Seasonal defrosting of the Phoenix landing site

摘要

The retreat of the seasonal polar cap at the Phoenix landing site is analyzed through observations of high resolution images and thermal modeling of the CO_2 frost mass. Numerical simulations indicate that the onset of CO_2 frost formation occurs around L_s= 220° at 68°N; however, the polar hood prevented imaging of the surface during this time. In the late winter/early spring a continuous layer of frost coated the surface obscuring the underlying polygonal patterns; however, rocks are clearly evident indicating that the frost depth was relatively shallow. Rare dark fans originating from rocks are also observed in an image from early spring. Frost dissipated from the polygon centers first and lingered longer in polygon troughs. Polygon centers were first exposed on L_s = 13.9°, and the polygon troughs were frost free by L_s ～57°. Frost distributions varied slightly between geologic units; however, in general, global observations at similar latitudes are consistent with what is seen in the Phoenix region. A series of hypotheses is also explored to explain the preference for the CO_2 frost to linger in the troughs: lower thermal inertia material in the troughs, aeolian redistribution of frost, burial of the ice table via aeolian redistribution of dust, and shadowing. Each of these hypotheses has drawbacks to explaining this phenomenon as stand-alone components. Shadows do play a role in keeping the CO_2 frost on the surface; however, this effect is small and cannot alone account for the observations.