft-ignored details of motion begin to matter: sliding friction, chemical bonding, damping of kinetic energy, etc. We are just beginning to fathom the subtle interplay of these minuscule forces.
The size of an asteroid should determine which force dominates. One indication is the observed pattern of asteroidal rotation rates. Some collisions cause an asteroid to spin faster; others slow it down. If asteroids are monolithic rocks undergoing random collisions, a graph of their rotation rates should show a bell-shaped distribution with a statistical “tail” of very fast rotators. If nearly all asteroids are rubble piles, however, this tail would be missing, because any rubble pile spinning faster than once every two or three hours fly apart. Recently, several astronomers discovered that all but five observed asteroids obey a strict rotation limit. The exceptions are all smaller than about 150 meters in diameter, with an abrupt cutoff for asteroids larger than 200 meters. The evident conclusion—that asteroids larger than 200 meters across are rubble piles—agrees with recent computer modeling of collisions. A collision can blast a large asteroid to bits, but those bits will usually be moving slower than their mutual escape velocity (the lowest velocity that a body must have in order to escape the orbit of a planet). Over several hours, gravity will reassemble all but the fastest pieces into a rubble pile.
Originally, scientists predicted small asteroids to be hard and rocky, as any loose surface material (called regolith) generated by impacts was expected to escape their weak gravity. Aggregate small bodies were not thought to exist, because the slightest sustained relative motion would cause them to separate. But observations and computer modeling are proving otherwise. Most asteroids larger than a kilometer are now believed to be composites of smaller pieces. Those imaged at high-resolution show evidence for copious regolith despite the weak gravity. Most of them have one or more extraordinarily large craters, some of which are wider than the mean radius of the whole body. Such colossal impacts would not just gouge out a crater—they would break any monolithic body into pieces. In short, asteroids larger than a kilometer across may look like nuggets of hard rock but are more likely to be aggregate assemblages—or even piles of loose rubble so pervasively fragmented that no solid bedrock is left.
The rubble hypothesis, proposed decades ago by scientists, lacked evidence, until the planetologist Schumaker realized that the huge craters on the asteroid Mathilde and its very low density could only make sense together: a porous body such as a rubble pile can withstand a battering much better than an integral object. It will absorb and dissipate a large fraction of the energy of an impact; the far side might hardly feel a thing. At first, the rubble hypothesis may appear conceptually troublesome. The material strength of an asteroid is nearly zero, and the gravity is so low one is tempted to neglect that too. The truth is neither strength nor gravity can be ignored. Paltry though it may be, gravity binds a rubble pile together. And anybody who builds sandcastles knows that even loose debris can cohere. Oft-ignored details of motion begin to matter: sliding friction, chemical bonding, damping of kinetic energy, etc. We are just beginning to fathom the subtle interplay of these minuscule forces.
The size of an asteroid should determine which force dominates. One indication is the observed pattern of asteroidal rotation rates. Some collisions cause an asteroid to spin faster; others slow it down. If asteroids are monolithic rocks undergoing random collisions, a graph of their rotation rates should show a bell-shaped distribution with a statistical “tail” of very fast rotators. If nearly all asteroids are rubble piles, however, this tail would be missing, because any rubble pile spinning faster than once every two or three hours fly apart. Recently, several astronomers discovered that all but five observed asteroids obey a strict rotation limit. The exceptions are all smaller than about 150 meters in diameter, with an abrupt cutoff for asteroids larger than 200 meters. The evident conclusion—that asteroids larger than 200 meters across are rubble piles—agrees with recent computer modeling of collisions. A collision can blast a large asteroid to bits, but those bits will usually be moving slower than their mutual escape velocity (the lowest velocity that a body must have in order to escape the orbit of a planet). Over several hours, gravity will reassemble all but the fastest pieces into a rubble pile.
Originally, scientists predicted small asteroids to be hard and rocky, as any loose surface material (called regolith) generated by impacts was expected to escape their weak gravity. Aggregate small bodies were not thought to exist, because the slightest sustained relative motion would cause them to separate. But observations and computer modeling are proving otherwise. Most asteroids larger than a kilometer are now believed to be composites of smaller pieces. Those imaged at high-resolution show evidence for copious regolith despite the weak gravity. Most of them have one or more extraordinarily large craters, some of which are wider than the mean radius of the whole body. Such colossal impacts would not just gouge out a crater—they would break any monolithic body into pieces. In short, asteroids larger than a kilometer across may look like nuggets of hard rock but are more likely to be aggregate assemblages—or even piles of loose rubble so pervasively fragmented that no solid bedrock is left.
The rubble hypothesis, proposed decades ago by scientists, lacked evidence, until the planetologist Schumaker realized that the huge craters on the asteroid Mathilde and its very low density could only make sense together: a porous body such as a rubble pile can withstand a battering much better than an integral object. It will absorb and dissipate a large fraction of the energy of an impact; the far side might hardly feel a thing. At first, the rubble hypothesis may appear conceptually troublesome. The material strength of an asteroid is nearly zero, and the gravity is so low one is tempted to neglect that too. The truth is neither strength nor gravity can be ignored. Paltry though it may be, gravity binds a rubble pile together. And anybody who builds sandcastles knows that even loose debris can cohere. Oft-ignored details of motion begin to matter: sliding friction, chemical bonding, damping of kinetic energy, etc. We are just beginning to fathom the subtle interplay of these minuscule forces.
The size of an asteroid should determine which force dominates. One indication is the observed pattern of asteroidal rotation rates. Some collisions cause an asteroid to spin faster; others slow it down. If asteroids are monolithic rocks undergoing random collisions, a graph of their rotation rates should show a bell-shaped distribution with a statistical “tail” of very fast rotators. If nearly all asteroids are rubble piles, however, this tail would be missing, because any rubble pile spinning faster than once every two or three hours fly apart. Recently, several astronomers discovered that all but five observed asteroids obey a strict rotation limit. The exceptions are all smaller than about 150 meters in diameter, with an abrupt cutoff for asteroids larger than 200 meters. The evident conclusion—that asteroids larger than 200 meters across are rubble piles—agrees with recent computer modeling of collisions. A collision can blast a large asteroid to bits, but those bits will usually be moving slower than their mutual escape velocity (the lowest velocity that a body must have in order to escape the orbit of a planet). Over several hours, gravity will reassemble all but the fastest pieces into a rubble pile.
Unlike Mercury and Mars, Venus has a dense, opaque atmosphere that prevents direct observation of its surface. For years, surface telescopes on Earth could glean no information about the surface of Venus. In 1989, the Magellan probe was launched to do a five-year radar-mapping of the entire surface of Venus. The data that emerged provided by far the most detailed map of the Venusian surface ever seen.
The surface shows an unbelievable level of volcanic activity: more than one hundred large shield volcanoes, many more than Earth has, and a solidified river of lava longer than the Nile. The entire surface is volcanically dead, with not a single active volcano. This surface is relatively young in planetary terms, about 300 million years old. The whole surface, planet-wide, is the same age: the even pattern of craters, randomly distributed across the surface, demonstrates this.
To explain this puzzling surface, Turcotte suggested a radical model. The surface of Venus, for a period, is as it is now, a surface of uniform age with no active volcanism. While the surface is fixed, volcanic pressure builds up inside the planet. At a certain point, the pressure ruptures the surface, and the entire planet is re-coated in lava in a massive planet-wide outburst of volcanism. Having spent all this thermal energy in one gigantic outpouring, the surface cools and hardens, again producing the kind of surface we see today. Turcotte proposed that this cycle repeated several times in the past, and would still repeat in the future.
To most planetary geologists, Turcotte's model is a return to catastrophism. For two centuries, geologist of all kinds fought against the idea of catastrophic, planet-wide changes, such as the Biblical idea of Noah's Flood. The triumph of gradualism was essential to the success of geology as a serious science. Indeed, all features of Earth's geology and all features of other moons and planets in the Solar System, even those that are not volcanically active, are explained very well by current gradualist models. Planetary geologists question why all other objects would obey gradualist models, and only Venus would obey a catastrophic model. These geologists insist that the features of Venus must be able to be explained in terms of incremental changes continuously over a long period.
Turcotte, expecting these objections, points out that no incremental process could result in a planet-wide surface all the same age. Furthermore, a slow process of continual change does not well explain why a planet with an astounding history of volcanic activity is now volcanically dead. Turcotte argues that only his catastrophic model adequately explains the extremes of the Venusian surface.
Unlike Mercury and Mars, Venus has a dense, opaque atmosphere that prevents direct observation of its surface. For years, surface telescopes on Earth could glean no information about the surface of Venus. In 1989, the Magellan probe was launched to do a five-year radar-mapping of the entire surface of Venus. The data that emerged provided by far the most detailed map of the Venusian surface ever seen.
The surface shows an unbelievable level of volcanic activity: more than one hundred large shield volcanoes, many more than Earth has, and a solidified river of lava longer than the Nile. The entire surface is volcanically dead, with not a single active volcano. This surface is relatively young in planetary terms, about 300 million years old. The whole surface, planet-wide, is the same age: the even pattern of craters, randomly distributed across the surface, demonstrates this.
To explain this puzzling surface, Turcotte suggested a radical model. The surface of Venus, for a period, is as it is now, a surface of uniform age with no active volcanism. While the surface is fixed, volcanic pressure builds up inside the planet. At a certain point, the pressure ruptures the surface, and the entire planet is re-coated in lava in a massive planet-wide outburst of volcanism. Having spent all this thermal energy in one gigantic outpouring, the surface cools and hardens, again producing the kind of surface we see today. Turcotte proposed that this cycle repeated several times in the past, and would still repeat in the future.
To most planetary geologists, Turcotte's model is a return to catastrophism. For two centuries, geologist of all kinds fought against the idea of catastrophic, planet-wide changes, such as the Biblical idea of Noah's Flood. The triumph of gradualism was essential to the success of geology as a serious science. Indeed, all features of Earth's geology and all features of other moons and planets in the Solar System, even those that are not volcanically active, are explained very well by current gradualist models. Planetary geologists question why all other objects would obey gradualist models, and only Venus would obey a catastrophic model. These geologists insist that the features of Venus must be able to be explained in terms of incremental changes continuously over a long period.
Turcotte, expecting these objections, points out that no incremental process could result in a planet-wide surface all the same age. Furthermore, a slow process of continual change does not well explain why a planet with an astounding history of volcanic activity is now volcanically dead. Turcotte argues that only his catastrophic model adequately explains the extremes of the Venusian surface.
Unlike Mercury and Mars, Venus has a dense, opaque atmosphere that prevents direct observation of its surface. For years, surface telescopes on Earth could glean no information about the surface of Venus. In 1989, the Magellan probe was launched to do a five-year radar-mapping of the entire surface of Venus. The data that emerged provided by far the most detailed map of the Venusian surface ever seen.
The surface shows an unbelievable level of volcanic activity: more than one hundred large shield volcanoes, many more than Earth has, and a solidified river of lava longer than the Nile. The entire surface is volcanically dead, with not a single active volcano. This surface is relatively young in planetary terms, about 300 million years old. The whole surface, planet-wide, is the same age: the even pattern of craters, randomly distributed across the surface, demonstrates this.
To explain this puzzling surface, Turcotte suggested a radical model. The surface of Venus, for a period, is as it is now, a surface of uniform age with no active volcanism. While the surface is fixed, volcanic pressure builds up inside the planet. At a certain point, the pressure ruptures the surface, and the entire planet is re-coated in lava in a massive planet-wide outburst of volcanism. Having spent all this thermal energy in one gigantic outpouring, the surface cools and hardens, again producing the kind of surface we see today. Turcotte proposed that this cycle repeated several times in the past, and would still repeat in the future.
To most planetary geologists, Turcotte's model is a return to catastrophism. For two centuries, geologist of all kinds fought against the idea of catastrophic, planet-wide changes, such as the Biblical idea of Noah's Flood. The triumph of gradualism was essential to the success of geology as a serious science. Indeed, all features of Earth's geology and all features of other moons and planets in the Solar System, even those that are not volcanically active, are explained very well by current gradualist models. Planetary geologists question why all other objects would obey gradualist models, and only Venus would obey a catastrophic model. These geologists insist that the features of Venus must be able to be explained in terms of incremental changes continuously over a long period.
Turcotte, expecting these objections, points out that no incremental process could result in a planet-wide surface all the same age. Furthermore, a slow process of continual change does not well explain why a planet with an astounding history of volcanic activity is now volcanically dead. Turcotte argues that only his catastrophic model adequately explains the extremes of the Venusian surface.