Astronomy 23/223 In-Class Exercise:

Crater Counting on Mars

 In a 1986 paper, K. L. Tanaka used cratering statistics to develop a detailed stratigraphy of Mars that now forms the basis for our geological stratigraphy of that planet. This stratigraphy is especially useful because it is constrained by crater abundances. So if you want to know how old any part of the martian surface is, you can simply count the numbers and sizes of craters within a given area, and estimate the age of that area as it relates to Tanaka’s time table for Mars.

Table 1. (Tanaka, 1986)

Crater Density Boundaries for Martian Series

 Series N(1) N(2) N(5) N(16) N(4-10) Upper Amazonian <160 <40 Middle Amazonian 160-600 40-150 <25 <33 Lower Amazonian 600-1600 150-400 25-67 33-88 Upper Hesperian 1600-3000 400-750 67-125 88-165 Lower Hesperian 3000-4800 750-1200 125-200 <25 165-260 Upper Noachian 200-400 25-100 >260 Middle Noachian >400 100-200 Lower Noachian >200

N = cumulative number of craters greater than or equal to each crater per one million km².

 On the following pages, you will find two images. In the first plot, you are looking at crater counts from before and after the intrusion of a volcanic knob, and so you can use the table above to determine the relative age of the knob’s emplacement. In the second plot, determine the relative ages of areas A, B, and C, using both N(16) and N(5) ages, and give the geologic time period in which each one formed.

 Reference: Tanaka, K.L. (1986) The stratigraphy of Mars. Proceedings of the 17th Lunar and Planetary Science Conf., Part 1. Journal of Geophysical Research, 91, supplement, E139-E158.

 1. This area of Mars has a prominent knob (probably volcanic) in its landscape. We'd like to know the relative age of the area before and after the knob was created. Use the table from Tanaka (1986) to determine the age of each, and give the name of the geologic time period in which each groups of units. 2. Determine the relative ages of units A, B, and C, and identify the geologic time period in which each was formed.

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