Surface processes I: Impact cratering
Yours truly pleased to be in Chicxulub Pueblo.
Although it looks placid and unremarkable on the surface, Chicxulub is the site of one of the great pivotal events in Earth History.
Lunar craters from Wikipedia
These impacts occur as hypervelocity - up to:
- 17 km s-1 for asteroidal material
- 70 km s-1 for cometary material
Lowest impact velocities would roughly equal planetary escape velocity
- 11 km s-1 for Earth
- 5 km s-1 for Mars
Gravitation anomaly image of Chicxulub impact.
- Chicxulub 64.5 Ma. (right)
- Tunguska, Russia 1908
- Comet Shoemaker-Levy 9 1994
- Chelyabinsk meteor, 2013 - a 20 m diameter object.
Whereas most geologic processes operate gradually, effects of impacts are essentially instantaneous.
Slow acceptance of the idea
The rise of geology as a science occurred in the late 18th and early 19th centuries through the work of people like James Hutton and Charles Lyell. Their big contribution was the concept of uniformitarianism. The uniformitarian philosophy holds that the processes that we see acting on Earth's surface today are the ones that have shaped it in the past. Summed up by the slogan:
"The present is the key to the past."
To geologists raised in this philosophy, the idea of instantaneous catastrophic changes seems intellectually ugly. Thus, the reality of impact cratering gained very slow acceptance. Into the 1960s, the prevailing view was that lunar craters were volcanic. Two misconceptions perpetuated this view:
- No impactors were out there: Science had long resisted the reality of meteorites at all.
- Circularity of craters: If impacts were creating craters, if followed that they would strike from all angles, producing everything from circular craters to elongate ellipses. The fact that almost all craters were circular appeared to argue for some other cause.
- Nature defeats human hubris: The first asteroids were discovered by Piazzi and Herschel in 1801 and 1802. Then a spectacular shower of thousands of meteorites fell near the village of L'Aigle, France, in 1803 and was witnessed by hundreds of people. The existence of potential impactors became difficult to deny.
- Impact dynamics revealed: Low velocity impact experiments yielded craters of various shapes, depending on the impact angle, as the impactor mechnically deformed the substrate. In 1916, E. J. Öpik demonstrated that hypervelocity impacts were fundamentally different. High-speed impactors vaporized on impact. The primary force of impact was the uniform outward explosion of vaporized material, regardless of the angle of impact, yielding circular craters.
Barringer Crater from Wikipedia
- Suspected of being an impact scar in late 19th century. Studied by U.S. Geological Survey geologist Grove K. Gilbert in 1891, who determined it to be volcanic, despite an abundance of iron meteorite fragments in the area.
- Ultimately shown to be an impact structure by Eugene Shoemaker in 1960 on the basis of:
- Non-vaporized meteorite fragments
- Whereas the flows comprising a volcanic crater wall obey the Principle of Superposition, those of an impact crater wall do not. In fact, Barringer Crater's wall shows inversion of adjacent stratigraphy where it has been "flipped back."
- Presence of impact altered rocks including:
Lake Manicouagan Wikipedia
Terminal Free-Fall: A small enough impactor can be slowed to a terminal free-fall velocity by the same factors that impose terminal velocity on falling volcanic material. On Earth, objects smaller than 20 m can be slowed in this way. On Mars, much smaller objects might strike the surface at cosmic velocities, whereas on Venus, much larger impactors may be slowed significantly.
What would you expect to see on Titan, with a surface atmospheric pressure of 1.45 bars and escape velocity of 2.65 km s-1?
What actually occurs during an impact. Although instantaneous on a geologic time scale, impacts actually unfold over time. We conventionally divide an impact event into three stages:
- Contact and Compression: When the impactor touches the surface, shock waves with pressures up to 100 GPa (gigapascals) are generated. (For comparison, other Earth surface processes would generate fewer than 5 GPa. A rock would have to be buried over 2000 km to experience 100 GPa otherwise.) When the pressure wave passes, the impactor and adjacent target are vaporized. In this process, the kinetic energy of the impactor is transferred to the target.
Note: The shock wave generated expands in all directions regardless of the angle of the impactor's path.
- Excavation: The shock wave propagates outward in all directions, compressing the rock through which it passes. This displacement produces a transient cavity. Displaced material near the surface is pushed over the transient cavity's edge as ejecta. The heat of the impact melts some target material.
- Modification: After the shock wave passes, modification of the transient cavity occurs through:
- The accumulation of impact breccia - rock fragments cemented together by the glass that forms as molten material quickly solidifies. This accumulates in the bowl of the crater. With very large impacts, much molten material can pool up in the bowl of the crater, imparting a flat-bottomed profile (E.G. lunar crater Plato.)
- Slumping of cavity walls, more typical of larger craters. (E.G. the walls of the lunar crater Copernicus.) The largest impact basins can have extensive concentric rings.
- Recoil of rock beneath larger craters, producing central peaks. These can be up to a tenth the diameter of the crater. E.G.: The central peak of the lunar crater Tycho. (See also Sierra Madera, TX.)
Crater types:Crater morphology typically varies according to size:
- Microcraters: On airless bodies, the high speed impact of even tiny fragments can yield impact craters, including the 10 μm impact crater on a piece of lunar glass at right.
Endurance Crater on Mars from Wikipedia
- Simple craters: Smaller impacts form simple bowls that broadly resemble their transient cavities. (E.G. Endurance crater on Mars, right. Barringer Crater, AZ - above) Note: In many simple craters, the contact between bedrock and ejecta in the crater wall is easy to see.
Lunar crater Copernicus from Damien Peach's View of the Solar System
- Complex craters: Larger craters are marked by extensive slump-block terracing of crater walls and the presence of a central peak or peaks. The transition size between simple and complex craters varies with gravity and target rock properties. On Earth in soft rock, a 2 km diameter crater may have complex characteristics. The crater floor may be flooded by impact melt or secondary volcanic eruptions. On the moon, the transition occurs around 10 km. E.G. Copernicus crater on the moon (93 km diameter)
Valhalla Basin on Callisto from Exploring the Planets - Information page
- Multi-ring basins: In large enough impacts, the recoil of ductilely deforming mantle material interacts with impact dynamics to form complex basins with concentric rims. (E.G. Valhalla basin on Callisto.)
Lunar microcrater from panspermia.org
Schiller Crater on the Moon from The Thunderbolts Project
Double-layered Martian crater from International Business Times
Contrasting crater saturation of young and old terranes (which is which?) on Enceladus from
But what GOOD are craters? - Impact Chronology:If impactors struck planetary bodies at a known constant rate, and each impact yielded a recognizable crater, then it should be easy to determine the age of a region from the concentration of craters. Thus, cratering provides a useful window into the relative ages of regions. On the image at right, two distinct terranes meet on Enceladus. Which is clearly older, the one on the left or right?
Impact chronology relies on knowledge of both the number and size of craters in a region. Plotted against one another, these yield distinct trends for regions of different ages. The example at right shows the cratering frequency of distinct lunar regions, whose names are the basis for periods of lunar history. Of course, this system breaks down in regions so saturated with craters that every new impact destroys the record of older ones.
Note: The y-axis shows cumulative frequencies of craters. Thus, the blue point at lower center is telling us the number of craters with a diameter => 100 km / square km. The blue point at the top of the blue line is telling us the number of craters with a diameter => 10 km / square km. Naturally, as the size of the craters counted gets smaller, the number of crater / square km increases.
We also note that for any given crater diameter, older terranes have more craters than younger ones. Thus, pre-Nectarian lunar landscapes are older than Copernican/Eratosthenian ones.
Caveats: Note that, for a given body:
- Rates of impact are neither well known nor constant over time.
- The properties of the target body (like atmospheric density) can change over time, altering the rate at which craters are produced.
- One impact can yield many craters: a primary crater and secondary craters produced by the impact of ejecta of the first.
- Craters can be destroyed by geologic forces, including erosion, subsequent impacts, and tectonic processes. Indeed, a surface can become so densely covered with craters that no new crater can form without obliterating one that was already there. We say it is saturated.
- Those geologic forces can change over time as well.
Record of Late Heavy Bombardment origin-life.gr.jp
- Earth formation - 4.56 ga
- Giant impact and formation of the moon - 4.50 ga
- Late heavy bombardment - 4.0-3.8 ga.
- Origin of life - ~3.8 ga.
Recent Martian impact from NASA
Key concepts and vocabulary.
- Impact examples
- Barringer Crater
- James Hutton ande Charles Lyell
- Eugene Shoemaker
- Impact processes:
- Terminal free-fall
- Contact and compression
- Transient cavity
- Shocked quartz
- Central peak
- Crater types:
- Simple crater
- Complex crater
- Multi-ring basin
- Low-angle impact
- Double-layer crater
- Impact chronology
- Cumulative frequency
- Late Heavy Bombardment
- Caleb Fassett and David Minton. 2012. Impact bombardment of the terrestrial planets and the early history of the Solar System. Nature Geoscience 6, 520Ð524
- Simone Marchi, Clark R. Chapman, Caleb I. Fassett, James W. Head, W. F. Bottke, and Robert G. Strom. 2013. IGlobal resurfacing of Mercury 4.0Ð4.1 billion years ago by heavy bombardment and volcanism. Nature 499, 59Ð61