Impact Hazards Run for your lives...!

Goals What is the evidence for impacts in the solar system? What is the evidence for impacts on Earth? What is the evidence for mass extinctions due to impacts? What are the odds of future impacts?

Concept Test The Moon shows far more craters on its surface than the Earth. We can conclude that: The Moon has been hit by far more objects than the Earth. The Earth’s magnetosphere protects us from the vast majority of impacts. The Earth has encountered just as many potential impactors but most have burned up in our atmosphere. The Earth has been hit just as often as the Moon, but geological processes have covered over or erased the majority of craters. The Earth’s larger gravity acts as a slingshot and redirects most potential impactors away into the Oort Cloud.

Earth Clearwater Lakes Crater Mjolnir Crater Manicouagan Crater Wolfe Creek Crater

Craters

SL9 Comet SL9 caused a string of violent impacts on Jupiter in 1994, reminding us that catastrophic collisions still happen. Tidal forces tore it apart during previous encounter with Jupiter

SL9 Geometry

IRTF

HST imaged impact plume rising high above Jupiter’s surface.

Galileo

Chain of craters on Callisto, of another comet torn apart by tidal forces from Jupiter.

Mass Extinctions Large dips in total species diversity in the fossil record. The most recent was 65 million years ago, ending the reign of the dinosaurs. Impacts?

Evidence Iridium Shocked crystals Tektites Soot Worldwide iridium layer laid down 65 million years ago. Very rare in Earth surface rocks but often found in meteorites. Shocked crystals Tektites Soot

Comet or asteroid about 10km in diameter approaches Earth

show that a large impact occurred An iridium-rich sediment layer and an impact crater on the Mexican coast show that a large impact occurred at the time the dinosaurs died out, 65 million years ago. Map of variable gravity strength; white lines show the outlines of land masses and the Mexican regions.

Other Impact-Extinctions? Gravity Map Credit – OhioState University Other Impact-Extinctions? Radar Map Credit – Ohio State University Permian-Triassic Exctinction. 250 million years ago. Crater of similar age found in Antarctica.

Concept Test Fossil evidence suggests a mass extinction occurring 65 million years ago. Which of the following is NOT a piece of evidence supporting the idea that an impact caused this mass extinction? Unusually large abundances of iridium and other rare metals in a layer of clay that dates to 65 million years ago. An impact crater along the coast of Mexico that dates to 65 million years ago. Grains of quartz that must have formed under very high pressure are found in a layer of clay that dates to 65 million years ago. Fossilized dinosaur bones that contain fragments of rock that must have been shot out by the impact. A layer of 65 million year old soot at the K-T boundary.

Facts Asteroids and comets have hit the Earth. A major impact is only a matter of time: not IF but WHEN. Major impact are very rare. Extinction level events ~ millions of years. Major damage ~ tens-hundreds of years.

Tunguska, Siberia: June 30, 1908 The ~40 meter object disintegrated and exploded in the atmosphere Several atomic bombs worth of energy.

Meteor Crater, Arizona: 50,000 years ago (50 meter object) Crater is 1 km in diameter. Impact was 20 megatons Privately owned National Landmark. Meteor Crater, Arizona: 50,000 years ago (50 meter object)

Concept Test On average, how often should an asteroid or comet strike Earth with as much energy as that of a hydrogen bomb (like the Tunguska impact)? Once every million years. Once every few centuries. Once every 100 million years. Once every 10,000 years. Once in the lifetime of the planet.

Chance of death by asteroid: About one in 100,000 From 7-April-2004 congressional testimony of Astronaut Ed Lu, B612 Foundation: 10% chance during our lifetime of a 70 m asteroid, impacting with energy of 10 megatons or 700 Hiroshima bombs. (Senate Subcommittee on Science, Technology, and Space.) Note that 50 meters was the size of the object that created Meteor Crater. Impacts will certainly occur in the future, and while the chance of a major impact in our lifetimes is small, the effects could be devastating.

What Happens When an Impact Takes Place? Bolides (up to 5 MT) Great fireworks display, no damage Tunguska-class (15 MT) impact Damage similar to large nuclear bomb (city-killer) Average interval for whole Earth: 100 yr. Minor risk relative to other natural disasters (earthquakes, etc.) Larger local or regional catastrophes (e.g.10,000 MT) Destroys area equivalent to small country Average interval for whole Earth: 100,000 yr. Moderate risk relative to other natural disasters Global catastrophe (> 1 million MT) Global environmental damage, threatening civilization Average interval for whole Earth: 1 million years Major risk relative to other natural disasters

Hazards of Global Catastrophe Kills more than 1.5 billion people Energy threshold calculated to be near 1 million MT Primary global effect is from stratospheric dust and smoke. Average interval 500,000 to 1 million yrs. Unique in capacity to destabilize civilization Can be compared with global nuclear war Only known natural hazard that can destroy civilization

Terrestrial Impact Frequency year Hiroshima century Tunguska ten thousand yr. million yr. K/T billion yr. 0.01 1 100 10,000 million 100 million TNT equivalent yield (MT)

Terrestrial Impact Frequency Hiroshima year Tunguska century Tsunami danger ten thousand yr. Global catastrophe million yr. K/T billion yr. 0.01 1 100 10,000 million 100 million TNT equivalent yield (MT)

Comparison with Other Risks Statistical risk of death from impacts is about 1 in a million per year, or about 1:20,000 lifetime risk Much less (in U.S.) than auto accidents, shootings Comparable with other natural hazards (e.g. earthquakes, floods) Near threshold for hazards most people are concerned about Well above threshold for U.S. governmental or regulatory action Severity of disasters (billions of people killed) is greater than any other known hazard we face Apparently unique in its threat to civilization Places this disaster in a class by itself Average interval between major disasters (hundreds of millennia) is larger than for any other hazard we face Causes some to question credibility of hazard

The asteroid with our name on it We haven’t seen it yet (maybe). Deflection is more probable with years of advance warning. Control is critical: breaking a big asteroid into a bunch of little asteroids is unlikely to help. We get less advance warning of a killer comet…

The asteroid with our name on it? Asteroid Apophis: 320 meter diameter Close approach: 2013, 2029 Possible collision: 2036 Potential significant local/regional damage Plan of action Use radar to get better orbit in 2013 Decide if we need a deflection mission by 2021, in time to do the mission in 2029.

Torino Scale

www.spaceweather.com Nov-Dec 2006 Earth-asteroid encounters Potentially Hazardous Asteroids (PHAs) are space rocks larger than approximately 100m that can come closer to Earth than 0.05 AU. None of the known PHAs is on a collision course with our planet, although astronomers are finding new ones all the time. On 28 Nov 2006 there were 834 known Potentially Hazardous Asteroids Nov-Dec 2006 Earth-asteroid encounters ASTEROID  DATE (UT) MISS DISTANCE MAG.  SIZE 2006 UQ216 Nov. 7 5.6 LD 21 ~15 m 2006 WB Dec. 4 6.9 LD 17 ~130 m Notes: LD is a "Lunar Distance." 1 LD = 384,401 km, the distance between Earth and the Moon. 1 LD also equals 0.00256 AU. MAG is the visual magnitude of the asteroid on the date of closest approach.

Concept Test The greatest threat to life due to a major impact event is: Being hit by the impactor itself. Being within the formation of the impact crater. Being within the initial fireball of the impact. The resulting environmental and climactic effects of the impact.

Impact Models http://www.lpl.arizona.edu/impacteffects Use asteroid Eros. 13 x 13 x 33 km (or 22 km diameter sphere) Density 1.24 g/cm3 Impact: LA (60 miles) Washington DC (2,200 miles)

What are we doing about potential impacts? Stay tuned to http://neo.jpl.nasa.gov/programs/

Homework #21 Due Monday 24 November: Special Presentation, Read www.planetary.org/parks