The Optical Properties of Nanophase Iron: Investigation of a Space Weathering Analog

Introduction: It is known that space weathering, in particular the nanophase iron (npFe) created via vapor and/or sputter deposition, has distinct and predictable effects on the optical properties of lunar soils [1,2]. In addition to the attenuation of absorption bands, weathering introduces a “characteristic continuum” which is controlled by the amount of npFe present [3]. The shape of this continuum may also be controlled by the size of the npFe grains. It is thought that small npFe grains result in reddening, while larger grains only darken the material [4]. To investigate this phenomenon we have created a lunar weathering analog by impregnating silica gel powders with npFe following the method of [5,6]. Methods: Nanoporous silica gel chromatography substrates (courtesy of GRACEDavison) were impregnated with ferric nitrate solutions then heated. The three powders used in this study have pores with narrow gaussian distributions centered at 6 nm, 15 nm, and 25 nm. All the powders are 100-200 μm. The gels were impregnated with ferric nitrate solutions of various normalities (0.001-1.0). This allowed us to create samples with a wide range of Fe contents. However, it is difficult to control exactly how much Fe is introduced into each sample, thus even samples created with the same normality may end up with different npFe concentrations. After impregnation the gels were thoroughly dried in air before calcining in air at 550°C for 60-80 hours, producing silica gel with nanophase hematite (Fe2O3) in the pores. Spectra of these samples compare well with similarly prepared samples of Morris et al [6]. The hematite-bearing powders were then placed in a furnace at 850°C under reducing conditions (flowing hydrogen) for 4 hours and then cooled (~500°/hr) while hydrogen flow was maintained, resulting in npFe-bearing powders. Spectra of all samples were obtained by the RELAB facility at Brown University. Results: Figures 1-3 are the visible and near-IR spectral data that we have obtained thus far for the three silica powders. The lightest blue spectrum in each graph (SGX.0R) represents the control sample that contains no iron. Successive spectra contain increasing amounts of iron obtained by varying the strength of the ferric nitrate solution. Measurements are planned (neutron activation analysis) to ascertain the iron concentration in each of the experimental runs. The sharp peak at 1365 nm is due to OH bound to the silica host. All three sets follow the same general trends seen in weathered lunar soils [3]. Initially there is a steep curvature in the visible while longer wavelengths remain relatively unaffected. With additional iron, the spectra become very red and increasingly linear and then begin to darken. Unlike the lunar soils however, we find that at higher iron concentrations, rather than becoming simply flat and dark, the analog powders become convex. These convexities were predicted by the modeling results of Hapke [2], and thus are probably an inherent property of npFe. This effect may be masked in natural soils by other darkening agents.