Solar System Fluff

Effects of an Asteroid Impact on Earth

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Some asteroids have orbits that cross the orbit of the Earth. That means that the Earth will be hit sometime. Recent studies have shown that the Earth has been hit an alarmingly large number of times in the past. One large impact is now thought to have contributed to the quick demise of the dinosaurs about 65 million years ago. What would be the effects of an asteroid hitting the Earth?

Known impact sites on the Earth's continents. See also LPI's Terrestrial Impact Site for pictures of the craters.

What follows is a condensation of an excellent article by Sydney van den Bergh called ``Life and Death in the Inner Solar System'' in the May 1989 issue of the Publications of the Astronomical Society of the Pacific (vol. 101, pages 500-509). He considers a typical impact scenario of a 10-kilometer object with density = 2.5 times that of water, impacting at a speed of 20 kilometers/second. Its mass = 1.31 trillion tons (1.31 × 10¹⁵ kilograms). A 1-kilometer object has a mass = 1.31 billion tons.

Explosion

Obviously, something this big hitting the Earth is going to hit with a lot of energy! Let's use the energy unit of 1 megaton of TNT (=4.2× 10¹⁵ Joules) to describe the energy of the impact. This is the energy one million tons of dynamite would release if it was exploded and is the energy unit used for nuclear explosions. The largest yield of a thermonuclear warhead is around 50--100 megatons. The kinetic energy of the falling object is converted to the explosion when it hits. The 10-kilometer object produces an explosion of 6 × 10⁷ megatons of TNT (equivalent to an earthquake of magnitude 12.4 on the Richter scale). The 1-kilometer object produces a milder explosion of ``only'' 6 × 10⁴ megatons (equivalent to an earthquake of magnitude 9.4 on the Richter scale).

On its way to the impact, the asteroid pushes aside the air in front of it creating a hole in the atmosphere. The atmosphere above the impact site is removed for several tens of seconds. Before the surrounding air can rush back in to fill the gap, material from the impact: vaporized asteroid, crustal material, and ocean water (if it lands in the ocean), escapes through the hole and follows a ballistic flight back down. Within two minutes after impact, about 10⁵ cubic kilometers of ejecta (10¹³ tons) is lofted to about 100 kilometers. If the asteroid hits the ocean, the surrounding water returning over the the hot crater floor is vaporized (a large enough impact will break through to the hot lithosphere and maybe the even hotter asthenosphere), sending more water vapor into the air as well as causing huge steam explosions that greatly compound the effect of the initial impact explosion.

There will be a crater regardless of where it lands. The diameter of the crater in kilometers is = (energy of impact)^(1/3.4)/10^6.77. Plugging in the typical impact values, you get a 150-kilometer diameter crater for the 10-kilometer asteroid and a 20-kilometer diameter crater for the 1-kilometer asteroid. The initial blast would also produce shifting of the crust along fault lines.

Meteor (Barringer) Crater in northern Arizona (about 1 kilometer across). Select here for a view from the rim.
Chicxulub Crater in Yucatan, Mexico (from the one that may have killed off the dinosaurs).

Tsunami

The oceans cover about 75% of the Earth's surface, so it is likely the asteroid will hit an ocean. The amount of water in the ocean is nowhere near large enough to ``cushion'' the asteroid. The asteroid will push the water aside and hit the ocean floor to create a large crater. The water pushed aside will form a huge tidal wave, a tsunami. The tidal wave height in meters = (distance from impact)^-0.717 × (energy of impact)^0.495/ (10^10.17). What this means is that a 10-km asteroid hitting any deep point in the Pacific (the largest ocean) produces a megatsunami along the entire Pacific Rim.

Some values for the height of the tsunami at different distances from the impact site are given in the following table. The heights are given for the two typical asteroids, a 10-kilometer and a 1-kilometer asteroid.

Distance (in km)	10 km	1 km
300	1.3 km	43 m
1000	540 m	18 m
3000	250 m	3 m
10000	100 m	3 m

The steam blasts from the water at the crater site rushing back over the hot crater floor will also produce tsunamis following the initial impact tsunami and crustal shifting as a result of the initial impact would produce other tsunamis---a complex train of tsunamis would be created from the initial impact (something not usually shown in disaster movies).

Global Firestorm

The material ejected from the impact through the hole in the atmosphere will re-enter all over the globe and heat up from the friction with the atmosphere. The chunks of material will be hot enough to produce a lot of infrared light. The heat from the glowing material will start fires around the globe. Global fires will put about 7 × 10¹⁰ tons of soot into the air. This would ``aggravate environmental stresses associated with the ... impact.''

Acid Rain

The heat from the shock wave of the entering asteroid and reprocessing of the air close to the impact produces nitric and nitrous acids over the next few months to one year. The chemical reaction chain is:

N₂ + O₂ Į> NO (molecular nitrogen combined with molecular oxygen produces nitrogen monoxide)
2NO + O₂ Į> 2NO₂ (two nitrogen monoxide molecules combined with one oxygen molecule produces two nitrogen dioxide molecules)
NO₂ is converted to nitric and nitrous acids when it is mixed with water.

These are really nasty acids. They will wash out of the air when it rains---a worldwide deluge of acid rain with damaging effects:

destruction or damage of foliage;
great amounts of weathering of continental rocks;
the upper ocean organisms are killed. These organisms are responsible for locking up carbon dioxide in their shells (calcium carbonate) that would eventually become limestone. However, the shells will dissolve in the acid water. That along with the ``impact winter'' (described below) kills off about 90% of all marine nanoplankton species. A majority of the free oxygen from photosynthesis on the Earth is made by nanoplankton.
The ozone layer is destroyed by O₃ reacting with NO. The amount of ultraviolet light hitting the surface increases, killing small organisms and plants (key parts of the food chain). The NO₂ causes respiratory damage in larger animals. Harmful elements like Beryllium, Mercury, Thallium, etc. are let loose.

Temperature Effects

All of the dust in the air from the impact and soot from the fires will block the Sun. For several months you cannot see your hand in front of your face! The dramatic decrease of sunlight reaching the surface produces a drastic short-term global reduction in temperature, called impact winter. Plant photosynthesis stops and the food chain collapses.

The cooling is followed by a much more prolonged period of increased temperature due to a large increase in the greenhouse effect. The greenhouse effect is increased because of the increase of the carbon dioxide and water vapor in the air. The carbon dioxide level rises because the plants are burned and most of the plankton are wiped out. Also, water vapor in the air from the impact stays aloft for awhile. The temperatures are too warm for comfort for awhile.

In the early 1990s astronomers requested funding for an observing program called Spaceguard to catalog all of the near-Earth asteroids and short period comets. The international program would take 10 years to create a comprehensive catalog of all of the hazardous asteroids and comets. The cost for the entire program (building six special purpose telescopes and operation costs for ten years) would be less than what it costs to make a popular movie like Deep Impact or Armageddon.

In mid-1999 NASA and the US Air Force began a Near-Earth Object search program using existing telescopes to locate 90% of the NEO's larger than 1 kilometer in diameter in ten years. Of the approximately 1000 asteroids larger than 1 kilometer in diameter that are thought to pose a threat of impacting Earth, about 75% have been found so far. To find out more about the United States' program go to NASA's Asteroid and Comet Hazards site and JPL's Near-Earth Object Program (both will appear in another window). You can try out your hand at making big craters at the Solar System Collisions website and the Earth Impact Effects Program website (but, please try not to wipe out the entire Earth).

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last updated: January 3, 2008

Is this page a copy of Strobel's Astronomy Notes?

Author of original content: Nick Strobel