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Comet PANSTARRS over Phoenix, March 12, 2013 COMETS GLG-190 – The Planets Chapter 14 LECTURE OUTLINE Introduction and history Orbits Comet Structure Comet Dust Origin COMETS THROUGH HISTORY Greek komitis: “long-haired” – Aristotle (384-322 BCE) describes hair-like tail Found in written records (Near East and China) to at least 1140 BCE (top right) Believed to be bad omens… Assassination of Julius Caesar, 44 BCE Battle of Hastings, 1066 (below right) Fall of Jerusalem, 70 BCE, "foretold" by comet (4 years earlier!) Eruption of Vesuvius, 79 CE Conquest of Constantinople by Turks, 1456 Outbreak of Black Plague that killed 100,000 in London, 1665 When beggars die, there are no comets seen; The heavens themselves blaze forth the death of princes – Julius Caesar, Act II, Sc. II EARLY STUDIES Comets considered atmospheric phenomenon Great Comet of 1557 observed by Tycho Brahe Halley’s Comet 1910 Proved orbit was beyond Moon comets not atmospheric Edmund Halley (pronounced “hawley”) Noted Great Comets of 1456, 1531, 1607, and 1682 had almost same retrograde orbits Different appearances of same comet orbital variation from gravitational perturbations by Jupiter and Saturn Predicted comet’s return in 1758 Died 1743, before seeing that he was right (next appearance in 2061) Halley’s Comet 1986 I came in with Halley's comet in 1835. It's coming again next year (1910), and I expect to go out with it. The Almighty has said no doubt, ‘Now here are these two unaccountable freaks; they came in together, they must go out together.’ – Mark Twain ORBITS OF COMETS Highly eccentric unstable orbits Affected by gravities of Jupiter and Saturn Long Period Comets (period >200 yrs) Short Period Comets (<200 yrs) Long period comets comprise 90% of total) Enter planetary region from no preferred direction 50% are retrograde consistent with random distribution From Oort Cloud (e.g., Hyakutake) Three short period comets types Most from Kuiper Belt (e.g. Halley) Some appear to be icy “asteroids” Main Belt comets Some are longer period comets thrown into closer orbits by Jupiter’s gravity (e.g., Tempel 1) Main Belt comets Origin of short period comets Main Belt comets have circular orbits and aphelions inside Jupiter’s orbit Some comets have orbits extending far from the sun (>1000 AU) Most comets have very eccentric inclined orbits OORT CLOUD AND KUIPER BELT LIFE OF A COMET STRUCTURE Nucleus Solid, centrally located part Made of dust and frozen gases When heated by sunlight, gases sublimate and produce atmosphere surrounding the nucleus Atmosphere Coma: gas and dust cloud around nucleus Coma materials swept into elongated tails: plasma and dust NUCLEUS SIZE Size measurement is difficult sizes mostly unknown Reliable measurements (flybys) indicate small diameters (above) Largest known are: Hale-Bopp (60 km), 29P/Schwassmann-Wachmann (31 km), 109P/Swift-Tuttle (26 km), and 28P/Neujmin (21km) NUCLEUS STRUCTURE Nothing known directly about interior structure Comets sometimes split apart some nuclei are fragile Nucleus has non-volatile outer mantle Comets sublime less gas than expected for their sizes Jets of material breaking through crust observed in close-up images Mantle origin(s) Rubble mantle loss of volatiles leaves behind rocky “rind” (top) Irradiation mantle high energy particles damage molecular bonds in outer few meters of nucleus creating carbon-rich, hydrogen-depleted “skin” (bottom) GAS COMA Molecules liberated from nucleus by solar heating (sublimation) are exposed to direct solar radiation Most are broken apart (dissociated) resulting ions are easy to observe using strong spectral lines at optical wavelengths Green coma of Comet 17P/Holmes (note bluish ion tail) Halley's Comet DUST COMA Dust coma imaged in infrared Dust dragged from nucleus by sublimating gas Grains have all sizes… Most grains are 1 m (0.001 mm) Active, near-sun comets can eject grains 10s of cm across Grains become part of the tail if they exceed escape velocity of nucleus (effectively in free orbits around Sun) Particles pushed out of dust coma by solar radiation pressure to form dust tail Dust particle collected from comet in Kuiper Belt by Stardust mission ION (PLASMA) TAIL Gas ionized by solar radiation is susceptible to solar magnetic field and wind Solar wind passes comets at 500 km/s ion tail is swept almost exactly in anti-solar direction Carried by solar wind ions are swept into long, distinctive ion tail Most common ion, CO+, scatters blue light better than red ion tail often appears blue Solar magnetic force is very strong and produces ropes, knots and streamers that distinguish ion tail from dust tail Comet Hale-Bopp. White: dust tail; blue: plasma tail (note thicker “knots”) DUST TAIL Dust tail is white or slightly pink because dust grains reflect sunlight slightly better at longer wavelengths than shorter wavelengths Particles in tail are in individual orbits around Sun dust tail is curved as comet swings around Sun Both plasma and the dust tails can attain great lengths, up to 1 AU (1011 m) in most spectacular cases Most meteors are dust particles from comets Orionid meteor shower Meteor shower occur when Earth’s orbit intersects streams of material left along orbits of comets Earth intersects orbit of Halley’s Comet twice… Eta Aquarids in May Orionids in October Other comet orbits intersect Earth’s orbit just once… Perseids in August come from comet Swift Tuttle DEEP IMPACT MISSION Studied composition of interior by crashing impactor into nucleus of 9P/Tempel Impactor hit (July 4, 2005) plume of material released and studied by main spacecraft instruments Comet more dusty than expected (ices lost over time?) DEEP IMPACT RESULTS Material excavated contained more dust and less ice than expected Material was finer than expected (more like talcum powder rather than sand) Other materials included clays, carbonates, sodium, and crystalline silicates which were found by studying the spectroscopy of the impact (right) Observations also revealed that comet is about 75% empty space STARDUST MISSION Launched February 1999 Comet Wild 2 rendezvous on January 2004 Captured material using aerogel Dust and carbon-rich material from comet and samples of interstellar dust Earth return January 2006 Re-entry capsule parachuted to Earth's surface Much of dust formed close to young sun and somehow ended up in outer solar system COMET COMPOSITION Some of darkest objects in Solar System (albedos < 0.05) Composition from spectra of comas, and Deep Impact and Stardust results Mass ratio of gases to dust and rock is probably 1 Dominant volatile is water, followed by CO and CO2 Dust consists of silicate minerals (olivine, pyroxene) and carbon-rich CHON (carbon-hydrogen-oxygen-nitrogen) grains Surprising that comets contain hightemperature silicate minerals Similar to meteorite minerals Formed near to Sun and were somehow mixed into cold comet-forming regions of outer Solar System Two-micrometer grain of forsterite, a high-T mineral, collected by Stardust spacecraft from Comet Wild 2. ROSETTA MISSION ESA (European Space Mission) to rendezvous and land on Comet 67P All previous comet missions have been flybys at speeds >20,000 km/hour with closest approach of 240 km Will make long term measurements as comet approaches Sun (from 3.5 AU to 1.4 AU for at least six months) Map nucleus (Aug. 2014) Release Philæ Lander (Nov. 2014) Measurements on surface as comet goes around Sun (Nov. 2014 to Dec. 2015) COMET ORIGINS Formed in outer part of protoplanetary disk Low temperatures high volatile content (water and carbon monoxide in nucleii) Contain silicate grains from hotter parts of solar system mixing Rearrangement of giant planets ejected most planetesimals into interstellar medium Small fraction (10%?) of ejected bodies perturbed by passing stars into Oort Cloud (long-period comets) Planetesimals not ejected that formed substantially beyond orbit of Neptune make up Kuiper Belt (shortperiod comets)