RADAR ALTIMETRY OF THE LARGE MARTIAN CRATERS, L. E. Roth, G. S. Downs, and R. S. Saunders, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91103; G. Schubert, Department of Earth and Space Sciences, University of California, Los Angeles, California 90024.

The Goldstone Mars data (1,2) have been used to obtain detailed morpho- metric information on martian craters. Some qualitative results of this effort have been reported previously (3). Radar profiles of a total of 131 impact craters have been recognized. (The following symbols are used in the subsequent discussion: Ri (km) is crater depth and Dr (km) is rim-crest diameter (4)). The craters range in size from Dr " 20 km to Dr " 500 km, and are all located within a subequatorial belt of the cratered highlands, bounded by approximate latitudes of -14' and -21'. This belt constitutes about 8% of the total surface area of the planet. The current data coverage does not exceed approximately 25% of the area of the belt, i.e., about 2% of the area of the planet. Radar profiles of large craters with the visually flat floors show sub- dued crater rims (rarely higher than 300-400 meters), usually shallow interior walls, and little structural detail in the interior and the exterior walls. In contrast, the floors of the same craters often display topographic relief greater than the error in the range measurement. The largest craters (Dr > 125 km) show differing degrees of the floor level adjustment. The two extreme cases are the craters Bakhuysen (Dr = 140 km, (-23.0°, 344.0')) whose floor is almost 2500 m below the surrounding terrain, in sharp contrast with Denning (Dr = 140 km, (-17.8O, 326.5O)) whose floor is almost at the same level as the surrounding terrain. Depths of the scanned craters have been determined with varying degrees of confidence. The data confirm the overall shallowness of themartiancraters, an observation made as early as the time of Mariner 4 and since then noted by several investigators (4-10). The measured radar depths of most of the 87 objects comprising the higher-confidence grouping cluster between 1 krn and 2 km. Ca~sequ$ntly, the least-squares fit to the relationship of the log depth versus the log diameter, is markedly less steep than a similar curve derived from the Mariner 9 UVS altimetry (7). Thus the depths of a mixed degradation stateandgeographically (D > 20 km)areonlyaweak function restricted radar sample of martian craters r of diameter. If the crater depth measurements are split into two subgroups, one with diameters less than 125 km and the other with diameters greater than 125 km, and the depthldiameter ratios for each subgrobp are fitted in the least-squares sense,

a pronounced break in slope, a "knee," at the junction of both fits can be observed. Since the 125 km diameter division was arbitrarily chosen,wecannot be sure that a knee in the depthldiameter relation,actually occurs at this diameter. However, these results do suggest that a knee exists somewhere in the diameter range from 100 km to 200 km. It is seen that for Dr > 125 km the average depths of ;th_e sampled martian craters become virtually independent of the crater size. , Therefore, a power-law least-squares fit of the depth/ .* - %.

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Roth, L. E. et al. diameter relationship for a mixed-age crater population, taken over the full range of diameters obsc.ures a possibly significant change in the depthldia- meter curve for Mars. It sllouldbenoted, however, that confidence in the accuracy of the radar range measurements decreases as the dimensions of the measured object become comparable to the dimensions of the radar footprint. Thus the least-squares fits in Eqs. (1) and (2) may suffer fromcontamination by the less reliable data points, while Eq. (3) gives abetterrepresentation of the depth/diameter relationship for the largemartiancraters. Clarifica- tion of this matter will have to be postponed until more dataareavailable. Because of the proximity of the radar depth/diameter curve to the lower boundary of the envelope containing the depth/diameter ratios of Pre-Imbrian lunar craters (5,7) it has to be concluded that the large martian craters contained in the radar compilation are substantially shallower and hence sub- stantially more modified than the lunar cratersofcorresponding age and size. This should come as no surprise since the martian craters appear to have been excavated to shallower depths (10) and have unquestionably been exposed to a more erosive environment and a more vigorous gravitational settling. In addition, the radar results show certain large martian craters to have been modified by some processesthattiltedthecrater floorsrelativeto the hori- zontal plane, thus enhancingtheir shallowvisual appearance. Themost prominent among the craters with tilted floors are Gusev (Dr = 170 km, Ri = 1.6 km, (-14.7O, 184.5')) and Williams (Dr = 125 km, Ri = 8 km, (-18.00, 164.0')). Gusev is located within the range of the Apollinaris Patera lava flows, andit serves as a terminus of the Ma'adim Vallis. The floor tilt may thus be related to the filling history of the crater. Williams is locatedatthe foot of the eastward-facing fault blocks in Memnonia (11) and its floor tilt isapparently associated with the regional tectonic trends. The Apollo Lunar Sounder radar images revealed the centers of the floors of some large lunar craters to be domed upward by several hundred meters. No such effect has been observed in the Coldstone Hars data. The shallow depths of the large martian craters, the modest slope ofthe depth/diameter curve at large diameters, and the absence of crater floor doming can be interpreted in several ways: (1) Compared to their lunar counterparts, the large martian craters have undergone a more complete post- impact structural adjustment. As a result, the doming, presumably an inter- mediate phase in such a process, has vanished and the floors of the large martian craters are structurally flat. (2) The large martian craters have retained a structural bulge, but it has been obliterated by a blanket of eolian/fluvial/volcanic deposits. If the latter is true, then modification by erosion and deposition is a significant process of landform and crater degradation on Mars. (3) The shallow initial depths remain the dominant fac- tor in the evolution of large martian craters, while the post-impact modifi- cation by pr.ocesses listed above is of secondary importance.

References: (1) Downs, G. S., Goldstein, R. M., Green, R. R., Morris, G. A., and Reichley, P. E. (1973). --Icarus 18, 8-21; (2) Downs, G. S., Reichley, P. E., and Green, R. R. (1975). Icarus 26, 273-312; (3) Roth, L. E., Elachi, C., Saunders, R. S., and Schubert, G. (1978). Lunar and Planetary Science -IX, 976-978; (4) Pike, R. J. (1974). Geophys. Res. Lett. 1, 291-294; (5) Pike, R. J. (1971). Icarus 19, 384-395; (6) Burt, T., ~everka,T., and Cook, K. (1976). --Icarus 29, 83-90; (7) Cintala, M. J. et al. (1976). Proc. Lunar Sci. Conf. a,3575-3587; (8) Malin, M. D. et al. (1977). J. Geophys. Res. -82, 376-388; i'F) Schubert, G. et al. (1977). Icarus 32, 131-146;(10)~intala, If. J and Mouginis--Mark, P J. (1980), Geophys, Res. Lett. 7, 329-332; (11) Plescia, J. B. et al. (1980). Lunar and Plan. Sci. a,891-893.

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