Determination of iron metal optical constants: Implications for ultraviolet, visible, and near‐infrared remote sensing of airless bodies

Abstract

[1] Iron metal is an opaque phase common in planetary materials both as an igneous mineral and as a spaceweathering by-product. In either form, iron metal has a large influence on the interpretation of ultraviolet, visible, and nearinfrared spectra of planetary surfaces obtained from Earth-or space-based observatories. Therefore, the optical properties of iron are a critical input necessary for accurate theoretical radiative-transfer mixing models for inversion of maturity and mineral proportions from reflectance spectra. Here we report new measurements of the optical constants of iron in the ultraviolet, visible, and near-infrared portions of the electromagnetic spectrum ($160 to 1700 nm). These values are determined from an iron metal film vapor-deposited onto a fused silica prism. Optical constant determination is carried out using an ellipsometer that performs the measurement within the prism, sensing the side of the metal film unexposed to the ambient atmosphere. The data we report have important implications for modeling planetary spectra and for comparison of laboratory measurements of extraterrestrial samples with remotely sensed data sets. Citation: Cahill,

Surveying the South Pole-Aitken basin magnetic anomaly for remnant impactor metallic iron

“…In general, metallic iron is relatively featureless in the NIR, with no prominent absorption features that could reveal its presence in a surficial mixture (Cahill et al, 2012;Cloutis et al, 1990Cloutis et al, , 2010. Furthermore, metallic iron is one of the mineral species, along with ilmenite, unlikely to be directly accounted for in Clementine spectral reflectance (CSR) NIR iron-determination methods due to the positioning of the wavelength bands employed by these algorithms (i.e., 750 and 950 nm; Blewett et al, 1997;Fischer and Pieters, 1994;Lawrence et al, 2002;Le Mouélic et al, 2000;Lucey et al, 1998Lucey et al, , 2000Lucey et al, , 1995Wilcox et al, 2005).…”

“…To demonstrate how spectral reflectance varies with metallic iron abundance we performed a test with radiative transfer theory (Hapke, 1993(Hapke, , 2001(Hapke, , 2012Lucey and Riner, 2011) using the optical constants of orthopyroxene (Trang et al, 2013), metallic iron (Cahill et al, 2012), and plagioclase (Lucey, 2002), respectively. The spectrum of a noritic anorthosite assemblage (75 vol.% plagioclase; 25 vol.% orthopyroxene; Mg' of 75) was modeled and then the assemblage was modified in two ways.…”

Simulated space weathering of Fe- and Mg-rich aqueously altered minerals using pulsed laser irradiation

“…Because it is the most abundant mineral in the sample, we use the real index of refraction and density of cronstedtite as input into the model for the Fe-rich assemblage. The optical constants used in our models are derived from Cahill et al (2012) for iron and Rouleau & Martin (1991) for the amorphous carbon.…”

Widespread hematite at high latitudes of the Moon

“…Then, we restrict the particle size of the hematite end-member to be between 1 and 10 μm for the fine hematite scenario. We also test a third scenario assuming nanophase hematite (<1 μm) embedded in the top tens to hundreds of nanometers of layer of surface grains [( 52 ) and references therein] by updating the absorption coefficient (α) in Eq. 1 ( 2 ) where n h , k h , and ρ h are refraction indices, absorption coefficients, and densities of host materials, respectively; similarly, n Hm , k Hm , and ρ Hm are refraction indices, absorption coefficients, and densities of hematite, respectively; λ is wavelength; M Hm is the mass fraction of hematite; z is expressed as …”