29. Meteors, meteorites, asteroids and comets 1

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1 29. Meteors, meteorites, asteroids and comets 1 The solar system contains one star, 8 or 9 planets (depending on whether we count Pluto as a "real planet"), some hundreds of thousands of minor planets (asteroids), many smaller bodies (meteoroids and comet nuclei), and dust grains, all orbiting the sun. All this "solar system junk" provides us with important clues to the origin of the solar system and to the processes that have altered it since then. Many of these also provide us with interesting and unpredictable "events" in the night sky. Comets A comet is a small body (the comet nucleus) that is composed entirely or in part of volatile material, so that when it comes close to the sun it develops a big atmosphere (the coma) and one or two tails: a dust tail and an ion tail. (Volatile in this context means that it evaporates easily. Water is volatile; iron and rock are not volatile.) Comet nuclei are typically ten or twenty kilometers across, and are generally not perfectly round. The nucleus of Halley's comet, the only one actually photographed (by the European Giotto mission in 1985/86), is about twice as long as it is wide. One may think of a comet as a giant dirty snowball (mostly made of ices) or a snow-covered mudball (mostly mud with ice on top). When the comet nucleus comes close to the sun, the "snow" (water ice, methane ice, ammonia ice) evaporates and forms a big cloud around the nucleus; this cloud is the coma. There are dust particles in this cloud; sunlight pushes these particles away from the comet, forming the dust tail. (Yes, sunlight does the pushing -- light has momentum and can make things move, if there is enough light and the thing being pushed has low mass per unit area exposed to the light) There are also ions in this cloud, and the solar wind with its magnetic fields sweeps out these charged particles, making the ion tail. One can tell the dust tail from the ion tail because the dust tail is fuzzy and curved, while the ion tail is usually lumpy and straight. Both tails point away from the sun at all times, so comets sometimes move tail-first! (In this sketch A is a cloud of hydrogen surrounding the coma, B; deep within B is the nucleus, invisible from Earth because it is too small and dim to see directly. C is the ion tail and D is the dust tail.) EXERCISE On the picture above, indicate the direction to the Sun. Are there any clues to the direction the comet may be moving? Describe them, and what they suggest could be the direction of motion of the comet.

2 29. Meteors, meteorites, asteroids and comets 2 Do comets ever collide with Earth? There was a comet visible during Fall 1992, comet Swift- Tuttle, that is predicted to have a 1/10,000 chance of colliding with Earth next time around (in 2126 AD). Although that may seem like good odds for us, it actually represents a pretty close call: it means that the comet will pass within about one hundred Earth-radii of Earth, while most comets miss us by hundreds of thousands of Earth radii. If it does hit, what will happen? In 1911, the Earth passed through the tail of Halley's comet, with no ill effects. So we are OK unless the nucleus hits us. In 1908 an explosion over part of Siberia near the Tunguska river flattened trees over more than a thousand square km of forest. This was probably caused by a piece of a comet (about 1 km across, or less than 1% of the mass of the nucleus of a comet) hitting the Earth's atmosphere and vaporizing, creating an explosion with about the same energy as a 10 megaton bomb. (To compute the energy of an impact: E = 1/2 m v 2 where m is the mass of the incoming object and v is its speed. Typically, an object from space will hit earth with a velocity between the escape velocity from Earth km/s -- and the orbital velocity of Earth -- about 30 km/s. Objects hitting at 11 km/s are in orbits similar to ours; the higher speed of impact comes when we run into things that are falling into the sun.) Earth's orbit object falling towards the sun Earth object that will fall to Earth with impact v around 11 km/s Comet Swift-Tuttle is associated with the most spectacular meteor shower (see below), the Perseids, visible in mid-august every year when we pass through the comet's orbit and run into lots of left-over debris. Since the comet recently passed through this part of its orbit, the Perseids should be spectacular for the next year or so! This summer, a comet will collide with Jupiter. This comet, Shoemaker-Levy 1992 (one of several comets named after Carolyn Shoemaker and David Levy, because they are very good at discovering comets), came rather close to Jupiter in early This "near miss" had two effects on the comet: (1) It caused it to break up into a couple of dozen small pieces, and (2) it captured it into a sort-of orbit around Jupiter, with a period of about 2 years. In this "orbit" it will return to Jupiter and collide with it in July, 1994; the biggest fragment will hit on July 20, on the 25th anniversary of the first time humans walked on the Moon. While the effect will be to release an amount of energy comparable to a number of hydrogen bombs exploding in Jupiter's atmosphere, it will be hard to see the effects of this collision from Earth, because Jupiter is entirely gaseous inside (so the fragments will deposit their energy well under the surface) and because these fragments will hit Jupiter on the side away from Earth. A comet nucleus loses a lot of material every time it comes near the sun. After ten or twenty perihelion passages, the comet will have little volatile material left, and it will be "but a shadow of its former self." Thus if a comet is in an orbit that brings it close to the sun once every century or so (like Halley's comet, 78 years, or Comet Swift-Tuttle, 134 years), after a couple of millennia it will be mostly gone. The solar system is much older than this, so how come there are still comets? There are, because "new comets" arrive from the outer solar system quite regularly. The short-period comets that are so famous, like Halley's comet or Swift-Tuttle, are comets that came

3 29. Meteors, meteorites, asteroids and comets 3 in from much farther out and then passed close enough to one of the giant planets to have their orbits altered to ones with periods less than about one hundred years. The orbits of comets are very eccentric ellipses. We will only see a comet with a good, bright coma and tail if the comet nucleus comes within about 1 AU of the sun. A few comets have orbital periods less than a century; these comets' orbits have semi-major axes (from Kepler's third law!) of less than about 20 AU. However, most "new" comets have orbital periods of tens of thousands of years or more, so their semi-major axes must be at least 500 AU. From this clue, and the fact that "new" comets appear regularly, it is possible to calculate the number of potential comets "waiting" in orbits far from the sun. This storehouse of future comets is called the "Oort cloud", and represents the outer boundary of the solar system. In fact the Oort cloud is about 1/3 the distance to the nearest star, and the gravity of passing stars is thought to alter the orbits of new comets so that they fall into the inner solar system. Minor planets or asteroids A minor planet, or asteroid, is an object that is smaller than a planet, and that does not develop the kind of atmosphere and tail that a comet produces. Thus minor planets are either composed of less volatile material or they are in orbits that keep them far enough from the sun at all times that they never develop a coma or a tail. The biggest minor planets are 1 Ceres, 940 km in diameter, 2 Pallas, 540 km, and 4 Vesta, 510 km. The larger asteroids are spherical, and are held together by gravity, just like the major planets. The smallest ones are held together by the strength of the material, like giant boulders, and are not spherical because their gravity isn't strong enough to crush the rock to make them round. The first minor planets that were discovered were found because people were looking for "the missing planet" between Jupiter and Mars. Why "missing planet?" Well, there is a cute little formula that fits the orbits of the first 6 planets pretty well. It is called the "Titius-Bode' law or sometimes just "Bode's law". If Mercury = #1, Venus = #2, Earth = #3, and Mars = #4 then the orbits of all four of these planets match up fairly well with the following recipe: Write 0, 3, 6, 12, 24, after the first two numbers, each number is twice the one before. Add 4 to each number: 4, 7, 10, 16, 28, Divide the result by 10 to get an estimate for the semimajor axis. Check it yourself: Planet actual predicted Mercury = # Venus = # Earth 1.00 Mars 1.6 Jupiter's orbit has a semi-major axis of 5.2. However, the Titius-Bode law for #5 predicts an orbit size of only 2.8 AU., while for #6 it predicts 5.2 AU. #7 doesn't do too badly for Saturn, the sixth planet, and #8 isn't bad for Uranus, the seventh planet. So it works rather well except that there is no planet in the position that Bode's law predicts planet #5 should be. If Mars is #4 and Jupiter is #6, where is #5? That was the puzzle that sent people searching for a planet between Jupiter and Mars. What they found was a number of much smaller bodies -- the minor planets -- residing between Mars and Jupiter. About 5000 asteroids have been detected, although not all of these have been studied well enough yet to have orbits and other properties catalogued. Present estimates are that there are probably about 100,000 asteroids that could be detected by existing telescopes (that is, with diameters down to about 1 km). Most (>99%) if not all of the asteroids with diameters greater than 100 km have been catalogued. Asteroids come in different types, corresponding to different composition. One clue to the composition of an asteroid is its albedo, A, which describes how easily its surface reflects light.

4 29. Meteors, meteorites, asteroids and comets 4 The fraction of light reflected is A, and the fraction absorbed is 1-A. So A = 1 means all the light is reflected, and the asteroid appears bright, and smaller A means the asteroid has a darker surface. The asteroids with the highest albedos are probably covered by some kind of snow or ice; those with the lowest albedos are probably covered by, or composed of, dark rock or metal. EXERCISE How bright an asteroid appears in your telescope will depend on three things: how far away it is, how big it is, and its albedo. The asteroid 3 Juno is the third brightest asteroid but only the tenth-largest. The asteroid 4 Vesta is the fourth brightest and the third largest. They are both at about the same distance from us. Which has a larger albedo -- Juno or Vesta? Could the asteroids have once been part of a planet that was destroyed somehow? This was once a popular science fiction idea. It does seem likely that at least some of the asteroids originate from some larger bodies that were broken up by collisions, because otherwise it is very hard to explain the variety of composition of the asteroids. However, in spite of the large number of asteroids known or suspected to exist, the total mass of the asteroids is quite small -- about 1/200,000 times the mass of the Earth. Even if all the asteroids were to be assembled into one body, it would not be big enough to count as a planet; it wouldn't even be twice as big as Ceres! Why are there a few asteroids between Mars and Jupiter instead of a planet? Several possible explanations, all of which probably are important at some level: (1) Bode's "law" is not a simple consequence of known physical processes. What we do know is that when there are several planets in a solar system, these will influence each other's orbits in such a way that pairs of the planets will tend to have orbital periods that are related simply, such as P1/P2 = 4/7, or 3/5, or 5/8. No matter what spacing there were between the planets, it would probably be possible to come up with some sort of recipe for describing them numerically, given this tendency for pairs to have simply related orbits. So we should probably not worry too much about the "missing planet"; it isn't really "missing". (2) The gap between Mars and Jupiter is also the boundary between the terrestrial (small, dense, rocky) planets and the Jovian (big, low density, hydrogen-rich) planets. This hints that in the inner solar system conditions were right for forming rocky planets; in the outer solar system, conditions were right for forming giant planets. Perhaps between Mars and Jupiter conditions weren't right for forming planets at all. (3) The asteroids we now observe may be only a small fraction of those that were in the system when it formed. When there are many small bodies in orbit around a bigger body, these smaller bodies will also interact through gravity. Unless they are arranged in some very special way so that these effects cancel out (as may be the case for the planets), these interactions will cause changes in their orbits. Because of this, from time to time two will come close enough so that their mutual gravity will be strong enough to really change their orbits. This may send one of them plunging towards the sun and eject the other one from the system altogether: Before After Both interactions among asteroids (pictured) and between asteroids and Jupiter can remove asteroids from the asteroid belt between Mars and Jupiter.

5 29. Meteors, meteorites, asteroids and comets 5 Do asteroids ever hit Earth? What happens then? A 10 km asteroid hitting Earth would create a big crater, cause devastating tidal waves and/or extensive fires (depending on where it landed), and probably trigger a lot of volcanic activity. It would also affect our climate for some years, causing a perpetual winter because so much dust and soot would be in the atmosphere that there would be much less sunlight reaching the surface. It is now generally thought to be most likely that such an impact accounts for the extinction of the dinosaurs (and a lot of other species!) about 65 million years ago. Meteors, meteorites, meteoroids and meteor showers A meteor is what we see when we observe a streak of light in the sky at night. It is caused by anything from a speck of dust, a grain of sand, or a snow-flake-sized bit of ice up to a severalpound chunk of rock, ice, or metal entering our atmosphere with a speed of ten or twenty kilometers per second. Most are really small particles that vaporize completely from the friction as they zip through the atmosphere. Some bits of dust from meteors survive and slowly settle to the ground. Vaporized meteors may have contributed quite a lot of the water in our oceans, according to calculations by L. Frank at the University of Iowa, although this suggestion is not (yet?) generally accepted by scientists working in this field. A meteor shower is an occasion when many more meteors are seen per hour than on a normal night. Meteor showers occur when Earth passes through the orbit of a comet. The meteors of a meteor shower all appear to come from one spot in the sky, called the radiant. This is because they all come from the same direction in space, and the "illusion" that they come from a point in the sky is exactly the same as the illusion that railroad tracks converge to a point in the distance. A meteoroid is a meteor waiting to happen -- that is, it is a chunk of material in space that, if it encounters Earth, will become a meteor. A meteorite is what we call any chunk of rock or metal that has come through our atmosphere and fallen on the surface of the Earth. Most of those that are found by people who didn't observe them fall are composed of nickel and iron in a blend that doesn't occur naturally on the surface of the Earth. Thus they are easy to recognize as "unusual", and they are also very tough, so they survive a long time after they fall. However, if we consider only those meteorites that are picked up by people who observed them falling, then most are actually rocky or a mixture of rocky material and nickel-iron. Such meteorites are much harder to distinguish from ordinary rocks, and are much more fragile than the nickel-iron ones, so they are harder to identify once they fall on the ground. There is one place on Earth where any rock on the surface is likely to have fallen from space, and that is the Antarctic (and Arctic) ice fields. Essentially all the rocks on the surface there are meteorites, and collecting Antarctic meteorites has greatly increased the number of rocky meteorites available for study. The rocky meteorites, or chondrites, are particularly important for the investigation of the early solar system, for several reasons. First, some of these contain material that is 4.6 Gyr old *, i.e. material that solidified into its present state as part of the process that formed the solar system. It is these meteorites that give us our best determination of the age of the solar system. * When we say that a meteorite or a part of a meteorite is 4.6 Gyr old, what do we mean? We don't mean the age of the individual atoms; these were presumably formed in stars that "died" some time before the material stuck together to form the meteorite. We also generally don't mean how long ago the meteorite reached its present size and shape; that would mostly just tell us how long ago they fell. What we do mean, for the meteorite as a whole or, more often, for a chondrule or grain in the meteorite, is how long ago did this material solidify and/or stop being involved in a lot of chemistry. It is this kind of age that is measured by radioactive dating techniques.

6 29. Meteors, meteorites, asteroids and comets 6 The chondrites get their name from the fact that when one looks at a very magnified picture of the inside structure of one of these, one sees a lot of small spheres called chondrules. (There are pictures of various types of meteorites in the textbook.) The chondrules are the oldest bits in meteorites. They include even smaller bits of material that are thought to be very similar to the grains that are observed to be scattered through space between the stars -- interstellar grains. Interstellar grains are observed through their effects on starlight -- they scatter light from stars, and they scatter blue light more than red light, so they make stars look redder and fainter. (The same thing happens to sunlight passing through Earth's atmosphere -- blue light scatters and makes the sky look blue, red light doesn't get scattered as easily and so the sun looks red at sunset or sunrise.) Interstellar grains also polarize starlight -- sort of like putting Polaroid sunglasses between us and the star. By studying how starlight is reddened and polarized and comparing that with how particles in the laboratory or Earth's atmosphere affect sunlight, it is possible to figure out how big and roughly what shape typical interstellar grains are. By counting what kinds of atoms there are in the places where grains are formed (near some red giant stars and in space itself), and by considering which substances become solid most easily, it is possible to make an educated guess as to what interstellar grains should look like. The best guess is that many are carbon in some form, while others are silicon compounds and mixtures (silicates) just as many Earth rocks are. Recently it has been possible to study the composition of meteorites on a very tiny scale. To everyone's surprise, a major component of meteorites turns out to be very tiny diamonds! These diamonds are very tiny -- typically 50 Ångströms in size. One Ångströms is m or 10-8 cm so 50 Å is about cm. A typical human hair is 50 microns in diameter; one micron is 10-6 m. How many of these tiny diamonds, lined up, would span one human hair? Carbon is now known to arrange itself in different ways, to make very different kinds of solids. One way is loosely, in sheets, to make graphite -- a black, soft substance sometimes used as the "lead" in pencils. Another is in a tight lattice arrangement; this makes a hard, clear substance, the diamond. A third way, recently discovered, is for 64 carbon atoms to link together to form a spherical molecule called "Buckminster-fullerene" or "Bucky-balls". In this form, carbon makes a pink/red solid or solution. This form of carbon was actually first discovered (just a few years ago) by people doing experiments to try to get laboratory material with the same response to light that interstellar dust has. The Bucky-balls don't, in fact, turn out to explain the effects of interstellar dust on starlight, but they turn out to be tremendously exciting material for "new technologies" here on Earth. For example, one Japanese research group reported recently that Bucky-balls that have trapped one non-carbon atom inside the sphere respond to bright light by becoming opaque essentially instantly. Imagine welder's glasses that are easy to look through until you try to look at the bright flame, or eclipse-viewing glasses that protect you from the bright sun but let you look at the faint corona while the sun is blocked from view! Where do meteors and meteorites come from? There are several possibilities: They could be bits and pieces of the asteroids that are mostly found between Mars and Jupiter. They could be bits of comets. They could be bits of the moon or Mars thrown into space by some process such as the impact of a large meteorite or asteroid. Or they could just be small chunks of material left over from the formation of the solar system, bits that never got stuck together into larger objects. Which of these processes actually contribute meteorites? Some clues: (1) There are "meteor showers" that occur when the Earth passes through the tail of a comet. These meteors must be bits of comet material. However, there are no meteorites associated with meteor showers, so it appears that most of the meteors in meteor showers are small particles or icy particles that don't survive their passage through Earth's atmosphere. (2) There are meteorites that are made of nickel and iron. Nickel and iron settle out into the centers of warm, large solid bodies like Earth. These meteorites then must once have been part of

7 29. Meteors, meteorites, asteroids and comets 7 some object large enough and/or warm enough that the nickel and iron could settle into the center. The large object(s) must then have been smashed into smaller bits somehow. There is one collection of objects in the solar system that appears to have gone through a process of big objects "differentiating" into nickel-iron, rock, and ice and then the bigger object(s) being smashed into a collection of smaller objects of a range of compositions -- that is, the asteroid belt (see below). (3) There are some meteorites that are indistinguishable from moon rocks, and others that look very much like the rocks on the surface of Mars. (Here, "look very much like" means that they are similar in a very detailed way -- structurally (lots of little grains, or whatever) and chemically (how much iron, etc.).) (4) There are the very "primitive" chondrites that seem to contain a lot of material that is 4.6 Gyr old, suggesting that they formed at the time the solar system formed and have simply orbited the sun ever since. So the answer to "where do meteors and meteorites come from?" appears to be "all of the above" -- some meteors from comets, some meteorites from the asteroids or from the moon or Mars, and some just left over from the material that formed the solar system. EXERCISE 29.4 What does all this tell us about the origin of the solar system? Meteorites tell us the most likely age of the solar system,. There are a lot of small objects in the solar system, and many of them contain a lot of "ices": for example, the (objects) in the Oort Cloud, some of the (objects) between Mars and Jupiter, and Pluto. Rocky bodies that differentiated into nickel-iron surrounded by rock were formed between the orbit of (planet or group of objects) and the Sun. Beyond (planet or group of objects) we find icy bodies and large, hydrogenrich, planets. Ices all have a lot of hydrogen. Probably, then, the fact that we find icy bodies and gas giant planets in the same part of the solar system is not a coincidence; rather, the Jovian planets could have been built up in part through an accumulation of icy bodies. We can do chemical analyses on those objects that fall to Earth; this includes (objects) and also some dust from (objects). These chemical analyses, plus other information (such as the presence of tiny, 50Å, (form of carbon)) in some chondrites, tell us quite a bit about conditions in the early solar system.