Astronomy 23/223 Homework:

Crater Counting on Mars

 Throughout our class discussions of the Moon, Mercury, Venus, and now Mars, a recurring theme is the use of crater counts to determine relative ages. In NASA Technical Memorandum 79730, the Crater Analysis Techniques Working Group (1978) spells out the achievements and goals of this approach. So, in this week’s problem set, we’ll use some real data and actually see how crater analysis is done. The only change I’ve made to actual real data is to reduce the number of craters in the small size ranges to make the problem more tractable. 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 Hesparian 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.

The in-class exercise we did will help you understand how this system works.

 In this problem set, we use some crater count data taken from a study by George McGill, who is a faculty member at U Mass. In this study (McGill, in press), George examined an area of Mars that spans the highland/lowland boundary in north-central Arabia Terra; it is bounded by 27.5 and 47.5N, and 330 and 335W. Through meticulous study of radar images in that area, he counted 634 craters and carefully measured the size of each one. From these data, he determined the relative ages of various features in his “field area.” In this problem set, we will use a small part of his data to look at the relative ages of two types of features. We are grateful to George McGill for generously making this unpublished data available to us! From the data set, George made of list of all his craters arranged by size. I’ve truncated his list so that it contains only those craters with diameters greater than 2 km, and broken it into two groups. Group #1 contains only 5 craters, which happen to be the largest ones in the quadrangles studied. Group #2 contains the remaining craters with diameters larger than 2 km. From these 68 samples, you can create a cumulative size-frequency distribution plot, estimate the age of the features he’s interested in, and determine which group is older. To do this, plot the following data (please use a spreadsheet; I’ll show you how if you don’t know!):

 1. The data below is arranged in decending numerical order and cummulative numbers have been assigned. The cummulative number is simply assigning each crater a subsequent integer based upon its size. The largest crater is 1 and so forth. This is done because we don't know exactly how many craters are on Mars, so we count the number of craters we do know cumulatively so our small data sample could, hopefully, represent a sample to the number of craters on Mars.....The cummulative number now needs to be normalized by craters to an area of 1 million km2. George’s area measured 75,750 km2. To determine the normalization factor, use this equation: factor = 1,000,000 / George’s area Now, multiply this factor times each of the cumulative numbers of craters column. normalization is used in order to match our experimental data to the accepted values in Tanaka's table. 2. Plot the data on an x-y plot, using the log of crater diameter as the x axis, and the log of the cumulative number of craters as the y axis. 3. Determine the y value of the line where it crosses N5 and N16, and then match this value to Tanaka’s time scale to determine when in Mars’ geologic time the units were formed. As you have learned in previous assignments, you may need to make a best fit line and/or extrapolate on the data. 4. Decide which group of samples, the small craters or the large ones, is older.

 You should be able to cut and paste these numbers directly into a spreadsheet program.

 Group 1 Crater Cummulative diameters number 103 1 79 2 58 3 45 4 43 5

 Group 2 Crater Cummulative diameters number 26 1 18 2 13.65 3 11.5 4 10.7 5 9.95 6 9.65 7 9.6 8 8.95 9 8.25 10 7.75 11 7.7 12 7.35 13 7.1 14 7.05 15 7 16 6.35 17 6.3 18 6.15 19 5.85 20 5.45 21 5.2 22 5.05 23 4.85 24 4.75 25 4.7 26 4.55 27 4.25 28 4.2 29 4.15 30 4.05 31 3.95 32 3.8 33 3.65 34 3.6 35 3.5 36 3.35 37 3.3 38 3.25 39 3.25 40 3.2 41 3.15 42 3 43 2.95 44 2.9 45 2.85 46 2.8 47 2.75 48 2.7 49 2.65 50 2.6 51 2.55 52 2.5 53 2.45 54 2.4 55 2.35 56 2.3 57 2.2 58 2.1 59 2.05 60 2.02 61 2 62

References Used:

 Crater Analysis Techniques Working Group (1978) Standard Techniques for Presentation and Analysis of Crater Size-Frequency Data. NASA Technical Memorandum 79730, 20 pp. McGill, G.E. (2000) Crustal history of north-central Arabia Terra, Mars. 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.

 Home | Syllabus | Course Schedule | Images and Data | Homework | Homework Answers In-Class Exercises | In-Class Answers | Exam Review | Exam Answers | Student List | Contact Darby | Summer Internships | More on Astronomy| Bus Schedule This page was created by Darby Dyar and is maintained by Darby Dyar and Rebekah Robson-May. Last updated on 28 May, 2004 .