The Vulnerability of the Earth: Lessons for Us All. In the next few lectures, we will be studying We will come to understand some features of the Solar System which make it a perilous place for life forms. For instance: We will learn that Venus is a planet which has suffered from a global greenhouse effect of catastrophic proportions. Our own planet may be subject to such a calamity, if we do not carefully monitor and control our influence on the atmosphere. We will learn that Mars is easily pushed about by the gravitational influence of the larger planets, with consequent changes in its climate which have led to catastrophic floods and so forth. Fortunately, the Earth's orbit is more stable than that of Mars, but in other planetary systems we cannot be certain that all planets will be so fortunate. Truly benign locations for life-bearing planets may be few and far between. We already know that the planets with thin atmospheres (like Mars), and objects with no atmosphere (like the moon and Mercury), are deeply pitted and cratered from the impact of rocky meteors and asteroids which occasionally run into them. One might have hoped that this was all a record of the distant past, when the solar system was filled with rocky chunks during its first formative stages, but the implication of the previous lecture was that there are still many potentially dangerous in the solar system. While it is certainly true that there are fewer rocky chunks moving around the Solar System now than there once were, we are still far from immune to collisions! Combining a couple of these thoughts, we are faced with an unpleasant reality. There are millions of asteroids (small rocky chunks) in the Solar System, most of them found between the orbits of Mars and Jupiter. In short, they are in just the region where they will be greatly perturbed by the influence of Jupiter's gravity, just as Mars has been. Unfortunately, the asteroids are even more susceptible to this, partly because they are closer to Jupiter to begin with, but also because they are astronomically tiny in mass. Consequently, they can be easily `pushed around' -- or rather, pulled, by competing gravitational tugs -- and may be flung off in new directions. The gravity of Jupiter may be almost literally throwing rocky chunks into the inner solar system, and possibly even into our path. Just how safe are we, and what can we do about it if we are not? Does a catastrophe await us?

From Catastrophism to Gradualism - and Back Again.

Some of you will know the famous movie Fantasia, a full-length animated feature film in which Walt Disney put cartoon images to classical music in imaginative ways. In one dramatic sequence, we see dinosaurs trudging across a dry, bleak landscape, suffering from desert-like conditions which led to their demise. Eventually they collapse in thirsty agony, accompanied by Igor Stravinsky's ballet music for 'The Rite of Spring.' The episode ends with a total eclipse of the sun followed by horrendous earthquakes in which whole mountains come shooting up out of the ground. Eventually great tsunamis flood the landscape, and the sequence ends peacefully. The science of this depiction is enormously suspect in a couple of ways - for instance, there is absolutely no reason that an eclipse would spark enormous earthquakes, as the movie seems to imply. But it is relevant in that it addresses two related questions: why did the dinosaurs die out? more generally, what role did catastrophic events play in the geological and biological history of the Earth? Something over a century ago, it was widely believed by scientists that the history of the Earth was marked by brief episodes of great activity. According to the theory of catastrophism, the formation of mountain ranges occurred rather as depicted in Fantasia, during brief periods of great geological activity, with much longer quiescent periods in between. For various reasons, this theory was supplanted by one of gradualism, a theory introduced and championed by the geologist Lyell and according to which essentially all the geological structures on Earth, even the most impressive mountain ranges, were the product of slow, steady changes acting for aeons. Biological evolution, by the way, is still an area of contention. Many biologists accept the notion of a steady, slow process of the evolution of lifeforms, but there are proponents of what has come to be called `punctuated equilibrium' in which there are rapid changes, perhaps imposed by some near-catastrophic change in the external environment. (Some of the excellent essays of Stephen Jay Gould address these issues.) Such considerations lead to a natural question: what led to the extinction of the dinosaurs? Was it a catastrophic influence, or gradual change? One of the problems with our interpreting the demise of the dinosaurs 65 million years ago is that we do not really know just how prompt an event it was. What we know for certain is that the rock strata laid down for many tens of millions of years before the extinction episode were filled with dinosaur fossils; after that, we find essentially none at all in the layers of clay, sand, and mud which were subsequently deposited on top and compressed to form solid rock. But the transition, although abrupt, is too poorly defined: we cannot tell, looking at the compressed rock layers, whether the dinosaurs all died out in a day, a year, a decade, a century, or yet longer. All we can say is that it was relatively swift, in geological terms. This uncertainty admits a lot of possibilities! Gradualists, and the Walt Disney Corporation, might argue that there was a slow change in the enviroment, perhaps over several centuries or more, which led to the demise of the dinosaurs - something like a general drying-up of the terrain. On a briefer scale, you can imagine something like a period of enhanced volcanic activity, for instance, with a consequent change in the global climate - a period of `smog' in which the dinosaurs were gradually poisoned, as it were. But there is also the possibility that there was a single apocalyptic event, like the impact of a huge asteroid or comet onto the Earth, which led almost overnight to the mass extinction of many whole species. Indeed, the overwhelming evidence now supports the view that there was a catastrophic event of exactly this kind, 65 million years ago. Catastrophism is apparently still an important agency in the astronomical, geological, and biological evolution of the Earth. The public perception of this prospect was recently heightened by the spectacular collisions in August 1994 between Jupiter and the fragments of comet Shoemaker-Levy. It is interesting to note that the precise time and place of the impact was in fact calculated and predicted by astronomers. There is a lesson in this: believe what the astronomers say! If we find a dangerous asteroid which crosses the Earth's orbit and could run into us, it is a threat which should be taken seriously. There are in fact possible countermeasures, if we are given sufficient warning.

Comet Shoemaker-Levy.

This famous comet, described on pages 383-386 of your text, was discovered by the team of David Levy (a Queen's University alumnus who did an English M.A. here, studying the poetry of Gerard Manley Hopkins), Eugene Shoemaker, and Carolyn Shoemaker. When it was first detected, as an image on a photographic plate, it looked very strange - like a long skinny blob. Better images were taken which revealed that it looked like a `string of pearls,' a series of chunks of comet all following the same path in space rather like the cars of a train. (See the figure on page 384.) Later analysis revealed that this comet had been intact until a recent passage close to Jupiter, and that the gravitational tidal forces of that planet had torn it into bits. (Remember just how flimsy comets are!) But the tug of Jupiter had done more than just that. Jupiter's gravitational influence had changed the orbit so that the pieces were doomed to plunge, one after another, into the deep clouds of Jupiter itself. Unfortunately, from our persepctive, the impacts were on the far side of Jupiter, but the planet rotates quite rapidly, and the points of impact would very quickly be carried around to where we could see them. The interesting question, of course, was what would happen at the time of impact. Given the enormous distances and our limited telescopic powers, it was not possible to determine exactly what size the chunks were. Were they 1 km across, or 5 km across? Moreover, no one really knew how dense they were. Were they dense icy chunks, or very fluffy, like big snowballs? Given these unknowns, it was hard to predict how much energy would be released in the collisions. Some astronomers even predicted that essentially no disturbance would be seen, that the chunks would vanish into the clouds just as a stone thrown through fog on a misty day has no effect on the fog. The observations were made from all over the world, and caught a lot of public attention. The Hubble Space Telescope was also turned to Jupiter, and the spacecraft Galileo, then on its way to Jupiter, provided pictures which actually showed the points of impact. (Galileo was already far enough out into the solar system that it could `peek around the corner' to see part of the far side of Jupiter). In the end, the impacts were really colossal, as is shown by photographs on page 385 of your text. The energy which was released far outstripped anything we could generate on Earth, even with the simultaneous explosion of every bit of dynamite and nuclear weaponry on the planet.

Recent Comet Extravaganzas.

Some of you may be aware that we have recently enjoyed visits from two very spectacular comets: Hyakutake, in 1996, and Hale-Bopp, in 1997. We have been given quite a treat, because one bright comet a year is considerably more than you might expect to see on average! But of course we may now pass through a long period in which we see very few at all. As interesting as these phenomena are, however, I will not spend any time on them. (I invite you to search on the web, or to look into a magazine such as Sky and Telescope, which can be found in the University library, if you would like to know more about the spectacular sights we saw at the time of these cometary visitations.) Instead, I will focus my discussion on one specific aspect of cometary visits - that which pertains to the fragility of life on Earth. I want now to present the increasingly persuasive evidence that cometary impacts on the Earth might lead to widespread catastrophic extinctions, and indeed that they have done so in the past.

The Big One: The Extinction of the Dinosaurs.

As I noted above, scientists have been wondering for many decades just what could have led to the abrupt extinction of the dinosaurs. Speculations have ranged from gradual changes in the climate, through global pestilences which killed off some critical plants in the food chain, to excessive volcanic activity which effectively poisoned the atmosphere, and so on. What we know for certain is that the rocks older than 65 million years, the Cretaceous layers, are filled with dinosaur fossils, whereas the younger layers, from the Tertiary period, are devoid of such fossils. Moreover, the transition is very abrupt. We go from dinosaur-fossil-rich rock layers to layers with essentially none in a very small vertical distance in the geological strata, a distance which corresponds to a very short span of geological time. Some years ago, an exciting and telling discovery was made. Between the Cretaceous and Tertiary strata, there is a thin layer of soft clay-like material, embedded in which we find a lot of particles of what seems to be soot and ash. Moreover, the material in this layer (which is called the KT layer, or the KT boundary, `K' being the first letter in the German word for the Cretaceous) has a surprisingly high abundance of the element iridium. What does this tell us? Well, most of the iridium in the Earth is in the deeper rocks of the mantle rather than in the crust. This is because ridium is said to be a lithophobic material, which simply means that various geochemical processes lead to it being concentrated in deeper layers in the Earth. Given that, the relatively high abundance of iridium that we find at the KT boundary is surprising. One possible inference is that an asteroid or comet hit the Earth and was disrupted, showering the globe with its material in the form of pulverized dust which was distributed by the winds. Please note one important point: the comet or asteroid is not ultra-rich in iridium, no more than any other object in the solar system. You should certainly not think of it as a ball of pure iridium! -- it has no more than the usual proportion of that element, and is predominantly made up of other things. (Iridium is not a common element, and is vastly outweighed by carbon, silicon, iron, and so on -- the more familiar elements.) But it happens that the Earth's crust is relatively low in iridium, thanks to geochemical processes. Although the Earth, overall, has the expected amount of iridium, it has been chemically captured by material which is now deep in the mantle, leaving the crustal material depleted. The amount of iridium in the KT layer, then, is anomalously high relative to its surroundings, and is strikingly different in this tell-tale respect. The iridium-rich layer has been found world-wide at the KT boundary, which tells us right away that some global phenomenon must be responsible. (If you found only a local effect, you might argue that a small volcano or lava flow from deep layers brought up some anomalously iridium-rich material.) There are a couple of possibilities, the first being the impact of an asteroid or comet perhaps as large as 10 km in diameter. Let us do a quick calculation. For simplicity, treat the asteroid as a cube, 10 km on a side, like a giant squared-off boulder. Since it is 10,000 metres long on each side, it has a volume of a trillion cubic metres (10-to-the-twelfth power). Moreover, since rock is about 3-5 times as dense as water, which weighs one metric tonne per cubic metre, the chunk has a mass of about 3-5 trillion tons. Meeting the Earth from a random direction as it orbits the sun, the lump will arrive with a velocity of perhaps 50 km/sec relative to us. At such a speed, every gram of matter has as much energy of motion ('kinetic' energy) as would be released by 25 grams of TNT. So the moving chunk of rock is invested with as much energy, purely by virtue of its motion, as perhaps one hundred trillion tons of TNT. (The released energy would be somewhat less than I have calculated if the impacting object was a comet rather than an asteroid, because comets are not solid rock and are therefore not as massive in total. But this difference is not very important: it would still be a catastrophic event in every sense of the word.) And this is precisely the problem. When the rock hits the ground, it is brought to a sudden halt -- all its motion is stopped, more or less instantaneously. But the energy does not vanish! Just as a stone thrown at a window leads to the fracturing of the glass, so too the asteroid fractures the ground and itself, sending shock waves through the ground, the air, the oceans, and so on. In fact the energy released would be millions of times that of the combined nuclear arsenals of all the nations on Earth, and the effects would be catastrophic for life. It is worth emphasising that the impact does not have the extra complications we associate with the use of the nuclear arsenals. There would be no radioactive fallout and radiation sickness to worry about, any more than there would be in a bad traffic accident which leaves your car a wreck and you injured. It is the simple release of kinetic (not nuclear) energy that matters, but this is little comfort when power of such a magnitude is involved. I showed you a dramatic video realization of the potential effects of such an impact. The expectation is that there would be immediately destructive effects - big tsunamis, huge forest fires and so on - followed by a long cold nuclear winter because the atmosphere would be almost opaque owing to the dust from the comet or asteroid, the material thrown up from the Earth's crust, and the clouds formed by the water which would boil off the oceans. (The term `nuclear winter' comes from a discussion some years ago about the likely effects of a nuclear war, one of which would be the development of a thick smog above the Earth thanks to the large-scale burning of cities, petro-chemical establishments, industrial centres, and so on. This would lead to a pronounced decrease in the amount of sunlight reaching the ground.) Following the impact, it would not be surprising to see enormous disruption to the food chain, with the mass extinction of many species, in response to such a catastrophe. Gradually, however, the atmosphere would clear and the Earth would return to its usual hospitable conditions. Any surviving species would begin to flourish again. By the way, Hollywood has done a reasonable job of the necessary special effects in the movie 'Deep Impact.' The tsunami hitting New York City is very impressive! Unfortunately, they err in suggesting that we can be saved by breaking up the comet into tiny chunks which rain down harmlessly into the Earth's atmosphere. The total amount of matter would still be arriving at exactly the same speed as before, so the total energy released would be the same, merely distributed over a wider area, and we would still be in very serious trouble.

Did It Really Happen? If So, Where?

The iridium-rich layer does not provide absolutely conclusive proof of the impact hypothesis. Until lately, some geologists have supported the notion of enhanced volcanic activity as the origin of the layer and the cause of the mass extinctions. The obvious example of the impact of comet Shoemaker-Levy onto Jupiter, and its enormous energy release, gives us additional evidence that such a thing can happen, but do not prove that it did. Positive proof would, however, be provided if only we could find some geological trace of the impact site. If the asteroid hit us, where is the crater? While such a big impact would certainly produce a huge mark on the Earth, you must remember that the Earth's constant geological activity would gradually obliterate the evidence. For instance, the asteroid might have hit on a continental margin which later was subducted (pushed under) the edge of the continent by continental drift. As it happens, however, geologists now believe that they have identified the site: the asteroid appears to have landed in the area of what is now the Caribbean Sea, on the edge of the Yucatan Peninsula. The evidence includes characteristic round geological features (crater rings) many miles across on the sea floor, evidence of huge tsunamis which threw enormous chunks of rock high up onto the islands of Haiti and Cuba, and so forth, plus the fact that rocks brought up from the sea floor show evidence of shocks and heating which can be reliably dated (using radioactive tracers) to exactly 65 million years ago. One of the discoverers of this site, Allan Hildebrand, is a Canadian scientist who used to work at the Geological Survey of Canada in Ottawa. In a recent talk at Queen's, he described in graphic terms the effect the impact would have had. In addition to kilometer-high tsunamis and so on, the entire sky above most of North and South America would have been enormously hot because of the ejection of high-speed particles of rock and dust hurled from the impact site. The frictional heating as these rocks pass through the air would make them the equivalent of countless numbers of individual `shooting stars', but their combined effect would have been to make the sky look white-hot. The plants of North and South America would literally have burst into flame and burnt to the ground. Catastrophic indeed!

The Statistics.

It is not very difficult to work out some simple statistics. We know, roughly speaking, how many asteroids of various sizes there are in the solar system -- or at least a minimum number. (More are being discovered all the time, now that serious and regular searches are being made). We also know the sorts of orbital paths most of them follow, and can calculate the way in which these orbits get scrambled by the gravitational effects of Jupiter and, to a lesser extent, the other planets. In this way, we can estimate what the odds are of us being hit by a big asteroid in any given period of time. When you do this, it turns out that on average a big asteroid like the one which caused the KT extinction will hit the Earth about once every hundred million years. So it is not surprising that we have not been hit lately. (Of course, this does not mean that we are guaranteed to be safe for the next 35 million years! The statistic is merely an average: we could be more or less fortunate than this.) Smaller asteroids are more numerous, so they arrive with greater frequency. Allan Hildebrand tells me that his calculations imply that roughly once every three million years we might expect to receive an impact big enough to really strain our capacity to survive as a civilization - in other words, an impact that might cause some of the expected results of a nuclear war (global cooling, flooding, large-scale destruction) but without the catastrophic species-destroying effects of a KT-like impact. An event which causes severe damage over a fairly local region might happen once every few centuries (although, of course, the odds are good that such an impact will occur in the ocean or miss densely-inhabited regions). Finally, of course, the most common `asteroids' are merely tiny pieces of rock - and these hit us every day. They are meteors. We accumulate hundreds of tons a day in this form, without perceptible damage.

What Can We Do About It?

The obvious step is to search for and keep track of all Earth-crossing asteroids so that we can identify problem cases and try to do something before they hit. This is the sort of work that Gene Shoemaker and others do, which indeed is how they found the comet that eventually ran into Jupiter. (By the way, Gene Shoemaker was tragically killed in a car accident in Australia in late 1997.) The point of this is that we might be able to `nudge' a problem asteroid into an orbit where it is no longer a threat to life on Earth, if we are given enough warning. The `nudge' might come, for example, from sending rockets with nuclear explosives on board to the vicinity of the asteroid: an explosion to one side would set the asteroid off in a very slightly different direction, so that (over the millennia) it would miss us by a long way. Other ways of deflecting the asteroid's orbit are possible, and arguably better, but this is a fairly obvious tactic. It is the one which is used both in 'Armageddon' and in 'Deep Impact,' recent Hollywood treatments of the issue. There is no merit in waiting until the threat it is nearly on us! I used a helpful analogy in class. If someone throws a baseball at high speed straight towards your face, tapping it on one side when it is six inches away from your nose will not save you. But if you could give it even a gentle sideways tap early on, preferably just as it leaves the pitcher's hand, then it will probably miss you by a long way. That is the sort of preventative philosophy we must adopt. A final word: it would be wrong to live in acute fear of this sort of fate. Statistically, the chances of a big impact even in the next ten thousand years is very small indeed. But it would be wrong to ignore the problem as though it does not matter. (Remember that piece of video I showed you, where some amateur had caught footage of a big meteor `skipping off' the atmosphere of the Earth and moving back out into space. That would not have been a KT event, but it would have caused severe local damage, with the energy released being equivalent to that of a small atomic bomb.) Previous chapter:Next chapter


0: Physics 015: The Course Notes, Fall 2004 1: Opening Remarks: Setting the Scene. 2: The Science of Astronomy: 3: The Importance of Scale: A First Conservation Law. 4: The Dominance of Gravity. 5: Looking Up: 6: The Seasons: 7: The Spin of the Earth: Another Conservation Law. 8: The Earth: Shape, Size, and State of Rotation. 9: The Moon: Shape, Size, Nature. 10: The Relative Distances and Sizes of the Sun and Moon: 11: Further Considerations: Planets and Stars. 12: The Moving Earth: 13: Stellar Parallax: The Astronomical Chicken 14: Greek Cosmology: 15: Stonehenge: 16: The Pyramids: 17: Copernicus Suggests a Heliocentric Cosmology: 18: Tycho Brahe, the Master Observer: 19: Kepler the Mystic. 20: Galileo Provides the Proof: 21: Light: Introductory Remarks. 22: Light as a Wave: 23: Light as Particles. 24: Full Spectrum of Light: 25: Interpreting the Emitted Light: 26: Kirchhoff's Laws and Stellar Spectra. 27: Understanding Kirchhoff's Laws. 28: The Doppler Effect: 29: Astronomical Telescopes: 30: The Great Observatories: 31: Making the Most of Optical Astronomy: 32: Adaptive Optics: Beating the Sky. 33: Radio Astronomy: 34: Observing at Other Wavelengths: 35: Isaac Newton's Physics: 36: Newtonian Gravity Explains It All: 37: Weight: 38: The Success of Newtonian Gravity: 39: The Ultimate Failure of Newtonian Gravity: 40: Tsunamis and Tides: 41: The Organization of the Solar System: 42: Solar System Formation: 43: The Age of the Solar System: 44: Planetary Structure: The Earth. 45: Solar System Leftovers: 46: The Vulnerability of the Earth: 47: Venus: 48: Mars: 49: The Search for Martian Life: 50: Physics 015 - Parallel Readings.


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