Astronomy 23/223 In-Class Exercise:

Solution: Crater Counting on Mars

 Use each drawing the determine where the N=5 and N=16 lines cross each set of data. Once you have found that intercept, measure across horizontally to determine the Y intercept, and read that number. Use than number to determine the age from Tanaka’s (1986) chart.

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.

 1. In this diagram, the intercept of the “all craters” line for N(16) is around 800. Find the number 800 in the column labeled “N(16)” and you will find that this represents an age of Lower Noachian. Note that you can’t really get useful information out of the N(5) age, because the N(5) intercept would be about 1100. All that the N(5) intercept tells you is that this area is a lot older than Middle Noachian. 2. In this figure, the ages of each unit work out as follows: Unit A N(16) intercept = 66, so age is Upper Noachian. N(5) intercept = 180, so age is Lower Hesparian. Unfortunately the two lines don't agree. Unit B N(16) intercept = 39, so age is Upper Noachian. N(5) intercept = 141, so age is again Lower Hesparian, but this unit is probably younger than Unit A because its crater densities are lower. Unit C N(16) intercept = 16, so age is Lower Hesperian. N(5) intercept = 96, so age is Upper Hesperian. What's going on? This method is not perfect, because it is based on a lot of assumptions, not the least of which is that the relationship between age and lunar crater densities applies to Mars! Perhaps this area is particularly susceptable to dust storms or has experienced water from an outflow channel; eithe rof these possibilities might obscure the smaller craters preferentially. To get a more accurate answer in a real study, you might want to go back to a higher resolution image to look at smaller craters, or recount the ones you looked at. At any rate, you can at least draw some conclusions about the relative ages of the three units. Certainly A is the oldest, B is in the middle, and C is the youngest. If you were able to compare these with other units in the area, you might begin to put together an udenrstanding of the geology in this vicinity.

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