Cosmic Impacts and Life on Earth
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These are lecture notes from an experimental non-majors biology class taught in 1995 by
Bruce Walsh, University of Arizona.
Other lectures of potential interest:
Origins of life
Extinctions
Genetics class lecture notes
- Two separate tales to be joined
- Walter and Luis Alverez (1977)
- looking at world-wide clay deposits
- Dave Raup and Jack Sepkoski (1980)
- Looking at extinction rates of marine invertebrates
The Alverez's tale
The Alverez Iridium anomaly
- Around the world is a thin band of clay at the Cretaceous-Tertiary (KT) boundary
- This band is highly enriched for Iridium (Ir)
Iridium
- A rare-earth element
- Rare in EarthÕs crust
- Common in meteorites
- Can be extruded by Volcanoes
What was the source?
- Extra-terrestrial impact Vs. volcanoes
- comet/asteroid impact
- massive series of Volcanic eruptions
Impact rather than Volcanic origin suggested
- microtektites
- very small glass spheres caused by very high velocity impact (impact, Not volcanic)
- strewn fields worldwide
- shock quartz
- quartz grain with multiple sets of shock lamellae,
- impact, not volcanic
- Presence of both at Ir anomaly layer strongly supports impact over volcanic origin of Ir
Where was the crater?
- North American marine site suggested
- distribution of microtektite strewn fields
- greater abundance and larger grain size of shocked minerals in North American
Crater appears to be underwater, just off the Yucatan peninsula
Current view
- Impact of a large object (> 10k) at K/T boundary
- Threw up very large dust cloud which remained for weeks
- Impact(s) may have triggered rounds of volcanic eruptions
Earth is at risk of future impacts
- 1/300,000 in our lifetime.
- earth-crossing asteroids, comets
- Shoemaker-Levy 9
- periodic comet that struck Jupiter in a series of impacts last July
Part two of our tale: Raup and Sepkoski
- Back to Raup and Sepkoski
- Looked at extinction rates using families of marine invertebrates
- found an apparently periodicity of 26 MY for increased extinction rates
- True periodicity?
- Still unclear is this periodicity is real as statistical issues in showing true periodicity
- If true periodicity, what is the cause?
- too long for most natural cycles,
- too short for others
- rotations around the galaxy (200 MY)
The teaser
- Strong Iridium anomalies also seen at
- Devonian/Carboniferous boundary
- Permian/Triassic boundary
- Ordovician/Silurian boundary
- K/T boundary
Impact theory
- Cratering rates on earth show weak evidence of similar periodicity
- Extra-terrestrial impacts cause of most/all mass extinctions?
The Nemeis (Death Star) model
- Sun has a small dark companion star
- when it moves close to the Oort cloud, causes a cometary shower to be sent into the inner solar system
