EART 160: Planetary Science

1. EART 160: Planetary Science On Tuesday 2007TU24 will pass 500,000 km from Earth Asteroid is 150 m long Collision w/ an object this size every 37,000 y

2. Last Time • Paper Discussion • Asphaug et al. (2006) • Meteorites • Age of the Solar System • Types of Meteorites • Asteroids • Main Belt • Earth-crossing Asteroids

3. Today • Paper Discussion • Stevenson (2001) • Planetary Surfaces • Impacts • Morphology • Mechanics • Ages of Planetary Surfaces • Frequency and Consequences

4. IGPP Talk Today The Largest and Smallest Impacts on Mars Oded Aharonson Caltech EMS B210 3:30 pm (Tea and Snacks at 3:00)

5. The Four Planetary Processes • Impacts • Craters, Ejecta • Gradation • Weathering, Erosion, Mass Wasting • Modification by Wind, Water • Volcanism • Melting, Eruption, Formation of New Rock • Tectonics • Compression, Extension • Faults, Folds

6. Mercury Weird Terrain (antipodal to Caloris)-- Tectonics Caloris Basin – large impact Smooth plains inside – volcanism?

7. Venus Radar Map Highlands and Lowlands Tectonics? Arachnoid -- Tectonics Pancake Dome -- Volcanism “Mmmm. Pancakes.” -- Homer Simpson Splotchy craters

8. The Moon Bright cratered highlands Dark smooth lowlands Impacts and Volcanism Surface covered in regolith – fine dust created by constant pummeling Fear that Lunar Lander would sink! Wrinkle Ridges -- Tectonics

9. Mars Smooth Lowlands Weathering Crustal Dichotomy -- Tectonic? -- Volcanic? -- Giant Impact? Tharsis -- Volcanism Hellas – Impact Basin Valles Marineris -- Tectonism Landslide -- Gradation Cratered Highlands

10. Earth Meteor Crater Plate Tectonics -- Only known example! Crustal Recycling wipes out impact craters We can study Earth geology in situ to understand processes Then apply those concepts to images of other planets Mt. St. Helens -- Volcano

11. Icy Satellites Tvashtar Plume on Io Plumes, Tiger-stripes on Enceladus Iapetus Two-Tone Eq. Ridge Conamara Chaos on Europa

12. Impact Cratering • Ubiquitous – impacts occur everywhere • Dating – degree of cratering provides information on how old a surface is • Style of impact crater provides clues to the nature of the subsurface and atmosphere • Impacts produce planetary regolith • Impacts can have catastrophic effects on planets (not to mention their inhabitants)

13. “The next big event for astronomers will be Friday April 13th 2029. Scientists predict that the asteroid Apophis (~400m diameter) will be coming only 32,000 kilometres from the Earth, which is close enough to hit a weather satellite and even be visible without a telescope.”

14. Crater Formation 1. Contact/compression • Impactor is (mostly) destroyed on impact • Initial impact velocity is (usually) greater than sound speed, creating shock waves • Shock waves propagate outwards and downwards • Heating and melting occur • Shock waves lead to excavation of material • Transient crater is spherical • Crater later relaxes 2. Excavation 3. Modification Note overturned strata at surface

15. Timescales v 2r • Contact and compression • Time for shock-wave to pass across impactor • Typically less than 1s • Excavation • Free-fall time for ejected material • Up to a few minutes d • Modification • Initial faulting and slumping probably happens over a few hours • Long-term shallowing and relaxation can take place over millions of years

16. Crater Sizes • A good rule of thumb is that an impactor will create a crater roughly 10 times the size (depends on velocity) • We can come up with a rough argument based on energy for how big the transient crater should be: v 2r 2R Does this make sense? • E.g. on Earth an impactor of 0.1 (1) km radius and velocity of 10 km/s will make a crater of radius 2 (12) km • For really small craters, the strength of the material which is being impacted becomes important

17. Craters of different shapes • Crater shapes change as size increases: • Small – simple craters (bowl-shaped) • Medium – complex craters (central peak) • Large – impact basins • Transition size varies with surface gravity and material properties BASIN: Hellas, Mars COMPLEX: Euler, 28km, 2.5km deep SIMPLE: Moltke, Moon, 7km

18. Shape transitions Schenk (2002) Europa, scale bar=10km Note change in morphology as size increase Lunar curve • Depth/diameter ratio decreases as craters get larger • Gravity on icy satellites similar to that on the Moon • Transition occurs at smaller diameters than for Moon – due to weaker target material? (ice vs. rock) Ganymede complex simple basins

19. Ejecta • Symmetrical apron of debris surrounding an impact crater • Extent controlled by: • size / mass of impactor • surface gravity • atmosphere Bright ejecta on a dark surface Mare Serenetatis, The Moon Image Credit: NASA

20. Occur when a weak impactor (comet?) gets pulled apart by tides Crater Chains Crater chain, Callisto, 340km long

21. Shoemaker-Levy 9

22. Jupiter, the vacuum cleaner of the Solar System Impacts, fragments of Shoemaker-Levy 9

23. Rampart Craters (Mars) • Probably caused by melting of subsurface ice leading to slurry ejecta • Useful for mapping subsurface ice Tooting crater (28 km diameter) Tooting crater, 28km diameter Stewart et al., Shock Compression Condens. Matt. 2004

24. Oblique Impacts • Impacts are most like explosions – spherical shock wave leads to circular craters • Not understood prior to the space age – argument against impact craters on the Moon • Only very oblique (>75o?) impacts cause non-circular craters • Non-circular craters are rare impact Mars, D=12km Herrick, Mars crater consortium

25. Atmospheric Effects • Small impactors burn up in the atmosphere • Venus, Earth, Titan lack small impact craters • Venus’ thick atmosphere may produce other effects (e.g. outflows) After McKinnon et al. 1997 Radar image of impact-related outflow feature

26. “Airbursts” • Venus “dark splotches” Tunguska, Siberia 1908 • Result of (weak) impactor disintegrating in atmosphere 300km across, radar image • Thick atmosphere of Venus means a lack of craters smaller than about 3 km (they break up in atmosphere)

27. How often do they happen? (Earth) Hartmann

29. Crater Basics Ejecta blanket • Typical depth:diameter ratio is ~1:5 for simple (bowl-shaped) craters Depth Mars, MOC image

30. How do we date surfaces? • geometric saturation • hexagonal close packing • How many craters could possibly fit? • equilibrium • Also called saturation • new craters wipe out equal number of old ones • 5-7% of geom. saturation • production population • theoretical size-freq. dist. of all primary craters. young old • Crater densities – a more heavily cratered surface is older • The size-distribution of craters can tell us about the processes removing them • Densities reach a maximum when each new crater destroys one old crater (saturation). Phobos’ surface is close to saturated. • Lunar crater densities can be compared with measured surface ages from samples returned by Apollo missions

31. N = cD-b N = Total # craters larger than a diameter D D = diameter of crater b = slope b > 2: small craters occupy more area than large craters b < 2 : large craters occupy more area b = 2: same fraction of area occupied by all sizes Saturation Slope, b, depends on impactor population Effect of secondary craters? Increasing age Size-Frequency Distibution

32. Relative Ages • It is easy to determine the relative ages of different surfaces (young vs. old) • Determing the absolute ages means we need to know the cratering rate (impacts per year) Number of craters >1km diameter per km2 • We know the cratering rates on the Earth and the Moon, but we have to put in a correction (fudge factor) to convert it to other places • So the uncertainties tend to be large, especially for “intermediate-age” surfaces

33. Evolving impactor population • One complication is that the population of impactors has changed over time • Early solar system had lots of debris  high rate of impacts • More recent impact flux has been lower, and size distribution of impactors may also have been different • Did the impact flux decrease steadily, or was there an “impact spike” at ~4 Gyr (Late Heavy Bombardment)? Hartmann; W are numerical simulation results, boxes are data from Moon/Earth

34. Crater Counts saturation • Crater size-frequency plots can be used to infer geological history of surfaces • Example on left shows that intermediate-size craters show lower density than large craters (why?) frequency size • Smallest craters are virtually absent (why?) • Most geological processes (e.g. erosion, sedimentation) will remove smaller craters more rapidly than larger craters • So surfaces tend to look younger at small scales rather than at large scales

35. Complications • Rate of impacts was certainly not constant, maybe not even monotonic (Late Heavy Bombardment?) • Secondary craters can seriously complicate the cratering record • Some surfaces may be buried and then exhumed, giving misleading dates (Mars) • Subsurface impact basins (Mars) • Very large uncertainties in absolute ages, especially in outer solar system Pwyll crater, Europa (25 km diameter)

36. Cratering record on different bodies • Earth – few craters (why?) • Titan – only 2 craters identified so far (why?) • Mercury, Phobos, Callisto – heavily cratered everywhere (close to saturation) • Moon – saturated highlands, heavily cratered maria • Mars – heavily cratered highlands, lightly cratered lowlands (plus buried basins) and volcanoes • Venus – uniform crater distribution, ~0.5 Gyr surface age, no small craters (why?) • Ganymede – saturated dark terrain, cratered light terrain • Europa – lightly cratered (~0.05 Gyr) • Io – no craters at all (why?)

37. Where do impactors come from? • In inner solar system, mostly asteroids, roughly 10% comets (higher velocity, ~50 km/s vs. ~15 km/s) • Comets may have been important for delivering volatiles & atmosphere to inner solar system • In outer solar system, impactors exclusively comets • Different reservoirs have different freq. distributions • Comet reservoirs are Oort Cloud and Kuiper Belt • Orbits are perturbed by interaction with planets (usually Jupiter) • There may have been an “impact spike” in the inner solar system when the giant planets rearranged themselves (not quite as unlikely as it sounds)

38. Devastation

39. 65 million years ago . . .

48. Next Time • Planetary Surfaces • More on Cratering • Gradation • Weathering and Erosion • HW 2 is due • See me if you need help!