Impacts and Erosion 14 September 2015

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

Processes that HEAT planets

Possible Subduction and Crustal Recycling on Europa

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

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

Meteor Crater (Arizona) Tycho (Moon) Impact Craters

The Impact Process 1.Detonation - shockwave propagates through target, projectile vaporized 2.Excavation - target is heated, vaporized, liquified, solid material is ejected (possibly above escape velocity!) 3.Rebound - in larger craters, the target may “bounce back” viscously, forming a central uplift 4.Relaxation - crater walls subside, melt pools in the crater

Few Hundred Confirmed Craters on Earth

Meteor Crater

Ariel view of Meteor Crater

Manicouigan Crater Crater Lake

Moon Near Side Moon’s Giant Basins

Moon Far Side Lunar Farside

Lunar Highlands

Mare Imbrium Lunar Mare

Crater modification

Mars Rampart Crater 3 Mars Rampart Crater

Venus Craters 1 Venus Crater with Ejecta

Ganymede Crater Chain

Shoemaker-Levy 9

SL-9 Aftermath

Hyperion

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

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 30km Image of a dust devil caught in the act Martian dune features

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

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

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

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)? Martian Outflow channels 50km flow direction 150km Baker (2001)

Martian Gullies A very unexpected discovery (Malin & Edgett, Science 283, , 2000) Found predominantly at high latitudes (>30 o ), 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?

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

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 Malin and Edgett, Science 2003 meander channel

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

Mercury - lots of craters, little volcanism Venus completely re-surfaced by volcanism ~600 MY ago Earth - heavily re-worked surface Moon - volcanism stopped 3.2 BYago Lots of craters Mars - some volcanism, south heavily cratered Terrestrial Planets