Lunar and Planetary Science XXXII (2001) 1197.pdf SPECTRAL PROPERTIES OF CARBONATES IN THE 2.0-2.5 µm REGION: IMPLICATIONS FOR MARS. J. Strong, E.A. Cloutis, and B. Russell, Department of Geography, University of Winnipeg, 515 Portage Avenue, Winnipeg, Manitoba, Canada R3B 2E9 (j. [email protected]; [email protected]; [email protected]). Introduction: Earth-based Martian spectra have carbonates have spectral features in the 2-2.5 µm range shown absorption features thought to arise from the that are fairly similar to calcite and magnesite (Table 1). presence of carbonate minerals [1]. The present work Figure 2 shows the reflectance spectra of nahcolite, focuses on investigating the spectral properties of a manganocalcite, artinite, and trona. The important number of hydrous and anhydrous carbonate minerals to spectral feature to notice here is the absence of strong determine whether they are plausible candidates for the bands in the 2-2.5µm range. The lack of bands in this absorption features reported in these spectra. region may be due to lower overall reflectance due in A number of factors suggest the existence of carbon- part to the strong O-H fundamental band near 2.8 µm; ates on Mars. These include weathering models [2], the bands in the 2-2.5 µm region appears as inflections proposed models of climatic evolution [3] and detection on the low wavelength side of this intense fundamental of preterrestrial carbonates in some SNC meteorite [4]. band. Spectral evidence for the existence of carbonates on Figure 3 shows the reflectance spectrum of trona and Mars has been tentative and specific mineralogies have the dehydrated spectrum following heating. In the not yet been identified. Our work is designed to help unheated spectrum the absorption bands in the 2-2.5 µm resolve this issue by undertaking a thorough study of the range are virtually absent as expected (see above). Once spectral properties of a range of hydrous and anhydrous dehydration is accomplished the spectrum changes to µ carbonate minerals. This paper focuses on the 2-2.5 m reveal the presence of more pronounced bands in the 2- range of some hydrous and anhydrous carbonate miner- 2.5 µm region because most of the water is probably lost als, since they should exhibit absorption bands in this and the reflectance increases, allowing for the carbonate range due to overtones of the C-O stretching fundamen- bands to become visible. The lack of a large amount of tals [5]. water on the Martian surface has been postulated on the basis of volatile loss due to energetic asteroidal and Experimental Procedure: A variety of hydrous and cometary impacts [7]. If this scenario is valid then we anhydrous carbonate minerals have been characterized expect hydrous carbonates to have lost the majority of by reflectance spectroscopy, atomic absorption/emission, their water content; thus, the 2.3 and 2.5 µm bands wet chemistry and X-ray diffraction. The reflectance should appear in Martian spectra if carbonates are spectra were acquired at the RELAB facility at Brown present, even if these carbonates are "hydrous". University as follows: 0.3-26 µm relative to halon (5 nm ° ° spectral resolution; i=30 , e=0 ) and 2.5-26 µm relative Discussion: Those carbonates that exhibit absorp- -1 ° ° to brushed gold (4 cm spectral resolution; i=30 , e=30 ) tion bands in the 2-2.5µm region show slight differences [6]. Dehydration of the trona sample was accomplished in terms of band position and intensities. The wave- by heating for 25 minutes at 115°C. Weight loss upon length positions of the band minima for the samples are heating was comparable to the expected total water shown in Table 1. Table 1 lists only the wavelength content. Absorption band minima positions were deter- positions of samples that had well-defined minima in the rd mined by fitting a 3 order polynomial to 10-20 data 2-2.5 µm region; thus, carbonate minerals exhibiting points on both sides of a visually determined minimum. strong water bands were excluded since no minima in this region could be determined. Some of the samples Results: The reflectance spectra of the <45 µm (such as manasseite) that show spectral features similar fractions of the carbonates are shown in Figures 1-3. to the "typical" calcite spectrum exhibit more complex Figure 1 shows the reflectance spectra of calcite and absorption bands in the 2-2.5µm range that may indicate magnesite, two single-cation anhydrous carbonates. the presence of multiple overlapped bands that our curve Calcite serves as a good example of a "typical" carbonate fitting method does not resolve. in the spectral features it exhibits in the 2-2.5µm range. Although there are small shifts in band positions, all Both calcite and magnesite exhibit similar spectral of the samples exhibit at least two absorption features features in the 2-2.5 µm region: two prominent bands (probably composed of two or more partially overlap- that occur at approximately 2.3 and 2.5 µm (note that the ping bands) in the 2-2.5 µm range that are approximately bands are at slightly different positions). Many other equal in intensity. Some of the samples show slightly more complicated spectral features in the 2-2.5µm, but Lunar and Planetary Science XXXII (2001) 1197.pdf CARBONATE REFLECTANCE SPECTRA: J. Strong et al. they still exhibit the "typical" bands similar to calcite at Winnipeg (to EAC). Thanks to Drs. Takahiro Hiroi and Carlé approximately 2.3 and 2.5 µm. Pieters for providing access to the NASA-supported RELAB spectrometer facility, and Dr. Frank Hawthorne and Mr. Neil Ball of the University of Manitoba for the X-ray diffraction Implications for Mars: Calvin and King [8] found analysis. that artinite and hydromagnesite exhibited absorption Figures: Reflectance spectra (1.4 -2.6 µm) of <45 µ µ bands in the 2.3 m range (centered at 2.28 m). Ab- µm size fractions of: calcite and magnesite (Fig. 1; upper); nah- sorption bands at wavelengths extremely close to those colite, manganocalcite, artinite, and trona (Fig. 2; middle); noted by Calvin and King are present in most of the trona and dehydrated trona (Fig. 3; lower). carbonate samples for which reflectance spectra were measured. Absorption band positions in this region (2- 2.5 µm) indicate that various carbonates share similar spectral features to those noted by Calvin and King [8], 0.95 and may even be an indicator of specific mineralogy 0.9 based upon the complexity of bands in this region and 0.85 their wavelength positions (Table 1). Magnesite 0.8 Table 1. Wavelength positions of resolvable absorp- 0.75 Absolute reflectance Calcite tion bands (µm). 0.7 Magnesite: 2.302 2.503 0.65 Calcite: 2.339 2.536 1400 1900 2400 Wavelength (nm) Dolomite: 2.321 2.517 Gaspeite: 2.318 2.522 Huntite: 2.323 2.506 Siderite: 2.341 2.543 Coalingite: 2.325 2.484 Dawsonite: 2.292 2.358 2.558 0.8 Gaylussite: 2.454 2.555 Trona Hydromagnesite: 2.307 2.501 0.6 Nahcolite Hydrotalcite: 2.32 2.52 Artinite Manasseite: 2.259 2.325 0.4 Pirsonnite: 2.103 2.486 Absolute reflectance Mn-calcite Pyroaurite: 2.326 2.515 0.2 Brugnatellite: 2.322 2.482 2.531 1400 1900 2400 Dehydrated trona: 2.356 2.553 Wavelength (nm) References: [1] Blaney, D.L. and McCord, T.B. (1989) JGR, 94, 10159-10166. [2] Gooding, J.L. (1978) Icarus, 33, 483-513. [3] Carr, M.H. (1996) Water on Mars, Oxford U. Press, NY. [4] McSween, Jr., H.Y. and Treiman, A.H. (1998) 0.8 in Planetary Materials, Min. Soc. Amer., vol. 36, Chap. 6. [5] Trona, heated Gaffey, S.J. (1986) Am. Mineral., 71, 151-162. [6] RELAB 0.6 User’s Manual (1999) Brwon U., Providence, RI. [7] Watkins H. and Lewis, J.S. (1985) Workshop on Water on Mars, LPI Trona, 0.4 unheated Tech. Rep. 85-03. [8] Calvin, W.M., and King, T.V.V. (1991) Absolute reflectance LPSC XXII, 169-170. 0.2 1400 1900 2400 Acknowledgments: This study was supported by research Wavelength (nm) grants from the Natural Sciences and Engineering Research Council and the Canadian Space Agency Space Science Program, and a discretionary grant from the University of.