| Solar System Leftovers:
The Nature of Comets.
Chapter 13 of your text provides a lot of discussion of comets and asteroids -- the `debris' of the solar system. Please read that chapter carefully. You will learn in there why Halley's comet is the famous one. As we learned in earlier discussions celebrating
comets, it was the first one recognized as periodic -- that is, an object which returns at regular intervals. Its fame transcends the fact that it is unfortunately not necessarily always very spectacular. In fact, in 1985, at the time of its most recent return, it was really rather inconspicuous from our point of view. This is because when it was closest to the sun, and thus at its intrinsically brightest stage, it was on the far side of the sun from our perspective on the Earth.
The important points, described in more detail in the text, are these:
Despite their grandeur (at times), comets are really puny. They may be only a few km across, and it would take a trillion of them to make up the mass of the Earth. The reason they can look so spectacular is that as they approach the sun they are heated and `boil off' a cloud of gas (the coma ) which surrounds the tiny nucleus (central part). This huge gas cloud reflects a lot of sunlight, and so it is bright. The gas also fluoresces, like a fluorescent light bulb -- in other words, it absorbs ultraviolet light, and then reemits the energy in the form of visible light. This fluorescence mechanism makes the comet even brighter when it is close to the sun. A diagram on page 378 of your text shows the kind of structure it has then.
The tails of comets make them look like they are travelling through space like a snake, with a long tail trailing behind. This is not the case! The tail always points away from the sun, pushed out there by the solar wind. The comet may be moving in some completely different direction.
Comets are thought to be like "dirty snowballs." Imagine a layer of snow falling to depth of, say, 15 cm over all of Ontario. Now roll it up loosely into a ball, collecting dirt and loose bits of gravel with it. This is roughly what a comet might be like (except that it would not contain just frozen water, but also frozen carbon dioxide, methane, and so on).
Since the comets are literally evaporating and losing material every time they come close to the sun, they eventually disappear. The breakup of comets in this way has actually been observed in modern times, and what is left is a bunch of loose `gravel' moving more-or-less along the orbital direction the comet originally had. We will discuss the consequences in a moment.
Since they are low in mass, comets are easily perturbed in their orbits by the massive planets like Jupiter. In this way, a comet which might ordinarily have come in and gone past the sun in a long orbit can experience so strong a tug from Jupiter that it winds up in a completely new orbit, much closer to the sun. Comets like this are quickly doomed, because they evaporate much faster than those that only come in close to the sun once in a while.
Halley's comet has had the good fortune to survive many passages of the sun because it comes in "from below" and never passes very close to Jupiter or Saturn, the biggest planets. But it too is bound eventually to evaporate and disappear. Halley will last perhaps only another 40,000 years.
If comets are short-lived, why are there any left at all? Why have they not all disappeared long ago? The answer is that there must be some source of new comets to replenish those which get disrupted. This source is believed to be an enormous reservoir of comets, the Oort cloud (see p. 380), orbiting the sun out beyond the orbit of Pluto. These little inert lumps would never evaporate or break up, but would merely orbit out there in splendid isolation forever, except that occasionally something perturbs their orbits -- perhaps the extra gravitational effect they feel during the passage of a relatively nearby star -- so that they fall in towards the sun to become the more conspicuous comets that we all think of.
Much of what we know about comets comes from the fact that we were able to send space probes very close to Halley's comet the last time it was here (in 1985-86). That was not easy! Halley's comet goes around the sun in a direction opposite to that of the Earth, so it was simply not possible to send a space probe fast enough to catch up to it and move along with it or land on it. We were unable, therefore, to sample its constituents directly. Instead, probes were sent out to meet it nearly head on -- at very high speed! Since there was no actual collision with the small cometary nucleus, the probes survived to send back pictures, one of which is shown in one of my PowerPoint presentations.
Meteors: Some Basic Definitions.The textbook draws distinctions between meteors (the flashes of light we see in the sky), meteoroids (the small chunks of gravel out in space), and meteorites (any small pieces that survive the passage through the atmosphere to be collected here on the ground). As the small solid pieces enter the atmosphere, their high speed gives rise to frictional heating which causes them and a surrounding column of air to glow incandescently. (What you see from the ground is the glowing air rather than the tiny hot pebble that created it.) That is all that a "falling star" or a "shooting star" really is -- they have nothing at all to do with stars, of course! Most of the pieces are little more than grains of rock, with very few of them even as large as a pea. Of course, some of the infalling rocks survive their passage through the atmosphere. To do so, they have to be fairly big. This automatically means that they are rare, because of a very simple "law" of nature: For every big thing, there are lots of little things. As you can see, this is expressed in pretty informal terms -- which is why I put the word "law" in quotation marks! It's not really a very rigorous scientific statement, but I mean it to be taken quite seriously. In many natural contexts, the statement is strikingly borne out. Consider: for every blue whale, there are millions of people, trillions of ants, and almost uncounatble numbers of bacteria. for every Douglas fir tree, there are thousands of bushes, and millions of blades of grass. for every mountain, there are millions of boulders, trillions of stones, quadrillions of pebbles, and uncountable numbers of grains of sand. [as we will learn in Phys 016] for every very massive star, there are millions of stars like the sun and billions of very low-mass dwarf stars. Of course, this simplistic thinking does not work in every context! For instance, it would be wrong to say that "..for every seven-foot-tall basketball player, there are hundreds of six-footers [like your Phys 015 professor], thousands of five-foot-tall people, millions of four-foot-tall people, billions of three-foot-tall people,..." Obviously they argument cannot be applied unthinkingly! Still, it applies to the meteors. For all the millions of pebbles and bits of dust which fall onto the Earth, there is the occasional bigger chunk which can survive a passage through the atmosphere and make it to the ground. In class, I showed some pictures of the damage which can result when moderate-sized rocks do so. We saw a house with a hole punched through the hall ceiling, a car with a bashed-in trunk, and so on. I also showed some examples of even larger chunks, which would do even more damage. In later sections, I will be focussing on the theme of our fragility here on the Earth. This vulnerability is a result of the fact that we are in a "cosmic shooting gallery," a Solar System within which there are large numbers of quite big rocks still orbiting around, some of which may eventually hit us with catastrophic consequences. But there are a few other less threatening subjects to deal with first.
Meteor Showers.Many meteors arrive at random, and are just individual chunks of rock travelling in varied directions through the solar system. But there is also a phenomenon known as a meteor shower, an event in which we see many meteors all seeming to come from a particular direction in space. (See the figures on page 387.) These are understood to be caused by the Earth's passing through the orbit of a now-defunct comet, so that we pass through the strewn `gravel' left over in the comet's orbit when it was finally disrupted. There is a table of such showers on page 386 of the text. Some of these showers can be very dramatic! Indeed, the Leonid meteor shower in 1966 was particularly heavy, with an estimated rate of about 30 bright meteors per second visible from some locations. The Leonid shower originated from a comet which had an orbital period of about 33 years, so it was reasonable to infer that we might see another rich shower in November of 1999, as we again passed through the clump of material. As I reported in class, that prediction was modestly borne out. Although the meteor rates did not reach the levels reported in 1966, there were sites which reported several thousand meteors an hour -- a couple every second.
Asteroids.Not all meteors falling into the Earth's atmosphere come from bits of gravel left over from disrupted comets, notwithstanding the connection which we have just discussed. In addition to those, there are chunks of rock which were never in a comet at all. The biggest of these are called asteroids. Indeed, collisions between asteroids, or the impact of a small rock onto a larger asteroid, can `chip off' pieces of the asteroid to create yet more small meteoroids. There are ways of distinguishing where the various meteorites originated, using their different chemical compositions as an indicator. Such information proves useful in our developing notions of how the solar system formed and evolved. Keen science fiction readers will remember that there is an `asteroid belt' between Mars and Jupiter. This busy area was featured in a lot of early novels, as pioneering rocket pilots had to swerve and weave to avoid being hit by one of the myriads of asteroids. In fact, the danger is absolutely infinitesimal! Although there are many asteroids, they are still so widely separated in the vastness of space that a collision is extremely improbable. Unfortunately, however, asteroids and meteoritic material are not confined to this restricted region of space, partly because the orbits of such small objects are easily perturbed by the gravitational tugs of the real heavyweight planets (especially Jupiter and Saturn). In fact, there are many Earth-crossing asteroids, objects that come closer to the sun than the Earth is and thus have a statistical chance of running into us. As we will see, the bigger ones of these may represent a real danger to life on Earth. In class, I showed some representative orbits for the so-called 'Apollo' asteroids, which are Earth-crossers. One of these is named Icarus, after the mythological character who glued bird feathers to himself with wax, so that he could fly. He foolishly ignored the advice of his father Daedalus, and flew too close to the sun -- note the appropriateness of the name, therefore! -- with the tragic consequence that the wax melted and he fell to his death. The fact that new asteroids are being found all the time is underscored by the existence of seven asteroids named after the astronauts killed in the explosion of the Challenger Space Shuttle in the 1980's. You may remember that one of them, Christa McAuliffe, was a teacher going into space as part of an ill-fated NASA public-relations exercise.
Meteorites: The Collected Debris.In the lecture, I showed some pictures of meteors which have hit the ground and been recovered. There are actual pieces in the Miller Hall geology museum here at Queen's, some of which were collected by my brother John in the fall of 1994 after a meteor fall near Montreal. In class, I described some of the rich history of this subject, including: the fact that the Kaaba, the rock in the centre of Mecca which is revered in Islam, is at least in part meteoritic in origin; the story of how Peary, the Arctic explorer, took it upon himself to remove to a New York museum a massive meteorite which had some religious or totemic significance for the Inuit people; and the aforementioned pictures of occasional impacts of meteorites into cars and buildings on the ground. One interesting point is that many meteors are quite undistinguished, looking just like rocks. But others can really stand out: they may, for instance, be composed of almost pure iron or nickel, and be easily spotted as anomalous. (This may partly explain why they may serve as religious symbols in various societies.) Of course, they may stand out in other striking ways, such as being unexpectedly present on top of the Greenland or Antarctic ice caps, both of which are proving to be fertile hunting grounds for meteorites.
Meteorites: The Geological Record.We can do more than merely find meteorites. In certain locations, we can identify traces on Earth of meteor falls from millions of years ago. One of the best such areas is the Canadian Shield, a geological region consisting of old hard rocks which can preserve traces of impacts from long ago. (One such impact crater is pictured on page 388 of your text.) In many locations, of course, active weathering and continuing geological activity erase any trace of the impact fairly quickly. Naturally, meteors which fall into the sea leave no permanent record, unless they are big enough to leave a scar on the sea-bed. In class, I presented a tabulation of some of the many identified impact sites. Perhaps the best known one is the Barringer meteor crater in Arizona, a hole which is about a kilometer in diameter and a hundred metres deep. There is no ambiguity, I should reassure you, no danger that we have mistaken an old volcanic crater for an impact site in any of these firmly identified cases. The violent impact leads to the formation of some very characteristic geological features, including what are called ``shattercones.'' Radioactive age-dating of the rocks allows us to figure out fairly precisely when the impact took place. There is, by the way, a meteor crater about 10 km north of Kingston - the Holleford crater. It is now much eroded, so that it is not tremendously striking, but if you know what you are looking for it is still quite dramatic, and surprisingly large.
Tunguska: A Recent Example.In our discussion of the formation of the Solar System, we described how there was a gradual accumulation of gravel into planetesimals and protoplanets. While this was going on, collisions between chunks were of course common, and many of the big craters on the moon and Mercury (for instance) date back to the time when the solar system was full of independently-moving chunks. Nowadays, the chunks are rarer, but impacts are not completely impossible. Indeed, in 1908, a remote area of Siberia was struck by something which levelled trees for miles around and released thousands of times as much energy as the atomic bomb dropped on Hiroshima. Because of the isolated nature of the impact site, little news of it reached the rest of the world, although atmospheric shock waves were felt all around the globe, and colourful sunsets were caused by the dust thrown up by the impact. In the lecture, I presented some figures which give some sense of just how big an impact this was, complete with quotes from reindeer herders who were knocked unconscious or blown head over heels even though they were many kilometers from the impact site. These reports and other anecdotal records were collected years later by scientists visiting the Tunguska site and surrounding territories. (You can learn more about this in the June 1994 issue of Sky and Telescope magazine, available in the University library . The article is very well illustrated, and I encourage you to read it.) Another of my figures demonstrated just how catastrophic it would have been if the impacting body, now thought to have been a small comet or asteroid, had landed in a populated area, such as Manhattan Island in New York City. The picture on page 391 of your text gives some idea of the energy released in such an event, and there is a figure on page 392 which illustrates how infrequently we are struck by objects of sufficient mass to do serious damage - but just how catastrophic those rare events can be. 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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(Tuesday, 20 October, 2020.)