Lab 5_CraterDating

Lab 5_CraterDating - Lab 5: Crater Dating Introduction...

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Lab 5: Crater Dating Introduction Impact craters may also be used to estimate relative ages of planetary surfaces. If we assume that impacts are a completely random process (they occur at any place on a planet surface with equal probability), then if there enough impacts to form a valid statistical sample, we can use the number of impact craters in a surface to estimate age. For example, if the rate of formation of impact craters is 2 craters per million years, and a planet has 5,760 craters, then the age of the planet's surface is number of craters divided by the rate of crater formation: 5,760 / 2 per million years = 2,880 million years = 2.88 Ga (billion years) The rate of crater formation has changed with time and cannot be extrapolated from current crater rates. Also the number of craters also depends on the surface area, a large area will, in general, have more craters than a small area. The area problem is taken into account by using crater densities, which are calculated as the number of craters divided by the area of the surface in which they are formed. Even without knowing the rate of crater formation, if two areas on a planet have different crater densities, then the area with the higher crater density is the older . If an absolute age of a surface can be determined independently of the crater age, then the crater density can be calibrated to absolute age. The only planetary body for which this calibration is available is the Moon. Using crater density data from the Moon and absolute ages of lunar samples returned to Earth, the crater-count graph paper (below) has been calibrated in terms of absolute age. A further complication in using craters to calculate ages is that craters come in a wide range of diameters and not all sizes are equally probable. In general, small craters are much more common than large craters. Observations suggest that as the diameter decreases by a factor of 8 , the number of craters of that size increases by about 1,000 . For example, for every crater about 16 km in diameter, there are 1,000 craters 2 km in diameter. For every crater 128 km in diameter there are 1,000 craters 16 km in diameter and 1,000,000 craters 2 km in diameter. Thus, when counting craters and calculating crater densities, craters are divided into size ranges based on a factor of 2 (8 = 2 x 2 x 2), as shown in the figure below. In keeping with the observed distribution of crater sizes, crater counts are plotted on
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Lab 5_CraterDating - Lab 5: Crater Dating Introduction...

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