Martian double ring basins: New observations

Eighteen previously unknown martian basins have been detected on Viking photographs. The smallest basins have diameters 50 to 100 km less than the smallest known basins on Mercury or the moon. On the latter two planets and the earth, basin morphology varies with increasing diameter: central peak (CP) basins have both central peaks and fragmentary peak rings; peak ring (PR) basins have only concentric rings of peaks: and multi-ring (MR) basins have two or more concentric rings. The new Viking results show that on Mars the morphology sequence is PR-CP-PR-MR, and two CP basins occur within the larger PR basin diameter interval. The reasons for the existence of the anomalous small PR basins and the two large CP basins are uncertain, but may be related to unique crustal properties on parts of Mars, or alternatively to unique properties of some impacting bodies. The diameter distribution of martian basins exhibits three distinct slope segments, with inflections at the same diameters that changes in basin morphology occur. Embayed and ghost craters, rilles, and mare ridges occur on basin floors, suggesting that basins have been the sites of igneous extrusions as on the moon. Peak rings within martian and lunar basins are almost always composed of isolated peaks or short mountainous arcs, but on Mercury the rings are commonly complete. This remarkable difference may reflect lateral homogeneous physical properties in Mercury's crust or increased efficiency in peak ring production due to high modal velocities for impacting bodies,