A MARTIAN EQUIVALENT OF FRAGILE EARTH ENVIRONMENT

Juventae and other chasmata of Valles Marineris, Mars, which lead to outflow channels, are partly filled with relatively young interior layered deposits (ILDs). The strongest hypotheses of ILD origin include lacustrine, subaerial volcanism, or sub-ice volcanic processes. Data from instruments on the Mars Global Surveyor spacecraft may support a sub-ice volcanic origin. In the frigid Martian climate, any accumulating groundwater within the chasmata would pond and freeze, forming cold-based ice dams. Sub-ice volcanic eruptions on Earth melt overlying ice and form edifices of hyaloclastic tuffs that can be capped by flat-lying subaerial lava. Such volcanic eruptions also generate large jökulhlaups from unstable meltwater lakes. ILD mesas in Juventae Chasma are formed of friable wind-eroded material lacking resistant caps and may be hyaloclastic ridges formed below an ice sheet. The mesas are younger than upper Hesperian chaos material as they overlie chaotic blocks on the floor of Juventae Chasma. Mars Orbiter Laser Altimeter data (at 500 pixel/deg.) indicate that the largest ILD mesa in Juventae Chasma is about 2 km high, with the highest point reaching an absolute elevation of about +80 m (on an E-W profile, the surrounding plateau walls reach +1900 m). Infilling the depression of Juventae Chasma and the head of its outflow channel Maja Valles to this altitude indicates that all source locales, areas of chaos in Juventae Chasma and basins downstream, lie below 80 m of elevation. Maja Valles and its streamlined features lie above this elevation, indicating an uphill gradient, which would have resulted in a barrier to water flow. It may be that sub-ice eruptions generated jökulhlaups whose direction of flow was controlled by the gradient of a temporary ice sheet, as the slope of the bedrock and the slope of the overlying ice surface determine the gravity potential.