TODAY’S CHANGING LANDSCAPE OF MARS

The present is the key to the past, except when it isn’t, but if we do not understand present-day processes, then we have scant hope of understanding past geologic processes. MRO/HiRISE is ideal for observing present-day activity on Mars because ~30 cm/pixel images can be acquired multiple times per year over interesting locations anywhere on Mars, to document topographic and small-scale changes. More than 700 new impact events forming one or more craters have occurred since 2006, some of which expose shallow clean ice or trigger dust avalanches. The thin martian atmosphere has proven surprisingly effective at modifying the surface. Wind moves sand dunes and ripples today over much of Mars, and that motion must also be eroding the surface. Carbon dioxide condenses onto the surface seasonally and creates a bizarre zoo of polar defrosting features called spiders, flows, spots, and fans. Early spring avalanches occur over very steep north polar scarps, and the south polar residual cap is constantly changing form. The seasonal frost is also associated with active gully formation in the middle latitudes, now documented at ~100 locations. Most of these resemble water-carved gullies on Earth, but form when it is far too cold for liquid water. Debate continues on whether water played any role in the recent past. Linear gullies found on large sand dunes in the southern latitudes may form from sliding blocks of CO₂ ice. Seasonal frost or ice likely also drives alcove formation on smaller sand dunes in the high northern latitudes. Recurring Slope Lineae (RSL) occur over the warmest times and places (the opposite of gully activity), and are the subject of ongoing debates. They behave like seasonal water seeps and mark rare locations where hydrated salts can be seen from orbit in the middle afternoon, but are found only on “angle of repose” slopes. New topographic slumps are seen in association with some RSL fans. There are other episodic changes on steep slopes likely to result from dry mass wasting, like dust avalanches and rock falls. We haven’t yet seen changes definitively associated with magmatism or tectonics, although some of the mass wasting may be triggered by seismic activity. Topographic landform change associated with sublimation of mid-latitude ice has not yet been documented in before-and-after images, but may be seen in the near future.