1. PYTS/ASTR 206 – Craters 1 l Homework #1 returned n Grades were well distributed – Average was a high C wAverage question results 1 – 5 were 74%, 72%, 77%, 57%, 59% n We’re happy to talk about the homework – tomorrow! wSolutions posted after this lecture wNo discussions with us for 24 hours l Homework #2 posted on website after this lecture n One week to finish

2. PYTS/ASTR 206 – Craters 2 PTYS/ASTR 206 – The Golden Age of Planetary Exploration Shane Byrne – shane@lpl.arizona.edu Craters

3. PYTS/ASTR 206 – Craters 3 In this lecture… l Introduction to craters l Characteristics of craters n Bowls, rims and ejecta blankets n Nuclear test results n Simple vs complex craters l Crater formation n Impacts and Energy n Excavation n Relaxation n e.g. Meteor crater, Chicxulub l Atmospheric effects n E.g. Tunguska l Crater populations n Dating a planetary surface

4. PYTS/ASTR 206 – Craters 4 l Where do we find craters? – Everywhere! n Cratering is the one geologic process that every solid solar system body experiences… Mercury Venus Moon EarthMarsAsteroids

5. PYTS/ASTR 206 – Craters 5 l Morphology changes as craters get bigger n Pit → Bowl Shape→ Central Peak → Central Peak Ring → Multi-ring Basin Moltke – 1km 10 microns Euler – 28km Schrödinger – 320km Orientale – 970km

6. PYTS/ASTR 206 – Craters 6 l Origin of impactor craters n Asteroid fragments leave the main asteroid belt wFrom collisions with each other wBecome Near-Earth Asteroids n Kuiper Belt Objects leave the Kuiper belt wFrom collisions with each other wBecome Jupiter Family Comets l Steady trickle of the objects n Less common today than billions of years ago

7. PYTS/ASTR 206 – Craters 7 l Simple vs. complex Characteristics of craters Moltke – 1km Euler – 28km Melosh, 1989

8. PYTS/ASTR 206 – Craters 8 Meteor Crater – 1.2 km l Common crater features n Overturned flap at edge wGives the crater a raised rim wReverses stratigraphy n Eject blanket wContinuous for ~1 R c n Breccia wPulverized rock on crater floor Melosh, 1989

9. PYTS/ASTR 206 – Craters 9 l Craters are point-source explosions n Was fully realized in 1940s and 1950s test explosions l Three main implications: n Crater depends on the impactor’s kinetic energy – NOT JUST SIZE n Impactor is much smaller than the crater it produces wMeteor crater impactor was ~50m in size n Oblique impacts still make circular craters wUnless they hit the surface at an extremely grazing angle (<5°) Meteor Crater – 1200m Sedan Crater – 300m

10. PYTS/ASTR 206 – Craters 10 l Lunar craters – volcanoes or impacts? n This argument was settled in favor of impacts largely by comparison to weapons tests n Many geologists once believed that the lunar craters were extinct volcanoes l Which of these is a volcanic caldera?

11. PYTS/ASTR 206 – Craters 11 l Lunar craters – volcanoes or impacts? n This argument was settled in favor of impacts largely by comparison to weapons tests n Many geologists once believed that the lunar craters were extinct volcanoes l Which of these is a volcanic caldera? VOLCANIC Impact Raised Rim – from explosion No Raised Rim – formed by collapse

12. PYTS/ASTR 206 – Craters 12 SimpleComplex Bowl shapedFlat-floored Central peak Wall terraces Little meltSome Melt depth/D ~ 0.2 Size independent depth/D smaller Size dependent Small sizesLarger sizes Pushes most rocks downward and outward Move most rocks outside the crater Size limited by strength of rocks Size limited by weight of rocks

13. PYTS/ASTR 206 – Craters 13 l Crater size depends on impactor energy l Size of a simple crater depends on the strength of target rock n Small craters are in the so called ‘strength regime’ n The stronger the rocks, the smaller the crater n The weight of the rocks isn’t important l Size of a complex crater depends on the weight of the target rock n Large craters are in the so called ‘gravity regime’ n Weight of target rocks depends on gravity and target-rock density n The strength of the rocks isn’t important Moltke – 1km Euler – 28km

14. PYTS/ASTR 206 – Craters 14 l Rock strength and density don’t vary much n …but gravity varies quite a bit n Earth: D T ~ 3km n Moon: D T ~ 18km l When do you switch from the strength regime to gravity regime? n Transition diameter (D T ) n Y=rock strength n ρ=rock density n g=planetary gravity Moltke – 1km Euler – 28km

15. PYTS/ASTR 206 – Craters 15 l An example: n Typical rock strength is 10 8 Pa n Typical rock density is 3000 kg m -3 n What’s the transition diameter from simple to complex craters on Mars? wMartian gravity is 3.72 ms -2 n About 8.9 km l What about an impact into martian ice n Strength 10 7 Pa & Density 1000 kg m -3 n About 2.7 km

16. PYTS/ASTR 206 – Craters 16 l How to build a crater l Three stages n Contact and explosion n Excavation n Collapse l Total energy is ½mv 2 n m is the mass n v is the impactor velocity n v is at least 11 km s -1 (Earth’s escape velocity) n v is at most 72 km s -1 (A head-on collision with a comet) Formation of craters

17. PYTS/ASTR 206 – Craters 17 l Contact stage n Impactor hits the surface – traveling at several km s -1 n Shockwave start propagating through the impactor and target n Impactor penetrates the surface n Shockwave reaches the other side of the impactor – impactor explodes wLike an underground point-source explosion Trinity Nuclear Test – 0.03 seconds

18. PYTS/ASTR 206 – Craters 18 l Excavation stage n Bowl shaped cavity forms n Material ejected in a cone wParticles on balastic trajectories wCone appears to expand

19. PYTS/ASTR 206 – Craters 19 l Simulations can extend lab work Oslo University, Physics Dept.

20. PYTS/ASTR 206 – Craters 20 l Some blocks of ejecta can be very large n Can form secondary craters

21. PYTS/ASTR 206 – Craters 21 l Collapse stage n Initial bowl-shaped crater collapses n Produces Breccia lens l Small craters form shallower bowls n Depth/diameter goes from 0.5 to 0.2 l Large craters become complex n Floor rebounds to form a central peak Melosh, 1989

22. PYTS/ASTR 206 – Craters 22 l Meteor crater as an example n Occurred about 50,000 year ago n Impactor was an iron asteroid ~50m in diameter n Crater is about 1200m in diameter n Energy ~30,000 kilotons of TNT n Hiroshima ~ 15 Kilotons l In a modern city? n Depends on terrain n Compete destruction & death wOut to several km n Out to 10s of km wMostly destroyed wFew survivors l Is this common? n Every 10,000 years or so n Most of them over the oceans

23. PYTS/ASTR 206 – Craters 23 l Chicxulub as an example n Occurred about 65 Million year ago n Impactor was an asteroid ~10 km in diameter n Crater is about 200 Km in diameter n Local region was devastated for ~1000km l Debris blasted into orbit n Reenters atmosphere and causes global wild-fires n Heat radiation from hot debris boils animals alive n Evidence from global soot layer enriched in iridium n Sunlight diminished – plants die l Corresponds to the KT boundary n Cretaceous – Tertiary n Break in the fossil record where 75% of species went extinct

24. PYTS/ASTR 206 – Craters 24 l Pressure builds up on impactor as it passes through the atmosphere n Just like we use the atmosphere to slow down spacecraft n ‘ram pressure’ can exceed the asteroid strength n Asteroid fragments explosively n Atmospheric shock wave can level trees 100s of km away l Tunguska 1908 – A once per century event n 80m diameter stony asteroid, 22 km s -1 n No Casualties n Next one more likely to be a problem – population increase Atmospheric effects

25. PYTS/ASTR 206 – Craters 25 l Strong objects can survive the forces needed for deceleration n E.g. iron meteorites Found on Mars by the Mars Exploration Rover

26. PYTS/ASTR 206 – Craters 26 l Larger impacts are rarer than smaller ones n Time between large events is long (on average) n Impact rates have slowed down a lot! Crater Populations http://rst.gsfc.nasa.gov Threshold for global significance

27. PYTS/ASTR 206 – Craters 27 l Impact craters accumulate over time n If nothing removes the craters…. wLike on the Moon n And we know the rate they form at… wApollo samples provided the connection between crater counts and age n Then we can convert the crater counts to an age. l Calibrating our dating mechanism l Apollo samples can be dated in the lab l These dates are compared to crater counts l We can scale the lunar results to other planets

28. PYTS/ASTR 206 – Craters 28 l This mechanism works only up to a point l When a surface is saturated no more age information is added n Number of craters stops increasing with age

29. PYTS/ASTR 206 – Craters 29

30. PYTS/ASTR 206 – Craters 30 l Big craters are rarer than small craters l Number of craters plotted against size looks like this n Uses a log-log plot n Straight lines are isochrons l Red lines are ‘primary craters’ l Blue lines are primary and secondary craters n i.e. more than you’d expect n Secondaries dominate the small crater population

31. PYTS/ASTR 206 – Craters 31 l Impacts continue today Malin et al., 2007

32. PYTS/ASTR 206 – Craters 32 In this lecture… Next: Craters l Reading l Chapter 7.6 to revise this lecture l Chapter 9.2 for next lecture l Characteristics of craters n Bowls, rims and ejecta blankets n Nuclear test results n Simple vs. complex craters l Crater formation n Contact, Excavation, Relaxation n e.g. Meteor crater, Chicxulub l Atmospheric effects n E.g. Tunguska l Crater populations n Dating a planetary surface