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

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

2 The sky is falling! I would rather believe that two Yankee professors would lie than believe that stones fall from heaven. Thomas Jefferson

3 The sky is falling! Chapman & Morrison (1994)

4 Terminology 1) meteoroid 2) meteor 3) meteorite

5 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 Crater Morphology:

target for a large impactor Simple craters are bowl shaped Complex

6 Kinetic Energy = 1/2 Mass Velocity 2 Impactors hit Earth at speeds of about Earth's orbital speed ~30 km/s HUGE Kinetic Energy!!

7 Impacts vs. Nukes At v = 30 km/sec, what diameter rock does it take to make as big an explosion as the atomic bomb dropped on Hiroshima? a) 1m E = 1/2 mv 2 = Joules Rock Density: ~3000 kg/m 3 Sphere Volume: 4/3 π r 3 b) 10m c) 100m d) 1km

8 The Impact Process (another reference) 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

Excavation - target is heated, vaporized, liquified, solid material is ejected (possibly above

9 Contact and compression stage Most of projectile s energy, momentum transferred to target rocks t cc 2R/v i Rankine-Hugoniot equations can be solved for pressure At few km/s, energy per unit mass that of TNT

2R/v i Rankine-Hugoniot equations can be solved for

10 Rarefraction wave vaporizes/melts/ fractures rocks Ejection/excavation stage t cf (D/g p ) 1/2 ~10 s for Meteor crater ~13 min for Mare Imbrium From Melosh EAS 4803/ CP 26:22

11 Collapse/modification stage Steep rim of transient crater collapses into interior t few x (D/gp)1/2 Final depth d 0.2D Rim height h 0.04D Diameter D 20R Kring (2007) EAS 4803/ CP 26:23

12 Cinder Lake Crater Field, AZ From Melosh EAS 4803/ CP 26:21

13 Few Hundred Confirmed Craters on Earth

14 Only ~300 craters total over whole of Earth. Why so few? A. Earth is 2/3 covered with oceans B. Earth has thick atmosphere so few smaller impactors hit the surface C. Much of Earth's crust has been turned over many times by volcanism & tectonic activity

Earth has thick atmosphere so few smaller impactors hit the

15 Meteor Crater

16 Manicouigan Crater

18 Moon: Far Side

19 Lunar Highlands

20 Mare Imbrium Lunar Mare

21 Cratering of Moon Younger regions were flooded by lava after most intense cratering was finished heavily cratered areas = old

22 The ~$100 billion plot High impact flux early Most impacts over by 3.5 billion years ago Low current impact rate Dating of Moonrocks brought back by Apollo astronauts gives us accurate ages of different areas on the Moon. Counting numbers of craters tells us the impact rate in that area at that time.

23 Current Impact Rate Size distribution of craters Size distribution of impactors Crater size ~10 x impactor size

24 Younger surfaces

25 Using craters to date surfaces Time Melosh (2011)

26 Equilibrium cratered surface

27 Using craters to date surfaces N cum = cd -b, b 2 Melosh (2011)

28 Impact flux has changed over time Highest during planet formation (planetesimals, embryos = impactors) Clustered Lunar impact melt ages suggest LHB but are the data biased?

29 Crater modification

30 Mercury Heavily cratered like the Moon old surface, geologically dead

31 MESSENGER at Mercury Volcanic pits? Lava flows? Tectonics? Looking more and more like the Moon.

32 Mars Rampart Crater

33 Mars Rampart Crater 3

35 Mars Exhumed 1

36 Mars Oblique Impact

37 Cratering on Mars Counting craters on Mars is complex Enough activity to bury, erode, and exhume craters, but not enough to erase them globally Rampart or pedestal craters thought to be the result of volatile (ice) rich target - a big muddy splat! Entire range of surface ages - from ~4Ga in the south to seemingly untouched in the northern icecap

38 Venus Global

39 Venus Craters 1

40 Venus Craters 3

41 Craters on Venus Thick (90 bar) atmosphere filters out most small impactors High (~480 C, or ~860 F) surface temperatures mean it s easier to melt rock, and molten rock is liquid longer Venus appears to have a fairly uniform surface age of ~700Ma What happened back then?

42 Ganymede Crater Chain

43 Shoemaker-Levy 9

44 SL-9 Aftermath

45 Recent Jupiter impact

47 Phobos

48 Hyperion

49 The BIG Facts about impact cratering: 1. Impactors hit at HYPERVELOCITIES (~escape speed of planet) so they explode when they hit the surface and make round craters with little indication of the direction they hit. When the impact <45 degrees from the surface there is some indication of "shadow zones" of little ejecta. VERY oblique trajectory is need to produce any asymmetry in the crater. 2. Kinetic Energy of impactor -> Excavation, ejection, dissipation (shock waves, heat, melting, etc). 3. Diameter(crater) ~ 10 x Diameter(impactor) - useful rule of thumb 4. Crater Density vs. Age - profile of cratering rate provided by lunar rocks, applicable (more or less) across the solar system - critical for dating surface by crater density. 5. Crater size distribution - N(craters) Diameter(crater) -2 (interesting exceptions of Earth and Venus where there seem to be a lack of small craters mostly due to the small impactors "burning" up in the atmosphere, and to a lesser extent erosion) 6. Impactor size distribution is also Diameter(crater) -2

CRATERS in the SOLAR SYSTEM. REMOTE ACCESS ASTRONOMY PROJECT UNIVERSITY OF CALIFORNIA, SANTA BARBARA and CENTER for PARTICLE ASTROPHYSICS

CRATERS in the SOLAR SYSTEM. REMOTE ACCESS ASTRONOMY PROJECT UNIVERSITY OF CALIFORNIA, SANTA BARBARA and CENTER for PARTICLE ASTROPHYSICS CRATERS in the SOLAR SYSTEM REMOTE ACCESS ASTRONOMY PROJECT UNIVERSITY OF CALIFORNIA, SANTA BARBARA and CENTER for PARTICLE ASTROPHYSICS Craters in the solar system Jatila van der Veen, UCSB Physics Department