Class 10: Other mass extinctions & the Nemesis theory

Other mass extinctions – impacts? Search for the corresponding craters. Periodic mass extinctions. The Nemesis theory.

Jon Burnett, Oct 1/03, Wales

I : The Big Five extinctions…

There were 5 extinctions that killed 70% or more of species… 65 Myr (Cretaceous-Tertiary) 202 Myr (Triassic-Jurassic) 250 Myr (Permian-Triassic) 367 Myr (Late Devonian) 438 Myr (Ordovician-Silurian)

But there were other periods of mass extinction as well…

II : Other mass extinctions

Raup-Sepkoski

Seems to be a periodicity in the mass extinctions… Appears to be a mass extinction approx.

every 26-30 million years. Regularly spaced… like ticks of a clock.

Warning: this claim is still very controversial! It can sometimes be very hard to prove or

disprove that a particular signal is periodic.

III : Are all mass extinctions caused by impacts?

This would be a very extreme view… If true, impacts would be one of the most

important forces in evolution. Still very very controversial and vigorously

debated.

Impact theory would be boosted if we could find craters corresponding to the other mass extinctions.

But remember how hard (sometimes impossible) it is to find impact craters on Earth… and we must accurately date them.

Morasko, Poland

In fact, this is a whole group of craters…

Late DevonianSiljan, Sweden (368 Myr)

52 km diameter

Permian-TriassicArjguinha, Brazil (245 Myr)

44 km diameter

Triassic-JurassicManicouagan, Canada (214 Myr)

100 km

Jurassic-CretaceousMorokweng, South Africa (145 Myr)

70 km

(not exposed)

Cretaceous-TertiaryChicxulub, Mexico (65 Myr)

170 km

Eocene-Oligocene transition(35 Myr ago)

Chesapeake Bay crater… 35Myr

IV : What could cause periodic impacts and extinctions? Let’s assume that large impacts are periodic

(Shiva Hypothesis). One/several of them occur like clockwork every 26-

30 million years. Only plausible scenario…

Oort cloud suffers a regular disturbance every 26-30 million years.

Disturbance shifts the orbits of many distant comets, making them plunge into the inner solar system.

A small number of those comets hit the Earth, causing the mass extinctions.

But what could disturb the Oort cloud like this?

Possibility I As the Sun orbits the galaxy, we “bob” up and down

within the galactic disk. We probably pass through middle of galactic disk

once every ~30 million years. Maybe this, somehow, disturbs the Oort cloud. Comets rain inwards…

Possibility II (Nemesis Theory) The Sun has a companion star in orbit. Once every 30 million years, that star comes close to

(plunges through?) Oort cloud. Comets rain inwards… We’ve yet to find this star (people have looked!).