火星衝突クレーターの特異なエジェクタ地形と劣化過程

抄録

 Mars has fascinated us in terms of discovering vital activities and examining planetary surface evolutions. Recent explorations detected subsurface ice and methane, which reveal the importance of investigating ancient environments and their evolution. Impact craters and their ejecta morphologies are natural probes for obtaining knowledge on ancient surface and subsurface environments, because detailed morphologies of impact craters and their ejecta are sensitive to impact conditions and degradation processes. Martian enigmatic ejecta morphologies and their currently-proposed origins are summarized, and some degradation processes effective on Martian craters are introduced. Ejecta of Martian impact craters are different from those of lunar or Mercurian impact craters. They possess clear distal ridges and cliffs, and are diverted by preexisting obstacles, while edges of lunar or Mercurian ejecta are ambiguous. Their unique morphologies suggest that the ejecta are emplaced by radial ground-hugging flows generated at the time of the impact. Processes and agents of the fluidization draw attention, because the atmosphere and/or subsurface volatiles are suggested for its formation. Degradation processes on Mars vary, and include superimposed impacts, viscous relaxation, mass movements, lava infilling, fluvial incision, gully formation, lacustrine erosion and deposition, ice-related processes, and airfall deposition. Due to high ancient erosion rates, degraded crater morphologies also provide useful information on the ancient environment.