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Chapter+14+Space+Impacts+Basics1.pdf

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Department
Geography
Course Code
Geography 2240A/B
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Philip Egberts

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Chapter 14 Impacts from Space - The Basics 1.0 Introduction I've chosen to begin this chapter with a schematic figure of impacts and growth of Earth from those impacts. There was no more violent period in Earth's history than when it was being built by the accretion of impacting objects, and there is very little doubt that if human civilization lasts long enough it will be put in jeopardy - and possibly destroyed - by an impact event. One million years ago an asteroid impacted at Zhamanshin in Kazakhstan, making a crater 13.5 km in diameter (Because impact was in a mountainous area that has since been severely eroded, photographs are of very poor quality). It (apparently) did not produced quite enough disruption to equal what we refer to as a 'nuclear winter' (Fig. 2), and it certainly was not big enough to cause global mass extinctions; however, we recognize that an impact like that, today, could easily end our civilization - which is very fragile - but certainly not our species. To emphasize our vulnerability, note that in 1989 astronomers were startled to see an asteroid roughly half the size of the one that caused the Zhamanshin crater moving away from Earth, having come within 700,000 km. To put that in perspective, Moon is about 380,000 km from Earth. The asteroid's approach had not been detected at all. If it had approached only two hours later, its orbit and Earth's orbit would have intersected (i.e. impact would have occurred). As far as we know, the Zhamanshin asteroid was the largest to impact on land in the last million years; considering that 3/4 of Earth is water, we can make no statement about impacts for which we cannot study the evidence. According to impact projections, we should expect one Zhamanshin-size (1 km diameter object) impact with Earth every 1 million years; if the last one was about 1 million years ago, statistics say we're due now. Of course, if you want to believe the rigor of such statistics (which is dangerous), maybe the 1989 close-approach was it, and we dodged the bullet! We need a few definitions. Look at the mini-glossary as Appendix I to this chapter. 2.0 Asteroids and the Asteroid Belt Let's consider the importance of what we call the asteroid belt. It is from the asteroid belt that Earth receives most (but not all) of its large space debris. We know that the asteroid belt, located between Mars and Jupiter (Fig. 3), consists of material that was never incorporated into a planet (so it is not the debris of either an exploded small planet or two planets that crashed together). While we speak of the 'asteroid belt', upon closer examination we see the asteroids are actually concentrated in a number of distinct belts, separated by clear gaps (Fig. 4). In a search to explain that arrangement, it was discovered that the gap distances are directly related to the orbit frequency of Jupiter. Many of the asteroids have the same or certainly similar composition as the parent ingredients from which the planets were assembled (I will say more about that in the next chapter). We have collected many meteorites in this category of primitive material, and we normally refer to them as chondrite meteorites (named for the tiny, 1 mm or so, bead-like spherules they contain called chondrules). If you proceed into geochemistry and petrology classes, you will use the composition of chondrites as standards against which to judge the degree of differentiation or evolution from primitive composition (i.e. chondrites) that particular rocks exhibit. Many of the larger asteroids underwent roughly the same sort of differentiation history as Earth and other large space objects. In other words, there was sufficient contained radioactive material that the planetoids or asteroids melted and formed a core and mantle – a simple gravity-controlled process. With so many large bodies orbiting in relatively close proximity it is inevitable that collisions take place - many more in the past than now. Head-on collisions at 16,000 km/hr certainly spray debris widely! Some of the debris gets knocked into the gaps, from which it is whipped out in an eccentric orbit (accelerated by the gravity of Jupiter), with a looping trajectory toward Sun - and thus potential intersection with Earth's trajectory. From study of those fragments that do hit Earth, we are commonly able to get a close look at the different parts of other differentiated bodies and correlate our observations with what we know of Earth. We are gradually getting around to the point where we have enough information to consider impacts of space objects with Earth, and we need to think just a bit more about those asteroids which have been thrown out of the asteroid belt gaps and into wide elliptical orbits. A large number of those asteroids have, in the past, impacted the planets and their satellites; we see the evidence clearly in the cratering on Moon, for example (Fig. 5). By definition, Near Earth Objects (NEOs) are those asteroids and comets whose orbits intersects Earth’s orbit or comes very close to it (Fig. 6). Those NEOs that actually intersect Earth’s orbit are specifically referred to as Apollos; those which intersect the orbit of Mars rather than Earth (but still come very close to Earth) are referred to as Amors. The largest impact feature we have identified on Moon has a diameter of 2200 km; an asteroid measuring some 50-150 km in diameter would have created that. Earth has a gravitational force that is 24 times that of Moon, thus Earth MUST have collected many more of the largest objects than did Moon. If Earth were struck by any object (asteroid or comet) with a diameter larger than 5 km diameter (the size of the nucleus of Halley's Comet) it has been estimated that the atmosphere would be temporarily destroyed. Apparently this happened several times in the very early history of Earth. Also, apparently, several times prior to 3.8 billion years ago, objects also vaporized our oceans. The largest crater we see preserved anywhere on land is Sudbury (Fig. 7), Ontario (at about 200 km diameter - although it’s a highly defor; 1.85 billion yrs. ago). Next is Chicxulub, Mexico (approximately 170-180 km, but some scientists suggest as wide as 300 km; 66 million yrs ago); we'll consider the Chicxulub controversy later. Mapped structures on ocean floors and even under ice sheets are thought to be impact craters, but study of them is difficult. A bit further on in this chapter, we are going to look at some of statistics of asteroids that get close to Earth. 3.0 Comets Before we begin a discussion of actual impacts of space objects with Earth, we need to recognize that the impactors may be something other than asteroids; they may also be comets. The Solar System of planets is surrounded by something like a trillion comets in a crude envelope called the Oort cloud (Fig. 9). Only a very few of the comets in that cloud ever get diverted into orbits that bring them close to Earth (Those that have a frequency od appearance of 200 years or less appear to have originated in a relatively close zone of the Oort Cloud called the Kuiper Belt). The most famous of those is Halley's Comet, named for Edmund Halley who first calculated its orbit. It comes sweeping past every 74 to 79 years, which means it should go whipping by about 2061 or so. Then of course there was Hale Bopp (Fig. 10) that went through the Solar System in 1997; the nucleus was 40 km in diameter- easily large enough to sterilize Earth if it impacted. 4.0 Cosmic Dust, Shooting Stars, and 'Others' We will conclude our list of materials that enter from space with that stuff that's either very small - or our knowledge about them is very small. Astronomers say that every day Earth receives somewhere between 100 and 1000 tonnes of space junk - most of it in the form of dust (Fig. 11). Interestingly, the dust particles are so small they slip through the atmosphere without undergoing any change. On the other hand, what we call shooting stars is junk of about 1 mm diameter which is just large enough that the friction of entering the atmosphere raises the temperature to the melting point. The melt that forms as we see the meteor streak across the sky cools to little glass droplets that are added to Earth's surface. Now the stuff we really don't know much about! Recently, several large, icy bodies were discovered in the outer solar system. In fact, there appears to be a mix of asteroid-like and comet-like objects out there. We don't know how many, we don't know their properties, and there may be whole classes of objects yet to be identified. These low-density, fluffy balls of ice are sometimes called "space balls". 5.0 Impact! It doesn't require a really massive object to make it through the atmosphere. Dust gets through, then anything between dust and 1 gm doesn't; but, as the Abbott text says, anything more than 1 gm will probably land on the surface. (Fig. 12) shows a typical entry for an ordinary meteorite. You must assume that the force of gravity has been pulling the object for some considerable distance, thus it is probably traveling at several times the velocity of a rifle bullet. If it is traveling at the speed of sound or greater (about 1200 km/hr in dry air), it is likely to be accompanied by a sonic boom, just like a jet plane traveling at greater than the speed of sound. By the way, a sonic boom is just the result of a violent compression of air. If the in-coming object is greater than the size of a basketball and is traveling at greater than the speed of sound, the sonic boom will be heard on Earth's surface. All the way down, of course, the atmosphere's density increases and the friction increases, thus most objects slow. Many start out at the top of the atmosphere at much greater than the speed of sound but are traveling at only 300 or so km. /hr when they actually impact. Appreciate also that the friction not only slows the object, but the temperature induced by friction can greatly reduce its size through melting; temperature can reach 3000 C. Of course, size is important in all considerations. Just as tiny dust particles make it through the atmosphere with almost no change in properties, so very large objects (greater than 350 tonnes) plunge through so quickly the atmosphere has little effect on them. Until 2000, the Geological Survey in Ottawa maintained a web site that included a list and pictures of all known impact craters; that has now been transferred over to the University of New Brunswick: http://www.unb.ca/passc/ImpactDatabase/CILocSort.html. You can click your way around and see North American sites, or even very particular photos of sites together with their known characteristics. To-date, approximately 172 impact craters have been identified on Earth, but new ones are being found every year. 6.0 Classification of Meteorites Very quickly, let's look at the classification of meteorites (Fig.13). These are not very sophisticated terms! Aerolites or Stones (92.8% of all meteorites) Primitive chondrites: They are mostly composed of silicate minerals - particularly plagioclase, pyroxene and olivine; contain chondrules; some contain amino acids; probably formed from primitive materials in the solar nebula early in solar system history. One type of chondrites is called "carbonaceous chondrites" and they are characterized by a high volatile content and carbon compounds. They are though to be the most primitive of all chondrites. Differentiated chondrites (called ‘Achondrites’): Silicate minerals similar to those found in terrestrial rocks; some are basaltic, some are composed primarily of olivine and some of clinopyroxene; probably represent fragments broken from the outer layers of a differentiated, large asteroid. Siderolites or Stony-irons (1.5% of all meteorites) About 50% nickel-iron alloy and 50% silicate minerals (olivine, pyroxene, plagioclase); probably represent fragments broken from a differentiated asteroid that would be comparable to the mantle-core interface region of Earth. Siderites or Irons (5.7% of all meteorites) Primarily iron and nickel metal; there is a very large classification list depending upon the amount of nickel, gallium, germanium and iridium present; represent the metallic core of a large differentiated asteroid. Some of the very best discoveries of meteorites have been recovered from the ice sheets in Antarctica - where they can't be mistaken for anything else and where weathering has taken place at a very slow rate. 6.1 Distribution Compositional zoning is evident in the asteroid belt, with more primitive carbonaceous chondrites apparently increasing in abundance with distance from the Sun. This zonation is probably a secondary effect due to early heating of the inner belt, rather than representing a primary zoning of the original solar nebula. Chondrules We described chondrules as small spherical objects (Figure 14), typically about 0.5- 1.5 mm diameter, and appear to have crystallized rapidly from molten or at least partly molten drops. The table and graph of Figure 15 show the mean chondrule composition versus the matrix composition. Note that chondrules - despite being called the most primitive matter we have from the solar nebula - contain very little iron and certainly no free metal alloys. Thus, by the time they formed, there must already have been some differentiation of material in the solar nebula. It is suggested that the precursor of chondrules was silicate dust in the outer regions of the solar nebula. The dust was probably melted or partially melted by solar nebula flares thrown out of the mid-plane by turbulence as Sun gradually stabilized. 7.0 Crater Morphology OK, objects of various size strike at Earth virtually all day every day; fortunately, most is dust size. However, we on Earth’s surface seldom get battered even by medium sized objects, and we have Earth’s atmosphere to thank. The density of our atmosphere ‘shocks’ most objects, breaking them into small bits that are then melted or vaporized by friction with air molecules well before ground-level. The big guys (Fig.16), however, just blast right through – and those are the ones that make craters and do amazing damage. Relatively large space objects strike Earth at velocities between 20 and 30 kilometers per second, which translates to 72,000 to 108,000 km/h. Any moving object has energy, called kinetic energy, which can be found from the following formula: Kinetic energy = ½ mass x velocity . 2 Large objects come in so fast we say they produce a hypervelocity impact crater; hypervelocity simply means that the object’s space velocity was not really affected significantly by Earth’s atmosphere, thus was very high. When a large meteoroid strikes, nearly all its kinetic energy transforms into heat. On worlds with no atmosphere, even microscopic meteoroids strike with enough energy to melt rocks and form craters. Figure 17 is a photo of a tiny melt crater – so small it can be seen only with a powerful microscope – found on one of the samples brought back from Moon. Craters are always much larger than the impacting body that forms them – typically about 15-20 times larger on Earth. The crater forms in seconds as the asteroid stops and its kinetic energy is transformed to heat (see Fig.18). The thing which has confused geologists for decades is that the path of the meteor before impact has almost no effect on the shape of the crater, thus craters are round regardless of the angle of impact. Only the most grazing impacts produce oval craters. The impacting object is largely destroyed by the impact. Following the impact, the hot gases in the crater area expand explosively, and great quantities of material are hurled out of the crater, producing an ejecta blanket, which is thickest near the rim (Fig.19). The ejecta blanket often is much lighter in color than the older, more weathered rocks on the near-by surface, so new craters may be easily identified. Within the crust of the planet, the shock waves from the impact travel outward and subject the rocks to enormous and very sudden stress. Rocks at the rim of the crater are turned upward and even overturned. The shape or morphology of craters changes with crater diameter. Only the smallest impact craters have a bowl-shaped form. As crater diameter increases (i.e. as the size of the meteoroid/asteroid increases), slumping of the inner walls and rebounding of the depressed cr
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