Theme 10 – Leftovers: Impacts and Mass Extinctions ASTR 101 Prof. Dave Hanes

The Effect of Impacts

Two Competing Hypotheses

The Disappearance of the Dinosaurs

Just How Abrupt Was It?

First Evidence Suggesting a Catastrophic Event Discovery of the thin KT Layer [K = Cretaceous] relatively high in relatively high inIridium contains lots of ash contains lots of ash and soot is seen world-wide, is seen world-wide, typically several cm thick is considerably thicker is considerably thicker in southern parts of North America

Different fossils (including micro-organisms) are seen above and below the layer. Much more Iridium in the Much more Iridium in the layer than you would expect (but still just a few parts per billion!) layer than you would expect (but still just a few parts per billion!)

A Helpful Analogy The Crime Scene At a murder scene, we find a tiny trace of my DNA (perhaps from a drop of sweat) although I claim never to have been there It’s not the dominant material present, but it is surprising to find it at all. The KT Boundary Earth has its fair share of Iridium, but most of it is in the core, thanks to differentiation: there’s very little in crustal material. So where did the extra Iridium in the KT layer come from?

Three Obvious Possibilities Perhaps a period of enhanced volcanic activity dredged Iridium up from deeper layers (and the volcanism killed off many species, including the dinosaurs); or Perhaps a big impact (by an asteroid, say) blasted out a lot of material from deep within the Earth, including Ir; or Perhaps an incoming object, with its own share of Iridium, broke up on impact, and sprinkled its own material (and additional material from the Earth’s crust) all over the planet.

Other Important Evidence 1. Evidence of extensive fires 1. Impact site found, surrounded by ‘ejecta blanket’ 1. Near the site, “shocked” quartz crystals (which are evidence of direct impact) 1. Evidence of huge tsunamis 1. The impact site has the right age!

Evidence for Huge Fires - lots of soot and organic material in the KT layer

The Impact Site (the 100-km diameter crater suggest an impact by a 10-km body

Ejecta and Tsunamis

The Chicxulub Story… Alan Hildebrand

Before and After (but much overgrown and eroded today)

One Question: Would There Be Global Damage? Would the impact rupture the Earth as a whole, or knock it out of its orbit? A 10 km comet would have about 1 trillionth of the mass of the Earth. If it is brought to a halt on impact, the Earth itself will pick up about one trillionth of the comet’s original speed in that new direction (thanks to the Conservation of Linear Momentum) – obviously negligible. So the Earth’s orbital motion would be utterly unaffected.

But a Lot of Energy is Pumped into the Biosphere! The incoming object (10 km diameter) will have a mass of at least a trillion tons It is coming in at perhaps 50 km/sec On impact, it releases as much kinetic energy as about 100 trillion tons of TNT

A Large Hydrogen Bomb equivalent to ~10 megatons Chicxulub is equivalent to about ten million such hydrogen bombs!

Does the Atmosphere Protect Us? No. At its velocity, the object meets the atmosphere just a fraction of a second before impact with the solid surface. (Does your skin protect you from a bullet?)

Immediate Consequences The compression of the air causes enormous heating. Animals and plants in the vicinity crumple up “…like cellophane in a fire” (B. Bryson) There are atmospheric shock waves from the supersonic flight of the asteroid. (Remember Chelyabinsk?) The impact causes tsunamis and a big ‘ejecta blanket,’ and vaporizes a lot of sea water.

Hollywood’s Treatment Deep Impact

Subsequently Pulverized material (from the asteroid and the crustal rocks) is thrown into the air – many billions of tons If falls back in pieces like uncountable numbers of meteors, glowing red hot On continent-wide scales, the whole sky lights up like a blast furnace Grasslands and forests over much of the Americas erupt in flame (hence the soot)

Long-Term Consequences ‘Nuclear Winter’ : the foreseeable consequences of a nuclear war. Smoke and soot fills the air, blocking the sun; we enter a long ‘deep freeze.’

Can It Happen Again? Yes. We are in a ‘cosmic shooting gallery’ Because asteroids are low-mass, their orbits are constantly being perturbed by the gravity of Jupiter and the other planets So we cannot safely predict their orbits into the remote future, even if we could find all the dangerous objects now

There are Recent Examples by objects of various sizes We have already considered: Tunguska – 1908 Shoemaker-Levy – 1994 Chelyabinsk Near-miss captured in home movie (1950s): Toutatis

Fortunately: For every big asteroid, there are millions of mid-sized ones and trillions of pebbles. So big impacts are relatively rare.

Don’t be Complacent! An event on the KT scale is likely to happen, on average, once every 100 million years (My) The most recent was 65 My ago This does not mean that we are safe for the next 35 MY!

Other Mass Extinctions? The Permian-Triassic event (the “Great Dying”) 252 million years ago was (in the words of Wikipedia): “…the Earth's most severe known extinction event, with up to 96% of all marine species and 70% of terrestrial vetebrate species becoming extinct. It is the only known mass extinction of insects … Because so much biodiversity was lost, the recovery of life on Earth took significantly longer than after any other extinction event, possibly up to 10 million years. ” It has been suggested that this extinction was also the immediate consequence of an impact, but the evidence is inconclusive (partly because seafloor rocks are completely recycled on 200-million-year timescales).

How Can We Protect Ourselves? The critical point is early discovery. Hence NEO (‘Near Earth Orbiter’) surveys and programs, to identify all the potential problem objects!

‘ Nudge’ it Out of its Current Path

…or Use Gravity