Steady-state flow of solid CO2: Preliminary results

To help answer the question of how much solid CO₂ exists in the Martian south polar cap, we performed a series of laboratory triaxial deformation experiments at constant displacement rate in compression on jacketed cylinders of pure, polycrystalline CO₂. Test conditions were temperatures 150 < T < 190 K, hydrostatic confining pressures 5≤ P ≤40 MPa, and strain rates 4.5×10⁻⁸ ≤ ε ≤4.3×10⁻⁴ s⁻¹. Most of the measurements follow a constitutive law of the form ε = Aσⁿexp(−Q/RT), where σ is the applied differential stress, R is the gas constant, and the other constants have values as follows: A = 10^{3 86} MPa⁻ⁿs⁻¹, n = 5.6, and Q = 33 kJ/mol. Solid CO₂ is markedly weaker than water ice. Our results suggest that the south polar cap on Mars is unlikely to be predominately solid CO₂, because the elevation and estimated age of the cap is difficult to reconcile with the very weak rheology of the material.