Johan Warell*, A. Sprague, R. Kozlowski, A. Önehag*,

1. IRTF infrared spectroscopy of Mercury: Complementing MESSENGER compositional observations Johan Warell*, A. Sprague, R. Kozlowski, A. Önehag*, G. Trout, B. Davidsson*, J. Helbert, D. Rothery *Department of Physics and Astronomy, Uppsala University, Sweden SERENA-HEWG meeting, Santa Fe, 2008

2. Motivation • Search for spectral features in the wavelength range 0.8 - 5.5 mm • absorption bands due to the Fe2+ electronic transfer in mafic silicates near 1 and 2 mm • extension of MESSENGER MASCS spectroscopy beyond 1.45 mm • volume scattering emission features between 2.8 and 5.5 mm, particularly relevant for an iron-poor lithology • further study of the anomalous ”5 mm flux excess” • spectral effects due to variations in temperature

3. IRTF/SpeX • NASA 3-m IR telescope facility on Mauna Kea • US national facility open for large community • high altitude: low H2O, O2, CO2, N2 etc • daytime studies close to Sun allowed • on-site or remote observations • SpeX • medium-resolution spectrograph and imager • 0.8-2.5 mum R 250 single-order prism mode • 2.3-5.5 mum R 2500 cross-dispersed mode • 0.3 x 15” slit • photometry and imaging: Z, K, L bands • pixel scale 0.12” • seeing 0.7-1.4”

4. IRTF observing runs • Granted • 8-11 May 2008: Eastern M10 hemisphere • 5-8 July 2008: W. M10, E. MESS1 hemisphere • Applied for • 3-6 September 2008: E. MESS2 hemisphere • 24-27 October 2008: W. MESS2 hemisphere • Planned • 2009, remaining longitudes

5. May 2008 run Calibration targets include solar analog and IR standard near Mercury

6. Mercurian targets E Mariner 10 hemisphere Albedo map (Warell & Limaye 2001) • Surface locations • N central to extreme N lat. across disk • S central to extreme S lat. across disk • Equatorial lat at limb and terminator long. • Coverage of a range of geologic units and • MESSENGER flyby 2 surface regions Sample images in Z, K, L (May 9)

7. Lunar target • Petavius central peak • Well characterized • location for Mercury data • validation • Shocked plagioclase • (anorthosite) (Pieters 1986) • Mercury compositional • analog May 9, 2008

8. Previous NIR spectroscopy • Near-linear spectrum of Mercury (McCord & Adams 1972, McCord and Clark 1979) • iron-poor mineralogy and/or metallic iron • predominantly feldspathic composition • Shallow absorptions detected at 1.1 mm (Warell et al. 2006) • Ca-rich clinopyroxene (CPXA) mineralogy inferred • no 2-5 mm small-scale features indicative of mineralogy CVF photometer spectra by McCord &Adams (1972) and Vilas & McCord (1976). CCD spectrum by Vilas (1985). From Blewett et al. (1997). Continuum-removed Mercury spectra. (Warell et al. 2006)

9. Previous NIR spectroscopy • Hapke (2002) theory applied to Mercury’s spectrum • intimate mixtures, 1-2 regolith components, grains and agglutinates • feldspar-rich, FeO-poor lithologies: 75% labradorite and 25% enstatite • FeO: 1-2 wt%, npFe: 0.2 wt% (half that of mature lunar pure anorthosites) • backscattering efficiency increasing with wavelength • optically active grain size about 30 mm Mercury’s CCD reflectance spectrum combined with a CVF spectrum of McCord & Clark (1979). The best-fit solid line model is an intimate mixture of 75% labradorite and 25% enstatite with backscattering efficiency increasing with wavelength. From Warell & Blewett (2004).

10. 5-mm flux excess • Emery et al. (1998) reported a surprising rise in flux with decreasing wavelength from 6 to 5 mm (KAO/HIFOGS) • Possibly an effect of small grain size and regolith temperature gradient • Theoretically modeled by Henderson and Jakosky (1997). • IRTF/SpeX 2002 data showed a clear flux excess, while 2003 data do not, which may indicate grain size variations • Thermophysical modeling to be made with a revised version of the Davidsson et al. (2008) model • surface roughness, grain size, composition, thermal inertia

11. Regolith maturation: effects of Ostwald ripening? • The extreme temperature range on Mercury may result in latitudinal variations in the size distribution of npFe and the spectral properties of the soil (Noble and Pieters, 2003) • increased abundance of <5 nm npFe darkens, reddens and flattens continuum • nm-sized npFe particles formed at maturation may grow to larger sizes at high temperatures (>500 K) • increased abundance of >10 nm npFe primarily darkens continuum (most important near equator, particularly “hot poles”) • Effects should be manifested in variations of spectral slope and albedo with longitude and latitude VISNIR spectral slope as a function of latitude, consistent with Ostwald ripening. 0.5-m SVST high-resolution imaging (Warell 2003)

12. Pyroxene reflectance spectra • 2-mm band in OPX/CPX useful for compositional determination • displays discriminative variations with temperature • OPX and CPXB 1-mm and 2-mm CF bands due to M2-site Fe2+ • Ca2+ strongly partitioned into CPXB M2 sites • CPXA 0.9-mm and 1.15-mm CF bands in M1-site Fe2+ • OMCT absorptions <0.5 mm <45 mm grain size bidirectional reflectance spectra (Cloutis and Gaffey 1991)

13. Effect of temperature • Temperature effects in pyroxenes (Burns 1970) • higher T  broader CF bands • increased amplitude of thermally induced vibrations of cation around site center • higher T  increased band center wavelength • uniform expansion of site can decrease CF splitting energy • differential thermal expansion causes complex wav changes • 1-mm band (Aaronson etal. 1970, Sung et al. 1970) • general behaviour in OLI, OPX, CPX • broadening with increasing T • central wav unchanged with increasing T • 2-mm band (Singer and Roush 1985) • OPX: broadening same as 1-mm band, increased central wav with T • CPXB: broadening same as 1-mm band, decreased central wav with T OPX CPX OLI Singer and Roush (1985)

14. Observing temperature effects • Locations at the limb near the sub-solar point are 200-400 K warmer than when located near the terminator • Combined data from two suitably selected elongations • same surface features will be visible at similar phase angles • illuminated in inverted geometries • Provides opportunity to study temperature effects which are independent of surface composition and structure • the wav of absorption bands in reflectance • visibility of features in the reflectance-thermal transition region • the depth of any volume scattering bands in emittance • the magnitude of the infrared excess • BED optical constants as a function of T needed for modeling

15. PYX formation temperature • In principle possible to deduce pyroxene formation temperature from a combination of band center wavelengths and geotherms • However, contours are mainly orthogonal limiting possibility to determine temperature with precision 1-mm band 2-mm band Cloutis and Gaffey (1991)

16. Summary • New IRTF 0.8-5.5 mm spectroscopy a continuation of efforts in 2002 and 2003 • Anticipated verification of MESSENGER compositional results • Check on detailed pyroxene chemistry • Study of importance and effects of temperature on compositional interpretation • New BED data and improved thermal model

18. Pyroxene reflectance spectra • Combination of 1- and 2 mm band data increases compositional discriminability • Band center wavelengths a function of Fe, Ca, Mg abundance • Requires correction of reradiated thermal signal of Mercury Left: Pyroxene spectra (adopted from Burns 1988). Right: Mineral absorption band centers (Adams 1975)