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Chapter
4:
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Chapter Breakdown: |
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"I can hear the sizzle of newborn stars, and
know anything of meaning, of the fierce magic emerging here. I am
witness to flexible eternity, the evolving past, and I know we will
live forever, as dust or breath in the face of stars, in the shifting
pattern of winds." --Joy Harjo |
| "Watch the dust grains moving in the light near the window. Their dance is our dance. We rarely hear the inward music, but we're all dancing to it nevertheless, directed by what teaches us, the pure joy of the sun, our Music Master." --Rumi |
Earth has a lot in common with Mars and Venus. They are small rocky worlds with atmospheres and many familiar surface features that lend themselves to study by terrestrial analogy. Perhaps it is not surprising that we should share many similarities with these two, our closest planetary neighbors. After all, we share a common galactic heredity in the collapsing molecular cloud that became the sun 4.5 billion years ago, spinning off the planets in the swirls of its afterbirth, and we've matured together under the steady incubation of the same slowly warming star.
We also find worlds of difference between Earth and our neighbors, and our young science is still seeking a deep theoretical understanding of many of these variations. Some differences in chemical composition and climate conditions result from their different distances from the powerful influence of the Sun. Yet other differences seem to be due to freak accidents. Several recent discoveries suggest that immense, planet-crunching collisions were common during the turbulent youths of these worlds, and have continued to occur with decreasing frequency over their long lives. Such mammoth collisions may have given Venus its anomalous backwards spin, smashed off a big piece of Earth to make our peerless moon, and possibly created the "crustal dichotomy" which divides the northern and southern hemispheres of Mars.
To what extent are the dominant characteristics of our home planet and its neighbors due to systematic, and therefore scientifically predictable, trends related to their place of origin and evolution, and to what extent are even the most important planetary qualities due to fluke events? The answer might help us unravel our own past, and assess the likelihood that planetary systems elsewhere might resemble our solar system, with Earth-like worlds.
How do planets, and planetary systems, form and evolve? We want a general answer to this question, not one that just works for our own system. We need to know this, to determine whether our condition of existence is one of cosmic solitude, or whether we live in a fertile universe with many companions for us among the stars. But we, stuck here in this solar system and barely able to leave this planet, have only three worlds with which to try to decipher everything.
Pioneer's gift of a more complete description of Venus' atmosphere greatly improved our prospects for doing meaningful comparative planetology of the "terrestrial planets" those small rocky Earth-like worlds of the inner solar system that orbit within 140 million miles of the sun. The other major group of planets is the "Jovian planets", giant gas-balls inhabiting the outer solar system, from Jupiter's orbit nearly a billion miles from the Sun, out to Neptune's, 6 times more distant. When we study the physical evolution of planetary surfaces and interiors, we often include Mercury and the Moon as well as Venus, Earth and Mars among the terrestrial planets. However, for comparative studies of Earth-like atmospheres, it is only the last three that we deal with, since Mercury and the Moon are airless.
In any decent scientific study of a complex process, you would want more than three trials. However, when looking for insight into the nature and evolution of thin atmospheres around small, rocky worlds, we are restricted at present to the three planets within 50 million miles of our present location.1 So lets take a global look, with comparative planetology in mind, at Venus, Earth and Mars.
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Variations on a Theme: FeedbackIf either thing responds to increased activity in the other by acting to suppress or dampen the other's activity, then this creates negative feedback. Negative feedback causes stability because when thing one starts to get out of hand, thing two acts to counter this, pushing the system back towards some point of stability. But this is fairly abstract. Let me give some concrete examples of positive and negative feedback:
My favorite example of positive feedback is the feedback loop that creates the sound commonly known as "feedback". Picture Jimi Hendrix jamming on his upside down white Stratocaster guitar2. Jimi's fingers (or sometimes his teeth) pull at the strings, causing them to vibrate. An electronic pick-up on the guitar turns these vibrations into an electrical signal that travels down a wire into Jimi's amplifier, which does just that, amplifying the signal. The signal causes his speakers to vibrate, making waves in the surrounding air, which have the same frequency as the vibrating strings, but a higher amplitude. That is, the sound has the same pitch but it is louder. OK, that's how an electric guitar works, but where is the feedback? This comes in when Jimi, reaching a critical emotional peak in the song, turns his volume knob all the way up, hits the perfect note and walks right over to his amplifier holding his guitar inches from the speaker. Now something wonderful happens. The sound waves coming out of the speaker are so intense at this point that they will cause anything near them to vibrate at the same frequencies. So the strings on Jimi's guitar start vibrating all by themselves, in direct response to the speaker, no strumming or biting necessary. But every vibration of every string is still sending electronic signals to the amp, and these signals are still being amplified, causing the speakers to vibrate with even greater intensity, which moves the strings ever harder. This is positive feedback. The more the strings vibrate, the more the speakers vibrate. The louder the sound from the speakers, the more they move the strings. The amp goes crazy. Obviously it can't keep getting louder forever, or it would shake the whole Earth apart. Maybe at times you thought this was happening, but it didn't, did it? The volume eventually reaches a peak because some component in the amp is maxed-out and can't get any louder. Now the tubes are really humming, rich with resonance yet on the edge of dissonance, tense yet infinitely melodic. You shouldn't necessarily try this at home. The results can be quite grating, but in the hands of someone like Jimi who has mastered technology in the service of art, it is one of the most powerful and beautiful sounds the world has ever known.3
How about negative feedback? The common example is a thermostat. Some element senses temperature and turns a heater on, or a cooler off, if it gets below a certain point. This leads to stable behavior: the temperature hovers near the critical point where the thermostat is set and doesn't deviate greatly from it. If you made a mistake and wired your system backwards so that the heater turned on instead of off when it was above the critical temperature, you would get positive feedback and it would soon get way too hot. But here is a somewhat less prosaic example of negative feedback:
There is a short story by Thomas Pynchon called "Entropy" in which there is a party going on. One of the party-goers gets too intoxicated and heads for the shower to revive himself. He manages to turn on the water but then passes out in the shower, sitting right on the drain. The water starts to rise around him, but he is not in danger of drowning. Once the water reaches a critical level, he becomes buoyant and floats off the drain. The water runs out until the level is too low for him to float and he again blocks the drain, causing the level to rise once more. This is a negative feedback loop in which the level of water is regulated near a critical point defined by the buoyancy of our wasted anti-hero. His behavior may be a bit unstable, but the opposing forces of his buoyancy and gravity make a negative feedback loop with a stabilizing effect on the level of the water which should eventually revive him.
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"Forgive me for the banality of this reflection,
but there is something very wrong with the human race"
--Doris Lessing, Under My Skin |
At this point in our history, when we are just beginning to detect other planetary systems around nearby stars, learn something of their nature, and fantasize about someday exploring them up close, it is fun to try to imagine what outsiders might notice about our own system upon inspection.
The presence of atmospheres on the three largest terrestrial planets and the unusual nature of the Earth's atmosphere would be obvious to the casual extraterrestrial observer when watching at many wavelengths over a range of timescales. The blinding reflectivity of Venus would alert them, when they were still far from the Sun, to the presence of a thick atmosphere and global clouds on that world. They would have to come in closer to detect the more subtle atmosphere of Mars, by noticing that the limb (the edge), of the red planet appears fuzzy and that over a Martian year the polar caps of frozen CO2 grow and shrink with the seasons.
Seen in visible light, Earth's atmosphere reveals itself by its lovely hazy blue against the blackness of space. Its lively weather and life-sustaining temperature range are revealed in the rapid and complex daily movements of condensed water clouds across the globe. An infrared comparison, revealing a CO2 atmosphere on two out of the three planets, might be their first clue that something really strange is happening on Earth. Patient aliens observing over Earth's history with infrared eyes might have noticed a long-term decrease in CO2 content. Turning their attention to the ultraviolet, they would note a dramatic increase in ozone and other oxygen compounds over the last two billion years. The resulting extreme departure from chemical equilibrium might alert them to the fact that Earth is inhabited or perhaps even, by their definition, alive.
Yet, a puzzling set of rapid changes in the last few decades might cause them some concern about the nature of this life: they would see the clouds of Earth becoming acidified, the unique stratosphere being chemically eroded, and the anomalously low CO2 level being slowly pumped back up towards "normal" for this solar system. The conclusion is obvious. The inhabitants of the third planet suffer from acute Venus envy. They are doing their best to transform their world into one that more closely resembles their sunward twin.
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"One world is enough for all of us."
--Sting |
The Earth seems to be, in many respects, a most unusual planet. But, one must be careful with such statements. Aren't we awfully close to home here? We are awfully proud of our vast oceans, but a Martian might feel that an oxidized red surface is where it's at, and a Venusian might think that global sulfuric acid clouds are the coolest thing. Who are we to say that our world is so special? It's good to keep this inherent lack of objectivity in mind. Nonetheless, a planetary comparison with at least attempted lack of bias does reveal a number of striking features found, at least in these parts, only on our home world.
Our atmosphere stands out with respect to both its composition, and its role in the complex system of chemical, radiative, geological and biological feedbacks known these days as "the Earth system". It is irresistible to ask which of Earth's other apparently unique features are related by cause and effect to this oddball atmosphere, and which way the causality might go (which came first). Among these distinctive features are: an active hydrosphere, plate tectonics, one giant moon, a strong intrinsic magnetic field and, of course, life.
At first glance, it seems strange that one small planet should contain so many oddities. But are these really independent developments? If some, or all of them can be attributed to the same causes, then this would seem less unlikely. How is Earth's unique, biologically altered atmosphere related to these other distinctive planetary qualities? Well, for one thing our unusually large moon may have helped maintain a climate within "healthy" limits for life. Mars has two puny moons but nothing like ours. As a result, its spin has wobbled like a top, changing the angle of sunlight at the surface and causing extreme climate oscillations over millions of years. Our moon seems to have protected us from this hazard with the stabilizing hold of its gravity. Also, the giant impact that formed the moon in the later stages of Earth's formation probably had important effects on Earth's subsequent evolution. As far as we know, no other planet suffered such an insult and lived to tell about it (more on this a bit later).
What about Earth's active hydrosphere, the endlessly cycling water of our world that rains, runs, and evaporates to rain again? It is clear, for reasons discussed above, that the maintenance of Earth's climate in a suitable range for liquid water is closely linked with the compositional evolution of our atmosphere. Since liquid water is absolutely crucial for "life as we know it", it is safe to say that atmospheric evolution has kept the Earth alive. But how has life itself contributed to this evolution? The biosphere and atmosphere have evolved together in a most intimate fashion. But, has life merely adapted to, and passively contributed to, the changing atmosphere? Or has the atmosphere been actively regulated by and for life, as the Gaians argue?
Whether the atmosphere and biosphere have had important effects on the evolution of Earth's possibly unique (at least in this solar system) style of plate tectonics is even less certain. However, two reasons that have often been proposed to explain a divergence in tectonic styles between Venus and Earth are (1) the likely lack of water in Venus' crust compared to Earth, which affects the physical properties of rocks, and (2) the large difference in surface temperature between these worlds. (We will discuss these ideas in more detail in the next chapter.) So, to the extent that life on Earth has affected, or possibly controlled, atmospheric and climate evolution, plate tectonics itself has conceivably been biologically modulated.
If you agree to that, you must agree that the Earth's interior thermal evolution has been affected by its changing atmosphere and biosphere, because plate tectonics is the main way that Earth cools its interior. Even such remote quarters as the molten iron outer core, which produces Earth's singular magnetic field, may not have been immune to the modifying effects of earth's quirky air, its unique, biologically touched, gaseous envelope.
There may be only one thing that is strange about the Earth, with all the others following suit. But how are we to know? We cannot examine the Earth's uniqueness without looking elsewhere. For we who wish to understand worlds, one world is NOT enough for all of us. Did Venus and Earth start out as identical twins, only to go their separate ways through life, or were they different right from the start? The best way to know would be to ask their mother. So let's take a look at our theories of the birth of planets.
| "You are dust" --Genesis |
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"We know where
we're going cause we know where we're from." --Bob Marley |
The planets of our solar system were born, 4.5 billion years ago, of the solar nebula, a flattened, spinning disk of dust and gas surrounding the young sun. For centuries, since Immanuel Kant and Pierre Laplace first described them in the 1700's, these planet-spawning disks have existed as theoretical, almost mythical entities. We cannot directly observe the local events of such an ancient epoch. Here we can only sift through the ashes, searching for clues. But stars are being born all over the galaxy, and we can observe similar stages of growth in some of our near neighbors. Over the last decade, thanks to the Hubble Space Telescope and impressive advances in ground-based astronomy, proto-planetary disks have become real objects, commonly observed around nearby young stars. This observational confirmation has been a major shot in the arm for theorists of planetary formation. As we long suspected, wearing a huge, flat, dusty disk is a phase that all, or at least many, young stars go through. It is a phase we went through. Every atom of your body, of this book, of Earth, used to be a part of the solar nebula disk. The disk didn't last long. Small bits of it rapidly coagulated, accreting into bigger bits, successively forming larger and larger objects until there were only planets and debris, and no disk left. In less than 100 million years it was all over. We are still searching for definite signs that planets are forming within the disks we see around other stars.
From its inception planetary science sought general laws that could predict properties of planets from "first principles" based on such variables as size and distance from the sun. We would like a theory that can tell us what kinds of planetary system should develop, given some information about the "initial conditions", the circumstances of birth. One such optimistic theory "equilibrium condensation", was first proposed in 1972. It explains how varying conditions across the disk of the solar nebula became frozen in as hereditary differences among the planets. Picture the temperature distribution in the solar nebula: The inner part of the disk, near the new sun, is quite hot and everything is vaporized. At the outer fringes, far from the sun, it is so cold even gasses like nitrogen and methane, can freeze. In between these two extremes there is a gradient in temperature, a trend of cooling with distance from the sun. The disk as a whole is also gradually cooling as the young sun settles down and planets begin to form.
According to the theory of equilibrium condensation, different types of material condensed out of the solar nebula at different temperatures, and therefore at different locations. Imagine that you jump into a wormhole, travel back in time4. billion years and re-enter the space-time continuum. If you entered close to the new sun, things would be quite hot in the nebula. Around the present-day orbit of Mercury, it would be well over 1000 degrees, and you might want to high-tail it away before your shields gave out. As you gained distance on the sun, the temperature outside your ship would steadily fall, and on your view-screen you might notice that it was snowing outside. As you watched, and traveled towards the cooler regions of the nebula, you would see the snow changing colors and form.
This imaginary trip of time and space travel is just like a drive up a high mountain on Earth. As you rise past a certain altitude it starts snowing outside your car. You have passed the "snow line" where it is colder than the freezing point of water vapor. At temperatures cooler than this the thermal energy of motion in the water vapor is no longer strong enough to resist the forces of mutual attraction between water molecules. Water molecules in the air gather together and form crystals. It snows.
The equilibrium condensation theory says that a planet at a given distance from the sun has a specific composition (is made out of certain stuff) for the same reason that the snow line occurs at a specific altitude on Earth's mountains. But the nebula contains many different kinds of vapor, not just water, each condensing at a different temperature, so there are many "snow lines". More refractory materials (those that melt and vaporize only at high temperatures) such as metal oxides and some silicate (rocky), materials start to snow out in the hotter inner regions of the nebula. More volatile ( easily melted or boiled) substances such as hydrated silicates (rocks with chemically bound water) and ices begin to condense only farther out where it is colder. When these "snowflakes" of metal, rock and ice later accreted, gathering themselves together to form growing worlds, the compositions of the resulting planets preserved this trend.
According to this theory, planets should become less metallic and icier as you go farther from the sun, and the mineral composition of the rocks should also vary predictably among the planets. Early advocates of this simple model pointed out that it did a pretty good job of explaining density differences among all of the planets. For example, Mercury is the closest planet to the sun and is also the densest. Equilibrium condensation explains this with the large amount of metal which would have condensed preferentially in the high temperature zone near the sun. The theory also correctly predicts that Mars should be significantly less dense than Earth, because it formed farther out and incorporated more lighter volatile materials. Since Venus formed so near to Earth, the theory predicts that their densities should be similar, which they are. This also might mean that Venus started out drier than Earth. If the proto-Venusian material was hotter, more baked by the sun, it might have received proportionately less water in the forms of ice or water-rich minerals. This is of obvious interest for our efforts to understand the divergent atmospheric evolution of Venus and Earth. Did Venus start out substantially drier?
But, as we've learned more about planets, they've grown harder to explain. The search for simple, profound laws of planetary evolution has been thwarted by our growing knowledge of how lumpy and bumpy, full of quirks and oddities, this solar system is, and how messy and disorderly the process of planet formation may have been.
A theory like equilibrium condensation requires a fairly orderly process of planetary accumulation to maintain distinct chemical zones while planets are growing. After solid grains condensed out of the nebula and began to collide and grow into larger bodies, did these "planetesimals" stay in well-behaved, nearly circular orbits, like runners confined to their lanes at a racetrack? This would preserve the planet's chemical differences as they grew. But computer models of planetary growth raise doubts about this.
These models track the positions and motions of thousands of particles and record the outcome of their interactions. In high-velocity collisions the particles fragment, making more new particles to be tracked. Particles colliding at low velocity stick together and make larger ones. This is how planetary growth starts. The numerous near-misses alter orbits, as the participants respond to one another's gravitational pull. That's a lot of information to take account of, so a realistic simulation requires a fast computer with a lot of memory.
As computers have become better and faster, our models of planet formation have improved rapidly, including a larger number of planetesimals and more realistically simulating their interactions. In the 1980's the new improved models started pointing consistently in one direction. Planetary growth was not orderly. It was messy and unpredictable. As planetesimals grow, their gravitational influence on other bodies also increases rapidly. When planetessimals are nearly grown into planets, when they reach around a thousand miles in diameter, things really go crazy. Their mutual gravitational interactions pull them into wild elliptical orbits and scatter them throughout the solar system. If this is a racetrack, the runners have all been dosed with psychedelics. They have forgotten the race and abandoned their lanes, and they are now swinging each other around and careening about the whole field.
And so gas begot flakes, and flakes begot clumps which begot lumps. Lumps gave birth to boulders, and boulders were small worlds unto themselves. Boulders were fruitful and joined to make bigger boulders the size of cities. And then were born the planetesimals. And near the sun, in its great heat, planetesimals were metal. And farther out they were rock. In the farthest reaches where cold and darkness covered the nebula, planetesimals were ice. And everything was ordered and good.
And then along came mutual gravitational perturbations and screwed everything up, and chaos and confusion reigned for 10 million years.
If these model results are correct, there are three important consequences for planetary origins. First, there must have been a lot of mixing among the planetesimals that made the terrestrial planets. Simple, smooth compositional trends like those predicted by the equilibrium condensation theory are out. In the recipe for making planets, one of the last steps may have been "stir well until thoroughly mixed", so they should all be made of basically the same stuff.
Second, the details of planetary formation may be impossible to predict from first principles. Any surviving differences in composition between the planets may be due to random collisions and orbital perturbations during planet formation. The density differences among the terrestrial planets, in this view, are just the result of incomplete mixing, like lumps in batter. It may all come down to luck, the throw of the protoplanetary dice. In one famous simulation result, Mercury actually formed out near the orbit of Mars and was later gravitationally perturbed to its present location.
Third, the final stages, just before planets reached their present sizes, would be characterized by colossal collisions. Each terrestrial planet would have experienced a terrible pummeling by huge bodies thousands of kilometers across - a sort of rite of passage, a cosmic hazing before matriculating into full fledged worlds. Such collisions would have had huge effects on the environments and physical evolution of the newborn worlds.
This last idea, a new realization of the important role that giant, catastrophic collisions may have had in determining the character of worlds, dovetailed in an interesting way with another important intellectual development that happened around the same time. This was a wake-up call of the wild that came 65 million years ago, but one we did not hear until the 1980's.
| "You must have chaos within you
in order to make a dancing star." --Friedrich Nietzsche |
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"You're not supposed
to be here at all. It's all been a gorgeous mistake." --Sinead O'Connor, "Jump In the River" |
It seems that we will never have a theory predicting in detail, how a solar system arises from a disk. Just as we can predict, when we roll dice, that a seven will come up a lot more often than a three, we will learn some general principles guiding the likely evolution of planetary systems. But, just as we have no power to say what the very next roll of the dice will bring, we will continue to be surprised by the result when the planetesimals roll.
Like a jazz tune where the chord changes are charted but the actual notes and their timing are left to the impulses of the moment, there may be a predictable sequence of changes on the path from disk to planets. But, just as the exquisite spontaneity of jazz arises from interactions among the players, interaction among numerous orbiting and gravitating bodies seems to lead to complexity and chaos, and unpredictable behavior.
Maybe you wonder why I have waited so long to introduce the terms "complexity" and "chaos". After all, I have been discussing the inherent lack of predictability in the process of planet-building. Isn't that what "chaos" is all about? Yes, sort of. But, the ascendancy of large impacts in planetary science started a few years before "chaos" and "complexity" became the big buzz-words they are today. Now the "chaos" inherent in complex systems is used to explain irregular heartbeats, stock-market vicissitudes and climate fluctuations. But planetary scientists did not need chaos theory to conclude that, if huge impacts were important in early planetary history, the chance location and timing of these events would have introduced a lot of randomness, a lot of contingency into the histories of the planets. That is, the details are dependent on when or whether specific events happened not, as we have discussed, on nice clean and reliable evolutionary principles.
But, if the random differences were due merely to this contingency, then they were still, in theory, predictable. One could imagine a computer powerful enough to keep track of all the orbiting, gravitating, accreting bodies, and a program good enough to predict every orbit, every collision and near-miss, and the outcomes of all these events. Such a computer could predict every impact event and, therefore, numerically predict the detailed evolution of a planetary system. What chaos theory tells us, though, is that it doesn't matter how good a computer you have, that complex systems like this are inherently unpredictable. The weather on Earth is another example of a complex system, and "the butterfly effect" is often invoked to describe its inherent unpredictability: a butterfly stirring the air today in Mexico City could affect the weather next week in Boston. Such "sensitive dependence on initial conditions" arises in physical systems that are complex, meaning that they are composed of numerous parts that all interact simultaneously, creating mutual feedbacks with unpredictable "chaotic" results. These systems are also called "nonlinear", in reference to the kinds of equations needed to describe them.
Chaos is inherent in the formation of a planetary system, with its many orbiting bodies all tugging on one other. And there is probably also chaos in the evolution of individual planets. A planet's interior contains heat that wants to get out, the heat left over its formation and augmented by natural radioactivity. Like a pot left to simmer on a stove, this heat leads to the churning, overturning and self-stirring that we call convection. Computer simulations of convective planetary interiors reveal that this process is often chaotic. The tiniest of differences in initial conditions can lead to very large differences later on. The surface geology of a planet often reflects the pattern of convection below, plate tectonics being the local example. Atmospheres are also convective systems, and their motions (the weather) are the epitome of chaos. On a longer timescale, a planet's climate depends on feedbacks between surface, atmosphere, oceans and clouds. Feedbacks and the nonlinear relationships they create often generate chaos. These are all ways, and they are not the only ones, that chaos can influence the evolution of an individual planet and make it impossible for us to predict the details of planetary evolution from deterministic principles and initial conditions.
What all this means is that if the experiment of the solar system were
allowed to run again starting from the same point, things could have gone
very differently. Given random differences in conditions of origin due
to the tumultuous final phases of planet formation, given the contingency
on large random events early in planetary evolution, and given the chaos
inherent in planetary formation and later evolution, we must ask: If the
solar system had its life to live over again, would we find anything like
our present Earth? This question should remain with us as we model, and
search for, the development of Earth-like worlds around other stars.
1 Also, Titan, a large icy moon of Saturn, has a thick atmosphere with many interesting parallels to those of the terrestrial planets.
2 Jimi was left-handed, but when he was learning to play did not have access to a left-handed guitar, so he held the guitar upside-down and strung it backwards, learned all the chords and scales upside down, and played that way his whole career.
3 Shows you how beautiful technology can be, right up there with planetary exploration, heart transplants and the Golden Gate Bridge.
4 I recently read about a woman in Israel who sued a local TV station for an inaccurate weather prediction. Most people have an intuitive sense of the chaos inherent in the Earth's weather systems, but perhaps such forecasts should carry the legal disclaimer: "weather predictions are subject to sensitive dependence on initial conditions".