852 LPSC XVIII

MODIFICATION STYLES OF THE MARTIAN IMPACT CRATERS; L. E. Roth, R. S. Saunders, and T. W. Thompson, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91 109.

It has long been noted (1,2) that the large mrtian impact craters have a more degraded appearance than lunar craters of comparable size. The Goldstone Wrs radar altimet ry (3) affords identification of additional differences in the crater modification styles between the two planets. Of particular interest in the present context are the magnitudes of Schroeter's ratio (4) for the lunar and martian craters, and their implications.

Schroeter's rule (4), formulated on the assumption of an explosive origin of lunar craters, postulates the equality of the volume of the rim (Vr) and of the true volume (excavated volume, truncated at the level of the surrounding terrain; Vt) of a crater. In an idealized case, and for a freshly formed crater, the Schroeter's ratio (Vr/Vt) should be equal to unity. The values of Schroeter's ratio of less than unity would indicate a rim-mass deficit (due to, e.g., a far-field ballistic dispersion of the ejecta or to the indadequate range resolution of the altimeter in question). The values larger than unity would indicate a rim-mass excess resulting from the action of the modification processes (e.g., slumping, eolian/fluvial infilling, the post-impact rebound, etc). On the moon the magnitude of the ratio varies from -0.4 for small and fresh craters to values over 1.0 for larger and/or older craters (4). Goldstone data permit determination of Schroeter's ratios for a number of martian craters larger than -100 km in diameter. The values of the ratio for &rs span an interval far wider than do the corrksponding values for the moon. Following represent the two extreme cases. The Ladon basin (25.0°, -17.0' , 475 km) has a near-zero rim volume and a finite true volume; Schroeter's ratio equals zero. Crater Denning (326.7O, -17.8'~ 140 lan) has a finite rim volume and a near-zero true volurne; Schroeter's ratio approaches infinity. This range suggests that a simple progression from values of less than unity to values larger than unity, typical of the lunar craters and indicating an ordered modificatioddegradation regime, is absent on Mars. Thus Schroeter's ratio, while having a limited morphogenetic significance for lunar craters (4) , is not inf onnative for characterization of individual mrtian impact craters. On the global scale, however, the wide range of values of Schroeter's ratio provides a measure for the heuristic conclusion that the martian landforms have followed a more chaotic, feature- or location- specific, evolutionary sequence. In other words, each large object has its own history, not necessarily shared by the comparable objects. The observation that for the mrtian craters 'the type of modification depends strongly on latitude.. .at low latitudes the process is simply one of loss of detail' (5) is probably true in the most general sense only.

?he enclosed figures further illustrate these ideas. Fig. 1 provides the setting for two heavily modified martian craters, Williams (164.0°, -18.0°, 125 km) and (Xlsev (184.5'~ -14.7'~ 170 km). Both have tilted floors, a topographic anomaly not observed elsewhere in the same latitude band on rs. Note a relatively sharp elevation rise (-3 lun over 10' lon.) immediately west of Williams. The floor of Williams tilts by -600 m over 2' lon. (Fig. 3), in an apparent concordance with the gradient of the rise. Proximity of Williams to the extensive compressional-stress fields centered around 180° lon. and -30' lat. (6), and to the area characterized by the highest incidence of craters with central peaks and of craters with breached, furrowed, and pitted walls (7) is perhaps not coincidental. If the rise is of

O Lunar and Planetary Institute Provided by the NASA Astrophysics Data System LPSC XVIII 853 MARTIAN CRATERS Roth, L. E. et al. l a tectonic origin, then the floor of Williams provides a diagnostic for the occurrence of verticaf crustal displacement. The tilt of the CXlsev floor, -400 m over 2' lon. (Mg. 2), is unrelated to the regional topographic trends. Gusev serves as the terminus of the Ma'adim Vallis channel system. The elevation of the Gusev floor decreases taward the junction, apparently reaching the lowst reading at the junction itself. If this point is indeed the lawest point of the Gusev floor, then the tilt emerged after the alluviation associated with the Ma'adim Vallis discharges ceased. Also, the elevation readings decrease across the law-albedo splotch covering the southwst portion of the crater floor, suggesting an eolian origin for the tilt. This cannot be confirmed since other cases of albedo-related elevation anomalies have not been identified.

References: (1) B. C. Murray, L. A. Soderblom, R. P. Sharp, and J. A. Cutts, J. Geophys. Res. 76 (1971), 313. (2) R. J. Pike, Icanus 15 (19711, 384. (3) G. S. Downs, P. ~T~eichle~,and R. R. Green, Icarus 26 (1975), 273. (4) R. J. Pike, J. Geophys. ks. -72 (1967), 2099. (5) M. H. rrr (1981), The surface of Mars, Yale U. Press. (6) A. F. Chicarro, P. H. Schultz, and P. Masson, Icarus 63 (1985), 153. (7) T. A. Wtch, R. E. Arvidson, J. W. Head, K. L. Jones, and R. S. Saunders (1976), The geology of Mars, Princeton U. Press.

Longitude (deg)

O Lunar and Planetary Institute Provided by the NASA Astrophysics Data System