PTYS 214 Spring 2018 Announcements Midterm #4 in one week! 1
Previously Mass extinctions K/Pg extinction Impact theory -- evidence? Other possible causes Other extinctions 2
Where did the K/Pg 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 Jupiter Outer SS Inner SS Pluto 3
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 Comet Wild 2 First samples: 2006 Stardust Mission! Comet dust resembles asteroid material What kinds of orbits would Earth-approaching comets have? 4
Kepler's Laws 1. Planetary orbits are ellipses with Sun at one focus ellipticity is described by eccentricity e (0--1) 2. Equal areas swept out in equal times: i.e., planets move fastest near periapse and slowest near apoapse 3. Square of orbital period P proportional to cube of semimajor axis a: i.e., larger orbits have longer periods apoapse periapse 5
Rosetta (ESA) Launched 2004 Target: 67p/Churyumov-Gerasimenko (CG) 4.1 X 4.5 km Landing: 11/12/2014 Comet Wild 2 6
67p/Churyumov-Gerasimenko 7
A natural experiment: Impact of Comet SL9 HST, May 1994 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 Plumes thousands of km high! Dark scars lasted for months HST, July 27, 1994 8
Known asteroids Jupiter s orbit Ecliptic Trojans Near Earth Objects (NEOs) Main Belt Trojans 9 What kinds of orbits would Earth-approaching asteroids have? Mars orbit
A/2017 U1 (Oumuamua) -- An interstellar object Object discovered 10/19/2017 after perihelion, perigee Passed within 0.25 AU of Sun; 0.16 AU of Earth ~ 230 x 35 m; eccentricity ~ 1.15 ==> unbound! comet or asteroid? 10
Near Earth Objects (NEOs) Asteroids in the neighborhood of the Earth, called Near Earth Objects (or NEOs) NEOs rarely get close to Earth enough to be considered a major hazard... 11
But the possibility exists! Peekskill Meteor, 9 Oct. 1992 40 seconds! 12
NEOs are potentially hazardous 1980 Chevy Malibu Peekskill Meteorite 13
Some definitions Asteroid Rock in space, orbits the sun, not a planet Meteoroid Very small asteroid Meteor Fireball seen during meteoroid entry Meteorite Former meteoroid found on the ground 14
June 30, 1908 The Tunguska Event Early morning: A big fireball appeared over Siberia (Russia) It exploded in the atmosphere over the Tunguska region with an estimated force of 1,000 Hiroshima bombs - 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 (in London!) 15
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 ) 16
What happened? It was the airburst of an object 6 to 10 kilometers above the Earth's surface Size estimates range from 60 to 200 m across 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 17
February 15, 2013: Chelyabinsk Early morning: Another fireball over Russia. As with Tunguska, it exploded in the atmosphere. Smaller than Tunguska: ~20--30 Hiroshima bombs. - The shock wave injured ~1500 people - Largest object known to have struck Earth since Tunguska. - Captured by numerous cameras. 18
Chelyabinsk crater 6-m-wide hole in the ice covering lake Chebarkul. - Created by 700-kg fragment 19
What happened? Like Tunguska, it was the airburst of a meteor in the atmosphere. Diameter: 20 m; Altitude: ~24 km Near ground zero, several loud explosions were heard and windows were shattered in thousands of buildings. Some structural damage was reported as well. 20
Impact Hazards Bolides (energy <5 MT; D< 30 m ) no crater Great fireworks display ( shooting stars ), no damage Average interval for whole Earth: ~100 years? 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 catastrophe (<10,000 MT; D<250 m) crater ~10km Destroys area equivalent to small country Average interval for whole Earth: >100,000 years Global catastrophe (>106 MT; D>1 km) crater >50 km Global environmental damage, threatening civilization Average interval for whole Earth: >1 million years 1 MT= 1 Mton TNT equivalent= 4.2 1015 J 21
Terrestrial Impact Frequency Chelyabnsk Tunguska century 10,000 years Hiroshima Frequency year Meteor Crater million yr Global catastrophe (for human civilization) End-Cretaceous billion yr 0.01 1 100 10,000 million 100 million TNT equivalent yield (MT) 1 MT= 1 Mton TNT equivalent= 4.186 1015 J 22
Statistical Independence Random events are not affected by history! 23
Statistical Independence Random events are not affected by history! Probability of rolling a six:?? 24
Statistical Independence Random events are not affected by history! Probability of rolling a six: 1/6 25
Statistical Independence Random events are not affected by history! Probability of rolling a six: 1/6 Probability of rolling two sixes in a row:?? 26
Statistical Independence Random events are not affected by history! Probability of rolling a six: 1/6 Probability of rolling two sixes in a row: 1/6 x 1/6 = 1/36 27
Statistical Independence Random events are not affected by history! Probability of rolling a six: 1/6 Probability of rolling two sixes in a row: 1/6 x 1/6 = 1/36 Probability of rolling a six after rolling a six:?? 28
Statistical Independence Random events are not affected by history! Probability of rolling a six: 1/6 Probability of rolling two sixes in a row: 1/6 x 1/6 = 1/36 Probability of rolling a six after rolling a six: 1/6!! 29
Statistical Independence Random events are not affected by history! Probability of rolling a six: 1/6 Probability of rolling two sixes in a row: 1/6 x 1/6 = 1/36 Probability of rolling a six after rolling a six: 1/6!! Probability of rolling a six after rolling 99 sixes:?? 30
Statistical Independence Random events are not affected by history! Probability of rolling a six: 1/6 Probability of rolling two sixes in a row: 1/6 x 1/6 = 1/36 Probability of rolling a six after rolling a six: 1/6!! Probability of rolling a six after rolling 99 sixes: 1/6!! 31
Statistical Independence Random events are not affected by history! Probability of rolling a six: 1/6 Probability of rolling two sixes in a row: 1/6 x 1/6 = 1/36 Probability of rolling a six after rolling a six: 1/6!! Probability of rolling a six after rolling 99 sixes: 1/6!! Probability of rolling 100 sixes:??!! 32
Statistical Independence Random events are not affected by history! Probability of rolling a six: 1/6 Probability of rolling two sixes in a row: 1/6 x 1/6 = 1/36 Probability of rolling a six after rolling a six: 1/6!! Probability of rolling a six after rolling 99 sixes: 1/6!! Probability of rolling 100 sixes: (1/6)100 ~ 10-78!! 33
Are impacts truly a random process? Known Earth-crossing asteroids as of 2013 i.e., can they be predicted? 34
Spaceguard Program In the United States it is funded by NASA Goal: Find 90% of NEAs with D > 1 km by the end of 2008 As of 2013: D>1 km 864 discovered (>90%) 35
Activity: Death Risks Rank the following causes of death in terms of risk from lowest to highest: A) Asteroid? B) Lightning? C) Terrorist attack? D) War? E) Shark attack? F) Car accident? G) Earthquake? H) Selfie? 36
Activity: Death Risks Answers*: E) Shark attack H) Selfie D) War A) Asteroid G) Earthquake C) Terrorist attack B) Lightning F) Car accident 1:8,000,000 1:5,000,000 ** 1:250,000 1:200,000 1:132,000 1:90,000 1:74,000 1:84 *CDC statistics, probably subject to large uncertainties **Anecdotal 37
Comparison with Other Risks Statistical risk of death from impacts: about 1:200,000 Much less than auto accidents (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) 38
Impact cratering is normally regarded as a destructive process, dangerous for life but is it always that way? 39
Impacts eject material at high speed Could an impact eject material into space? Could it eject rocks containing LIFE into space? 40
Near-surface rocks can be ejected at high speed without serious damage (low shock) Could microbes contained in ejected rocks be ejected alive? YES! Nicholson et al. (2009) Bacterial spores survive hypervelocity launch by spallation: Implicatons for lithopanspermia, 27th Lunar Planet. Sci. Conf. 41
Microbes could be transferred from one planet to another, but they must survive a host of hazards! 1. Launch 2. Space exposure 3. Landing 42
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 nourishment? 43
Spore-forming bacteria are tough Spore-forming bacteria Bacillus Subtilis survived a 6-year space flight, experiencing vacuum, cold, lack of water and radiation Dormant microbes may survive for tens of millions of years (can contamination really be ruled out?) Science 268 (1995) 1060-1064 44
Some micro-organisms can tolerate a lot of cosmic radiation, but not much UV, so they have to hide in rocks Others can withstand almost anything: Deinococcus Radiodurans Listed in The Guinness Book of World Records as the world s toughest bacterium 45
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 46
Homework Homework #17 available shortly on the web site 47