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earth sci chap 12.doc

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Department
Earth Sciences
Course
Earth Sciences 1086F/G
Professor
Prof
Semester
Fall

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Chapter 12: asteroids  - Asteroids are primordial objects left over from the formation of the solar system - Asteroids are the last remains of the planetismals that built the planets 4.6 billion years ago - Small, complex with irregular shapes and heavily cratered surfaces - Asteroids discovered in the gap between Mars and Jupiter known as the Main Asteroid Belt - Asteroid belt appears mostly empty - Asteroids are spread over such a large volume that it would be highly improbable that you could see one from another - Most travel in fairly circular orbits - After nearly 5 billion years, belt population bears little resemblance to the original one - ASTEROID DISCOVERY - Late 18 century astronomers were organized to search the sky for a missing planet predicted by the Titius-bode law - In the first few years, 4 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 - In march 1998 Lincoln Near Earth Asteroid Research (LINEAR) telescope was made o Highly sensitive electro-optical detector with a CCD (charge coupled device) which is an array of light-sensitive elements that can record very faint images o 50-75 asteroids/per up to 182 asteroids/per year - Estimated that there are between 750,000-1.7 million asteroids in the main aasteroid belt that are larger than 1 km in diameter - 100,000 hav been given numbers - 6139 asteroids not resident in the belt + are “near Earth” - When an asteroid is discovered it is assigned a number immediately THE MAIN ASTEROID BELT - ASTEROID DISTRIBUTION AND THE JUPITER EFFECT - Originally asteroids formed uniformly throughout the inner solar system - May have placed them as close as 30 million km from sun and as far as 800 million km from sun - When planets began to form their gravitational forced hanged that geometry dramatically - Asteroid material gathered up by the forming planets, leaving zones around their orbits devoid of asteroids - Asteroids that this did not happen were flung outward, ending up within the influence of Jupiter - Jupiter has such a large mass that its gravity affects the motions of small bodies anywhere close to it - Jupiter formed early in solar system’s history, first one to grow large, and formed where the asteroid belt now lies - Jupiter’s gravity confined the asteroids into a more or less restricted area and dictated their locations within that belt - Gaps in belt (no asteroids) = Kirkwood gaps - Distances between gaps corresponded to simple fractions of the orbital period of Jupiter o Within these gaps gravitational perturbations/disruptions are particularly strong - 2 maj forces acting on asteroids o Gravitiy of sun o Gravity of Jupiter - Over a long period each asteroid is influenced by these forces to change it position and swing either outward or inward from Jupiter - Asteroids entering Kirkwood gaps are booted out by jupiters gravitational disruption forces to outside the belt and move into the inner solar system - Originally may have been sufficient planetismals in the belt to form 2 earth-mass planets but over time they’ve been lost - ASTEROID CLASSIFICATION - Those closest to the sun are brighter and have higher metal content and those furthest from sun are darker in color and richer in carbon - Can FIRSTLY base classification scheme upon meteorite samples of the asteroids - 93 % of them are composed of minerals called silicates (silicon + oxygen + some other elements) - And most of the rest are pure metal (iron with a bit of nickel) - We have no assurance that the meteorites we look at represent the whole population of asteroids this technique is least reliable - Really large asteroids become differentiated, formed from primitive material in the solar nebula, heated to the point where their interiors melted and geochemical processes occurred o In some cases, temps became high enough for iron to sink to center, forming iron core and erupting basaltic lavas on surface o 4Vesta: survives to this day and has a basaltic surface o Other differentiated asteroids disrupted by collisions that stripped away their crusts and mantles and exposed their iron cores o Thus we get an assortment of compositional samples from those collisions - SECOND we can base classification upon characteristics of sunlight reflected off the surfaces of asteroids o Looking closely at the albedo (proportion of light reflected from an object, 0 = perfectly black, 1 = perfectly reflecting) o Object with lots of metal showing would be highly reflecting o One composed of carbon would have a very low number - THIRD 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 (light spectrographs) - CLASSIFICATION SCHEME - 3 main groups - - C type asteroids o High carbon content (carbonaceous) o 75% of known asteroids o Similar compositions to the sun minus the volatile elements - S-type asteroids o High silicon (silicaceous) o 17% of known asteroids - M-type asteroids o Metallic o Most of the remaining asteroids - P-type and D-type (closer to C-type than anything else) are characteristic of the outer asteroid belt o Both considered to be rich in organic matter and have ice water in their interiors o Represent the least altered (aka most primitive) of objects in the asteroid belt - Other groups are pretty much restricted to the inner portions of the asteroid belt and probably represent the most altered and differentiated material in the whole asteroid belt - MATCHING CHILDREN WITH PARENTS - Can Museum specimen meteorites (children) be matched up with known asteroids (the parents) - Most asteroids have relatively low albedos, aka dark, reflecting 3-7 percent of the light striking them - From lab studies of samples here on earth, we know these to be carbonaceous asteroids w dark carbon minerals on their surfaces - Investigators realized that if meteorites are truly pieces of asteroids they should exhibit similar spectral reflectance characteristics - Selected several basic kinds of meteorites including ordinary chondrites, carbonaceous chondrites and achondrites, irons, and stony-irons, and ground them into a powder - Obtained laboratory reflection spectra of the powders and compared them with those from the asteroids - Similarities between the asteroids and the meteorite samples are striking - 4Vesta spectrum matches the spectrum of a eucrite, igneous, basalt-like meteorite - First asteroid discovered (turned into a dwarf planet) = 1Ceres o Albedo of 6 percent o Spectrum very flat o Carbonaceous chondrites - 433Eros falls within the zone of the ordinary S-type chondrites o Reddish in colour o Spectrum shows combo of the minerals pyroxene, olivine, and metallic nickel- iron ally - Asteroids with the highest albedo (40 %) = E type asteroids (E for mineral enstatite) - 16Psyche has a flat spectrum with an albedo of about 10% o Spectrum shows no absorption features, aka metallic body o Nickel-iron is an opaque mineral returning only reflected light o These are called M-type asteroids ! o Enstatite chondrites have a nearly identical flat spectrum - Direct correlation between compositional differences in the asteroids and their distribution in the asteroid belt - More than 75 % of asteroids sampled were C-type asteroids o About 2/3s show evidence of water in their mineral structures o These asteroids range from the middle to the outer edge of the belt - Seems we cannot rely on our meteorite collections to tell us the true ratios of asteroid types - Rarity of carbonaceous chondrites on earth (by far the most plentiful asteroid type) illustrates the difficulty of removing them from the outer fringes of the belt where they normally reside to earth-crossing orbits - S-type asteroids are confined to the inner belt, but there is some confusion about the s- type asteroids, they do not closely match the spectra of ordinary chondrites - WONDERFUL VESTA - Careful study of its reflection spectrum shows changes over a period of hours o Combined results of rotation and varying mineral composition o Reveals heterogeneous surface with different mineral regions o Can be explained if we assume that it is a differentiated body such that different layers have different mineral compositions - General covering of eucrite material (type of basalt flow) - Irregular area of diogenite material (non-volcanic igneous rock formed within the body) - Roughly circular area with diogenite material + olivine rich area o Located on Vesta’s equator and extend northward - Diogenite area is impact zone where eucrite material was splintered off revealing diogenite mantle layer beneath - Second, more circular area is possibly an impact crater deep enough to reveal deeper olivine-rich layer - Undoubtedly the source body for eucrite and possibly also for the diogenite meteorites - Enourmous crater covering almost 75 percent of one side of Vesta - Rocky debris from this collision continues to reach earth as eucrites and diogenites - Dawn was launched to learn more - Asteroid Families - Resulting pulverization or melting of the smaller projectile means that only one asteroid (the larger body) forms the majority of observable fragments resulting from the collision - Hirayama surmised that the breakup of an asteroid into a collection of fragments, which he called a family, would result in similar orbital characteristics for these bodies - Corrections must also be made for minor orbital perturbations caused by nearby Jupiter - This results in what are called proper orbital characteristics - Able to recognize clusters of asteroids (Hirayama families) hypothesized that the members of anyone family were collisional fragments of the same original planetesimal - 19 families have been defined - NON-BELT ASTEROIDS WOOWOOWOWOWO - Some asteroids follow orbits that bring them close to terrestrial planets-especially earth o Termed near earth asteroids or near earth objects - All follow highly elliptical orbits - Subdivided into categories according to the dimensions of their orbits - All of them spend some time between 0.963 AU and 1.3 AU o Aka relatively close and space craft could visi them - We’ve visited 433 Eros and 25143 Itokawa - All NEAs have unstable orbits o Atens: most of these asteroids have orbits less than 1 AU of earth (orbit within the orbit of earth) o Apollos: most have an orbit that brings them through the orbit of Earth (cross earth’s orbit! ) o Amors: these commonly cross the orbit of mars: get close to the orbit of earth but don’t cross - All are dangerous because they interact with the gravitational influences of the planets - 1/3 will be thrown into the sun - Earth is hit by an Apollo object once ever 250,000 years on avg - About 1000 of Near Earth asteroids larger than 1 km in diameter (the minimum size impactor that could cause global effects on earth) - LONEOS is searching the entire sky visible once a month - Largest impact ever detected in modern time was atmospheric explosion of a rocky meteorite over the South pacific feb 1, 1994 - The distinction between comets and asteroids is not totally clear (some may be comets that have exhausted their ices and become trapped in short orbits that keep them in the inner solar system) - Also non-belt asteroids in the outer Solar system - Since they are Farther from the sun they move more slowly - Chiron = 170 km in d, orbit carries from just inside the orbit of Uranus to just inside the orbit of Saturn o First classified as an asteroids, now less certain o Astronomers now suspect that it may have a rocky crust covering deposits of ices such as solid nitrogen, methane, and carbon monoxide o Orbit is not stable - 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 (go degrees ahead of and behind the planet and are regions where the gravitation of Sun and Jupiter combine to trap small bodies) - Lagrangian points have trapped chunks of debris called Trojan asteroids (over 1000 known, only the brightest given names) - Growing evidence suggests that other planets may have Trojan asteroids trapped in their orbits - POTENTIALLY HAZARDOUS ASTEROIDS - = an asteroid of minimum 150 m diameter and comes closer than 0.05 AU to Earth o As of 19/08/2011 there were 1244 PHAs mapped - Torino scale = an impact potential scale of 0-10 used primarily to convey level of concern of an event to the general public - A 1 = category of objects not recently observed - A level of 8,9,10 means that collision is certain - 10 = suggests the future of global life may be @ stake - Apophis, asteroid previously listed as 4 for a little while now @ 0, if it were to hit London, whole city would be replaced by a large crater and all of human life between t dot and Windsor would be over o 400 m diameter body o Serious concern for impact o As more observatories made sightings Torino scale level was lowered to 0 o Aka ratings change as we get more/better observations and they are revised daily o Based upon the trajectory of the asteroid and earth’s orbit the impact was likely to occur in the eastern hemisphere - TOUTATIS - One of the largest known asteroids that crosses Earth’s path is 4179 Toutatis - Fast but highly irregularly moving asteroid - Approx every 4 years its orbit either comes close to or crosses earth’s orbit - Orit although erratic in motion is predictable - Most chaotic orbit studied to date - If any new trajecetory sends it towards earth; peace humans - ASTEROID EXPLORATION - First close up photographs of asteroid-like objects were taken in 1971 when Mariner 9 probe images Phobos and Deimos (satellites of mars, probably captured asteroids) - First true asteroid to be photographed in close-up was 951 Gaspra in 1991 followed in 93 by Ida and its satellite Dactyl imaged by the Galileo probe - First dedicated asteroid probe was NEARShoemaker o First spacecraft mission specifically designed to study an asteroid (Eros) o Launched on feb 1996 o Eros was the first of the Near-Earth-Object to be discovered and is the largest o Closest approach to earth was approx 0.15 AU o Eros is no threat to actually hit earth - Objective of mission was to encounter and orbit eros to collect imagery and gather data on surface features, composition, and rotation - Landed on eros - MATHILDE - NEAR did a fly-by of asteroid 253 Mathilde (main belt asteroid) - History of violent impacts - One hemisphere is scarred by an impact crater so large that scientists don’t understand why it wasn’t pulverized by the collision - This encounter gave scientists the first close up look of a carbon-rich C-type asteroid - Has the highest density of craters ever seen on object this size - Number of craters suggests that mathilde is a few billion years
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