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Cratering in the Solar System |
Take the Quiz! == page down to the bottom of these lecture notes
We will be spending a lot of time on cratering in the solar system. Why? Read the following excerpt from Rare Earth: Why Complex Life is Uncommon in the Universe as an event that took place roughly 65 million years ago is described:
The asteroid (perhaps it is a comet) is between 6 and 10 miles in diameter, and it enters the Earth atmosphere traveling at a rate of about 25,000 miles per hour. At such speed the body takes 10 seconds to pass through the atmosphere and then smashes into Earth's crust. Upon impact, its energy creates a non-nuclear explosion at least 10,000 times as strong as the blast that would result from humankind's entire nuclear arsenal detonating simultaneously. The asteroid hits the equatorial region in the shallow sea then covering the Yucatan and creates a crater as large as the state of New Hampshire. Thousands of tons of rock from the ground-zero impact area, as well as the entire mass of the asteroid itself, are blasted upward. Some of the debris goes into Earth orbit, while the heavier material reenters the atmosphere after a suborbital flight and streaks back to Earth as a barrage of meteors. Soon the skies over the entire Earth glow dull red from these flashing small meteors. Millions of them fall back to Earth as blazing fireballs and, in the process, ignite the verdant Late Cretaceous forests; over half of Earth's vegetation burns in the weeks following the impact. A giant fireball also expands upward and laterally from the impact site, carrying with it additional rock material that clots the atmosphere as fine dust is transported globally by stratospheric winds. This enormous quantity of rock and dust begins sifting back to Earth over a period of days to months. Great dust plumes and billowing smoke from burning forests also rise into the atmosphere, cloaking Earth in a pall of darkness. From space we begin to lose sight of the surface of the planet and can see only darkening gauze obscuring Earth's once green and blue surface. It is a vision from Dante's Inferno, a nightmare of red fires and black soot.
The impact creates great heat both on land and in the atmosphere. The shock heating of the atmosphere is sufficient to cause atmospheric oxygen and nitrogen to combine into gaseous nitrous oxide; this gas then changes to nitric acid when combined with rain. A prodigious and concentrated acid rain begins to fall on land and sea, and before it ends, the upper 300 feet of the world's oceans are acidic enough to dissolve calcareous shell material. The impact also creates shock waves spreading outward through the rock from the festering hole in the crust; Earth is rung like a bell, and earthquakes of unprecedented magnitude occur. Huge tidal waves spread outward from the impact site, eventually smashing into the continental shorelines of North America, and perhaps Europe and Africa as well, leaving, when they recede, a trail of destruction and a monstrous deposit of beached and bloated dinosaur carcasses skewered on uprooted trees. The surviving scavengers of the world rejoice. The smell of decay is everywhere.
For several months after this fearsome day, no sunlight reaches Earth's surface; the atmosphere is darker than the oil-fueled miasma that blanketed Kuwait following the Gulf War. After the initial rise in temperature from the blast itself, the ensuing darkness causes temperatures to drop precipitously over much of the planet, creating a profound winter in a previously tropical world. The tropical trees and shrubs begin to die; the creatures that live in them or feed on them begin to die; the carnivores that prey on the smaller herbivores begin to die. The Mesozoic era, which began 250 million years after the Cambrian Explosion ... comes to the end of its nearly 200-million-year reign.
... All of this havoc creates death: the death of individuals, the death of species, the death of entire families of organisms. This event is a planetary catastrophe. Had the impacting object been only twice the size it was, it might have sterilized the surface of planet Earth. It was a narrow escape for complex metazoans.
Ward, Peter D. and Brownlee, Donald, Rare Earth, 2000 (Copernicus, New York)
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Composition of
Nebula |
Condensation Sequence |
Remnant primitive material
Comets -- thousands in the Kuiper Belt, millions in the Oort Cloud
Chondrites (stardust)
Thousands and thousands of small planetesimals -- asteroid belt
Dozens of large planetesimals + nearly full-grown versions of terrestrial planets
Large planets (>1000 km) undergoing heating from radioactive material
Large planets differentiate
Jupiter keeps "stirring the soup"
The early solar system was no place to be as the debris from the formation of the solar system was being swept up. Between approximately 4.3 to 3.8 billion years ago, comets and asteroids plummeted the Earth and every other body in the solar system. The solid planets and moons show ample evidence of the maelstrom. Large asteroid- and cometary-type bodies were common, but so were small-planet-sized bodies. Scientists believe that a planet about the size of Mars collided with Earth about 4.5 billion years ago, splashing out material that quickly formed our moon. These types of collisions may also explain Venus' retrograde spin, and Uranus' 90 degree tilt.
Scientists can date a planet's surface (at least in a relative sense) by counting the number of craters as a function of size. A surface that has not seen any rejuvenation for over 4 billion years may have a million craters per million square kilometers, ranging in size from 0.25 kilometer to over 512 kilometers. In contrast, a surface that is less than a million years old may have less than 1 crater per million square kilometers and the maximum size would be less than 4 km. In comparison, the Earth has a surface area of about 500 million kilometers.
As the material was swept up in the nebula, the material becoming part of the rapidly growing planets, the impact rate declined. Since there were more medium-sized objects (say around 100 m in diameter) than large-sized objects, it took a bit longer for these objects to thin out. Small-sized objects (say, less than 100 m in diameter) were far more abundant, and the planets and moons are still experiencing impacts from these objects, even 4.5+ billion years after formation. Take a look at the daytime Planets class (see link just below) for the number of craters versus diameter graph, and think about how we could use this information to determine the age of a surface on any solid world. Remember as you view this graph that the actual ages were determined by radioactive dating of Moon rocks.
Here is the scenario of an impact by a 10-km asteroid or comet on the Earth:
The asteroid delivers its vast amount of kinetic energy (KE)--which had been stored initially as gravitational potential energy (GPE).
Because energy is conserved, the KE is converted to thermal energy or transferred as kinetic energy as the surface material gets airborne.
Shock waves propagate into the target rocks and the impacting body.
The rock becomes almost fluid.
A decrompression wave follows the shock wave. Decompressed material sprays out of the cavity as an expanding conical sheet.
The ejecta represents overturned layers.
In large impact structures, the rock walls slump inward soon after excavation of the initial cavity.
As slumping material converges inward, it produces a pronounced central peak.
A simple
animation of
an impacting event
While we will go into greater detail during our discussions of the individual worlds, here is just a sampling of the impact craters in the solar system, from the largest impact basins to the smallest secondary craters.
| Impact Basins | |
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| Caloris Basin, Mercury:
The largest surface feature on Mercury is the Caloris Basin, which resulted from a collision with an asteroid. The basin, which is more that 1000 kilometers across, is visible as the large circular feature at the bottom of the linked photograph. Similar features, such as the Mare Orientale, are seen on the Moon. The Caloris Basin gets very hot because it is near the "sub-solar point" - the point on Mercury's surface that is directly under the Sun when Mercury is closest to the Sun. |
Image of the Caloris Basin on Mercury (340 kb), NASA |
| Mare Orientale, Luna:
Mosaic of more than 2000 Clementine 750-nanometer images, showing the Orientale Basin of the Moon at full resolution of about 250 meters per pixel. The Orientale Basin is about 930 kilometers in diameter and is only partly filled by dark, mare lava. The relative paucity of mare basalt fill, coupled with the young age and topographic freshness of the basin, makes this feature the archetypal lunar multiring basin. |
Clementine's view of Mare
Orientale (204 kb), Lunar Planetary Institute |
| Valhalla, Callisto:
This 4000-kilometer-wide impact structure consists of at least 25 concentric rings or ring arcs, and is one of the most unusual impact structures in the solar system. The unusual number of rings may be due to impact into a thin, icy lithosphere, which fractured very easily when the basin collapsed. |
Voyager's View of the Multi-ring Basin
Valhalla (200 kb), NASA |
| Complex Craters | |
| Tycho, Luna:
This image clearly shows both the central peak and terracing in the walls of Tycho. Tycho is in the lunar highlands, and the terrain surrounding the crater is quite rugged. The crater floor is also fairly hummocky. Multispectral images obtained by the Clementine spacecraft show that the central peak has a different composition than the surrounding material, presumably because the central peak is composed of material that originated at greater depths in the Moon's crust. Lunar Orbiter image V-125M. |
Close-up of Tycho from Clementine (457 kb), LPI |
| Simple Craters | |
| Moltke crater, Luna:
Moltke Crater, 7 kilometers in diameter, is an excellent example of a simple crater with a bowl-shaped interior and smooth walls. Such craters typically have depths that are about 20 percent of their diameters. The hummocky material surrounding the crater is Moltke's ejecta deposit. (Apollo 10 photograph AS10-29-4324.) |
Moltke simple crater (370 kb), LPI |
| Secondary Craters | |
| Copernicus ejecta, Luna:
Material ejected from a crater sometimes produces additional craters nearby, called secondary craters. The overlapping chain of craters in this image is an example of such secondary craters. These craters are part of the crater chains shown in the center of the first Copernicus image. (Lunar Orbiter image V-144M. Apollo 17 view of Copernicus ejecta.) |
Apollo 17 view of Copernicus ejecta
(508 kb), LPI |
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Sketches
and descriptions of
simple versus complex craters MANDATORY VISIT! Find your 3-D red/blue glasses for some stereo images. The Far-Side (but not necessarily The Dark Side) of the Moon. Lunar and Planetary Institute Lunar Missions Home Page Clementine Images of Mare Orientale and other Lunar images. |
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