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 ≈ 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 m 3 –Density of iron is about 7000 kg/m 3, 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 MV 2 works out to about 2 x Joules (2 x 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 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 km diameter peak ring impact basin

Orientale 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 water- rich 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

Rampart Crater and Channels in Medusa Fossae Highland Lowland

Close-up of crater in Medusa Asymetric lobes of ejecta probably reflect oblique impact direction Impactor Path

Distribution of Rampart Craters 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.

A little crater math Formation time for craters scales with the gravity of the object – t = (D/g p ) 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  E 0.28

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 10 6 (P/  g p sin  ) 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 size- frequency distribution of impactors is a steep power law

Basic Cratering Principles This is what a steep power law looks like

Cratering Rate 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

Crater Saturation For very old surfaces enough craters have accumulated that new craters destroy old craters. For the Moon, saturation age is around 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 crater- counting 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 roll- off 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.

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.

How Young is the Lunar Crater Giordano Bruno? 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 150 m

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)