Distribution and relations of 4- to 10-km-diameter craters to global geologic units of Mars

By correlating the 1:25,000,000 geologic map of Mars of Scott and Carr (1977) with 4- to 10-km-diameter crater density data from Mariner 9 images, the average crater density for 23 of the equatorial geologic-geomorphic units on Mars was computed. The correlation of these two data sets was accomplished by digitizing both the crater density data and geologic map at the same scale and by comparing them in a computer. This technique assigns the crater density value found in the corresponding location on the geologic data set to a discrete computer file assigned each of the 23 geologic units. By averaging the crater density values accumulated in each file, an "average" crater density for each geologic unit was obtained. Condit believes these average crater density values are accurate indicators of the relative age of the geologic units considered. The statistical validity of these average values is strongest for the geologic units of the largest areal extent. The relative ages as obtained from the average crater density values for the seven largest geologic units, from youngest to oldest, are: Tharsis volcanic material, 21 ?? 4 craters/106km2; smooth plains material, 57 ?? 14 craters/106km2; rolling plains material, 66 ?? 16 craters/106km2; plains materials, 80 ?? 17 craters/106km2; ridged plains material, 128 ?? 25 craters/106km2; hilly and cratered material, 137 ?? 38 craters/106km2; and cratered plateau material, 138 ?? 27 craters/106km2. ?? 1978.