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Research in Space Science SA0 Special Report No. 268 AN ANALYSIS OF THE MARINER 4 PHOTOGRAPHY OF MARS Clark R. Chapman, James €3. Pollack, and Carl Sagan February 14, 1968 % Smithsonian Institution A s tr o phy s ica1 Ob s e rvat ory Cambridge, Massachusetts 02138 708-1 1 TABLE OF CONTENTS Section Page ABSTRACT...... ........................... vi INTRODUCTION ............................. 1 A NEW REDUCTION OF THE CRATERING STATISTICS. 3 CRATER EROSION AND OBLITERATION . 17 SOURCES OF IMPACT CRATERS. 23 5 MAJOR TOPOGRAPHICAL FEATURES AS POSSIBLE CRATERS.................................. 33 6 EROSION MECHANISMS . 35 7 CRATERS AS SOURCES OF DUS-T . 51 8 CRATERAGES .............................. 53 9 VARIATION OF CRATER DENSITIES WITH POSITION ONMARS.. ................................ 55 10 RECOMMENDATIONS FOR FUTURE WORK. 59 11 ACKNOWLEDGMENTS . 61 REFERENCES ............................... 62 APPENDIXA ............................... ~-1 APPENDIXB ............................... B-1 BIOGRAPHICAL NOTES . LIST OF ILLUSTRATIONS Figure Page 1 Low-quality reproduction of Picture 4 of the Mariner 4 photographic sequence. 5 2 Sketch of the position of cataloged craters (Appendices A and B), Picture 4 of the Mariner 4 photographic sequence. 6 3 Low-quality reproduction of Picture 7 of the Mariner 4 photographic sequence. 7 4 Sketch of the position of cataloged craters (Appendices A and B), Picture 7 of the Mariner 4 photographic sequence. 8 5 Low-quality reproduction of Picture 11 of the Mariner 4 photographic sequence. 9 6 Sketch of the position of cataloged craters (Appendices A and B), Picture 11 of the Mariner 4 photographic sequence. 10 7 Diameter-frequency plot for Pictures 7 through 14, Qualities A and B (left); Pictures 7 to 11, Qualities A and B (center); and Pictures 7 and 11, Qualities A and B (right) . , . , . 28 8 Influence of filling by dust or lava on the class member- ship of a crater.. 46 9 An attempt to localize approximately the positions of Pictures 7 through 14 and their largest craters on the Martian surface. 56 10 The variation of crater number density with frame number . 57 iv LIST OF TABLES Table Page 1 Parameters of Pictures 2 to 15 of the Mariner 4 photography ................................. 11 2 Crater percentages by class at several diameter intervals for Mars and the Moon 18 3 Minimum diameters for obliteration and equilibrium class membership ................................. 20 4 Comparison of predicted and observed numbers of Martian craters .................................... 30 .L 5 Estimates of D". from distribution functions ............ 32 6 Predicted values of F ........................... 40 7 Predicted and observed number of craters on Mars, 20 km f D 5 D*, Bf > 3 ......................... 43 8 Percentages of craters by class .................... 47 9 Mean ages of Martian craters (in units of billions of years) ................................... 54 V ABSTRACT A catalog of nearly 300 craters and crater-like objects has been pre- pared from several sets of contrast-enhanced high-quality positive trans- . parencies of the Mariner 4 photography. Craters were identified and counted by the same procedures used in the compilation of lunar crater catalogs; particular attention was given to crater class and quality, Counts of craters with diameters D < 20 km begin to show the effects of incompleteness. A direct inspection of the photographs as well as the constancy of crater class proportions with crater diameter interval above 20 km indicates that substantial erosion and obliteration of all but the largest craters have occurred during the history of Mars. The epochs of crater formation and crater erosion appear to be closely tied together in time. 4 A statistical curve-fitting program for the observed crater diameter- frequency relations and a differential number- density distribution law of ADmBgive B = 2.5 f 0.2 for D > 20 km or B = 3.0 f 0.2 for D > 30 km. The population of impacting objects assumed responsible for these craters is taken as having a differential number density varying as X-', where X is the diameter of the impacting object. The number of "live" comets and Apollo objects crossing the orbit of Mars is insufficient by more than 2 orders of magnitude to explain the observed number density of craters on Mars. For asteroidal objects with p = 2 or 3, the predicted and observed number densities cannot be brought into agreement unless we assume an early epoch of very high cratering rates on Mars. For p = 4 or 5, agreement between the predicted and observed number densities can be secured with a nearly uniform rate of asteroidal bombardment. The absence of saturation bom- bardment of Mars for very large craters points to a value of p significantly above 3. This is not inconsistent with expectations for asteroids with X > 1 km in the inner part of the asteroid belt. In this case, crater diameter scales with kinetic energy, W, of the impacting object as W 1/3 . vi J i For all reasonable values of p, impact damage contributes to crater erosion and obliteration. The fraction of the surface covered by craters is of the order unity. For all reasonable values of p, asteroidal bombardment is capable of accounting quantitatively for the observed values of both A and B, particularly if the zone of obliteration around Martian craters is larger than that for lunar craters. For near-saturation bombardment, the existing observations are of very little use in determining the value of p, or in dis- tinguishing between saturation bombardment and such other erosion mechan- isms as windblown dust of impact or of micrometeoric origin; liquid water on macro or microscales; mountain building; and flooding by lava. The dust produced by impact during the history of Mars is estimated to have depths between 0. 1 and several kilometers. The diameter of the largest crater 9 obliterated in 4. 5 X 10 years is calculated to be between 60 and 180 km; for a the lifetime against erosion, assumed to scale as D , we calculate a 0 for p = 2 or 3, and 1 sa5 2. 5 for p = 4 or 5. For p < 3, the mean ages of Martian craters are found to be approxi- mately equal to the age of the planet. However, for p significantly larger than 3, different craters will have different mean ages, ranging from about 9 2. 25 X 10 years for the very largest craters, down to some tens of millions of years or less for craters smaller than 20 km. In this case, surface features of the order of 10 km in width or smaller may have been quite prom- inent in the early history of Mars and undetectable on the Mariner 4 photo- graphs. Thus, the absence of such signs of running water as river valleys in the Mariner 4 photography is quite irrelevant to the question of the exist- ence of bodies of water in early Martian history. These conclusions on ages are independent of estimates of the ages of lunar maria. Some weak evidence exists for a correlation between high crater density and dark areas on Mars. vii Un catalogue de prGs de 300 cratLres et objets en forme de cratgre a dte' pre'pare' & partir de plusieurs jeux de copies diapositives de tr&s bonne qualite' et & contrastes accentue's, de la photographie originale prise par Mariner 4. Les cratGres furent identifie's et compte's en em- ployant les m6mes m6thodes que celles utilise'es dans la pre'paration des catalogues des cratgres lunaires; une attention particuligre a e'te' accor- de'e & la classe et, & la qualite' des cratGres. Le de'nombrement des cra- tGres de diamgtre D < 20 km commence montrer les e'vidences d'un carac- tgre incomplet. L'observation directe des photographies ainsi que le fait que la rgpartition en classes des cratgres ne varie pas avec l'in- tervalle de diamgtre conside're', pour des diamgtres sup6rieurs & 20 km, indiquent que tous les crat&res& part les plus grands ont e'te' soumis & une e'rosion et & une destruction importantes au cours de l'histoire de Mars. Les e'poques de formation et d'e'rosion des cratGres sont e'troite- ' ment relie'es dans le temps. En remplaqant la distribution observe'e des cratGres en fonction de leur diamgtre par une courbe continue obtenue par me'thode statistique et en supposant une densite' diffe'rentielle du nombre des cratgres, de la + + forme ADsB, on obtient B = 2,5 - 0,2 pour D > 20 km, ou B = 3,O - 0,2 pour D > 30 km. On suppose ensuite que la population des projectiles conside're's come responsables de la formation des cratGres, est telle que la densite' diff6rentielle du nombre des projectiles varie come X -B en fonction du diamGtre X du projectile. Le nombre des comGtes "actives" et d'objets du type "Apollo" qui traversent l'orbite de Mars, est trop faible par plus de deux ordres de grandeur pour pouvoir rendre compte de la densite' du nombre des cratbes observ6s sur Mars. Dans le cas d'objets du type ast6rsi'de pour lesquels f3 = 2 ou 3, les densite's cal- culges et observe'es ne sont en bon accord que si l'on suppose une e'poque initiale caractCrise'e par un trGs fort taux de formation des cratsres sur Mars. Pour @ = 4 ou 5, l'accord entre les densite's observe'es et viii calcule'es peut 6tre assure' moyennant un taux de bombardement asteroi'dal presque uniforme. L'absence de bombardement de saturation pour les plus grands cratGres semble indiquer une valeur de j3 supgrieure & 3 de faSon apprkeiable. Cette hypothhse n'est pas en contradiction avec les esti- mations faites pour les astgroTdes de diambtre supt?rieur & 1 km dans la partie infdrieure de la ceinture ast6roidale. Dans ce cas le diamktre du crat'ere varie avec l'dnergie cinetique W du projectile comme W 1/3 .
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