Life in a Shooting Gallery Meteors, Meteorites, and Meteoroids • Meteor: the streak of light seen in the sky • Meteorite: the rock found on the ground • Meteoroid: the rock before it hits Earth Meteorites are easily-studied remnants of the formation of the solar system Meteors “It is easier to believe that Yankee professors would lie than that stones would fall from heaven.” -- attributed to Thomas Jefferson From below … Meteors … and above Meteors • Most are the size of a grain of sand • They vaporize about 75-100 km up when they hit the atmosphere • Impact velocities >20 km/s • The trails are ionized gas • Best viewed after midnight Meteor Showers Occur when Earth passes through the orbit of a comet. Examples: • Orionids: comet 1P/Halley Oct 21-22 • Leonids: comet 109P/Tempel-Tuttle Nov 17-18 • Geminids: asteroid 3200 Phaeton Dec 13-14 • Perseids: comet 55P/Swift-Tuttle Aug 12-13 • Lyrids: comet Thatcher Apr 22-23 Orbits of Meteor Showers Fireballs and Bolides • Very bright meteors • May leave a persistent trail • Due to impacting object bigger than about 1m Geminid Fireball 12/9/2010. source: S. Korotkiy, Russian Academy of Sciences Grand Tetons Meteor 8/10/72. 3-14m Apollo asteroid. V=15 km/s; 15km altitude Bering Sea Bolide Dec 18, 2018. 170 kT explosion. 26 km height Source: NASA Peekskill Meteorite 10/9/92 12 kg Stony-Iron Classification S C M Primitive meteorites (chondrites) Unchanged since solar system formation • Stony: rocky minerals + small fraction of metal flakes • Carbonaceous (Carbon-rich): like stony, with large amounts of carbon compounds Primitive meteorites (chondrites) Majority of meteors • Accreted from solar nebula • Chondrules: droplets formed during accretion • Stony/Carbon-rich • > 3 AU, carbon compounds condense • Carbon-rich formed at outer edge • More stony meteorites hit Earth than carbon-rich Processed meteorites (achondrites) Fragment of larger, differentiated object • Metal rich: mostly iron/nickel • Stony-Iron: composition resembles terrestrial planet crust/mantle; some with basalts • Stony meteorites: • About 85% are primitive (or undifferentiated) Processed meteorites (achondrites) Fragment of large asteroid that differentiated • Rocky • Made from lava flows • Surface material • Metal-rich • Proof of differentiation Estimate: ~10 geologically active asteroids initially • Last remaining: the asteroid Vesta Biases • Irons most likely to survive impact • About 7% of falls; 50% of finds • Stony most likely to be overlooked C type Marília Meteorite: chondrite H4. Marília, Brazil, 10/5/1971 M type Willamette - AMNH Pallasite Chelyabinsk 2/15/2013 Chelyabinsk • Incoming speed ~ 19 km/s • Shallow entry angle • Mass ~ 10,000 tons • Radius ~ 20 m • Stony (S) type meteorite • LL Chondrite (low Fe and total metal abundance) • Orbit derived from observations • Originated in the Apollo group of asteroids Chelyabinsk Meteor Orbit Chelyabinsk Kinetic Energy • 200-990 kT of TNT • kT = 4x1012 Joules = 4x1019 ergs • Hiroshima atom bomb: 15 kT • Estimate from • a.) airburst shock waves (38-23 km altitude) • b.) radiated energy • Damage as far as 120 km from impact • Largest fragment: 1.5m diameter Chelyabinsk Fragment 112 gm; cube is 1 cm Origin of Chelyabinsk Meteorite Dates to 4.6 Gya • Parent body affected by impacts at 30 Myr and 100 Myr • Argon isotopes suggest impact 29 Mya • Surface exposed to solar radiation for 1.2 Myr Dates from radioisotopic analysis Yesterday’s Meteors Source: spaceweather.com Meteors and Asteroids • Most meteors originate in the asteroid belt • Meteors and asteroids • Have similar spectra • Have similar orbits • Differ primarily in size Orbits of Meteors Comets and Meteor Showers Near Earth Asteroids • Potentially Hazardous Asteroids • Earth Minimum Orbit Intersection Distance (MOID) of 0.05 AU or less • diameter larger than 150 m • 2037 known • Not all will hit Earth Halloween Asteroid 2015 TB 145 Passed at 0.003AU on 10/31/15 Radar Images of NEAs Near Earth Asteroids Near Earth Asteroids Near Earth Asteroids Near Earth Asteroids Risk: The Torino Scale Risk: The Torino Scale Risk: Palermo Scale Combines probability and danger to estimate risk • PS = log10 R • R is relative risk • R = PI / (fB × DT), • PI : impact probability of the event in question • DT : time until the potential event, measured in years. -4/5 • fB = 0.03 × E : annual background impact frequency. • = annual probability of an impact event with energy E (in megatons of TNT) at least as large as the event in question. • Cumulative Palermo Scale PS1 PS2 PS3 • PScum = log10 (10 + 10 + 10 + …) • PS=0: background risk • PS>0: more likely than random event • 0>PS>-2: worth monitoring • PS<-2: not an imminent hazard Reference: https://cneos.jpl.nasa.gov/sentry/palermo_scale.html Palermo Scale Watch List • 29075 (1950 DA): PScum=-1.42 • 101955 Bennu: PScum=-1.69 • 1979 XB: PScum=-2.79 • 2002 SG344: PScum=-2.84 • 2009 JF1: PScum=-2.88 • 2007 FT3: PScum= -2.95 The ESA Risk List 1136 Asteroids with a non-zero probability of hitting Earth* *Based on current data Source: https://neo.ssa.esa.int/risk-list Estimating Asteroid Sizes H magnitude • An absolute magnitude for asteroids • The brightness observed • 1 au from Earth, • 1 au from Sun, • at 0o phase angle (physically impossible) • D = 1329/ � 10-0.2H km • a: geometric albedo • Reference: https://cneos.jpl.nasa.gov/tools/ast_size_est.html This Month’s NEAs Velocity Asteroid Date(UT) Miss Distance Diameter (m) (km/s) 2021 EN4 2021-Mar-15 0.2 LD 17.3 4 2021 EJ3 2021-Mar-16 2 LD 2.3 10 2021 DT 2021-Mar-16 18.3 LD 7.3 34 2021 EW3 2021-Mar-16 6.5 LD 8.5 18 2021 EO2 2021-Mar-17 6.2 LD 6.3 9 2021 EY2 2021-Mar-19 9.4 LD 9.7 16 2021 DP2 2021-Mar-20 7.5 LD 4.3 22 231937 2021-Mar-21 5.3 LD 34.4 1024 2021 EV3 2021-Mar-25 19 LD 18 91 2021 CX5 2021-Mar-27 7.7 LD 5.6 48 2020 GE 2021-Mar-27 12.7 LD 1.5 8 2019 GM1 2021-Mar-31 15.1 LD 3.9 14 2015 MB54 2021-Apr-06 13.6 LD 3.7 57 2020 GE1 2021-Apr-07 12.2 LD 4.2 14 2014 FO38 2021-Apr-07 16.8 LD 8.3 20 2020 UY1 2021-Apr-15 16 LD 8.7 22 2017 HG4 2021-Apr-16 7.6 LD 4.1 10 Source: spaceweather.com Barringer Crater, AZ !.2 km diameter, 200 m deep. Age: 49,200 ± 1700 years The Barringer Crater Meteor • 50m diameter iron • Approached from north • Mass ~ 500 million kg ~ 500 thousand tons • Kinetic Energy: 20-60 MT The Canyon Diablo Meteorite • Fragments of the meteorite that created Barringer Crater, Arizona • Iron metorite: 90% Fe, 7% Ni, 1% S, 1% C • Total recovered weight: 30 tons • ESS fragment: 70 lb? Crater Sizes • Impact velocity is the vector sum of • Escape velocity from Earth 2��Å/�Å (11.2 km/s) • Plus intrinsic velocity • Intrinsic velocities range from 0 to 71 km/s ! 2 11 • Kinetic energy = "v , or > 6x10 erg/gram #p 3 • Impactor mass M = $ r (D/2) • r ~ 1 for a comet nucleus • r ~ 3 for a stony (type C or S) asteroid • r ~ 5 for a metallic (type M) asteroid • Rules of thumb: • on Earth: Rcrater ~ 20 Rimpactor • on Moon: Rcrater ~ 12 Rimpactor Crater Formation • Craters are almost always circular • Volume of debris rim ~ excavated volume • On impact the ground and the impactor are compressed • Reverse shock wave vaporizes the impactor • Reverse shock expels ejecta from crater, forming rim • Often central rebound The KT Event The KT Event • The Cretaceous-Tertiary Boundary • Marks the end of the Cretaceous and start of the Paleogene • Mass extinction event; 75% of species died • Impact origin proposed by L. Alvarez in 1980 • Dated to 66.01 Mya Evidence for An Impact • The world-wide Iridium layer Possible Causes of KT Mass Extinction Event • Asteroid/Comet Impact • Predicts large crater • Nearby Supernova • Predicts 244Pu (half life 83 Myr) • Vulcanism/Tectonic Activity All 3 predict Iridium excess Evidence for An Impact • The world-wide Iridium layer • Chicxulub Crater – 150 km diameter Gravity Maps Source: Sky anD Telescope Source: quora.com Result of the Impact • Ocean impact • 30 km deep hole in crust • Crater boundary granite • Sulfates (gypsum) vaporized • Shocked crystals • Aerosols caused 1-2 years of nuclear winter • Rapid ocean acidification • Extinctions perhaps abetted by volcanic activity • Evidence of tsunamis in Carribean • Ash layer: Global (?) wildfires • Effects felt planet-wide The KT Impactor • 10-15 km asteroid or 15-30 km comet • Most likely a carbonaceous chondrite • Family unknown • Energy release ~ 420 zJ (4 x 1023 J) • 108 Mt of TNT, 109 Hiroshima bombs • Blast wave would have killed everything within 1000 km Tunguska Event • June 30 1908 • 2150 km2 of forest flattened • No crater • Probably air burst at 5-10 km • Estimated diameter 60-200 m • Equivalent of 3-5 MT explosion • Pressure wave equivalent of mag 5 earthquake • Comet or asteroid? • Long path suggests stony-iron asteroid • Skipped off atmosphere • See Khrennikov, D.E et al. 2020, MNRAS 493, 1344 Tunguska aftermath Tunguska Event • Expect one about every 300 years • Since 2/3 of Earth is ocean, expect one on land every millenium. • Probably an ordinary chondrite that disintegrated and exploded about 9 km up • 5 hours later, it would have taken out St Petersburg Denouement of the Impactors Depends on • the mass and composition (friability) of the impactor • The velocity and angle of impact At 11 km/s, an impactor passes though the atmosphere in <10 seconds • Aerodynamic forces due to ram pressure µ rv2 Denouement of the Impactors Anything big (≳30m) survives Comets (low density; friable) • Disintegrate and explode high in the atmosphere Carbonaceous chondrites • Disintegrate and explode lower in the atmosphere Ordinary (stony) chondrites • Depends on the angle.