The size distribution of craters on a planetary body can be used to estimate relative, and even absolute, surface ages. As discussed in part II, in the absence of agents of change such as erosion, tectonics, and volcanism, a planetary surface tends to become saturated with craters. Since even currently inactive bodies like the moon were active at some point in their past, surface processes have removed craters at some point in every planet's past. Thus regions on a body with higher crater densities tend to be older than regions with lower crater densities. This activity will investigate crater densities and size distributions, and interpret those in terms of relative and absolute surface ages.

[complete activities are available elsewhere... Craters!, "Crater Count"]


Scientific context: The first part of the activity above revealed that small craters are much more common than large ones. This is because small bolides are more common than large ones, and, as seen in activity IIIA, crater diameter is proportional to bolide size. Smaller bolides would be more common than larger ones if most craters are caused by impacts of asteroid fragments. The asteroid belt is a region of small planetesimals, most of which orbit the sun between the orbits of Mars and Jupiter. Asteroids come in sizes from very small to very large, but the size distribution is not random. Rather, it is governed by the fact that asteroids often collide with each other. In such a collision, both asteroids break into smaller pieces. The size of these pieces follows a predictable distribution (which can be simulated in a laboratory) made up of many small fragments and a few larger ones. Over geologic time, therefore, asteroids continue to collide with each other and produce many small fragments. These bodies may eventually collide with the planets, producing a crater size distribution dominated by smaller craters.

The activity above also involved measurements of crater densities, which can be used for relative dating of planetary surfaces. Assuming that bolides strike all regions of a planet at approximately the same rate, all areas of the surface should have the same crater density unless agents of change have removed some of them during the planet's geologic history. A common way for large numbers of craters to be removed is through volcanism. Early in the Moon's history, for example, large lava flows flooded portions of the surface, completely burying any craters which were there at the time. These areas were essentially wiped clean of craters about 3.5 billion years ago andnd thus their crater density dates back to those lava flows. The lunar highlands, in contrast, were not flooded by lava flows. Their crater density dates back to when they were formed, about 4.1 billion years ago, and thus is much higher. Io, in contrast to the ancient Moon, is one of the most geologically active bodies in the solar system today. Its surface has no recognizable impact craters, and is continually being resurfaced by volcanic eruptions which cover any craters which might form. Regions of a surface with a higher crater density are older than regions with a lower crater density, and a surface like Io's, with no observable craters, is extremely young and implies current geologic activity.

Crater size distributions can also be used to estimate the absolute age of a surface. The cratering rate decreased with time in the early solar system, beginning when the planets finished forming about 4.5 billion years ago and there was much leftover interplanetary debris to cause impacts. The amount of debris decreased over time as collisions and impacts swept it up, thus decreasing the frequency of impacts. By measuring the crater density on different areas of the Moon, and measuring the actual ages of rocks returned from different regions by Apollo astronauts, scientists can calibrate cratering rate to actual surface ages. This relationship of cratering rate vs. time can then be extrapolated to other places in the solar system, since it can vary based on distance from the asteroid belt (Mars, located nearer to the asteroid belt than the Earth, may have a rate of crater formation roughly twice that at earth) and proximity to large planets such as Jupiter, whose gravitational field attracts impactors.


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This module was written by Cynthia Phillips, Dept. of Planetary Sciences, University of Arizona, Tucson AZ, and funded in part by the NASA Spacegrant program.

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