Elsevier

Planetary and Space Science

Volume 70, Issue 1, September 2012, Pages 134-147
Planetary and Space Science

Search for ultraviolet luminescence of soil particles at the Phoenix landing site, Mars

https://doi.org/10.1016/j.pss.2012.05.002Get rights and content

Abstract

The Optical Microscope (OM) on the Phoenix Mars lander (operated from May through October 2008) was used to search for visible-wavelength luminescence of soil particles excited by ultraviolet (UV) illumination (λ=360–390 nm). No luminescent particles were found, with the possible exception of a few potentially luminescent features comprising about 0.02% of the total soil volume. The luminescence quantum efficiency of bulk soil as well as individual soil particles at the Phoenix site is constrained to less than 0.04%. A similar UV experiment will be performed by the Mars Hand Lens Imager (MAHLI) on the upcoming Mars Science Laboratory (MSL) mission. We compare OM and MAHLI UV experiments to each other and suggest a strategy to search for UV-excited luminescence with MAHLI.

Highlights

► Microscopic images of Martian soil particles under UV illumination are analyzed. ► The UV luminescence quantum efficiency (QE) of soil particles is less than 0.04%. ► Expected luminescent materials are listed and luminescence search algorithms are presented. ► These studies prepare UV luminescence experiments onboard MSL (landing 6-Aug-2012).

Introduction

This paper discusses the Phoenix (PHX) Optical Microscope (OM) ultraviolet (UV) experiment, extending earlier analyses of OM color images. Those analyses led to a descriptive taxonomy of Phoenix soil particles based on their size, shape, color and visible reflectance properties (Goetz et al., 2010a). In the present paper we also include a brief discussion of the UV experiment to be performed by the Mars Hand Lens Imager (MAHLI) onboard Mars Science Laboratory (MSL) and provide general recommendations for future in-situ Mars UV experiments.

The OM UV experiment consisted of illuminating Martian soil with near-ultraviolet (near-UV) radiation and searching for visible (VIS) luminescent emission. The term “luminescence” (here used synonymously with “photo-luminescence”) designates a process by which one form of electromagnetic radiation (often UV) stimulates the emission of light of some other type (usually of longer wavelength). Luminescence can occur as fluorescence or as phosphorescence depending on whether the decay time is short or long. Solid-state physicists set the boundary between the two phenomena at ∼1 μs, thereby distinguishing allowed and forbidden radiative (electronic) transitions. However, in mineralogy the boundary is generally set at ∼0.1 s such that phosphorescence (“afterglow”) can just be resolved by the human eye (Gaft et al., 2005, et al., 2002). It is the latter definition that is adopted here. As an example, the luminescent (fluorescent) material BAM (BaMgAl10O17:Eu2+) that is used in the OM UV calibration target (Hecht et al., 2008) has a decay time of ∼1.1 μs (Zych et al., 2004). The OM UV experiment only addresses potential luminescence of soil particles and cannot distinguish between fluorescence and phosphorescence.

UV-excited luminescence of minerals is most often caused by trace abundances of cations (so-called activators) such as Mn2+, Cr3+, rare-earth elements (Ce3+, Dy3+, Eu2+) and specifically the complex cation UO22+. Other causes are traces of organic materials and point defects in crystal lattices (Warren et al., 1995). Most organic material appears to be luminescent in some sense. In particular, polycyclic aromatic molecules (Table 1) are relevant to the type of luminescence studied here, since they can be excited by UV (typically in the wavelength range 250–350 nm) and emit dominantly in the near-UV and blue part of the spectrum (λ∼350–500 nm). Many polyaromatic hydrocarbons (PAHs, including pyrene and fluoranthene that are mentioned in Table 1) have been found in the Murchison meteorite and are thus examples of abiotic extraterrestrial organic compounds (Pering and Ponnamperuma, 1971). Inorganic examples include the blue–violet fluorescence of plagioclases (activated by Eu2+, et al., 2002, Robbins, 1994) and opal/chalcedony green fluorescence caused by traces of UO22+ (et al., 2002, Warren et al., 1995; see Milliken et al., 2008, Squyres et al., 2008 on opaline silica deposits on Mars). In general, increasing activator concentration up to a specific level brightens the luminescence. However, above this characteristic content of activators the luminescence fades away, a behavior referred to as concentration quenching (Warren et al., 1994; Gaft et al., 2005). Often, luminescence requires not only an activator, but also a co-activator (also called sensitizer) that absorbs incident UV photons and transfers the energy efficiently to the activator ions. The best-studied example of this phenomenon is the red fluorescence of calcite that is activated by Mn2+ and co-activated by Pb2+ (or in some cases Ce3+, Robbins, 1994, Sidike et al., 2006).

It is reasonable to ask whether luminescent material can be expected on the surface of Mars since one of the best luminescence quenchers (triggers for non-radiative decay) is Fe2+ (Gaft et al., 2005; Warren et al., 1995). As a result, olivine ((Mg,Fe)2SiO4) is generally non-luminescent, although pure (strictly iron-free) forsterite (Mg2SiO4) sometimes does exhibit VIS luminescence (Gorobets and Rogojine, 2002, p. 44). The trivalent iron (Fe3+) also acts as a luminescence quencher (disregarding a few exceptions such as orthoclases in metasomatites, Gorobets and Rogojine, 2002, p. 45; Warren et al., 1995, p. 154). More importantly, Ni2+ quenches luminescence as efficiently as Fe2+. Since (average) Martian soils contain almost 10 wt% Fe2+ and a few wt% of Fe3+ (Gellert et al., 2006, Morris et al., 2006), and since the concentration of Ni (possibly in the form of Ni2+) is enhanced in bright Martian soils (∼650 ppm at the Gusev landing site, Yen et al., 2005), chances to detect luminescent materials on Mars are slim. In summary, the luminescence of a given mineral depends critically on the specific assemblage of trace elements (activator/co-activator ions and the host composition) implying that luminescence is not a reliable tool for identification of minerals. However, the occurrence of luminescent material on Mars would be a valuable geochemical marker for certain trace elements and associated mineral/organic phases.

The fixed-focus OM instrument has been described elsewhere (Hecht et al., 2008, Goetz et al., 2010a). Several sets of sample substrates mounted on a sample wheel (Hecht et al., 2008) received material that was scooped up from a workspace adjacent to the PHX lander. Each substrate was a 3 mm diameter circular plane disk. Each set included five varieties of substrates: (1) “sticky” silicone, (2) an empty well or “microbucket”, (3) a textured silicon-“nano-bucket”, (4) a substrate containing a weak magnet, and (5) a substrate with a strong magnet. Soil particles stick to these substrates either by adhesive (sticky silicone or nano-bucket) or magnetic forces (weak or strong magnet). The design of the magnetic substrates and their magnetic interaction with different Mars analog soils has been described by Leer et al. (2008). During image exposure the soil material was illuminated by one or more of a ring of 12 Light Emitting Diodes (LEDs), 3 each of red, green, blue (RGB), and near-UV (Fig. 1). Color micrographs of soils were generated by combining monochrome images acquired separately with red, green, and blue illumination. The typical RGB exposure utilized 2 LEDs of each color for ∼1 s each. The images were also corrected for small intensity changes in the blue, green and red LEDs over the course of 100 sols (+4.1%, +2.0%, and −8.7%, respectively; Goetz et al., 2010a).

For most samples, a long-exposure image (typically 31 s) was also acquired under illumination by 3 UV LEDs (λ=375 nm). Since the MECA box (Microscopy, Electrochemistry and Conductivity Analyzer, Hecht et al., 2008) containing the OM is not completely light-tight, the long exposure used for the UV-illuminated images required acquisition of a “dark image” with the same exposure time. All UV-illuminated images discussed here are therefore dark-subtracted and are referred to as “UV–D.” The differencing also reduces the effect of warm pixels and removes the common pedestal value (bias). The lower signal-to-noise ratio of these long-exposure difference images (as compared to the VIS images) has important implications for image analysis. It should also be noted that the need for 31 s UV-illuminated exposures was not anticipated prior to launch, and therefore all relevant calibration was performed in situ during the mission on the surface of Mars.

Following the convention of Goetz et al. (2010a), RGB color images are referred to by sol number and the last four digits of the spacecraft clock time of the red image. UV images (as well as dark-subtracted UV images) are referred to by the corresponding parameters of the UV image.

Section snippets

LED emission spectra

Fig. 2a shows the biconical lighting and viewing geometry for the OM. Light emitted by the LEDs is partly absorbed and partly backscattered by the soil sample on the substrate, then transmitted through a lens system and a Schott GG420 filter prior to detection by the CCD (Charge Coupled Device). Fig. 2b and c shows the emission spectra of the LEDs (left ordinate in Fig. 2b and c) compared to the transmission spectrum of the filter and the responsivity of the CCD (right ordinate in Fig. 2b).

UV–D difference signal

UV and D data acquired over the course of the PHX mission are summarized in Table 2. Fig. 5 plots the mean UV and D signals of soil/dust areas versus sol number. It can be seen that the UV signal is always slightly higher than the D signal. All D and UV images represent 31-sec exposures acquired with UV-LEDs switched off and on, respectively. The D image documents the small amount of stray light inside the MECA box. The UV image contains the D signal and an additional signal due to illumination

Future UV-excited luminescence experiments

Martian soil particles at the PHX landing site were found to be basically non-luminescent when illuminated by UV LEDs (λ∼375 nm). Future experiments of similar type and on similar samples will therefore require increasingly sophisticated strategies to elicit a luminescent response, without sacrificing the high spatial resolution that allows investigation of fine (e.g. silt-sized) particles in soils, inclusions in volcanic rocks (phenocrysts, xenolithic inclusions), and the microstructure

Summary and conclusions

PHX optical microscope data were examined for evidence of soil particles exhibiting near-UV-excited luminescence, as indicated by pixels with an anomalously high intensity in UV–D difference frames. This was done over the entire PHX mission (Section 3.1) as well as for a particular (high-quality) data set that was acquired on sol 99 (3.2 Upper bound to luminescence of bulk soil, 3.3 Search for luminescent soil particles).

The luminescence of PHX soil was inferred to have QE<0.04% (Section 3.2).

Acknowledgments

This research was supported in part by Deutsches Zentrum für Luft- und Raumfahrt e.V. (DLR) grant 50 QM 0602, the UK Science and Technology Facilities Council, the Wolferman Nägeli Foundation, and NASA/JPL. M.B. Madsen acknowledges support from the Danish Research Agency and from the Lundbeck Foundation. The manuscript was improved by thoughtful comments from Jeffrey R. Johnson, Johns Hopkins University, Applied Physics Laboratory.

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