Mass Extinctions - Interruptions in the Orderly Process of Evolution

Key points: Chaotic asteroid orbits and ongoing bombardment of the planets; the K/T extinction; other extinctions

Chaos and Asteroid Bombardment

We saw that there are no major collisions with large asteroids  expected for the near future. However, estimates indicate that most of the thousands of Earth-crossing asteroids are likely to collide with the earth in the next few tens of millions of years. Given that the Solar System is very much older than this, is the population of Earth-crossing asteroids replenished in some fashion?

Yes -- calculations indicate that Jupiter's gravitational field is strong enough to perturb asteroid orbits so that eventually some Main Belt asteroids will be deflected and become Earth-approaching asteroids.

Despite the 4.6 billion years for the earth to clear out all asteroids it might collide with, collisions are still frequent. With thousands of members, the solar system is so complex that chaotic orbits can propel asteroids into the inner solar system.

chaos2.gif (78683 bytes) This comet (blue) is on an orbit that takes it back and forth between Jupiter (yellow) and Mars. We show just 50 years of its motion. Every time it comes close to Jupiter it is perturbed into a new orbit, and eventually it is likely to collide with Mars. (animation by M. & G. Rieke, from Dance of the Planets)

Even though the comet has gone through many orbits, its behavior is difficult to predict accurately because of the large effects from Jupiter. Chaos applies to a system where virtually unmeasurable changes can lead to completely different final results. The comet is on a chaotic orbit.

The processes that can deflect the orbits of comets or asteroids lead to the possibility that Earth could collide with a significant sized object.

The frequency and power of impacts are compared with the effects of nuclear warfare to the left. The impact that killed the dinosaurs is labeled K/T, Zharmanshin is a large crater in Russia, and the Arizona crater is labeled "Meter Crater".

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Cartoon picture of comet hitting EarthWhat if a large comet or asteroid hits......


The Shoemaker-Levy Comet Impact with Jupiter gave us a way to test theories of extinction by asteroid impact.

Multiple nuclei of Comet Shoemaker-Levy 9
In the spring of 1993, a very curious comet was discovered. This comet was not  single, but a string of cometary pieces strung out in a line. Running the orbits backwards showed that these pieces most likely came from a parent body which fell apart in 1992 when it passed very close to Jupiter. (from Astronomy Picture of the Day, NASA/HST/H. Weaver, T. Smith

Further calculations showed that the pieces would hit Jupiter in July of 1994. Astronomers around the world prepared to observe the crash.

Orbit of Shoemaker-Levy 9 The orbit of the comet took it close to both Jupiter and Saturn, so it had been in a chaotic situation similar to the comet illustrated previously between Mars and Jupiter.
Artis's concept of impact of SL9, viewed from a distance  


The impacts occurred on the far side of Jupiter. Here are two artist's concepts of how they might have looked


Artist's concept of impact, close-up
Black scars left by impacts on Jupiter Each impact resulted in a bright flash on the limb of Jupiter. When the Jovian rotation brought them into view, there was a large, dark scar at each site. Our measurements of the spectrum of a flash showed that temperatures reached 5000K!

Series of HST pictures showing plume from impact Each nucleus crash had hurled material high above Jupiter. The flashes and high temperatures occurred when this material fell back onto the top of the Jovian atmosphere. Chemical reactions at these high temperatures left the dark scars.


Implications for Collisions with the earth

Artist's concept of KT impactor, starting with beautiful day  

The events on Jupiter agreed closely with the most popular theory for the extinction of the dinosaurs. In this theory, a dramatic impact on the Yucatan coast of a comet or asteroid about 10 km in diameter 65 million years ago caused havoc with life on Earth.  The time of this extinction is called the "Cretaceous-Tertiary" (also called K/T) Boundary.

A series of paintings by William Hartmann shows how it might have looked from a "safe" distance.






















The impactor is about to hit!
It does, causing a huge explosion
The impact site is though to be on north side of the Yucatan Peninsula The location of the impact is shown by shattered rock and a huge, shallow crater. (from
Animation of impact viewed from space; it throws up material that collides all over Earth The impactor threw material up into space. This material eventually fell back all over the earth (falling soonest near the impact site). The friction with the atmosphere heated it to many hundreds of degrees, setting all the plants on fire and literally cooking any exposed animal life -- and it is hard to hide a dinosaur! Some mammals survived because they were burrowing animals and were protected under ground.
Animation of a dinosaur getting fried by impact returning debris To the above left, we show the progressive spreading of returning material over the earth.  To the left, we show a dinosaur's perspective on events. (animations adapted by G. Rieke)
extinct1.jpg (33083 bytes) Disaster strikes! From David Hardy,

Although most dinosaurs were probably killed off in the initial event where returning impact debris superheated the upper atmosphere, the afterevents would have been very destructive too. There would have been huge tidal waves from the shock, and possibly the impact triggered large earthquakes that caused even more. Afterwards, the upper atmosphere would have been saturated with the soot and gases from the massive fires, resulting in years of climate change that included severe acid rain. Here is another version of the events. en00500_1.jpg (18578 bytes) (from L. Close,


Iridium rich layer at KT boundary Supporting evidence for this theory comes from a thin layer of material all around the earth which contains a large amount of the rare element iridium, plus soot from widespread fires. Because the earth is differentiated, iridium is rare on its surface (it is heavy and has sunk to the center). However, it would be well mixed in an asteroid/comet, so this layer appears to be rich in material from the impactor. (from

However, there are also a number of variations on the theorylilnk to an extra topic

Paleocene forest
"When the dust finally settled the mammals found a world in which most vertebrates larger than themselves were dead; the meek had inherited the earth." - John Allman, "Evolving Brains," 2000

In any case, the elimination of the dinosaurs left the evolutionary field open for mammals buttonex.jpg (1228 bytes) The reconstruction to the left shows primitive mammals in a scene just after the K/T extinction  (from

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There have been a number of mass extinctions previous to the K/T event.

extinct.gif (5467 bytes) The major extinctions are indicated to the left by peaks in the rate of disappearance of biological "families" of plant and animal types. The Permian-Triassic event coincides with a huge volcanic event in Siberia that produced a flow of lava covering 700,000 square miles. It is likely that the gas and dust from these eruptions produced a climate catastrophe. There may have also been an asteroid impact at about the same time to make things worse. It must have come close to wiping out all life on earth; 96% of marine invertebrates species and 70% of land vertebrates became extinct along with many insects. Ironically, this extinction cleared away other animal forms and gave the dinosaurs the opening that allowed them to rule the earth for 200 million years. The cause of many of the other extinctions is not clearly understood. They might be related to impacts, to other astronomical events like nearby supernovae, or to geological events that resulted in large-scale climate change, such as volcano eruptions buttonex.jpg (1228 bytes).

How safe are we from another extinction that might include uslink to a key question

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Walleserops trilobite, Devonian period (G. Rieke, Nat Mus. Nat. Hist.)

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Rodin's "The Thinker", from Cleveland Museum of Art,

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