Transcript Impacts and Mass Extinctions

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
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relatively high in
Iridium
contains lots of ash
and soot
is seen world-wide,
typically several cm thick
is considerably thicker
in southern parts of
North America
[K = Cretaceous]
Different fossils (including
micro-organisms) are seen
above and below the layer.
Much more Iridium in the
layer than you would expect
(but still just a few parts per
billion!)
A Helpful Analogy
The Crime Scene
The KT Boundary
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
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.
It’s not the dominant material
present, but it is surprising
to find it at all.
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
http://www.astro.queensu.ca/~hanes/ASTR101-Fall2015/ANIMS/Deep-Impact.mp4
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 - 2013
Near-miss captured in home movie (1950s):
http://www.astro.queensu.ca/~hanes/ASTR101-Fall2015/ANIMS/Meteor-Skim.mp4
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