22: IMPACTS AND BIO-EXTINCTIONS

Impact of a "planet buster" asteroid
(Don Davis)

A. History

• Based on geological evidence described below, this paper argued that the major extinction of lifeforms, including dinosaurs, 65 million years ago was produced by an asteroid impact.

• Although threats from extraterrestrial impacts had been speculated about before, this study provided the first direct evidence for a link to worldwide biological extinctions.

• Many geologists & paleontologists argue for other, non-astronomical, mechanisms such as the massive volcanic outbreak at the Deccan Traps which might have caused a sudden extreme greenhouse event.

• Earth moves continuously at high speed through a swarm of asteroids, meteoroids, and comet nuclei. E.g. "Apollo," "Aten," and other types of Earth orbit-crossing asteroids.

• The unambiguous conclusion is that major impacts are inevitable! The question is not if?, but when?

• The areal density of lifetime impacts on the Earth's surface for craters over 500-m diameter is higher than that on the Moon. See this illustration of the impact density.

• Finally, the energy deposited by big asteroids is large enough that the destructive effects can be global.

The Barringer "Meteor Crater," near Winslow, AZ. 1 mile diameter.
Created by the impact of a 50-m diameter metallic meteoroid about 50,000 years ago.

B. Direct Evidence for Major Impacts on Earth

Ancient Impacts

• Impact geology: shocked & melted rocks or other debris characteristic of sudden high temperatures or pressures which go beyond what is encountered in volcanic eruptions. This can be identified even in the absence of a definite crater nearby. Such events have been identified at various locations worldwide
  • E.g. Tiny spherules embedded in thin rock layers that originated from molten droplets of rock flung into the atmosphere by explosive impacts. Recent samples found in Australia date from an asteroid impact 2.6 billion years ago.

  • E.g. Debris from a Chesapeake Bay impact event 35 million yrs ago, which produced a 56 mile-wide crater and tidal waves estimated to be 1000 ft high. Click here for map of the swamped region.

• Fossil Craters
  • From spacecraft images, we are now aware of the ferocious impact history of the surfaces of all other solid bodies investigated so far in the Solar System (e.g. Mars, Jupiter's moon Callisto, and of course, our own Moon).
    
  • Owing to weathering and crust recycling, old craters are mostly erased on Earth's surface

  • But we have still been able to identify over 150 fossil craters in regions where older geological strata are exposed. E.g. at right is Manicouagan Crater, Canada. This Space Shuttle image shows a 43 mile diameter annular lake (in winter), part of a 62 mile diameter crater structure. Age: 210 million years.

Continuing Impacts

• Observed near misses (last 50 years)
  
  Some large meteoroids have been seen passing through our atmosphere. At right is a picture taken Aug 10, 1972 in Grand Teton National Park of a near-miss (click for enlargement). The object is about 10-m diameter and at an altitude of about 55 km, moving at 15 km/sec (33,000 MPH). It approached at such a shallow angle that it skipped off the atmosphere. If it had hit the Earth, it would have had H-bomb equivalent impact energy.
  Click here for a video

  There have now been many telescopically-observed near misses within twice the distance of the Moon.
  • A complete list of these, including predicted passages, is given here.

  • Some interesting examples:
    2011 CQ1: a small 1-m diameter meteoroid that came within 3,400 miles of Earth's surface on 4 Feb 2011.
    2005 YU55: a 400-m asteroid that passed within 202,000 miles (inside the Moon's orbit) on 8 November 2011. Largest known near-miss object until 2028.
    2012 DA14: a passage of a 45-m meteoroid at a distance of only 17,000 miles, inside the geosynchronous satellite belt(!), on 15 February 2013.
    This Twitter site routinely reports passages of asteroids within 0.2 AU (19 million miles) of the Earth. (Note that this includes objects up to four times farther than would be nominally considered "hazardous" for an Earth impact.)

• Witnessed impacts.
  • The biggest recorded hit: Tunguska, Siberia 1908
    • Energy release: equivalent to ~20 million tons of TNT. [One million tons, or one "megaton," is the explosive energy of a typical hydrogen bomb.] Tunguska was 1000 x the energy release of the Hiroshima atomic bomb ("only" 20,000 tons).

    • The explosion flattens an area the size of Washington, DC.

    • Likely cause: air detonation (altitude 10 km) of a stony meteoroid about 30 m in diameter

  • Tunguska Redux (Chelyabinsk meteoroid, 15 February 2013):
    
    • In a near-replay of the Tunguska event, a 20-m diameter meteoroid (see picture at right) exploded over Chelyabinsk, Russia, at an altitude of 23 km (14 miles). The shock wave overpressure from the explosion caused extensive structural damage and produced a number of injuries but no deaths.

    • The explosive energy of the meteoroid was equivalent to about 450,000 tons of TNT. It was smaller than Tunguska because the incoming body was smaller and had a lower velocity. It also exploded at a higher altitude, reducing effects at the ground level. The event, however, was a sobering reminder of the impact threat from space.

    • The incoming meteoroid had not been detected before impact and was unrelated to the (ironically) highly publicized approach of 2012 DA14, which passed Earth harmlessly 16 hours later. Mother Nature was just having a little fun raining on the smartypants asteroid prediction parade.

• Continuous atmospheric detonations. Air Force infrared monitoring satellites have discovered that several Hiroshima-sized meteoroid explosions occur each year in the atmosphere.
  • Context: imagine the political firestorm if the Chinese government were doing this!

• Predicted and witnessed impacts: the first and only (so far) of these was 2008 TC3, a 5-m diameter meteoroid that was detected only 20 hours before it hit the Earth in northern Sudan on 7 October 2008. No damage or injuries. A number of fragments were recovered.

C. Energetics

The impact energy deposited equals the kinetic energy (KE) of the incoming object, where (from Newton's dynamics)
• KE = 1/2 M V²

• V is large!
  Orbital velocity ~ 30 km/sec = 66,000 mph

• Mass is large!
  • M = Density x Volume = Density x 1/6 π D³, where D is the diameter of the object.

  • For rocks, Density ~ 5 gr/cc

  • ===> Mass ~ 2.5 D³ tons, where D = diameter in meters

• Combine, and convert to equivalent explosive energy in units of tons of TNT. [1 ton TNT = 4 x 10¹⁶ ergs]

===> Impact energy = 250 D³ equivalent tons of TNT, where D is in meters.
Note the very strong dependence on size. A factor of 10 increase in diameter results in a factor of 1000 increase in deposited energy.
A tiny object with D=4 meters packs the explosive energy of the Hiroshima bomb (20,000 tons).
If D = 1 kilometer, the size of a typical large asteroid, the released energy = 250 billion tons of TNT!

Numerical calculations of impact physics have been made by Sandia Laboratories, among others. You can find videos here,

See the images of the impact the Comet Shoemaker-Levy 9 fragments on Jupiter. Fragment "G" deposited about 6 trillion tons of TNT equivalent energy. For context when you view these pictures, remember that Jupiter is 11 times the Earth's diameter. The movie poster below shows an artist's concept of a much smaller impact on Earth.

D. Potential Impact Scales

• City Buster: 15-m diam meteoroid ===> 10⁶ tons TNT = 1 Megaton (MT). Serious local consequences, if explodes at ground level. Atmosphere provides a partial shield. Hydrogen-bomb scale, but without the radioactivity.

• People Buster: 1-km diam asteroid ===> 250,000 MT. No atmospheric shield. Hemispheric-scale effects. At threshold for global effects. Significant fraction of all humans killed.

• Planet Buster: 10-km diam asteroid ===> 250 million MT. Global effects. Ejected, vaporized rock and water fill atmosphere ===> global winter ===> major extinction of lifeforms, including virtually all humans.
  Note: you have already seen pictures of Planet Buster-class asteroids: e.g. Ida, Eros

E. Impact-Induced Bio-Extinctions

• Half of all species (not individuals) of plants and animals vanish

• Dinosaurs eliminated. Creates eco-niche for small mammals (our forebears)

An Extraterrestrial Origin for the K-T Event

• As first shown by Alvarez et al. in 1980, the sedimentary rock layers at the K-T boundary are abnormally rich in platinum group metals like iridium. Their composition is like meteorites/asteroids, not Earth rocks. Deposition is worldwide. The extraterrestrial origin of this extinction is now virtually certain.
  
• The energy release involved would have required a 10-km diameter asteroid impact.

• The site is now probably identified: Chicxulub crater, N. Yucatan. See map at right. Diameter 110 miles. Date (radioactive method) 65 Myr ago. Debris deposits are scattered throughout the Caribbean area. The image here shows gravity and magnetic anomalies associated with the buried/undersea crater. See this Washington Post article for details.

Earlier Major Impacts

There is some evidence, based on extraterrestrial isotopic signatures, that the largest known extinction (at the Permian-Triassic boundary), 250 million years ago, which extinguished 90% of all lifeforms on Earth, was also impact-related.
Evidence for the Late Heavy Bombardment on the Moon suggests that the Earth was subjected to a similar intense storm of large impacts about 4 Byr ago. This may have been sufficient to sterilize Earth's surface of any primitive lifeforms which had emerged in the first 500 Myr of terrestrial history.

F. Risk Level?

• E.g.: 5 great extinctions in 570 million years implies a rate of about one global extinction (planet-buster) event per 100 million years; perp would be a ~10-km diameter asteroid.

A people-buster event (1-km diam impactor killing 1/4 of human race) will occur once per 150,000 years.
• In human terms, very rare but very serious.

Click here for a plot of frequency of impacts as function of size.

The estimated risk to an individual (e.g. you) of asteroid impacts has declined by a factor of about 30 in the last decade due to improved surveys. The estimated net fatality risk (all impactor sizes) is now 1/700,000 chance per person per lifetime

Laughably small and forgettable, yes? Well.....no. Many people actively worry about events with comparable or smaller statistical probabilities: death from sharks, nuclear power plant explosions, tidal waves, or poisonous snakes, for instance.
I don't recommend that you add asteroid impacts to your list of serious personal anxieties...but you certainly shouldn't worry any less about them than you do about other those more familiar risks. And, of course, the real problem isn't the personal risk, it's the devastation that a 1-km class impactor would inflict on the race and on other higher lifeforms on Earth.

Risk ranking: Astronomers have created the "Torino Scale" (a combination of estimated impact energy with probability of a strike on Earth) as a threat index for potential Earth impactors.

G. Umbrellas

1. First, we must identify threatening "Near Earth Objects" (NEO's). A number of ground-based and space-based surveys, described here, have identified over 9000 NEO's. The census is effectively complete for the brighter, larger objects, but is deficient for those smaller but still dangerous objects under 500-m in diameter.
   • "Planet-busting" large objects (10-km) are relatively bright and already identified. There are relatively few, and none pose a foreseeable threat.

   • "People-busting" medium objects (1-km): it is estimated there are about 980 objects larger than 1 km in Earth-crossing orbits, of which 90% are now identified. No obvious near-term threat, but we can expect 1 impact per 150,000 years.

   • "City-busting" small objects (10--500-m): bad news. Too many (perhaps a million), too faint; search too expensive. We will have to "live" with these. (Tunguska and Chelyabinsk are in this class.) Over 5,000 with diameters larger than 100-m are known, but another 15,000 are expected in that range.

2. Second, we must develop technologies to eliminate them
   • Best method: a gentle velocity deflection when they are still at large distance from Earth. Trying to break up potential impactors with explosives could easily create more danger than it prevents.

   • Requires new, though feasible, space technologies, e.g. an asteroid tugboat

3. Following the asteroid 1997 XF11 public relations debacle, NASA established a Near Earth Object Program to oversee studies of potentially hazardous objects.

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Reading for this lecture:

Study Guide 22
Bennett textbook, Chapter 12
Optional reading:
Essay on Impact Risk (Clark Chapman, 2002; PDF).
Dave Barry column on a "near miss"
Rain of Iron and Ice John S. Lewis (Clemons & SciEngr Lib: QB721.L42.1996)

Reading for next (last) lecture:

Study Guide 23
Bennett textbook, Chapter 24

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Web Links:

Big screen drama! Not just one but TWO big budget movies about impacts appeared in 1998: Deep Impact and Armageddon. Former was OK, but Armageddon was hopelessly egregious crap. For comments on the science content of these flix, see Bad Astronomy.
Both movies lifted their plots from: Lucifer's Hammer by Larry Niven & Jerry Pournelle (the best comet-flattens-Earth novel)

Tutorial on Impact Geology (N. M. Short, Federation of American Scientists)
Background on the K-T Extinction & Alternate Interpretations (UC Berkeley)
Volcano-Greenhouse Alternative for K-T Extinction (McClean, VPI)
Earth Impact Database (UNB, Canada)
Terrestrial Impact Craters (info & images, C. J. Hamilton)
Google Earth Impact Structures Database
Resources on Impact Cratering in the Solar System
Documentary Video on Tunguska Event
Tunguska Event (Wikipedia)
Chicxulub Crater and the K-T Boundary (Wikipedia)
Near Earth Object Resources Page (International Astronomical Union)
Near-Earth-Object Program (NASA-JPL)
Near Earth Object Risk Table (Current) (JPL)
Asteroid Comet Impact Hazards (NASA-Ames)
Human Utilization of Asteroid Material (PERMANENT site)
Asteroid Watch Twitter Feed (JPL)
List of Recent and Upcoming Earth-Asteroid Encounters (spaceweather.com)
Earth Impact Effects Simulator (National Museum of Wales)
Earth Impact Effects Calculator (Univ. Arizona)
Near-encounter public relations flaps:
Asteroid 97XF11
Asteroid 99AN10
Asteroid 1950 DA

A major predicted flyby:
Asteroid 2004 MN4 (Apophis): This ~300-m diameter asteroid will pass 18,600 miles from Earth on April 13, 2029. Impact potential: 850 megatons; will be visible as 3rd magnitude object
Apophis (Wikipedia)
A Call to (Considered) Action on Asteroid 2004 MN4 from the B612 Foundation

Exit Mundi, A Collection of End-of-World Scenarios

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Last modified April 2013 by rwo

Opening painting copyright © 1998, Don Davis. Tunguska areal map from Clark Chapman/John Pike. Impact frequency plot copyright © Prentice-Hall. Chicxulub map copyright © 2001 Athena Publications. Text copyright © 1998-2013 Robert W. O'Connell. All rights reserved. These notes are intended for the private, noncommercial use of students enrolled in Astronomy 1210 at the University of Virginia.

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