Mercury s Surface Composition Kerri Donaldson Hanna Questions answered by studying surface composition What type of geologic history has Mercury undergone This would constrain the thermal evolution of the planet How much FeO is on the surface This would constrain the evolution models discussed last week How much space weathering has occurred on Mercury s surface This would constrain the space environment of the planet over its history Does any of the material in the exosphere come from Mercury s surface This would constrain the interactions between the exosphere and the magnetic field solar wind and or surface Common minerals on planetary surfaces Feldspars K Ba Ca Na Si 3O8 Two groups K Ba solutions and Ca Na solutions plagioclase Anorthite most abundant plagioclase on the Moon Pyroxenes Mg Fe Ca Mg Fe Si 2O6 Two groups orthopyroxenes and clinopyroxenes Fe rich Mg rich Ca rich NaAl rich and CaMn rich Olivines Mg Fe 2SiO4 Common in the mantle on Earth Solid solution between Mg rich and Fe rich Fe Ti Oxides FeO TiO 2 FeTiO3 Other minerals include sulfates sulfides carbonates amphiboles micas On Mercury no plate tectonics or hydrologic cycle should expect rocks and minerals that are associated with the crystallization of magma possible igneous intrusions and meteorite impact melting fracturing and mixing Mercury versus the Moon Originally Mercury thought to be similar to the Moon Bright craters and dark plains Smooth plains associated with impact craters and basins Mariner 10 Observations No observations made that could determine elemental abundances specific minerals or rock types on Mercury Mariner 10 observed day side albedos of Mercury and the Moon Dark plains would have a lower albedo than a bright crater Mercury s albedo lower overall than the Moon s by a few percent but in the visible it has a higher albedo Mercury s albedo varies across its surface and at different wavelengths from 400 to 700 nm Composition grain size and porosity plays key roles in explaining a planet s albedo Finely crystalline silicates low in Fe and Ti tend to be brighter and scatter more light off of the surface New measurements from SOHO paired with Mariner 10 data looked at phase angle and backscattering Results indicate Mercury s surface has smaller grains and more transparent than the Moon and the higher efficiency of reflecting light towards the sun indicates the presence of complex or fractured grains Re calibration of Mariner 10 Images Technique first used on lunar data Robinson and Lucey 1997 Use 375nm UV and 575nm VIS bands Ratio UV VIS Plot UV VIS versus VIS As FeO increases and soils mature spectrum reddens and UV VIS decreases As opaque minerals increase the albedo decreases and increases the UV VIS Rotate axis to decouple FeO maturity from opaque index Re calibration of Mariner 10 Images VIS image UV VIS image FeO maturity Opaque Index Brighter tone indicate increasing blueness Brighter tone indicate decreasing FeO and maturity Brighter tone indicate increasing opaque minerals Remote Sensing of Planetary Bodies Remote Sensing of Planetary Bodies Spectroscopy Visible light 0 4 0 7 m Near IR 0 7 2 5 m Mid IR 2 5 13 5 m Visible to Near IR spectroscopy Measuring reflected light Absorption bands are created from electronic transitions in the molecules bonded in the lattices of silicates Interested in 0 3 0 5 and 1 0 m bands associated with FeO Spectral contrast of features can be diminished due to space weathering Spectral slope indication of the maturity and composition Fit straight line from 0 7 1 5 m Slope of line increases as soil matures Look at ratios to determine soil maturity and FeO and opaque mineral content Again techniques used originally on the Moon Visible to Near IR results Weak 1 m band detected during 1 observation run only in bright materials Shape and width of 1 m band indicative of Carich clinopyroxene Mercury s spectral slope has a higher value than the spectral slope from immature to submature regions on the Moon Low FeO 0 3 and TiO2 0 2 Mid IR spectroscopy Measuring emitted light Absorption bands are caused by the vibration bending and flexing modes of the crystalline lattices Grain size and composition of mineral samples greatly affect spectra Compare key spectral features diagnostic of composition with spectra of rocks and minerals measured in the laboratory Reststrahlen bands fundamental molecular vibration bands in the region from 7 5 11 mm Emissivity maxima also known as the Christensen feature associated with a silicate spectrum and occurs between 7 9 mm Transparency minima associated with the change from surface scattering to volume scattering and occurs between 11 13 mm Good indicator of SiO2 weight percent in rock Highly depends on the quality of spectral libraries built from laboratory measurements of rocks and minerals Diagnostic Spectral Features CF RB TM Grain Size and Composition Effects in the Mid IR Varying the grain size changes the depth or existence of spectral features Varying the composition changes the location of spectral features Mid IR results Mercury s surface composition is heterogeneous Most spectra match models of plagioclase feldspar with some pyroxene Plagioclase more sodium rich than that on the Moon Pyroxene low Fe Ca rich diopside or augite or low Fe Mg rich enstatite Bulk compositions indicate an intermediate silica content similar to diorite or andesite on Earth No evidence for Fe and or Tibearing basalts as lava flows as seen on the Moon Observing Mercury and the Moon in the mid IR NASA Infared Telescope Facility IRTF using Boston University s Mid Infrared Spectrometer and Imager IRTF allows for pointing telescope near the sun MIRSI covers the 8 14 m spectral range Mercury daytime observations Moon day and night time observations Locations on the lunar surface with well known composition from near IR telescopic observations and How does a spectrometer work The Moon Grimaldi Basin Grimaldi and Laboratory Spectra Comparison Grimaldi spectra compare well in overall shape with the RELAB Impact Melt and Breccia spectra Grimaldi spectra also compare well with Salisbury et al NoriteH2 in particular 11 13 m region No perfect matches yet but indicates our results are reasonable 250 260 200 210 175 185 Mercury Spectral Deconvolution Ramsey 1996 and Ramsey and Christensen 1998 developed algorithm and provided in ENVI by Jen Piatek Inputs spectrum to be deconvolved spectral library of pure mineral spectra and wavelength region to be fit over