DARK HALO: ENIGMATIC FEATURES OF DARK STREAKS AT MARTIAN VOLCANOES. Takenori

Introduction: On the planets having atmosphere such as Mars, there are various types of interactions between the atmosphere and the ground surface. Such interactions cause observable changes in the surface patterns. Polar caps and aeolian features are typical examples. With the accumulation of satellite-based exploratory data, time-variable surface patterns have been recognized and investigated extensively, because they can be direct indicators of the changing surface environments. Here we report a new kind of time-variable surface pattern called " Dark Halo " near the top of high altitude volcanoes in Tharsis region. Figure 1 shows an example of " Dark Halo " on Pa-vonis Mons. The large caldera can be seen at the top of the volcano. Surrounding the caldera there exists " Dark Halo ". Dark Halo " is first described in detail by [1]. They interpreted it is a kind of a wind streak pattern, an aeolian product. Following researches [2, 3] took over their idea, and suggested contributions of rolls of the boundary layer and surface material interaction with the atmosphere. Observation and enigma: Recent continual and high resolution observations have revealed enigmatic features of " Dark Halo " :  " Dark Halo " is composed of assemblage of dark spire-shaped streak, hereafter we call " Spire Streak " (Close-up image of Figure 1).  Each " Spire Streak " starts from a point-like very narrow region.  In most cases, no topographical obstacles can be identified at initiation point.  Each " Spire Streak " starts at high position and increases its width downward.  " Dark Halo " and " Spire Streak " universally exist near the top of high mountains (Olympus, Ely-sium, Pavonis and Ascraeus) and on Alba Patera although the actual heights are not uniform.  " Spire Streaks " are aligned in radial direction from the top (caldera center). Each unit is spindle shaped with length of 30-50km and width at the middle part of 5 km. " Spire Streak " seems to start from higher position, and the initiation point is quite narrow region, which can be considered as a point. In many cases, there exist no recognizable obstacles at the initiation point. This is a remarkable difference from the wind streaks, which are caused by erosion/sedimentation of