Mars: Periglacial Morphology and Ice Stability
- At the Mars Phoenix landing site and in much of the martian northern plains, there is ice-cemented ground beneath a layer of dry permafrost. Unlike most permafrost on Earth, though, this ice is not liquid at anytime of year. However, in past epochs at higher obliquity the surface conditions during summer may have resulted in warmer conditions and possible melting. This situation indicates that the ice-cemented ground in the north polar plains is likely to be the most recently habitable place on Mars as near-surface ice likely provided adequate water activity ~5 Myr ago. The possibility of life on Mars is important both for Mars science (Science Mission Directorate (SMD) and Mars Exploration Program Analysis Group (MEPAG) goals and objectives) as well as preparation for human exploration (Human Exploration and Operations Mission Directorate (HEOMD) and Strategic Knowledge Gaps (SKGs) pertaining to biohazards and planetary protection). The high elevation Dry Valleys of Antarctica provide the best analog on Earth of martian ground ice. These locations are the only places on Earth where ice-cemented ground is found beneath dry permafrost. The Dry Valleys are a hyper-arid polar desert environment and in locations above 1500 m elevation, such as University Valley, air temperatures do not exceed 0°C. Thus, similarly to Mars, liquid water is largely absent here and instead the hydrologic cycle is dominated by frozen ice and vapor phase processes such as sublimation. These conditions make the high elevation Dry Valleys a key Mars analog location where periglacial processes and geomorphic features can be studied in situ. This talk will focus on studies of University Valley as a Mars analog for periglacial morphology and ice stability. We will discuss observations revealing a unique trend as the depth to ice-cemented ground varies linearly from near zero at the head of the valley to over 80 cm deep 1.5 km away at the valley mouth. This setting provides a natural gradient in physical permafrost properties, water vapor transport, and ice stability. We will also discuss geomorphic ramifications of this ground ice distribution as polygon size is shown to increase down the length of the valley and is correlated with increasing ice depth. Since polygons are long-lived landforms and observed characteristics indicate no major fluctuations in the ice-table depth during their development, the University Valley polygons have likely developed for at least 104 years to achieve their present mature-stage morphology, and the ice-table depth has been stable for a similar length of time. In addition, we will discuss geomorphic features (e.g., rock weathering and erosion, thermal contraction, sublimation till) as possible diagnostics for subsurface ice type. Finally, we will review a landing site selection study encompassing this information gleaned from the Antarctic terrestrial analog studies plus Mars spacecraft data analysis to identify candidate landing sites for a future mission to search for life on Mars.