Laser Raman spectroscopy of varnished basalt and implications for in situ measurements of Martian rocks

Abstract

Abstract. Earth-based and orbital spectroscopy and Viking Lander data strongly suggest that rocks on Mars are coated with indurated dust deposits. Rock coatings will influence mineralogical measurements to be acquired in situ from Mars rovers and other surface missions, in that most techniques will detect the coatings and/or coatings plus underlying minerals. In this paper, we present results of experiments on varnished basalt cobbles from the Lunar Crater Volcanic Field, Nevada, using laser Raman spectroscopic microprobe measurements obtained with a 514.5 nm Ar + laser and irradiation spot sizes For background, LRMS uses monochromatic radiation (i.e., a laser) as excitation and collects the radiation scattered from samples. The frequency of the scattered radiation is shifted relative to the laser in ways that are directly related to molecular vibrations in the sample (gas, liquid, and solid). Any given mineral has a "fingerprint" Raman spectrum typified by a number of peaks with characteristic frequencies and relative intensities [Wang et al., 1994]. In fact, LRMS has been shown to be useful for identifying major rock-forming minerals, such as silicates, carbonates, sulfates, sulfides, phosphates, oxides, and hydroxides. Additionally, this technique provides direct information to distinguish among polymorphous minerals (e.g., quartz, coesite, stishovite) and to characterize structural distortions due to such processes as shock-induced damage or metamictization.In this paper we first discuss the samples used for LRMS observations, including petrography and chemistry. We next summarize our LRMS measurements of igneous minerals within the host rock, move to analysis of the mineralogy of the varnish from LRMS data for thin sections, and end with a discussion of LRMS observations acquired from hand speci-28,705

Optical scattering properties of terrestrial varnished basalts compared with rocks and soils at the Viking Lander sites

“…In the southwestern United States, varnish ranges from a few to approximately 100 !lm in thickness and is dominated by manganese oxides, iron oxides, and fine-grained clay minerals [Israel et al, 1997]. For the study site, most rock surfaces exposed to the atmosphere are coated with dark gray varnish that has a glossy appearance and is dominated by manganese oxides and clay minerals.…”

Stand-off Raman spectroscopic detection of minerals on planetary surfaces

“…Advanced Raman spectroscopic techniques have been employed to investigate the mineralogy of meteorites [1 Á/3], and have been proposed for in situ mineral characterization on planetary surfaces [4,5]. Both micro-Raman and remote Raman systems are being considered for future exploration on planetary surfaces [6 Á/10].…”

Astrobiological instrumentation for Mars – the only way is down

“…Laser Raman spectroscopy is a favoured technique for the analysis of minerals in Mars meteorites (Edwards et al 1999) and potentially for analysis of Martian regolith in situ (Edwards & Newton 1999 ;Popp et al 2001) and concurrently characterizes the mineral environment of any biomolecules. The emphasis for Raman research on Mars has been placed either on rock crusts, such as the varnish on rocks of Martian geology (Israel et al 1997) or using point-count methods on the surface of lithosols (Haskin et al 1997). The JPL Raman spectrometer that was originally proposed for the original Athena payload for the Mars Exploration Rovers (MER) has been dropped from the MER payload (Wang et al 1998).…”