Martian dust particles as condensation nuclei: A preliminary assessment of mineralogical factors

Abstract

Condensation of frosts on Mars should depend not only on temperature and degree of vapor supersaturation but also on the nature of dust particles that would act as condensation nuclei. For a given particle size, the favorability of a condensate nucleator is determined by (1) degree of crystallographic misfit, or disregistry (δ), between substrate and condensate, (2) chemical-bond compatibility between substrate and condensate, and (3) abundance of substrate surface defects that would encourage assembly of condensate atoms or molecules. New data on ice-forming characteristics of candidate Martian materials, obtained by differential scanning calorimetry, confirmed previous evidence for systematic variations in ice-nucleation temperature, Tin, among geologic materials. By considering individual types of minerals separately, factors (2) and (3) can be held relatively constant and differences in nucleation effectiveness can be estimated by computation of (1). Both calorimetry data and cloud-chamber (literature) data indicate that Tin varies inversely with minimum absolute value of disregistry, /δ/. On Mars, H2OIc might be an important form of water ice whereas H2OIh is the common form on Earth. Phase H2OIc offers the lowest overall /δ/ values for heterogeneous nucleation of other condensates, including H2OIh, solid CO2, and CO2 hydrate, suggesting that the best overall mineral nuclei might be those that most effectively nucleate H2OIc. Based on /δ/ values, good nucleators of H2OIc should include nonexpandable clay minerals (e.g., kaolinite, chlorite), certain zeolites (e.g., clinoptilolite), goethite, and bassanite. Although goethite might preferentially nucleate H2OIc, hematite should preferentially nucleate H2OIh. Cryptocrystalline mineraloids (e.g., palagonite) should be generally poor nucleators and nucleating abilities of expandable clay minerals (e.g., nontronite, montmorillonite) should vary significantly with degree of c-axis expansion, as controlled by degree of interlayer hydration.