The Measured Permittivity of CO2 Ice Grown Under Martian Polar Conditions

Introduction: The relative electric permittivity, er, of dense carbon dioxide ice has been measured, with the ice being formed directly from its gas at temperatures around 130 K under 1 mbar of pressure. At 1 kHz er is 4.6 ± 0.2 and at 100 Hz er takes a value of 5 ± 1. There appears to be no data in the literature for this near-DC property of CO2 ice, especially for ice grown from the vapour phase at temperatures comparable to those at the surface of the Martian poles. Experiments at frequencies similar to those used by Ground Penetrating Radar (GPR) will be useful for future spacecraft missions aiming to reveal the stratigraphy and burial depth of Martian polar materials such as water ice. GPR to come: The European Space Agency (ESA) spacecraft, Mars Express, will employ a radar science experiment during its mission. This package is designed to be capable of detecting strong discontinuities in the electromagnetic impedance of the Martian near-surface at depths of several hundred metres. Models of the electromagnetic properties of potential Martian sub-surface components, such as water ice and various rock types, have been studied [1] using laboratory measurements of the permittivity values for these analogues. One such material has, however, not been considered in these experiments. Although CO2 snows 1 made by manual compaction have had their er values measured [2] at various bulk densities, there appear to be no data for er of void-free ice grown from CO2 gas under the low pressures and temperatures found in the Martian polar winters. This form of solid CO2, in contrast to rapidly condensed snows, could be a close analogue for the major component of the seasonal Martian icecaps. Accurate measurements of this substance's dielectric properties will be needed when Martian GPR data are processed. Formation: A dedicated vacuum chamber has been built at the PSSRI for the study of the mechanical properties of cryogenic ices. This chamber, equipped with an instrumented drilling system, can condense ices from the vapour phase into a liquid-nitrogen cooled sample holder. Temperature measurements are made with small Pt 100 thermometer elements and the sample holder can be fitted with a mesh-plate capacitor, shown in the following figure.