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5.16: Impact Craters

Looking toward the Moon we see a world very different from our own. Without weather, its ancient surface preserves a visible record of eons of too-close encounters with asteroids and other space debris. Most of the Moon is pocked with deep, circular features called impact craters. The lunar surface has been continually scarred by impacts of all sizes, ranging from large basin-forming impacts in the distant past to the tiny impacts of micrometeorites the continue to happen today. Most lunar craters are delineated by a circular, raised rim that surrounds a depressed floor. An impact may also have bright "streamers," streaks of lighter-colored material ejected from a bright crater that radiates outwards, and in some cases eject blown out of impact craters falls back to the surface to form secondary craters scattered nearby. The crater a given object forms is much larger than the impacting object — typically about ten to twenty times as big!


NASA image of simple and complex crater structure. Click here for original source URL

While most craters have circular edges, their internal shape varies greatly. Factors ranging from size and speed of the impactor to the density and depth of the surface being hit all literally shape the shape of a crater's floor. Small craters, less than a few kilometers across, are generally parabolic, or bowl-shaped. Larger craters have central peaks, and the largest impact basins have multiple concentric rings. The central peaks and rings are extraordinary evidence of the amount of energy involved in these impacts. When a large asteroid or comet hits the lunar surface, much of its enormous kinetic energy is converted into thermal energy. The resulting explosion sends out a shock wave that vaporizes most of the incoming object and some of the lunar surface. A large part of the target is also liquefied. A wave of fluidized rock travels outward, and at the point where the shock wave slows and the rock solidifies, a crater wall forms. In large craters, this wall is too steep to remain standing, and slumping occurs around the rim. Other waves are “frozen” into concentric rings. The central peak is the result of a wave that reflects off the crater wall and converges at the center. This is directly analogous to a water droplet hitting a pool of water. Most of the rock that was originally in the bowl of the crater sprays off from the impact point to form a blanket of ejected material and smaller secondary craters surrounding the crater. Resulting craters are often partially filled with melted and broken-up (“brecciated”) rocks. What is perhaps most amazing is that all of this violent activity takes place in only a fraction of a second!< br/> 
A crater’s shape and appearance can also give clues as to the planet’s surface properties. For example, some craters on Mars, called “rampart” craters, have eject blankets that look like they flowed across the surface instead of being thrown out in chunks. This may indicate the presence of ice beneath the surface that melted upon impact and formed muddy flows.

The prominent impact crater?Tycho?on the?Moon. Click here for original source URL

The almost uniformly circular nature of impact craters confuses many people. After all, when you throw a rock onto the sand, it might make an oblong "crater." If impacting bodies come from all different directions, why aren’t most craters elliptical? The catch is, the force released by that hand-thrown rock hitting the sand is nowhere near the force of an asteroid hitting the Moon’s surface. The reason is the extremely high amount of kinetic energy of the impactor. An asteroid may be traveling at tens of kilometers per second, several times the speed of sound. The resulting explosion of the rock vaporizing propagates outward in all directions symmetrically, creating circular craters.

Comparing the surface of the moon to the surface of the Earth, we find our world strangely empty of craters when compared to its nearest companion. This disparity is due to terrestrial weathering and plate tectonics eroding away craters on the Earth's surface. Looking around the Solar System, heavy cratering is a sign of a thin or absent atmosphere and a geologically dead world. By contasts, a planet or a moon without craters is likely to be active and interesting.

The number of craters we see on the Moon and via space probes and telescopes on other planets can help determine the ages of these alien surfaces. This is very useful when we don’t have samples of rock we can date using radioactive elements — which is the case for most planets. In general, if there are many impact craters on a planet’s surface, that surface is old, probably more than one or two billion years old. If there are very few impact craters on a surface, that surface is young, or less than a billion years old. Notice that the larger number of craters on the Moon compared with the Earth does not mean the Moon itself is older, but only that the surface is older.

A very important finding from lunar rocks is that the rate of impacts was much higher soon after the Earth and Moon formed than it is today. From 4.5 billion years ago until 4 billion years ago, the average number of impacts occurring per year was perhaps thousands of times higher than it is today. This early intense bombardment occurred because planets were sweeping up interplanetary debris left over after the planets formed. The cratering rate declined exponentially from its early peak. Only a small amount of that debris remains, but it's enough to cause occasional impacts on the Earth and other planets.

Our appreciation of the importance of impact cratering has grown as we realize how universal it is: every Solar System body we’ve studied has experienced impact cratering to some degree. Cratering has been the most influential process in creating the solar system as we see it today. Not only are most planetary surfaces visibly marked by craters, but impacts also led to the physical and dynamic states of many objects. For example, an impact probably caused the extreme axial tilt of Uranus. A significant portion of Mercury’s mantle may have been lost when a planetesimal slammed into it, leaving the planet with a relatively large core. Impacts have severely affected life on Earth, and the Earth-Moon system itself most likely formed during a giant impact event early in the Solar System’s history.


Multi-ringed impact basin Valhalla on Jupiter's moon?Callisto. Click here for original source URL.