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 IRTF infrared spectroscopy of Mercury: Complementing MESSENGER compositional observations

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

IRTF/SpeX NASA 3-m IR telescope facility on Mauna Kea –US national facility open for large community –high altitude: low H 2 O, O 2, CO 2, N 2 etc –daytime studies close to Sun allowed –on-site or remote observations SpeX –medium-resolution spectrograph and imager – mum R 250 single- order prism mode – mum R 2500 cross- dispersed mode –0.3 x 15” slit –photometry and imaging: Z, K, L bands –pixel scale 0.12” –seeing ”

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

May 2008 run ObjectMercuryMoon Date Wav. region (  m) Elongation (  ) Phase angle (  ) Fraction illuminated (%) Diameter (  ) 7.1 – Sub-Earth long (  ) 95 – 110 W6 – 7 E Sub-Solar long (  ) 8 – 12 W137 – 125 E Limb long (  ) 10 – 20 W E Terminator long (  ) 98 – 102 W E Calibration targets include solar analog and IR standard near Mercury

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)

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

Previous NIR spectroscopy Continuum-removed Mercury spectra. (Warell et al. 2006) 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  m (Warell et al. 2006) –Ca-rich clinopyroxene (CPXA) mineralogy inferred –no 2-5  m 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).

Previous NIR spectroscopy 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). 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  m

5-  m flux excess Emery et al. (1998) reported a surprising rise in flux with decreasing wavelength from 6 to 5  m (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

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)

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

Effect of temperature OLI OPX CPX Singer and Roush (1985) 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-  m 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-  m 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

Observing temperature effects Locations at the limb near the sub-solar point are 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

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 Cloutis and Gaffey (1991) 1-  m band 2-  m band

Summary New IRTF  m 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

Pyroxene reflectance spectra Combination of 1- and 2  m 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)