Phoenix Landing Site Geomorphology: Surface Stability and Implcations for the Mar- Tian Latitude-dependent Mantle

Introduction. The NASA Phoenix lander has provided a unique first-look at permafrost processes operating on the surface of the martian northern plains [1]. Critical results from the mission include the detection of an icy surface present beneath thin, overlying, dry sediments [1-3]; complex soil chemistry comparable to that observed in terrestrial cold and arid terrains [4, 5]; and weather/climate conditions consistent with the generation of a range of cold-desert land-forms [1-3]. Questions remain, however, as to the abundance and origin of the observed ice, the history of liquid water at the landing site, the age and stability of the landing site surface , and the relationship between the landing site and the martian latitude-dependent mantle (LDM). The martian northern plains are typical of martian LDM surfaces [6-9]. Data from Phoenix can be used to test hypotheses accounting for the origin of the martian LDM in the vicinity of the landing site by addressing questions raised by each hypothesis (indicated parenthetically): 1) Young, latitude dependent mantle: the LDM is meters-thick and formed from precipitation of snow and dust, with dust forming a sublimation lag, and with massive ice concentrated in underlying layers [6-13] (What processes have brought pebbles, cobbles, and boulders at the site to the surface?). 2) Young, latitude-dependent mantle—wet active layer: the LDM is meters-thick and formed from precipitation of snow and dust, with dust forming a sublimation lag—subsequently, formation of a wet active layer has mixed the sediment and brought rocks to the top surface [14, 15] (Why are the ages of the Phoenix landing site [16] and northern plains LDM surfaces [7, 12, 17] considerably younger than the last predicted period of active layer activity in the northern plains [18]). 3) No latitude-dependent layer—cold climate: Circumpolar deposits are secondary ice [19]—polygons form from thermal cycling of ice-cemented sediment and boulders accumulate at the surface by long-term reworking of an old regolith (What processes account for the <1 Ma age of the LDM surface?). 4) No latitude dependent layer—wet active layer: Martian polar surfaces are actually ancient and have been enriched in secondary ice by vapor diffusion—melting has occurred in the recent geological past, causing cryoturbation of the deposits that bring boulders to the surface [1, 19, 20] (Occurring only during peak obliquity excursions [18], are active layer processes sufficient to degrade craters?). Here we present a range of observations of the Phoenix landing site from the surface …