A geologic process An erosional force A chronological tool An influence on biology

Transcription

1 Impact Cratering: Physics and Chronology A geologic process An erosional force A chronological tool An influence on biology

2 Impact features are common All solar system bodies with solid surfaces show evidence of impact craters Geologically active bodies have erased many of their craters (eg. Earth/Venus)

3 'Dead' Terrestrial planets Mercury and the Moon preserve craters back to >4 billion years ago.

4 Venus/Mars Surfaces of varying ages Some erosion on Mars, much on Venus Venusian atmosphere blocks out small impactors!

5 Jupiter's satellites Craters preserved on ancient icy surfaces Well studied by Galileo probe

6 Other outer satelites Mimas (Saturn) Miranda (Uranus) Saturn's Mimas has one huge crater (the Death Star moon) All outer Solar System satellites cratered.

7 Small bodies too Asteroids are covered with craters of all sizes. Comets tough to tell...

8 Earth! Many have been eroded away Large ones seen best from the air.

9 Meteor Crater : Arizona Formed ~20,000 years ago Impactor was an iron meteoroid : many fragments have been recovered.

10 Meteor Crater : Arizona Excellent for studying crater morphology Go for a visit! Just south of Grand Canyon entrance.

11 Impact Crater morphology All craters have a bowl and ejecta blanket Bowl rim is raised relative to the surrounding terrain. Larger craters have 'central peaks'

12 Rim to rim diameter

13 Simple crater Simple bowl Raised rim Note ejecta blanket

14 Complex crater Central peak due to 'rebound' of rock behaving like a liquid (transparancy)

15 Movie of central peak formation Central feature depends on when collapse ceases

16 central peak Lunar crater Copernicus

17 central ring Lunar crater Schroedinger

18 central ring Lunar crater Schroedinger

19 Lunar basin Orientale Near leading edge of Moon outer ring 930 km in diameter one of many lunar basins, but best preserved multi

20 Impact Basin Morphology Very large craters (called basins) have a series of outer rings Orientale (on the Moon) shows these easily. Caloris on Mercury Hellas on Mars.

21 Valhalla multi ring basin (Callisto) Little vertical relief Lack of relief may be due to properties of ice, which relaxes after the impact.

22 Impact physics When an impactor strikes a solid surface it is like a point explosion This explains why nearly all impact craters are round. An 'ejecta curtain' forms and falls back to surface.

23 Ejecta blanket falls back to the surface (martian crater below)

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25 Crater excavation The crater bowl is created in the 'excavation stage' Most of the bowl material is 'folded over' and deposited upside down on the ejecta blanket > inverted stratigraphy

26 Crater ejecta Some ejecta is ejected fast and lands far away from the crater, making 'secondary craters' (Euler secondary field at left) A tiny fraction moves so fast it can escape back into space.

27 Lunar/martian meteorites Proof of fast moving ejecta! AT LEFT: Lunar meteorite ALH from Antarctica, an impact breccia Has GREEN fusion crust of anorthosite

28 Crater Sizes For 'typical' impact speeds of 10 km/s for rocky impactors and targets, the ratio between impactor diameter d and crater diameter D : D ~ 20 d (eg. 1 km impactor hitting the Moon creates a

29 Crater Scaling How does crater size vary with: impactor size? impactor density? impactor speed? Is crater diameter controlled by the impactor's: A. Kinetic energy? B. Momentum? See Lab 4!

30 Cratered landscapes How many craters are there on a planet? This is a meaningless question without adding: Bigger than what? Often 1km or 20km crater sizes used.

31 Cratering age estimates Take a 'blank piece' of planetary surface and wait; what happens? Craters form at some rate; wait 2X longer and there'd be 2 times as many craters. So if you can measure the # of craters N *and* know the crater formation rate R (# craters/unit time) then you can get the ''accumulation age' T from : T = N/R Right?

32 Cratering ages: complications Cratering rate is PER UNIT AREA # of craters proportional to area counted! As time goes on, can you 'fill' a surface? The concept and problem of saturation (transparencies)

33 Cratering ages: complications Cratered surfaces on the various planets and satellites (see handout) Variations in apparent age due to resurfacing. Could be due to volcanism filling in craters Cratering rate is KNOWN to have varied with time

34 Lunar cratering history Grossly simplified: Very high near formation of Solar System and declined rapidly