Theoretical Predictions of Ice Crystal Shape in CO2 Clouds on Mars

Details Theoretical Predictions of Ice Crystal Shape in CO2 Clouds on Mars Theoretical Predictions of Ice Crystal Shape in CO2 Clouds on Mars

Nov 2001

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001dps....33.1914w&link_type=abstract

American Astronomical Society, DPS Meeting #33, #19.14; Bulletin of the American Astronomical Society, Vol. 33, p.1070

Astronomy and Astrophysics

Astronomy

Scientific paper

Carbon dioxide ice crystals that form in the Martian atmosphere play very important roles in the climate of Mars, primarily through their radiative effects. Unlike water ice, small CO2 ice particles are very efficient scatterers of thermal infrared radiation [1]. On present-day Mars, CO2 clouds over the polar regions during winter have been observed to reduce the net cooling rate to space [2]. Theoretical studies have shown that the presence of CO2 clouds in a dense (>=1 bar) CO2 atmosphere on early Mars (≈3.8 Ga) could have had a net warming effect and maintained surface temperatures above the melting point of water [3,4], for certain assumptions regarding the optical depth and areal coverage of the clouds, and the size and shape of the cloud particles. Modeling studies to date have assumed spherical particles in their radiative transfer calculations. While this may be true for evaporating crystals, for growing crystals it is certainly not. In order to predict the actual shapes of crystals in CO2 ice clouds, I have derived values for the surface energy, edge energy, and surface diffusion distance for molecules on each of the singular crystallographic faces [5]. Using these values in a model of crystal growth, I find that CO2 ice crystals in the Martian atmosphere will be cubes, octahedrons, or a combination thereof (truncated octahedrons), depending largely on which crystal growth mechanism is active. The values of these physical parameters are also important for calculating the nucleation and growth rates of cloud particles in a microphysical model. I will present calculations of the scattering phase function for these shapes, at visible and infrared wavelengths, for comparison with the Mie phase functions for spheres. [1] Hansen, G. B., JGR v.102, p.21569, 1997 [2] Kieffer, H. H. and T. Z. Martin, JGR v. 82, p.4249, 1977 [3] Forget, F. and R. T. Pierrehumbert, Science v.278, p.1273, 1997 [4] Mischna, M. A. et al., Icarus v.145, p.546, 2000 [5] Wood, S. E., Ph.D. Thesis, UCLA, 1999

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