Types of Planetary Disasters

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The immediate, or direct, cause of all mass extinctions appears to be changes in the "global atmosphere inventory." Changes to the atmospheric gases (which may be changes in volume or in the relative constituents of the atmosphere) can be caused by many things: asteroid or comet impact, degassing of carbon dioxide or other gases into the oceans and atmosphere during flood basalt extrusion (when great volumes of lava flow out onto Earth's surface), degassing caused by liberation of organic-rich ocean sediments during changes in sea level, and changes in the patterns of ocean circulation. The killing agents arise through changes in the makeup and behavior of the at mosphere or through factors such as temperature and circulation patterns that are dictated by properties of the atmosphere.

Planetary disasters can occur for a great number of reasons. We shall examine a few, in no particular order of importance.

• Changing a planet's spin rate

We take Earth's 24-hour spin rate for granted, when in fact it appears to be unusual if we compare it to other planets and satellites in our solar system. Jupiter and Saturn, for instance, each far greater in mass and diameter than Earth, spin much faster. Many other bodies, however, such as Venus and Mercury (and even our own Moon), spin much more slowly. In lower-mass stars, planets in the habitable zone become "tidally locked" by the gravitational force of the larger star or planet they revolve around. When one side always faces the star in question, that particular face becomes very hot, whereas the other side is always facing cold space and becomes frigid. Either environment would be lethal to surface life and prevent its evolution.

Planets can change spin rates, and when they do, any life already adapted to a particular spin regime would be likely to face planetary disaster because of the major temperature changes it would encounter. Earth itself has been gradually slowing, a phenomenon that has probably altered the distribution of cloud cover over time.

• Moving out of the animal "habitable zone"

Animal life needs liquid water, so it requires a mean global temperature that allows liquid water to exist. Any movement of a planet out of an orbit that allows such temperatures will create a planetary disaster. Though such changes of orbit are unlikely, they could be caused by another planet in a stellar system. Such perturbations would be common in open star clusters.

• Changing the energy output of the sun (star)

Complex animal life on any planet is dependent on stellar energy. If stellar output either increases or decreases such that liquid water can no longer exist, the result will be disastrous to animal life or to the prospects for its evolution. Short-term and long-term changes in stellar energy output may be one of the most common forms of planetary extinction—and even sterilization. Some scientists are convinced that the end of life on Earth will be caused by an increase in the sun's energy output. This is nothing new. As we have seen, the amount of energy being produced by the sun—and indeed, by most stars—increases over time. On Earth, the maintenance of a relatively constant temperature has been attained through a gradual reduction in greenhouse gases as the amount of energy from the sun has increased, thus keeping temperatures in check. We seem to be nearing the end of this type of planetary temperature regulation, however. There are now very small volumes of carbon dioxide in the atmosphere compared to earlier periods of geological time, and the sun's energy output continues to increase. Some scientists have predicted that temperatures on Earth will become too high for animal life within several hundred million years from now. That event, when it comes to pass, will produce the last greatest mass extinction on Earth, its sterilization.

• Impact of a comet or asteroid

Any planetary system is rife with cosmic debris: asteroids and comets, the residue left over from planetary formation. Great quantities of this material will eventually strike all members of a planetary system, and the energy released can spell planetary disaster. Such disasters are now known to have caused mass extinctions on Earth. In 1980, Luis and Walter Alvarez, Frank Asaro, and Helen Michel from the University of California at Berkeley proposed that one of the greatest of all mass extinctions, the 65-million-year-old event that killed off the dinosaurs and many other species living near the end of the Mesozoic Era, was caused by the impact of a large meteor or comet striking Earth, as described at the beginning of this chapter. As evidence for this view mounted, most scientists realized that collision with a meteor or comet could cause a biotic crisis on any planet and that it has done so at least once (and probably other times as well) during Earth's past (see Figure 8.1).

Equivalent size of bomb 10 1 100 10,000 1,000,000

(tons ofTNT) kilotons megaton megatons megatons megatons

Approximate initial aa^ 1 m 10 m 100 m 1 km 10 km

Equivalent size of bomb 10 1 100 10,000 1,000,000

(tons ofTNT) kilotons megaton megatons megatons megatons

Approximate initial aa^ 1 m 10 m 100 m 1 km 10 km

Crater diameter on Earth or Moon

Effects Meteor breaks up in Earth's Devastates Wipes out globalWipes out atmosphere and may make no continent- agriculture, disrupts most crater during a terrestrial impact scale region civilization species

Crater diameter on Earth or Moon

Effects Meteor breaks up in Earth's Devastates Wipes out globalWipes out atmosphere and may make no continent- agriculture, disrupts most crater during a terrestrial impact scale region civilization species

Figure 8.1 The rate of meteor impacts at the top of Earth's atmosphere as a function of meteor size. Bottom scale gives crater size for typical impact velocity of about 15 kilometers per second. Top scale gives meteorite size and energy in terms of tons of TNT. Dotted line shows the scale of the Siberian meteoritic explosion of 1908. Dashed line shows the scale of the impact 65 million years ago that wiped out dinosaurs and other species. (After Hartmann and Impey, 1994; 1993 data from E. Shoemaker, C. Chapman, D. Morrison, G. Neukum, and others)

Many variables affect the degree of lethality resulting from a collision, such as the meteor's size, composition, angle of impact, and velocity and the nature of the impact target area. In the case of the Cretaceous event (also known as the K/T impact), for instance, the target rock was rich in sulfur, which exacerbated the impact's environmental effects. (The sulfur reacted with air and water to produce a highly toxic acid rain that lasted many months after the impact event itself.) Moreover, not only the geology of the impact site, but also its geography, may play an important part. An impact in a low-latitude site will have entirely different consequences from a similar body hitting a high-latitude site at a similar angle and speed, because the distribution of lethality across the globe may be produced by atmospheric circulation patterns. Finally, the nature of both the biota and the atmosphere at the time of impact are surely important. An impact in a highly diverse world of ecological "specialists"—animals and plants with little tolerance for environmental change—might produce more extinction than the same event in a low-diversity world of "generalists." And an impact in a greenhouse world might have different effects from one where greenhouse gas inventory or oxygen content was lower than that on Earth today.

In the early years after the Alvarez hypothesis was advanced, some investigators thought that a general synthetic model linking most or all mass extinctions to impact would emerge. This was the thinking behind the "Nemesis" hypothesis of astronomer Rich Muller from Berkeley, and it underlies the work of David Raup and Jack Sepkoski of the University of Chicago, who hypothesized in 1984 that mass extinctions show a 26-million-year periodicity. Since then, elevated levels of iridium (the platinum-group element used by the Alvarez team as a sign of impact) have been found from 11 different time intervals in the geological record. Yet most of these are at such low concentration that they are not indicative of larger impacts. The evidence to date suggests that only the major mass extinction at the end of the Triassic and that at the end of the Cretaceous Period (the K/T) were brought about by the effects of impact.

The presence of numerous impact craters on every stony planet or moon of the solar system is stark evidence of the frequency of these events, at least early in the history of our solar system. It is probable that impact is a hazard in most, or perhaps all, other stellar systems as well. Impacts are probably the most frequent and important of all planetary catastrophes. They could completely reset the course of the biological history of a planet by removing previously dominant groups of organisms, thus opening the way for entirely new groups or for the rise to dominance of previously minor groups.

• Nearby supernova

Another mechanism that could produce a mass extinction is the occurrence of a supernova in the sun's galactic neighborhood. Two astronomers from the University of Chicago calculated in 1995 that a star going supernova within 10 parsecs (30 light-years) of our sun would release fluxes of energetic electromagnetic and charged cosmic radiation sufficient to destroy Earth's ozone layer in 300 years or less. Much recent research on ozone depletion in the present-day atmosphere suggests that removal of the ozone layer would prove calamitous to the biosphere and to the species residing within. A depleted ozone layer would expose both marine and terrestrial organisms to potentially lethal solar ultraviolet radiation. Photosynthesizing organisms, including phytoplankton and reef communities, would be particularly affected.

Judging by the number of stars within 10 parsecs of the sun in the last 530 million years, and by the rates of supernova explosions among stars, astronomers have concluded that it is very plausible that one or more supernova explosions have occurred within 10 parsecs of Earth during the last 500 million years. They also believe such explosions are likely to occur every 200 to 300 million years. The probability of nearby supernovae would be much greater closer to the galactic center, as suggested in Chapter 2.

• Sources of gamma rays

Astronomers have detected sudden bursts of intense gamma radiation being emitted from various galaxies (gamma rays are the most dangerous radiation emitted by atomic bombs). Although very little is yet known about these short but extremely violent releases of energy, they would be lethal to any life on nearby planetary systems.

• Cosmic ray jets and gamma ray explosions

A new entry into the mass-death rogues gallery is lethal bursts of radiation produced by violent stellar collisions. Cosmic ray jets and gamma rays might both result from the same source: merging neutron stars. Astronomers Arnon Dar, Ari Laor, and Nir Shaviv have postulated that cosmic ray jets may account for several of the major mass extinctions and might explain the rapid evolutionary events that follow them. They propose that high-energy fluxes of cosmic rays follow the merger or collapse of neutron stars, themselves the residues of supernovae. These explosions are the most powerful in the Universe, releasing in a few seconds as much energy as the entire output of a supernova. When two of these objects coalesce, they create a broad beam of high-energy particles that, if it hit Earth, would be capable of stripping away the ozone layer and bombarding the planet with lethal doses of radiation.

The frequency of these events is the critical issue. New calculations suggest that these events may be both more frequent and more dangerous to life in any galaxy than previously supposed. Chicago physicist James Annis proposed in 1999 that gamma ray explosions are so lethal that a single such event could obliterate life over much or all of an entire galaxy. Annis has calculated that the rate of such explosions is about one burst every few hundred million years in each galaxy. For instance, Annis suggests that if the energy from such an event hit Earth, it would kill all land life on our planet, even if the explosion occurred at the center of our galaxy. If such violent and dangerous collisions are rare, they are but one more low-probability event. Yet both Annis and Dar argue that such collisions occur relatively often and were even more common earlier in the history of the Universe. They calculate that such effects would cause a major mass extinction on Earth every hundred million years.

• Catastrophic climate change: Icehouse and Runaway Greenhouse

Under certain circumstances, radical changes in climate can cause mass extinction. Major glaciations and greenhouse heating are examples, and both depend on the amount of carbon dioxide or other greenhouse gases in the atmosphere. These are the actual killing mechanisms brought about by the reduction or increase of stellar output or by a planet's orbit becoming either closer to or farther from its sun. Climate changes intense enough to threaten the biosphere with major mass extinction would involve great swings in mean planetary temperature, as well as relocation of oceanic current systems and shifts in planetary rainfall patterns.

The two most catastrophic such conditions can be called Icehouse (the Snowball Earth events are examples) and Runaway Greenhouse. In both cases, global temperatures move outside the 0-100°C range that allows the presence of liquid water on the planet. We will see possible examples of each when we examine the fates of Venus and Mars in the next chapter.

• The emergence of intelligent organisms

There is abundant evidence that the emergence of humanity as a globally distributed species armed with technology has triggered a new episode of mass extinction on Earth. It can be argued that the emergence of any intelligent species co-opting a planet's resources in the service of advanced technology and agriculture will necessarily cause a planetary mass extinction.

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