2Goals 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?
3Concept TestThe 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.
21show that a large impact occurred An iridium-rich sediment layer and an impact crater on the Mexican coastshow that a large impact occurredat the time the dinosaurs died out, 65 million years ago.Map of variable gravity strength; white lines show the outlines of land massesand the Mexican regions.
22Other Impact-Extinctions? Gravity MapCredit – OhioState UniversityOther Impact-Extinctions?Radar MapCredit – Ohio State UniversityPermian-Triassic Exctinction.250 million years ago.Crater of similar age found in Antarctica.
23Concept TestFossil 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.
24Facts 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.
25Tunguska, Siberia: June 30, 1908 The ~40 meter object disintegrated and exploded in the atmosphereSeveral atomic bombs worth of energy.
26Meteor Crater, Arizona: 50,000 years ago (50 meter object) Crater is 1 km in diameter.Impact was 20 megatonsPrivately owned National Landmark.Meteor Crater, Arizona: 50,000 years ago (50 meter object)
27Concept TestOn 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.
28Chance 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.
29What Happens When an Impact Takes Place? Bolides (up to 5 MT)Great fireworks display, no damageTunguska-class (15 MT) impactDamage 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 countryAverage interval for whole Earth: 100,000 yr.Moderate risk relative to other natural disastersGlobal catastrophe (> 1 million MT)Global environmental damage, threatening civilizationAverage interval for whole Earth: 1 million yearsMajor risk relative to other natural disasters
30Hazards of Global Catastrophe Kills more than 1.5 billion peopleEnergy threshold calculated to be near 1 million MTPrimary global effect is from stratospheric dust and smoke.Average interval 500,000 to 1 million yrs.Unique in capacity to destabilize civilizationCan be compared with global nuclear warOnly known natural hazard that can destroy civilization
33Comparison with Other Risks Statistical risk of death from impacts is about1 in a million per year, or about 1:20,000 lifetime riskMuch less (in U.S.) than auto accidents, shootingsComparable with other natural hazards (e.g. earthquakes, floods)Near threshold for hazards most people are concerned aboutWell above threshold for U.S. governmental or regulatory actionSeverity of disasters (billions of people killed) is greater than any other known hazard we faceApparently unique in its threat to civilizationPlaces this disaster in a class by itselfAverage interval between major disasters (hundreds of millennia) is larger than for any other hazard we faceCauses some to question credibility of hazard
34The 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…
35The asteroid with our name on it? Asteroid Apophis: 320 meter diameterClose approach: 2013, 2029Possible collision: 2036Potential significant local/regional damagePlan of actionUse radar to get better orbit in 2013Decide if we need a deflection mission by 2021, in time to do the mission in 2029.
38www.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 AsteroidsNov-Dec 2006 Earth-asteroid encountersASTEROID DATE (UT)MISS DISTANCEMAG. SIZE2006 UQ216Nov. 75.6 LD21~15 m2006 WBDec. 46.9 LD17~130 mNotes: LD is a "Lunar Distance." 1 LD = 384,401 km, the distance between Earth and the Moon. 1 LD also equals AU. MAG is the visual magnitude of the asteroid on the date of closest approach.
39Concept TestThe 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.
40Impact 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/cm3Impact:LA (60 miles)Washington DC (2,200 miles)
41What are we doing about potential impacts? Stay tuned to
42Homework #21 Due Monday 24 November: Special Presentation, Read