FRACTAL PROPERTIES OF OUTFLOWS FROM VENUSIAN IMPACT CRATERS

This article presents fractal analyses of 28 outflow margins from 18 V enusian impact craters. The fractal dimensions of the second parts of R-plots of the outflow outlines were measured by a three-step method. The fractality values for the same outflow measured from images which have only a small difference in resolution are very similar, while lar ge differences in image resolution may result in differences in fracta lity, possibly due to the fact that we are actually studying geologica l processes on different scales. The outflows were classified into thr ee general categories: single outflows, multiple outflows and outflow fields. Three conclusions were drawn on the relations between fractali ty and crater diameter, which may be related to the greater effects ca used by the immediate local environment on the outflows from small cra ters than on those from larger craters. Investigation of the relations between the regional topography and fractality indicates that there a re substantially less effects on outflows originating from large crate rs than on those from small craters. The smooth bending in the R-plot and the higher D-value for the multiple outflows could result from the mixing of various fractal or non-fractal units. When comparing our re sults with the fractality of terrestrial lava flows, outflows from cra ters of diameter greater than 50 km seem to resemble a'a-type lava flo ws in their fractal dimensions and outflows from craters of diameter b elow 50 km tend to be more pahoehoe-like. This preliminary result is b ased on 28 outflows, however, and the pattern should be investigated m ore carefully by further more extensive work.