Life in a Shooting Gallery
←
→
Page content transcription
If your browser does not render page correctly, please read the page content below
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. • fB = 0.03 × E-4/5 : 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 • PScum = log10 (10PS1 + 10PS2 + 10PS3 + …) • PS=0: background risk • PS>0: more likely than random event • 0>PS>-2: worth monitoring • PS
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 • Kinetic energy = !"v2, or > 6x1011 erg/gram • Impactor mass M = #p r (D/2)3 $ • 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
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. Direct hit may survive. Irons • Generally survive
Physics of the Airburst When the aerodynamic pressure rv2 exceeds the yield strength, the body deforms • Atmospheric pressure r = r0e-z/H • r0= 10-3 g/cm3; H=8 km • Pressure needed to induce deformation • PB ~ 10(3.1 + 1.97√rl) ; rl is density of impactor • Deceleration is due to atmospheric drag ! "# $ • !% =3 r CD / 4 rlD sinq ; D = initial diameter; !% !& =-vsinq Are airbursts dangerous: see https://www.livescience.com/64179-ancient-cosmic-airburst-middle-east.html
Airburst models Source: Prof. J.L. Lattimer
Crater Sizes Dcrater ~ 4.02 (ri/rT)1/3 D0.78 v0.44 sin1/3 q km • ri: density of impactor • rT: density of ground • D: initial diameter of impactor (km) • v: impact velocity (km/s); generally close to v0 • q: angle with respect to the normal • For an ocean impact, replace 4.02 with 3.74 Relation is from simulations and experiments For other planets, add a term (gÅ/g)0.22, where g is the gravitational acceleration at the surface
Impact Craters on Earth 190 catalogued in Earth Impact Database http://www.passc.net/EarthImpactDatabase/New%20website_05-2018/Index.html Biggest: (rim diameters; age) • Vredefort, South Africa. 160 km, 2.0 Gyr • Chicxulub, Yucatan. 150 km, 65 Myr • Sudbury, Ontario. 130 km, 1.8 Gyr • Popigai, Russia. 90 km, 35 Myr • Acraman, S. Australia. 90 km, 590 Myr • Manicouagan, Quebec. 85 km, 214 Myr • Morokweng, South Africa. 70 km, 145 Myr • Kara, Russia. 65 km, 70 Myr • Beaverhead, Montana. 60 km, 600 Myr • Tookoonooka, Queensland. 55 km, 128 Myr • Charlevoix, Quebec. 54 km, 342 Myr
Source: Earth Impact Database/PASSC
Notable Recent Craters • Carancas. Peru. 9/15/2007. 13m diameter. Cause of arsenic poisoning. • Sikhote Alin. Russia. 2/12/1947. ~100 craters up to 20m diameter. Fragments of iron meteorite. • Wabar, Saudi Arabia. Between 1750 and 1930. 110m diameter crater. Iron • Hiawatha Greenland. 12,000 yrs. 30km. • Chesapeake Bay. 40km crater, 35 Mya
From: Extinction, by D.M. Raup, 1991 (Norton)
Have people been hit by meteorites? Two documented cases: • 11/30/1954. Sylacauga, Alabama. Ann Hodges hit in thigh by 9 lb meteorite than came through roof of house. (https://www.smithsonianmag.com/smart-news/only-person-ever-hit- meteorite-real-trouble-began-later-180961238/) • 8/14/92. Mbale Uganda. Child hit in head by 3.6 gram stone in meteor shower. Slowed by trees; child uninjured Reported Animal Deaths: • Horse in New Concord OH (1860) • Dog in Nakhla Egypt (1911)
Other Impacts
Causes of Mass Extinctions • End Ordovician: • Asteroid collision reduced solar flux; ice ages • Sea level drop • Late Devonian: • Global anoxia; plant-induced CO2 reductions • Viluy Traps (Siberia) • End Permian: • Tectonically-induced climate change (10-20 oC global warming) • Siberian Traps • Asteroid impact(s)? • End Triassic: • Volcanic activity; gas hydrates; oceanic acidification • Central Atlantic Magmatic province • Climate change (3-6 oC global warming) • End Cretaceous (KT): • Chicxulub asteroid impact • Deccan traps
Large Igneous Provinces (LIPs) • Extensive geographic areas covered with igneous rock. • Area > 100,000 km2; Volume > 106 km3 • Extruded at divergent plate boundaries, mantle plumes • Deposited in 1-3 Myr • Flood Basalts (continental LIPs) • Called Traps from trappa (stairs) in Swedish • North Atlantic Igneous Province (56 Mya) • Deccan Traps (65 Mya)* 1.5 x 106 km2, 3x106 km3 • Central Atlantic Magmatic Province (201 Mya)* • Siberian Traps (251 Mya)* 7x106 km2, 4x106 km3 • Vilyuy Traps (373 Mya)* *associated with mass extinction event • Other examples: • Yellowstone • Lunar Maria • Venus
Periodic Extinctions? Raup & Sepkowski (1984; PNAS 81, 801) noted a possible 26 year periodicity in mass extinction events. • A suggested cause was a low mass stellar companion to the Sun, in an elliptical orbit that brought it through the Oort cloud every 26Myr • a=88000 au • It was given a name: Nemesis • At the time, it’s existence could not be ruled out. • Orbit likely not stable over age of solar system • WISE and 2MASS IR all sky surveys found nothing.
Periodic Extinctions? Another astrophysical possibility: • The Sun passes through the plane of the Galaxy about every 35 Myr (epicyclic motions) • Dust clouds tend to be in the mid-plane • Could increased galactic dust reduce solar irradiance? • May modulate galactic cosmic rays • Rampino et al. suggest some unknown interaction between galactic dark matter and the Earth’s core. • Unlikely because • Dark matter is likely distributed spherically, and • Requires new physics
Periodic Extinctions? Suggested periodicity criticized due to significant uncertainties. Rampino et al. (2020; Historical Biology) revisited the issue: • There are now more known extinction events • Ages are better known. 10 major extinction events appear to have a 27.5 Myr periodicity • 8 correspond to enhanced Large Igneous Provinces (flood basalts) • 3 correspond to large craters
Are Extinctions Periodic? Rampino et al tabulate (going back 300 Myr) • 10 tetrapod extinction events • 13 marine mass extinctions • 16 large igneous provinces (LIPs) • PASSC tabulates 66 large craters with age uncertainties
The power density (periodogram) for the events tabulated by Rampiro et al plus the craters in the PASSC database. The strongest peak is at 27.7 Myr. It is not very significant.
The number of events, by type, plotted as a deviation from the prediction of Te=8.9 Myr + 27.7 Myr * x Craters and LIPs clearly peak near 0 offset.
So, Are Extinctions Periodic? • The evidence is marginal • Mass Extinctions are not points in time; they unfold over millions of years
Is Cratering Periodic? The evidence is stronger • The reason for this is unknown • Reasons are more likely geological than astrophysical • The crater record is incomplete due to resurfacing and ocean impacts • Are LIPs caused by major impacts?
Effects of Large Asteroid Impacts Land impact • Crater/blast wave • Nuclear winter • Global wildfires Ocean impact • Tsunamis • Blast wave • Nuclear winter Can converging shocks crack the crust?
Mitigation Prediction • Underway Prevention • Nuclear Weapons? • No • Paint? • Maybe the Yakovsky effect can be put to use • Rocket engines? Can change orbits in predictable ways
You can also read
