Laboratory Simulation of tex2html_wrap_inline18 Ice Condensation on Mars by Radiative Cooling

S.E. Wood, D.A. Paige (UCLA), W.D. Smythe (JPL)

We have performed realistic laboratory simulations of the thermal and radiative environment at the surface of Mars to produce the first samples of carbon dioxide ice formed as it does on Mars, by radiative cooling from a 600 Pa, near-pure tex2html_wrap_inline18 gas. It is important to determine the physical characteristics of Mars' seasonal tex2html_wrap_inline18 polar ice caps because these determine their radiative properties which, in turn, control the polar energy balance and the seasonal variation in global surface pressure. It is not known whether they form as fluffy fine-grained deposits, dense solid ice, or something in between. Previous simulations have used conductive cooling, condensing tex2html_wrap_inline18 onto a substrate cooled by liquid nitrogen (Kieffer 1968, Ditteon and Kieffer 1979). This technique favors the growth of grains having the best thermal contact with the surface, resulting in large grain sizes and a coarse texture. On Mars, however, the latent heat released by condensation must be lost radiatively to space. To simulate this process in a lab it is necessary to separate the gas from the space simulator (a tex2html_wrap_inline26 -cooled black copper plate) using a material which is transparent at the appropriate thermal infrared wavelengths (> 10 /mu).

For this experiment, we have constructed a Mars simulation chamber, an airtight copper pot containing low thermal conductivity "soil" and tex2html_wrap_inline18 gas, placed inside a thermal/vacuum chamber for insulation. To enable radiative cooling of the gas and "soil" inside the Mars chamber, its top is an infrared window; thin (12 /mum) polypropylene film. To be sure that the latent heat of condensation is not lost conductively to the chamber, the walls of the Mars chamber are maintained at a temperature just slightly higher than the condensation temperature. As the tex2html_wrap_inline18 condenses, the pressure is maintained at 600 Pa automatically with a pressure control system. We will present measurements of the physical and radiative properties of the ice including; temperature, texture, grain size, and broadband emissivity.