The impact crater is one of the most common geological features in the Solar System, reflecting the violence that characterized the formation and early history of the planets.

Photo Credit: NASA

Copernicus Crater on Earth's Moon

"Cratering" simulates the process that formed craters. Viewers manipulate a small dipper to capture clumps of powder and drop them into a vat of fine powder, causing the explosion of random-sized craters in a terrain that resembles the crater-pocked surfaces found in our Solar System.

Photo Credit: Ned Kahn

"Cratering" artwork (photo credit: Ned Kahn)

The planets and moons were formed from the "solar nebula," a huge disk of gas and dust created by the collapse of an interstellar cloud. This process led to the formation of the star that is our Sun (a process described in more detail in "Cyclone "). After the solar nebula disk had formed, the elements heavier than hydrogen and helium tended to settle to the midplane, where they began to clump into larger bodies. The disk became filled with billions of kilometer-sized "planetesimals," which collided with each other and began to collect into even larger bodies. As they grew, their gravitational fields widened in a kind of "rich-get-richer" expansion, attracting planetesimals from an ever-broadening surrounding volume. These larger bodies became the rocky planets and moons of the inner Solar System and the mostly icy bodies found from the orbit of Jupiter outward.

The collision of a planetesimal into a moon or planet occurs at a speed typical of orbits in the Solar System, thousands of miles per hour. The energy released in such a crash is enough to completely vaporize the impactor, along with an equivalent mass of the planetary surface. This hot vapor sprays away from the impact site at high speeds, leaving a crater behind. The impact excavates piles of rock, which remain as a circular range of mountains around the rim of the crater. Sometimes the molten center of the crater will rebound, as when a drop of water lands in a bowl and a "plop" of water flies back out. This rebound can sometimes freeze up into a central mountain peak. Although the craters in the work titled "Cratering" are due to far more gentle processes, the basic idea is the same: an impact leaves a hole in the surface, and subsequent impacts further pock the surface until craters begin to form on older craters. Scientists can determine the chronology of a surface like that of the Moon by looking at how craters overlie each other.

Photo Credit: NASA

Earth's Moon

We know through radioactive dating from recent expeditions to the Moon that heavy cratering ended in the Solar System about four billion years ago, when most of the debris had either been collected by large bodies or ejected from the Solar System by Jupiter and Saturn. Thus, when we see a rock or ice surface that shows heavy cratering, we know that it is a very old surface. The surfaces of our Moon, of Mercury, and of several of the moons of the outer planets, as well as of sections of Mars reveal that they have remained unchanged for a very long time. Other bodies, such as the Earth and Jupiter's moons Io and Europa, have rather few craters, because they have continued to undergo geological processes. The cratering rate from planetesimals or comet-sized bodies today is quite low, but not zero. The impact of the Shoemaker-Levy comet on Jupiter in the summer of 1994 was a dramatic reminder of the possibility for continuing giant impacts in the Solar System. If a body several miles in diameter were to hit the Earth, life on the surface would be seriously threatened. There has been a recent spate of movies (such as Armageddon and Deep Impact) inspired by this possibility, but the odds that it will happen in your lifetime are extremely low.