Transcript document
Impact Mechanics and Morphology Impact Craters • Crater: From the Greek krater meaning bowl • Drop a rock into some sand (v = a few m/sec) – Physically what happens is that the impact sets up a shock wave moving at the speed of sound – It passes from grain to grain, moving the grains out of the way of the rock • With asteroids hitting a planet, things are different – Velocities are cosmic ≈ 10-70 km/sec – Velocities are MUCH faster than the speed of sound in rock (or sand) – The material does not have time to move out of the way. Energy and Impacts • Since the surface is not moving out of the way, the impactor (the asteroid) comes to a sudden stop. • What happens to the impactor’s kinetic energy? • Well…..lets do an example – Take an small iron asteroid about 30 meters in diameter, volume is about 15,000 m3 – Density of iron is about 7000 kg/m3, so the mass is about 100 million kilos – Say it is going slow in cosmic terms, 20,000 m/sec – What is the kinetic energy? • ½ MV2 works out to about 2 x 1016 Joules (2 x 1023 ergs, or 4.7 megatons) • From our example the impactor transforms 2 x 1016 Joules of kinetic energy into heat. This is essentially an explosion….creating a shock wave and vaporizing the impactor and some of the rock • The rock gets compressed, material at the surface jets out • The shock wave compresses the rock, fracturing and pushing it away • Material at the free surface is ejected • Part of the surface material is lifted up by the compression wave • Material down to about 1/3 of the transient cavity is excavated. The rest is just pushed down • The compression wave dissipates. The crater relaxes • Debris partially fills the crater and ejecta is deposited on the sides. • With very large impacts the thickness of the crust is small relative to the size of the shock wave. • The shock wave reflects of the crust/mantle boundary causing rebound the and the formation of a Central Peak • This provides insight on the depth of the crust/mantle boundary Alfrancus C - 10 km diameter simple impact crater Tycho - 85 km diameter complex impact crater • As complex crater diameter increases, the depth increases much more slowly. – Complex crater diameters range 20-400 km – Depths only range 3-6 km • At about 140 km diameter complex craters get a “ring” of mountains instead of a central peak. Schrödinger - 320 km diameter peak ring impact basin Orientale 900 km diameter multi-ring basin Products of Impacts • Secondary Craters – Found in lines way from the impact – Clumps that form V’s pointing in the direction of the impact (from other impacts off the picture) • Rays – Radiating out from the crater • Melt – In the crater • Ejecta Blankets – Out about 1 crater radius Copernicus ejecta rays and secondaries on Mare Imbrium Tycho rays This is characteristic of a fresh crater Lines of Secondaries • But identifying secondaries can be tough….. • You can pick out two groups of secondary craters in the top image. • Where are the groups of secondaries in the bottom image? Impacts into wet or icy material • Impacts into waterrich surfaces produce….mud. • High energy from impact instantly vaporizes the ice and fluidizes the ejecta. Mud splatters like….mud • Lobes of soft muddy sediment are deposited around the crater to form one or more “ramparts”. • Strong evidence for subsurface ice or groundwater. Rampart Craters in The Martian Highlands Lowland Highland Rampart Crater and Channels in Medusa Fossae Close-up of crater in Medusa Asymetric lobes of ejecta probably reflect oblique impact direction • Larger impacts penetrate the subsurface to greater depths. • Small craters near the equator do not penetrate subsurface ice horizons • Only large craters near the equator can penetrate the ice layers needed for rampart formation. • Near the poles subsurface ice is more abundant and near the surface, so smaller impacts can produce ramparts • In this way, craters provide a view into the subsurface. Distribution of Rampart Craters A little crater math • Formation time for craters scales with the gravity of the object – t = (D/gp) ½ • The ratio of the crater’s depth to the diameter for simple craters – depth = D/5 – This relationship does not work with complex craters • The crater’s size will scale with the impact energy – D 0.0135 E0.28 How do atmospheres effect things? • Remember that thick atmospheres like the Earth’s will interact with some impactors • Stresses can break up incoming meteoroids, friction can burn them up (meteors) • This effect is size dependent – (think about the physics of a 10 km impactor) – If the atmospheric mass displaced by the meteoroid is about equal to its mass, then it will probably not reach the ground – The critical radius of the object for any planet is R 8 x 106 (P/gpsin) in cgs units – P is atm pressure in bars (the e6 term is to convert the bars into the cgs system (a bar is 10e6 dyn/cm2), is the density of the meteoroid, and is the entry angle Meteorites on Mars Possible Stony-irons Basic Cratering Principles • Big stuff is less common than small stuff • The sizefrequency distribution of impactors is a steep power law Basic Cratering Principles • This is what a steep power law looks like • The overall cratering rate declined rapidly after solar system accretion • But, there was probably a significant bump during the Late Heavy Bombard, probably due to outer planet migration. • The last couple billion years have seen more-or-less stable cratering rates Cratering Rate Crater Saturation • For very old surfaces enough craters have accumulated that new craters destroy old craters. • For the Moon, saturation age is around 4.1-4.2 Billion years Cratering Principles • More craters = older surface • Differences in the position of the cratering curve show differences in surface age. – In this plot Nectaris is older (more craters) than Orientale The Dirt on Crater Dating • There is a complex sub-culture that uses crater counts to estimate the ages of surfaces. • For any planetary surface this works fine in the relative sense…..more craters means an older surface. • The problem is how old? Putting an absolute age on the surface is tough – Requires knowledge of the cratering rate, which is not at all precise – And probably not the same everywhere in the solar system • Shown here are plots of two areas on Mars superimposed on an estimated crater dating scheme. • This attempts to give an absolute age to the surface. • However, what we are looking at with cratercounting is “crater retention ages” which include two factors. – The degradation of craters by local surface processes (wind, water, lava flows, etc) – The degradation of craters from other craters. • These are crater counts for the most heavily cratered surfaces in the solar system. • These objects typically have 30 times the crater density as the lunar mare reference curve. • Crater counts for Earth with reference isochrons • Why the rolloff in smaller diameters? • Is the European shield really older than the North American Shield? Relative Age Dating with Craters Where is the oldest terrain on Mars? Venus? Crater Dating Applied: How Young is the Lunar Crater Giordano Bruno? • Is it possible that people witnessed the impact event that made this crater in the year 1178 or did it form long, long ago? • Small craters on its ejecta blanket were counted to derive a formation age of Giordano Bruno. http://www.psrd.hawaii.edu/Feb10/GiordanoBrunoCrater.html How Young is the Lunar Crater Giordano Bruno? • On the left chart, the crater size-frequency distribution for small craters counted on the ejecta blanket of Giordano Bruno falls between 1 to 10 million years, not younger. • On the right, Giordano Bruno plots at 4 million years on this lunar cratering chronology. http://www.psrd.hawaii.edu/Feb10/GiordanoBrunoCrater.html How Young is the Lunar Crater Giordano Bruno? 150 m • Complication: Some or all of the small craters could be secondary craters formed by the Giordano Bruno event. • Counting secondaries leads to an overestimate of the age. • But is Giordano Bruno 2 million or 832 years old? Example of cluster of secondary craters on the farside of the Moon http://www.psrd.hawaii.edu/Feb10/GiordanoBrunoCrater.html Cratering on Earth • What does this plot really show? – Erosion (erases craters) – Ground cover (craters are easier to see in deserts than jungles) – Geological exploration (More explored areas find more craters)