Unit 4 - From Asteroids to Meteorites (Chapters 12 - 13)

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Earth Sciences
Earth Sciences 1086F/G
Phil Mc Causland

Introduction April-17-14 12:39 PM Meteorites - Originally called betyls (Hebrew for home of God); they thought the rocks might be chunks off some god's house - It is said the sacred black Kaaba Stone, to which Muslims pay homage in Mecca, is a large meteorite (but it is out of bounds for critical examination) - The earliest instance of a witnessed meteorite fall was in November of 1492 by Ernst Chladni (now known as the father of meteoritics) who published a book containing data on several meteorites Asteroids - According to the Titius-Bode Law (see below), there should be another planet in the space between Mars and Jupiter - The discovery of the first object in that space was made by Giuseppe Piazzi, a monk who was also the director of the PalermoObservatory in Sicily → That asteroid, which he named Ceres, has now been classified as a dwarf planet - Since then, lots of objects have been found in that space between Mars and Jupiter, which, in combination, constitutes the Asteroid belt - The original idea was that these objects represented the material left over from a planet that was blasted into bits by a collision with a large comet — this idea was wrong - Recent work strongly supports the hypothesis that the material represents stuff that never assembled into a planet because ofthe enormous force of Jupiter - The total mass of everything in the Asteroid Belt is estimated at much less than that of Moon → Some material certainly has been scattered during the "planet shuffle" and may have participated in pummeling some of the inner planets during the Late Heavy Bombardment Definitions ASTEROID: A natural rocky object in space measuring 100 m to several hundred kilometres in diameter METEOROID: A natural rocky object in space measuring from a few millimetres to 100 m in diameter METEOR: The visual streak of light associated with passage of a small meteoroid through Earth's atmosphere; the heat energy producing the light is a result of friction between the object and molecules of gas in the atmosphere *Remember that a meteor is not the object, only the light phenomenon FIREBALL: The light associated with a large meteoroid or asteroid as it interacts with the atmosphere METEORITE: A fragment (any size) of either a meteoroid or asteroid that lands on Earth's surface *Remember it is not called a meteorite until it actually lands on surface The Titius-Bode Law - In 1702, a scientist named David Gregory noted the relationship between planet orbits, and wrote a little mathematical sequence to explain it (originally written in Latin, then German) - Here is how it works: 1) Take the simple mathematical series 0, 3, 6, 12, 24, etc. (Note that each successive number is double the previous one) 2) Add 4 to each of the above, getting: 4, 7, 10, 16, 28, etc. 3) Now divide each of the above numbers by 10 to get: 0.4, 0.7, 1.0, 1.6, 2.8, etc. → These are the predicted planet spacings in Astronomical Units Learning Objectives - We must know where meteorites come from ▪ Samples of asteroids and possibly comets ▪ Samples blasted from the surfaces of Moon and Mars - How representative of the sources are the samples? - Basic classification of meteorites ▪ Of all the different types, chondrites (particularly carbonaceous chondrites) have most meaning for us - It's important to learn whatever we can about asteroids because: ▪ They represent very primitive material left over from formation of the solar system ▪ Much water and organic material came to Earth from them ▪ Sooner or later a large asteroid impact is likely to put an end to many terrestrial species (including humans) Unit 4 - From Asteroids to Meteorites Page 1 Chapter 12: Asteroids April-17-14 12:35 PM - Asteroids are primordial objects left over from the formation of the solar system - The study of asteroids gives us a way to explore the ancient past of our planetary system, and can also give us better direction in our search for other star- planet systems - In the "Main Asteroid Belt," most asteroids travel in fairly circular orbits; however, because there is a high population (in celestial terms), and variations in their orbits may occur in response to nearby Jupiter, collisions do occur → A collision may fragment an asteroid in numerous smaller pieced, or may "glue" two asteroids together if the collision occurs at low relative speeds - The current asteroid belt bears little resemblance to the original one - There are also many asteroids not in that Main Belt, and some of those can be really dangerous; they fall into a category termed "Potentially Hazardous Asteroids" Asteroid Discovery - Asteroid discovery proceeded in several phases:  In the last years of the 18th century, a group of astronomers searched the sky for the "missing planet" predicted by the Titius-Bode law → Slowly, more and more asteroids were discovered  By 1891, astronomers were using successive photographs to look for the tell-tale streaks of asteroids moving across the field of view → Discoveries became almost common-place, but astronomers did not bother with them, calling them "vermin of the skies"  In March 1998, a new asteroid detective hit the skies: the Lincoln Near Earth Asteroid Research (LINEAR) telescope → A very sophisticated and highly sensitive electro-optical detector with something called a CCD (a charge coupled device) which is an array of light-sensitive elements that can record very faint images → This telescope has been very successful in detecting objects as of 2011 - When an asteroid is discovered, it is assigned a number immediately, so you can tell from the sequential number when any particular one was discovered (the assigned number followed by the chosen name) The Main Asteroid Belt Asteroid Distribution and the Jupiter Effect - Originally, asteroids formed throughout the inner solar system, and sufficient refractory minerals probably formed as far out as Jupiter → Therefore, in the early solar system, the asteroids were distributed more or less uniformly within the inner solar system - When the planets began to form, their gravitational forces changed that geometry dramatically — much asteroid material was gathered up by the forming planets, leaving zones around their orbits devoid of asteroids → Those asteroids that did not meet this fatal end were commonly flung outward, ending up within the influence of Jupiter - Jupiter has such a large mass that its gravity dramatically affects the motions of small bodies anywhere close it → It formed early in the solar system's history; millions of years before any planets could accrete where the asteroid belt now lies Gaps in the Asteroid Belt - As Jupiter's gravity confined the asteroids into a more or less restricted area, it also dictated their locations within the belt - There are especially prominent gaps where no asteroids exist, called Kirkwood gaps by Daniel Kirkwood → He noticed that the distances between gaps corresponded to simple fractions of the orbital period of Jupiter — within these gaps, the gravitational perturbations (or disruptions) are particularly strong Unit 4 - From Asteroids to Meteorites Page 2 - At this point, two major forces are acting on the asteroids: 1) The overall gravity of Sun (which affects everything in the solar system) 2) The gravity of Jupiter → Gradually, over a long period, each asteroid is influenced by these forces to change its position and swing either outward or inward from Jupiter, leaving behind a gap where it used to be - Asteroids or meteoroids entering the Kirkwood gaps are booted out by Jupiter's gravitational disruption forces at velocities so great they completely escape the limits of the Asteroid Belt → These gaps are like "open doors" through which meteoroids and asteroids can escape Jupiter's influence and move into the inner solar system and, indeed, come to Earth → Because collisions between large bodies are inevitable in the belt, objects keep slipping into the gaps and starting off on unique tracks - The remaining asteroids, if consolidated, add up to a body significantly less massive than Moon Asteroid Classification Classification Techniques - In most general terms, those closest to Sun are brighter and have higher metal content, and those furthest from Sun are darker in color and richer in carbon First. We can base a classification scheme upon meteorite samples of the asteroids: ▪ Almost 93% of them are composed of minerals called silicates (i.e., silicon plus oxygen plus some other elements) and the rest are pure metal (mostly iron with a bit of nickel) ▪ Since we have no assurance that the meteorites we look at represent the whole population of steroids, this technique is the least reliable → Really large asteroids became differentiated; after they are formed from primitive material in the solar nebula, they (just like the planets) were heated (by radioactive decay or other means) to the point where their interiors melted and geochemical processes occurred Second. We can base our classification upon the characteristics of sunlight reflected off of the surfaces of asteroids — we're looking closely at the albedo (albedo = proportion of light reflected from an object; the range is 0 [perfectly black] to 1 [perfectly reflecting]): ▪ An object with lots of metal showing would be highly reflecting while one composed of carbon would have a very low number Third. Astronomers and other scientists have constructed pretty sophisticated instruments which break down the reflected light into a whole spectrum, thus collecting rough element analyses of surfaces as well as simple albedo numbers — the same light spectrographs as the ones used to determine chemical compositions are used here Classification Scheme - Based upon these techniques, the three main groups are: 1) C-type asteroids — high carbon content or "carbonaceous," 75% of known asteroids; roughly similar composition to Sun, minus the volatile elements 2) S-type asteroids — high silicon or "silicaceous," 17% of known asteroids 3) M-type asteroids — metallic, most of the remaining asteroids [Other than these three, there are dozens of rare types in very small volumes] - New studies have led to increasing definition of asteroid groups ▪ Two groups: P-type and D-type (closer to C-type than anything else), are characteristic of the outer asteroid belt → Both are considered to be rich in organic matter and both are considered to have ice water in their interiors → These represent the least altered (i.e., the most primitive) of objects in the asteroid belt) Unit 4 - From Asteroids to Meteorites Page 3 *The notes that say "no meteorite type" are wrong - Other groups are pretty much restricted to the inner portions of the asteroid belt, and they probably represent the most altered and differentiated material in the whole asteroid belt Matching Children with Parents - Astronomers are in the process of attempting to match meteorites (the "children") with their asteroids (the "parents") - Most asteroids have relatively low albedos, meaning they are very dark, reflecting only 3% to 7% of the light striking them → These are known as carbonaceous asteroids, with dark carbon-rich minerals on their surfaces - Astronomers noted that is meteorites are truly pieces of asteroids, they should exhibit similar spectral reflectance characteristics ▪ To test this idea, they selected several basic kinds of meteorites ("ordinary chondrites," "carbonaceous chondrites," "achondrites," irons, and stony-irons), and ground them to a fine powder ▪ They then obtained laboratory reflection spectra of the powders and compared them with those from the asteroids → When the reflection spectra are compared with laboratory spectra of meteorite samples, one must bear in mind that the spectrum is not from a single mineral but from several at one time — this tends to "muddy" the resulting spectrum, since it is really composed of several overlapping spectra - The similarities between the asteroids and the meteorite samples are striking ▪ Asteroids with the highest albedo (40%) are designated E-type asteroids (E for the mineral enstatite) ▪ Asteroids with no absorption features indicates the possibility of a nickel-iron core — nickel-iron is an opaque mineral, returning only reflected light and these are called M-type asteroids (M for metal) - The conclusions are tentative but encouraging — having identified a few meteorites that seem to match some asteroids, we can begin to place them in their proper positions within the asteroid belt - As suspected, there is a direct correlation between compositional differences in the asteroids and their distributions in the asteroid belt - More than 75% of the asteroids sampled were classified as C-type asteroids ▪ Of these, about two-thirds show evidence of water in their mineral structures, which shows up as an absorption feature in the ultraviolet part of the spectrum ▪ These asteroids range from the middle to the outer edge of the belt - The S-type asteroids are surprising because the disparity between the carbonaceous chondrites, rare on Earth but plentiful in the asteroid belt, and the ordinary chondrites, common on Earth but relatively rare in the asteroid belt, indicates that chondrites probably came from one or at most a few parent bodies — The large numbers reaching Earth are apparently not indicative of large numbers of S-type asteroids in space → It seems we cannot rely on our meteorite collections to tell us the true ratios of asteroid types Wonderful Vesta - The most thoroughly observed asteroid is 4 Vesta, one of the brightest in the sky and at times just visible to the unaided eye - Its reflection spectrum reveals a heterogeneous surface with different mineral regions passing into view as it goes through its 5.3-hour rotation — these observations can be explained if we assume that Vesta is a differentiated body such that different layers have different mineral compositions → Observations show a general covering of eucrite material (a distinctive type of basalt flow), an irregular area of diogenite material (a non-volcanic igneous rock formed within the body), and a roughly circular area with a combination of diogenite material and an adjacent olivine-rich area → These areas are located on Vesta's equator and extend northward for some 30 degrees latitude → Vesta was struck several times in its history, and large impact craters mark its equator ▪ The diogenite area is an impact zone where eucrite material was splintered off, revealing a diogenite (pyroxene-rich) mantle layer beneath ▪ The second, more circular area is possibly an impact crater excavated through the eucrite and diogenite layers and on into a deeper, olivine-rich layer - Vesta is undoubtedly the source body for eucrite and possibly also for the diogenite meteorites → This hypothesis was confirmed by Hubble Space Telescope images of Vesta which show an enormous crater covering almost 75% of one side of Vesta - To learn more about Vesta, NASA launched the spacecraft Dawn (powered by ionized Xenon atoms) which has been "captured" into orbit around Vesta → It has taken amazing images of the surface revealing its complex impact history, crustal mineralogy, and morphology Asteroid Families - The destruction that occurs when planetesimals collide catastrophically produces fragments with a great variety of sizes - The resulting pulverization or melting of the smaller projectile means that only one asteroid (the larger body) forms the majority of the observable fragments resulting from the collision Unit 4 - From Asteroids to Meteorites Page 4 resulting from the collision - Japanese astronomer, Hirayama, surmised that the breakup of an asteroid into a collection of fragments, which he called a family, would result in similar orbital characteristics → On the basis of similar proper orbital features, Hirayama was able to recognize clusters of asteroids, which we now refer to as Hirayama families — he hypothesized that the members of any one family were collisional fragments of the same original planetesimal Non-Belt Asteroids Near Earth Asteroids - Not all asteroids orbit within the Main Asteroid Belt — some of the most interesting follow orbits that bring them close to the terrestrial planets (particularly close to Earth) → These objects are broadly termed the Near Earth Asteroids (NEAs) or sometimes just Near Earth Objects (NEOs) [Near Earth Object is a bit more inclusive than Near Earth Asteroids simply because it can include comets] - All follow highly elliptical orbits, and they are subdivided into categories according to the dimensions of their orbits → All of them spend at least some time between 0.963 AU and 1.3 AU (AU = astronomical unit; 1.0 AU is Earth's location from Sun) — so they are relatively close and space craft could visit them (we have already visited two) - All the NEAs have unstable orbits; none will last more than a few million years — this is disturbing because if a big one becomes unstable near us, it likely means an impact that could destroy many species on Earth ▪ ATENS: Most of these asteroids have orbits less than 1 AU — which means they orbit within the orbit of Earth ▪ APOLLOS: Most have an orbit that brings them through the orbit of Earth (i.e., they cross Earth's orbit), and these are particularly worth watching closely ▪ AMORS: These commonly cross the orbit of Mars; they get teasingly close to the orbit of Earth, but don't cross - All these Aten-Apollo-Amor objects are dangerous because they interact with the gravitational influences of the planets → Eventually, about one-third will be thrown into Sun, and a few may be ejected from the solar system, but many of these objects are doomed to collide with a planet → Earth is hit by an Apollo object once every 250 000 years, on average - Many dozens of Apollo objects have known orbits, and none of those will hit Earth in the immediate future - The bad news is that there are about 1000 of these NEAs larger than 1 km in diameter, the minimum size of an impactor that could cause global effects on Earth - The largest impact ever detected in modern time was the atmospheric explosion of a rocky meteorite over the South Pacific on February 1, 1994 ▪ It was seen by fisherman as a blast as bright as Sun ▪ It produced about five times the energy of the Hiroshima bomb and was probably a rocky body at least 15 m in diameter (probably a small Apollo object) - At least some of these objects may be comets that have exhausted their ices and become trapped in short orbits that keep them in the inner solar system — the Unit 4 - From Asteroids to Meteorites Page 5 - At least some of these objects may be comets that have exhausted their ices and become trapped in short orbits that keep them in the inner solar system — the distinction between comets and asteroids is not totally clear - There are also non-belt asteroids in the outer Solar System that, because they are farther from Sun, move more slowly → The object Chiron, located orbiting inside the orbit of Uranus to just inside the orbit of Saturn, warns us that the distinction between asteroids and comets is not clear-cut - The icy asteroids of the outer solar system seem to be widely scattered, but Jupiter ushers two groups of asteroids within its own orbit → These non-belt asteroids have become trapped in the Lagrangian points along Jupiter's orbit (Figure 12.9) — these points like 60° ahead of and 60° behind the planet and are regions where the gravitation of Sun and Jupiter combine to trap small bodies → Like cosmic sink-holes, the Lagrangian points have trapped chunks of debris now called Trojan asteroids and named after the heroes of the Trojan - Growing evidence suggests that other planets may have Trojan asteroids trapped in their orbits Potentially Hazardous Asteroids (PHAs) POTENTIALLY HAZARDOUS ASTEROIDS (PHAs): An asteroid of minimum 150 m diameter and comes closer than 0.05 AU to Earth - A level 8, 9, or 10 means that collision is certain; a rating of 10 suggests the future of global life may be at stake - Projected trajectories indicate that no subsequent Earth encounters in the 21st century are of any concern Toutatis: The Doomsday Asteroid - One of the largest known asteroids that crosses Earth's path is 4179 Toutatis — a fast but highly irregularly moving asteroid → Approximately every four years its orbit either comes close to or crosses Earth's orbit - Although it's orbit has been plotted, eventually the gravity of Earth or Mars or Jupiter (or even a collision with another asteroid) will change the trajectory of Toutatis → If the new trajectory sends it crashing to Earth, it will wipe out humans and many other species Asteroid Exploration - The first true asteroid to be photographed (the two satellites of Mars were photographed first but they are only thought to be captured asteroids) was 951 Gaspra in 1991 - The first dedicated asteroid probe was NEAR Shoemaker, which entered orbit around 433 Eros and eventually landed on
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