ASTR 1210 (O'CONNELL) STUDY GUIDE
22: IMPACTS AND BIO-EXTINCTIONS
Impact of a "planet buster" asteroid
"We can never anticipate
the unseen good or evil that may come
upon us suddenly out of
space." --- H. G. Wells
Until the 1950's, craters on the Moon and Earth were usually
interpreted as having a volcanic origin even if they were not located
in volcanically active regions. Recall that until the first
spacecraft reconnaissance in the mid-1960s's, there was no direct
evidence for craters on other planets (excepting the Moon). Then,
E. Shoemaker demonstrated (1960), by comparing the structure of
the Barringer Crater in
Arizona & others to nuclear bomb craters and discovering the presence
of shock-heated minerals
like coesite, that most isolated craters were formed by explosive
impacts, not vulcanism.
This revealed that the history of the Earth and solar system was even
more violent than had been supposed. But the main revolution in
our view of impact events was introduced by the publication by Alvarez
et al. in Science Magazine (1980) of
"An Exterrestrial Cause for
the Cretaceous-Tertiary Extinction".
There has been great controversy over the impact/extinction
- 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
- Although threats from extraterrestrial impacts had been
speculated about before, this study provided the first direct
evidence for a link to worldwide biological extinctions.
But the extraterrestrial proponents have indisputable facts on their
- 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
- 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
- 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
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 large 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.
- 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.
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.
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.
- Tunguska, Siberia 1908:
the biggest recorded hit
- 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
- Likely cause: air detonation (altitude 10 km) of a stony meteoroid about 30 m
- 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
(because of its small size and the fact that it approached from the
direction of the Sun) and was unrelated to the (ironically) highly
publicized approach of 2012 DA14, which passed Earth harmlessly only 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 (so far) only
one 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.
Where does the tremendous explosive energy of impactors originate? We
can use simple physics to predict that:
The impact energy deposited equals the
kinetic energy (KE) of the incoming object, where (from Newton's dynamics)
Want to see a real impact on these gigantic scales?
===> Impact energy = 250 D3 equivalent tons of TNT, where D is in
Note the very strong dependence on size. A factor of 10 increase in diameter
results in a factor of 1000 increase in deposited energy.
- KE = 1/2 M V2
- V is large!
Earth's orbital velocity ~ 30 km/sec = 66,000 mph
Maximum impactor velocities could be up to about 120,000 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]
(TNT is a powerful and widely-used chemical explosive, used as a
standard of comparison for other kinds of explosives.)
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
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. Both witnessed Russian
impactors (see above) were in this category.
- 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.
E. Impact-Induced Bio-Extinctions
The fossil record (at right) shows 5 great
extinctions in 570 million years. These are times where the fossil
record abruptly changes character and many species vanish from more
recent rocks. It is now believed that most of these were probably
induced by extraterrestrial impacts.
The last great extinction was 65 million yrs ago at
the so-called Cretaceous-Tertiary ("K-T") boundary in the fossil record.
- Half of all species (not individuals) of plants
and animals vanish
- Dinosaurs eliminated. Creates eco-niche for small mammals (our
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
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
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?
We can crudely estimate the frequency of large impacts from the
history of lunar cratering & bio-extinctions on Earth.
Adjusting for larger number of smaller impactors:
- 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.
event (1-km diam impactor killing 1/4 of human race) will occur once
per 150,000 years.
for a plot of the predicted frequency of impacts as function of
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
- In human terms, very rare but very serious.
- Even though Homo sapiens has existed for about 250,000
years, if the most recent of these events occurred before about 15,000
years ago, there would be no records of it.
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
catastrophes, 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.
- First, we must identify threatening "Near Earth Objects"
(NEO's). A number of ground-based and space-based surveys,
have identified over 10,000 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 almost
all have 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. A
recent study finds that the number of potential city-busting impactors
may have been underestimated by as much as a factor of 10.
- 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
- Requires new, though feasible, space technologies, e.g. an
- Following the asteroid 1997 XF11 public relations debacle, NASA established a Near Earth Object Program to oversee
studies of potentially hazardous objects.
Reading for this lecture:
Study Guide 22
Bennett textbook, Chapter 12
Reading for next (last) lecture:
Study Guide 23
Bennett textbook, Chapter 24
November 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.