mercury’s surface composition kerri donaldson hanna
TRANSCRIPT
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Mercury’s Surface CompositionKerri Donaldson Hanna
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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
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Common minerals on planetary surfaces
• Feldspars [(K,Ba,Ca,Na)Si3O8]• Two groups: K - Ba solutions and Ca - Na solutions
(plagioclase)• Anorthite most abundant plagioclase on the Moon
• Pyroxenes [(Mg,Fe,Ca)(Mg,Fe)Si2O6]• 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, TiO2, 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
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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
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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
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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
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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
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Remote Sensing of Planetary Bodies
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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)
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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
1.0m 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 -
1.5m• 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
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Visible to Near-IR results
• Weak 1m band detected during 1 observation run - only in bright materials
• Shape and width of 1m 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 TiO2 (0 - 2%)
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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
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Diagnostic Spectral Features
CFRB TM
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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
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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
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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 Apollo sample returns chosen
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How does a spectrometer work?
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The Moon - Grimaldi Basin
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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
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Mercury250 - 260 200 - 210 175 - 185
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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)
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Spectral deconvolution results for Mercury
Feldspar• An90-10 (Bytownite -
Oligoclase)K-spar
• Orthoclase or Sanidine
Pyroxene• Hypersthene,
Enstatite, and Diopside
Olivine• Mg-rich (Fo66-89)
TiO2
• RutileSmall amounts of garnet
• Mg- and Ca-rich garnets
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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 SiO2 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
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MESSENGER• Mercury Atmospheric and
Surface Composition Spectrometer (MASCS)• UVVS covers 88.4 – 254.2 nm• VIRS covers 216.5 – 1395.2 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