Impact Cratering

Virginia PasekSeptember 18, 2008

Astronomical Observations

• Galileo first noted craters on the Moon ~1610

• Robert Hooke, 1665, speculated about the origins of the lunar craters – Couldn’t be impactors; space was empty

• J. H. Schröter first formal use of word ‘crater’ in 1791– Concluded volcanic origins after seven years of study

• Beer and Mädler 1876• Scientists believed that the moon was covered with extinct volcanoes until ~1930

Space is not so Empty

• Meteorites – 1819 by Chladni– Supported by April 26, 1803 fall in L’Aigle near Paris

– Scientifically accepted by 1880

• Discovery of asteroids around 1820• Connection made between large impacts and meteorites in 1906 – Meteorite Crater

The Bowl-shaped Problem

• Incidence angle was a problem– Objects with low incidence angles should produce elongated impacts

Right?

High-velocity Impacts

• E. J. Öpik, 1916– Like explosions, high-velocity meteoroids produce circular craters for most incidence angles

• Again in 1919 by H. E. Ives of Langly Field– Even noted central peaks

• Countered by W. W. Campbell of Lick Observatory

• Third times the charm… Finally, two papers by A. C. Gifford in 1924 and 1930 fixed the bowl-shaped problem forever

World War II

• Recent and poorly documented– Much research still classified– Driven by threat of nuclear weapons and high-velocity impacts

– Dangers to satellites in low-earth orbit

Converging Lines of Study

• Astronomical study of the origins of the lunar craters

• The acceptance of meteorites as impactors circa 1880

• Military testing associated with WWII

= Study of Impact Cratering

What is Impact Cratering?

• The study of the physics of impact and explosion craters joined with astronomical and geological study of impact craters– Recent field of study - decades– Spurred forward by space travel and Apollo program

Craters Everywhere!

• Grieve, 1987, lists 116 impact craters on Earth

• Craters found on nearly every solid body in the Solar System

Cratering Mechanics

• Contact• Compression• Excavation• Modification

Contact and Compression

• Briefest of the stages– Lasts only a few times longer that it takes for the impactor to traverse its own diameter

• Transfers energy and momentum to underlying rocks

• Impactor is slowed and compressed• Surface is pushed downward and outward

• Material at the boundary moves at same velocity

Excavation

• Shockwave expands and weakens moving as fast or faster than speed of sound

• Attains shape of hemisphere as it expands through the target rocks

• High shock is confined to surface of hemisphere– Interior has already decompressed

• High pressure minerals such as stishovite and coenite form

Excavation

• Surface pressure is zero. Shock pressure from contact and compression.– Thin layer of surface rocks thrown upward at very high velocity

– Debris is lightly shocked or unshocked

– Only 1 - 3% of total mass excavated – May be origin of lunar meteorites and SNC meteorites from Mars

Modification

• Motion halts, then moved downward and back toward the crater

• Due to gravity and occasionally elastic rebound

• Simple craters– Debris and drainback

• Complex craters– Complete alteration of form, including floor rise, central peak, rim sinking into wide stepped terraces

– Mountain ranges and pits in the largest complex craters

• Begins almost immediately after formation of transient crater

Crater Morphology

• Simple craters• Complex craters• Multiring basins• Abberant crater types

Simple Crater Facts

• Common at < 15 km rim-to-rim diameter, D, on moon

• Rim height 4% of D• Rim-to-floor depth 1/5 of D• Ejecta blanket extends one D from rim• Secondary craters and bright ray ejecta• Floor underlain by breccia

– Contains shocked quartz i.e. coesite and stishovite

– Floor typically 1/2 to 1/3 of rim-to-floor depth

Simple Craters Schematic

Simple Craters on Earth

• First to be identified on Earth

• Not always completely circular– Faults

• Common at 3 km to 6 km diameter

Simple Crater on Moon

• Moltke crater, a simple crater, was photographed by Apollo 10 astronauts in 1969. The depression, about 7 km (4.3 miles) in diameter.

• Common up to 15 km diameter

Transition to Complex Craters

• Transition diameter scales as g-1, where g is the acceleration of gravity at the planet’s surface

• On moon, transition is about 20 km

• Because…

• Earth gravity 9.8 m/s2

• Moon gravity 1.6 m/s2

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T = k1

g

Complex Craters

• Formed by collapse of bowl-shaped crater

• Observed on Moon, Mars, Earth, and Mercury

• Uplift beneath centers– Structural uplift to crater diameter by

• Diameter of central peak approx 22% of rim-to-rim diameter on terrestrial planets

• Depth increases slowly– Depth from 3 - 6 km– Diameters from 20 - 400 km

• Diameter may increase as much as 60% during collapse

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hsu = 0.06D1.1

Complex Crater Schematic

Complex Crater on Mercury

Complex Crater on Moon

The far side of Earth's Moon. Crater 308. It spans about 30 kilometers (19 miles) and was photographed by the crew of Apollo 11 as they circled the Moon in 1969

More Complex

More Complex Facts

• Transition to central ring at approx 140 km diameter on Moon

• Still follows the g-1 rule• Central ring generally about half of rim-to-rim diameter for terrestrial planets

Central Ring Crater

• Barton crater on Venus

• Discontinuous central ring

• Very close to transition diameter– 50 km ring

Multiring basins

• Valhalla basin on Callisto

• 4000 km – Only central bright stop believed to be formed by impact

• Outward facing scarps

Multiring basins

• Orientale basin on Moon

• Youngest and best preserved

• Approx 930 km diameter

• 2 km depth• Inward facing scarps

Characteristics of Multiring Basins

• Most likely caused by circular normal faults– Normal fault is result of crustal extension

• Ring diameter ratios of roughly• No longer function of g-1 • Possibly influenced by the internal structure of the planet

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Valhalla

Multiring Schematic

The ring tectonic theory suggests that in layered media in which the strength decreases with increasing depth, one or more ring fractures arise outside the rim of the original crater (figure 5) (Melosh and McKinnon, 1978). This suggests that for the formation of multiring basins to occur there must be a high brittle-ductile thickness ratio in the impacted material i.e. where thick crust exists over a deeper ductile layer (Allemand and Thomas, 1999). www.spacechariots.biz/ creaters.htm

Aberrant Crater Types

• Unusual formation conditions– Either in impactor or planetary body

• Very low impact angles - 6° from horizontal– Circular crater with asymmetric ejecta blankets

– Elliptical craters with butterfly eject patterns

• Smaller impactors on Earth and Venus tend to form clusters of craters, reflecting atmospheric breakup

References

• Encyclopedia of Planetary Sciences, pp. 326, Impact Cratering by H. J. Melosh

• Impact Cratering: A Geologic Process, H. J. Melosh, Oxford University Press, 1989

• Encyclopedia of the Solar System, Ch 43, Grieve et al

• Google