1. PTYS 214 – Spring2011  Homework #7 available for download from the class website Due Tuesday, Mar. 29  Class website: http://www.lpl.arizona.edu/undergrad/classes/spring2011/Pierazzo_214 /  Useful Reading: class website  “Reading Material” http://en.wikipedia.org/wiki/Near-Eart_object http://neo.jpl.nasa.gov/stats/ http://users.tpg.com.au/users/tps-seti/swaprock.html Announcements

2. Extra Credit Presentation Sara Cohen Viktoriya Stoeva

3. Where did the K/P impactor come from? Asteroids: small bodies that are made of rock - Located in the Asteroid Belt (between Mars and Jupiter) Comets: small bodies made of rock and ice (“dirty snowball”) - Located in the Oort Cloud and in the Kuiper Belt Inner SSOuter SS Jupiter

4. Comets  Among the oldest bodies in the solar system  Origin: Kuiper Belt or Oort Cloud (outskirts of the Solar System)  Contain organic material  Very porous objects rich in ices We do not know a lot about them First samples: 2006 Stardust Mission! Comet dust resembles asteroid material Comet Wild 2

5. A natural experiment: Impact of Comet SL9  Comet Shoemaker-Levy 9 was torn into pieces as a result of a close approach to Jupiter in July 1992  Discovered in 1993, it collided with Jupiter at a speed of 60 km/s (135,000 mi/hr!) during the third week of July 1994 HST, July 27, 1994 Plumes thousands of km high! Dark “scars” lasted for months HST, May 1994

6. Near Earth Objects (NEOs) Known asteroids Jupiter’s orbit Mars’ orbit Ecliptic Main Belt

7. Near Earth Objects (NEOs) NEOs rarely get close to Earth enough to be considered a major hazard Asteroids in the neighborhood of the Earth, called Near Earth Objects (or NEOs)

8. But the possibility exists! Peekskill Meteor, 9 Oct. 1992 – 40 seconds of glory!

9. NEOs are potentially hazardous Peekskill Meteorite

10. June 30, 1908 The Tunguska Event - The atmospheric shock wave knocked people off their feet and broke windows up to 650 km (400 miles) away - For few weeks, night skies were so bright that one could read in their light Early morning: A big fireball raced through the dawn sky over Siberia (Russia) It exploded in the atmosphere over the Tunguska region with an estimated force of 1,000 Hiroshima bombs

11. Tunguska: No crater! 1927: The first expedition to the site found a region of scorched trees about 50 km across and no crater! - Most trees had been knocked down pointing away from the center (“ground zero”)

12. What happened?  It was the airburst of a meteor 6 to 10 kilometers above the Earth's surface  Near ground zero, trees were knocked down by the shock wave produced by a large explosion, similar to the effects observed in atmospheric nuclear tests in the 1950s and 1960s  Alternate Explanation: the Tunguska event is the result of an exploding alien spaceship or an alien weapon going off to "save the Earth from an imminent threat" No evidence was ever found by UFO sympathizers…

13. Asteroids Hazard 1 MT= 1 Mton TNT equivalent= 4.2  10 15 J Bolides (energy <5 MT; D< 30 m ) – no crater  Great fireworks display (“shooting stars”), no damage Small Impact (<15Mt; D< 50 m) – crater ~1 km  Damage similar to large nuclear bomb (city-destroyer)  Average interval for whole Earth: >1,000 years Local catastrophes (<10,000 MT; D<250 m) – crater ~10km  Destroys area equivalent to small country  Average interval for whole Earth: >100,000 years Global catastrophe (>10 6 MT; D>1 km) – crater >50 km  Global environmental damage, threatening civilization  Average interval for whole Earth: >1 million years

14. Terrestrial Impact Frequency year century million yr billion yr 10,000 years 100 millionmillion10,00010010.01 Hiroshima Tunguska End-Cretaceous TNT equivalent yield (MT) Global catastrophe (for human civilization) Time Meteor Crater 1 MT= 1 Mton TNT equivalent= 4.186  10 15 J

15. Spaceguard Program D>1 km Goal: Find 90% of NEAs with D > 1 km by the end of 2008 Mar. 16, 2011: 822 discovered (>85%) In the United States it is funded by NASA

16. Comparison with Other Risks Statistical risk of death from impacts:  1 in a million per year (about 1:20,000 over a lifetime) Much less than auto accidents, shootings (in U.S.) Comparable with other natural hazards ( earthquakes, floods ) It is a different kind of risk!  Average interval between major impact disasters is larger than for any other hazard we face (millions years)  A single event can kill millions of people (and other living things) !  Unique as major threat to civilization (comparable to a global nuclear war)

17. Impact cratering is normally regarded as a destructive process, dangerous for life… … but is it always that way?

18. Impacts eject material at high speed Could an impact eject material into space? Could it eject rocks with LIFE into space?

19. Could microbes contained in ejected rocks be ejected alive? Nicholson et al. (2009) Bacterial spores survive hypervelocity launch by spallation: Implicatons for lithopanspermia, 27 th Lunar Planet. Sci. Conf. YES! Near-surface rocks can be ejected at high speed without serious damage (low shock)

20. Microbes could be transferred from one large body to another, but they must survive a host of hazards! 1.Launch 2.Space exposure 3.Landing

21. Material ejected from planets in large impacts wanders around the solar system, rather than traveling directly from planet to planet Reaching another planet may take tens of millions of years! Can life survive this long without nurishment?

22. Spore-forming bacteria are tough Spore-forming bacteria Bacillus Subtilis survived a 6-years spaceflight, experiencing vacuum, cold, lack of water and radiation Science 268 (1995) 1060-1064 Dormant microbes may survive for tens of millions of years (can contamination really be ruled out?)

23. Some micro-organisms can tolerate a lot of cosmic radiation, but not much UV, so they have to hide in rocks Deinococcus Radiodurans Listed in The Guinness Book of World Records as the world’s toughest bacterium

24. Surviving reentry and landing is difficult …yet meteorites contain fragile organic molecules like amino acids ! So, It is highly probable that viable microbes could be carried from Earth to Mars or vice versa

25. Quiz Time !