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Impacts and Erosion

This article explores the four basic geological processes - impact cratering, volcanism, tectonics, and erosion - and their impacts on the Earth's surface. It also discusses the possibility of subduction and crustal recycling on Europa.

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Impacts and Erosion

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  1. Impacts and Erosion 14 September 2015

  2. Four Basic Geological Processes • Impact cratering • Impacts by asteroids or comets • Volcanism • Eruption of molten rock onto surface • Tectonics • Disruption of a planet’s surface by internal stresses • Erosion • Surface changes made by wind, water, or ice

  3. Possible Subduction and Crustal Recycling on Europa

  4. Impact Cratering High kinetic energy of impacting object • Excavation  Heat Shape round for all impact directions

  5. Crater Morphology: • Impactor in the inner solar system can have 10s of km/sec relative velocity • May vaporize or liquefy a significant amount of the target for a large impactor • “Simple” craters are bowl shaped • “Complex” craters are flatter, and have a central uplift or peak

  6. Impact Craters Tycho (Moon) Meteor Crater (Arizona)

  7. The Impact Process • Detonation - shockwave propagates through target, projectile vaporized • Excavation - target is heated, vaporized, liquified, solid material is ejected (possibly above escape velocity!) • Rebound - in larger craters, the target may “bounce back” viscously, forming a central uplift • Relaxation - crater walls subside, melt pools in the crater

  8. Few Hundred Confirmed Craters on Earth

  9. Meteor Crater Meteor Crater

  10. Ariel view of Meteor Crater

  11. Manicouigan Crater Crater Lake

  12. Moon Near Side Moon’s Giant Basins

  13. Moon Far Side Lunar Farside

  14. Lunar Highlands

  15. Mare Imbrium Lunar Mare

  16. Crater modification

  17. Mars Rampart Crater 3 Mars Rampart Crater

  18. Venus Craters 1 Venus Crater with Ejecta

  19. Ganymede Crater Chain

  20. Shoemaker-Levy 9

  21. SL-9 Aftermath

  22. Hyperion Hyperion

  23. Erosion and Deposition • Erosion and deposition require the presence of a fluid (gas or liquid) to pick up, transport and deposit surface material • Liquid transport more efficient • These processes tend to be rapid compared to other geological processes • So surface appearance is often controlled by these processes • Earth, Mars, Titan, Venus have erosional or sedimentary features

  24. Aeolian Features (Mars) • Wind is an important process on Mars at the present day (e.g. Viking seismometers . . .) • Dust re-deposited over a very wide area (so the surface of Mars appears to have a very homogenous composition) • Occasionally get global dust-storms (hazardous for spacecraft) • Rates of deposition/erosion almost unknown Martian dune features Image of a dust devil caught in the act 30km

  25. Aeolian features (elsewhere) Namib desert, Earth few km spacing Longitudinal dunes, Earth (top), Titan (bottom), ~ 1 km spacing Yardangs (elongated dunes) Mead crater, Venus

  26. Wind directions Venus Wind streaks, Venus Mars (crater diameter 90m) Global patterns of wind direction can be compared with general circulation models (GCM’s)

  27. Fluvial features • Valley networks on Mars • Only occur on ancient terrain (~4 Gyr old) • What does this imply about ancient Martian atmosphere? 100 km • Valley network on Titan • Presumably formed by methane runoff • What does this imply about Titan climate and surface? 30 km

  28. flow direction 150km 50km Martian Outflow channels • Large-scale fluvial features, indicating massive (liquid) flows, comparable to ocean currents on Earth • Morphology similar to giant post-glacial floods on Earth • Spread throughout Martian history, but concentrated in the first 1-2 Gyr of Martian history • Source of water unknown – possibly ice melted by volcanic eruptions (jokulhaups)? Baker (2001)

  29. Martian Gullies • A very unexpected discovery (Malin & Edgett, Science 283, 2330-2335, 2000) • Found predominantly at high latitudes (>30o), on pole-facing slopes, and shallow (~100m below surface) • Inferred to be young – cover young features like dunes and polygons • How do we explain them? Liquid water is not stable at the surface! • Maybe even active at present day?

  30. Lakes Clearwater Lakes Canada ~30km diameters Gusev, Mars 150km Titan, 30km across Titan lakes are (presumably) methane/ethane Gusev crater shows little evidence for water, based on Mars Rover data

  31. Erosion • Erosion will remove small, near-surface craters • But it may also expose (exhume) craters that were previously buried • Erosion has recently been recognized as a major process on Mars, but the details are still extremely poorly understood • The images below show examples of fluvial features which have been exhumed channel meander Malin and Edgett, Science 2003

  32. Sediments in outcrop Opportunity (Meridiani) Cross-bedding indicative of prolonged fluid flows

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