Impact Craters Flashcards
Source of impact craters?
Comets & asteroids
What is a comet?
A collection of large number of rock and metallic particles coated with frozen ice
Low density object originating from Kyper Belt
elliptical orbit around the sun that accelerates towards sun
EFFECTED by gravitational field of outer planets epically Jupiter
Roche limit - dissembled by tidal forces
What is an asteroid?
Found between Jupiter and Mars (asteroid belt) - a planet that formed and broke up due to Jupiters large gravitational field, however, continued to orbit the sun
How does Jupiter effect asteroids?
Dispersed by Jupiters massive gravitational field - frequent collision of ejecta into eccentric orbits
Chaos - Kidwood group
Main implications of impacts?
Crater depends on the impacters kinetic energy
Impacter is much smaller than the crater it producers
Oblique impacts will still make circular orbits -
How have we understood impacts on planets?
NASA Vertical ballistic gun
RHUL VBG - impact angle
Radiometric dating (only on moon)
How is crater and it’s features formed?
Impact - rarefaction wave - shockwave (content - compression)
Impact - asteroid or comet (causes ejecta - ejecta follows ballistic trajectory with orderly deposition)
Rarefaction wave causes shockwave generation and projectile deformation
Shockwave - characterised by an abrupt nearly discontinuous change of the medium - rapid rise in Pressure, Temperature and density - passage of shockwave producers shatter comes
What is a simple and complex crater?
Simple depends on the strength of the target - the stronger the rock the smaller the crater
Complex depends on the weight/gravity of the rock - weight depends on the target and rock density
How does impact angle effect the crater?
Oblique angles to 15 degrees - craters remain circular but ejects blanket becomes asymmetrical
Low angle impacts - craters become oval/elliptical when impacts angles are less than 15 degrees (butterfly ejecta pattern)
Post crater modifications?
Circular faulting producing terrains
Gravity slides
Central peaks (technically due to isostatic rebound)
Terrestrial impact craters?
Eroded circular structure >3km Presence of shock melted glass Generally your get than 500my Shatter cones - Concoidal fractures with the apex of come points to shockwave Remains of the meteorite Impactites - glassy or finely crystalline material Shocked minerals Geochemical anomalies Mass extinctions?
Craters on mercury?
Small impacts are bowl shape
Larger impacts - flatter floor, terracing (different elevations), Central peaks
Higher gravity therefore decreasing distribution of ejecta - features also appear more degraded
Craters on Venus?
50% of earth crossing asteroids
Impact 14% higher than earth (higher gravity and closer to the sun)
No craters
Craters on mars?
Two terrains highlands and flat
Southern heavily cratered highlands
Northern sparsely cratered flats
Ancient terrain
Name craters on the moon?
Impact basin - schrodinger crater King crater (David et al,. 2005) - sharpness of features, distinct halo of dark impact glass, central peaks - due to isostatic rebound (higher albedo - matching the rim), Central walls - possible slumping of the walls due to rebound of the central peak, crater excavation - ejecta blanket (anorthositic composition) - dune like features can be present indicating interaction with the ground, tektites - glassy melt ejecta, shock metamorphism, crystalline melt sheets (flows and pods volcanic and impact origin)
