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 3 O 8 ] Two groups: K - Ba solutions and Ca - Na solutions (plagioclase) Anorthite most abundant plagioclase on the Moon Pyroxenes [(Mg,Fe,Ca)(Mg,Fe)Si 2 O 6 ] Two groups: orthopyroxenes and clinopyroxenes Fe-rich, Mg-rich, Ca-rich, NaAl-rich, and CaMn-rich Olivines [(Mg,Fe) 2 SiO 4 ] Common in the mantle on Earth Solid solution between Mg-rich and Fe-rich Fe-Ti Oxides [FeO, TiO 2, FeTiO 3] 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 Brighter tone indicate increasing blueness FeO + maturity Brighter tone indicate decreasing FeO and maturity Opaque Index Brighter tone indicate increasing opaque minerals

Remote Sensing of Planetary Bodies

Spectroscopy Visible light ( m) Near-IR ( m) Mid-IR ( 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 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 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 Ca-rich 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 TiO 2 (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 mm Emissivity maxima (also known as the Christensen feature) - associated with a silicate spectrum and occurs between mm Transparency minima - associated with the change from surface scattering to volume scattering and occurs between 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 composition changes the location of spectral features  Varying the grain size changes the depth/or existence 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 Ti- bearing 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 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 Apollo sample returns chosen

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 μm region No perfect matches yet, but indicates our results are reasonable

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 Spectral library of 337 end-members created with reflectance spectra of fine and coarse grain minerals (ASTER, RELAB, USGS, ASU and BED) When minerals in an assemblage are present in library, algorithm determines abundances within 5% Previous successes for whole rocks, meteorite samples and plagioclase sands include: Hamilton et al. (1997), Feely and Christensen (1999), Hamilton and Christensen (2000), Wyatt et al. (2001), and Milam et al. (2007)

Spectral deconvolution results for Mercury Feldspar An (Bytownite - Oligoclase) K-spar Orthoclase or Sanidine Pyroxene Hypersthene, Enstatite, and Diopside Olivine Mg-rich (Fo ) TiO 2 Rutile Small amounts of garnet Mg- and Ca-rich garnets

What do and don’t we really know? Surface composition heterogeneous Feldspar-rich of moderate Ca and Na Low-Fe pyroxenes and olivines present Low FeO content (up to 3%) No observations contradict the scenario of early core formation accompanied by global contraction of the planet Extrusive lava flows on the surface are likely low in SiO 2 and enriched in K and Na Not clear about space weathering - different than the Moon Still not enough info to constrain evolution and thermal history models Still unsure of link between surface and exosphere

MESSENGER Mercury Atmospheric and Surface Composition Spectrometer (MASCS) UVVS covers 88.4 – nm VIRS covers – nm Mercury Dual Imaging System (MDIS) 12 filters over the 395 – 1040 nm spectral range Gamma Ray and Neutron Spectrometer (GRNS) Will measure cosmic-ray excited elements O, Si, S, Fe, and H Will measure naturally radioactive K, Th, and U X-Ray Spectrometer (XRS) Will measure K lines for Mg, Al, Si, S, Ca, Ti, and Fe