OBSERVATIONS OF ROCK ORGANIZATION ON HIGH LATITUDE PATTERNED GROUND TERRAINS ON MARS

Glacial and periglacial environments on Mars are becoming better understood as large volumes of high quality data are collected. The HiRISE camera aboard the Mars Reconnaissance Orbiter, for example, continues to acquire images at sampling rates of ~25 cm/pixel, allowing for individual objects as small as 1 meter to be resolved. These data sets reveal detailed aspects of periglacial terrain, allowing us to begin to understand the mechanisms of patterned ground evolution – terrains that are formed by changes in near surface ground ice in response to climate and seasonal variation. We do so by observing the behavior of rocks on the surface of Mars in patterned ground terrains.

Patterned ground is observed to pervade the geology of ice-rich Martian terrains. The Gamma Ray Spectrometer aboard Mars ODYSSEY detected 60-90% water ice by volume at high latitudes in the Martian soil. The Phoenix lander set down in these terrains and confirmed the presence of ice just beneath the surface in a landing site characterized by regular decameter-scale polygons.

Previous studies have focused on the character, formation mechanism, structure and distribution of Martian patterned ground. Now, with the aid of HiRISE, we are conducting a project to characterize how the patterned ground interacts with the surface at small scales, focusing especially upon the redistribution of rocks by pattern formation.

We show that patterned ground can organize rocks on the surface. We find that this organization requires horizontal as well as vertical movement of rocks. Rock organization takes multiple forms; for example, rocks can cluster in troughs and polygon interiors and in both local highs and lows. A formation mechanism that is consistent with our observations, is for rocks found in lows to be the result of down-slope movement, and for rocks in highs to be the result of rocks first collected in local lows, which then armor the surface against further erosion and end up as local highs.

We find that organization of rocks occurs on timescales faster than small-crater obliteration, allowing us to quantify the rates of patterning. Many craters in polar latitudes are extremely degraded and heavily influenced by patterned ground but have similar rock distributions to equatorial craters. Rocks are therefore only moving on scales relative to the patterned ground itself.