Glacial and Periglacial Geology
By Mike Mellon
Explanation of the theme.
Mars is globally covered with permafrost, where soil temperatures are permanently below the freezing point of water. Bitter cold temperatures dominate the Martian equatorial regions, with an annual-mean temperature of the soil colder than -50 C, and colder still at middle and high latitudes. Therefore, any water present in the Martian soil or atop the soil surface must be in the form of ice. Icy-soil and surface-ice deposits influence the formation of Martian landforms, much as they do on Earth. Glacial deposits form characteristic flow features that indicate thick piles of water ice in a slow viscous motion, even when they are covered by dust and soil hiding the bright ice. Periglacial landforms are features found commonly on the margins of glaciers, where icy soil plays the most important role. Incredibly regular polygonal patterns, sorted stone circles and ridges, collapse pits and scalloped scarps, and smooth ice-cored mounds, commonly dot terrestrial landscapes. They form naturally as ice in the pore space between soil grains undergoes seasonal melting freezing cycles, or thermal expansion and contraction cycles. Early images of Mars from the Viking orbiters revealed a wide array of large scale features of a potentially glacial or periglacial origin at low resolution. Mars Global Surveyor provided greater details, but many of the diagnostic shapes and textures are typically only a meter in scale. HiRISE provides us with a new window to observe these features.
Major science questions for this theme.
What is the distribution of water ice in the Martian subsurface? The distribution of periglacial landforms allows us to globally map ice deposits. What is the history of surface ice deposits and the Martian climate? Climate cycles on Mars, similar to Earth's ice ages, could have resulted in glacial deposits, potentially anywhere on Mars in the past, thus relic landforms hint at the past, and at the history of water on Mars. Is ice accessible for future exploration? Some periglacial landforms reveal hidden ice in the subsurface that can be sampled and analyzed by future landed spacecraft or even future human explorers.
Relationship to other science themes.
The comings and goings of ice in the Martian environment relates to a wide range of landforms and other natural processes and nearly every other science theme. Melting of surface and subsurface ice deposits may lead to the formation of rivers and streams and other fluvial features if enough water is released to erode the surface. Likewise, evaporation of buried ice results in collapse, erosion, and general mass wasting of the surface. Water frosts come and go on Mars, an integral part of seasonal processes and the longer climate cycles that form glacial and periglacial features. The polar deposits of Mars comprise enormous glacial masses of ice. Volcanic flows can interact with ice to create special landforms through steam explosions and chilling fluid lavas.
Features of interest potentially visible at HiRISE scale.
Much of the middle and higher latitudes of Mars exhibit polygonal patterns tens of meters across, rather like a giant honeycomb exposed at the surface. Thermal contraction polygons are the most common periglacial feature on Earth, the results of seasonal cracking of ice-cemented soils. Polygon size, intersection angles, and trough depth are all clues to their origin and age. Such characteristics can distinguish permafrost polygonal patterns from other polygon-forming processes such as giant mud cracks or lava cooling cracks.