Mars Science A Surveyor's Chronicles.htm

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Mars Science A Surveyor's Chronicles.htm
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Weathered basalt
Mars Science A Surveyor's Chronicles.htm
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What is TES? Thermal Emission Spectroscopy

• The TES Instrument has 3 parts
• Michelson Interferometer/spectrometer
• Bolometric (broadband) thermal radiance (4.5 to
  100 µm) channel
• Solar reflectance channel (0.3 to 2.7 µm )
  measures brightness of reflected solar energy.
• Covers the 6 to 50 µm (1655 200 cm-1)
  wavelength range
• Spectral Resolution 5 10 cm-1
• Spatial Resolution 3 km from MGS orbit
• Entered Mars orbit on board the Mars Global
  Surveyor (MGS) on 9/11/1997.
• Collected 4.8 x 106 spectra of Mars over 510 days
  during a series of 1284 aerobraking orbits prior
  to 350 km mapping orbit.
• PI Philip R. Christensen (ASU, grad student at
  UCLA)

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MGS - TES

• Six sensors in 3 x 2 array
• Spatial resolution of 3 km from MGS orbit
• About the size of a small microwave
• Weighs 32 lbs.
• Powered by solar sails on spacecraft uses 14.5
  watts!
• Also designed to study composition of atmosphere,
  atmospheric temp. field and polar energy balance.

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How does TES determine surface composition?

• Step 1 - Apply atmospheric correction to raw
  spectra
• Bandfield et al. 2000
• Estimated the spectral shape of the dust and
  water-ice cloud components.
• Spectral shapes were shown to be constant in
  space and time over a wide range of atmospheric
  conditions (dust loading and ice content).
• Dust spectrum found to contain little, if any,
  surface spectral characteristic.
• Smith et al. 2000
• Developed two different algorithms for
  surface-atmosphere separation.
• Both algorithms gave very similar results.
• Derived a suite of surface-only spectra for
  classic dark region of Cimmeria Terra
• The low albedo Dark Regions were looked at first
  since they are believed to be relatively free of
  surface dust coating.
• Tested the techniques using TES data collected
  under two very different atmospheric temperature
  and opacity conditions.

Bandfield, J.L., Christensen P.R., Smith, M.D.
(2000) Spectral data set factor analysis and
end-member recovery Application to analysis of
Martian atmospheric particulates. J. Geophys.
Research, 104, 9573-9588. Smith, M.D.,
Bandfield, J.L., Christensen, P.R. (2000)
Separation of atmospheric and surface spectral
features in Mars Global Surveyor Thermal Emission
Spectrometer (TES) spectra. J. Geophys. Research,
104, 9589-9607.
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Atmospheric correction Models

• Two atmospheric correction models
• Radiative transfer model
• Deconvolution model
• Comparison of surface emissivity spectral shapes
• Comparison of dust and water ice spectral shapes
  obtained by both models. The agreement is very
  good. Difference in dust spectral shapes are
  caused by CO2 hot bands at 1075 cm-1.
• Smith et al (2000)

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Radiance and Emissivity

• A blackbody emits energy with a Planck
  distribution
• Vibrating molecules emit and absorb infrared
  light - depends on bond energies.
• Unique spectrum for a given mineral.
• Infrared energy measured in terms of radiance
  Watts per unit of area
• The radiance from a mineral at one temperature
  will be different from that at another temperature

70? F
Quartz
(From ASU Thermal Emission Spectroscopy website)
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Radiance and Emissivity, cont.

• Need to remove the temperature effect.
• Divide the radiance spectrum of selective emitter
  by blackbody (perfect emitter) at same
  temperature.
• Result Emissivity spectrum (dimensionless)
• Values lt1 are wavelengths where molecules absorb
  energy
• For quartz, SiO2 molecules are responsible for
  absorption.

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Mixed Spectra

• Rocks are a mixture of minerals
• Emissivity spectrum from individual components of
  a mixture add together in a simple linear
  fashion.
• The linearity of the mixed spectrum allows it to
  be deconvolved.

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Step 2 Deconvolution of Mixed Spectra
Thermal Emission Spectra of Feldspars

• Assemble a spectral library of mineral end
  members (ASU has an online spectral library)
• Write a deconvolution algorithm which adds and
  subtracts endmembers and calculates the best fit
  (rms error)
• Depth of absorption feature is directly related
  to abundance (or smoothness see later)
• NB the mineral mix you come up with depends on
  the end members you start with!
• This is from a suite of feldspars with grain size
  of 710-1000 µm
• Vertical lines call attention to various spectral
  features which change with composition.
• From ASU Thermal Infrared Mineral Spectroscopy
  Library

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Two distinct surface types found on Mars

• The spectral fits were modeled by two groups
• Bandfield et al. (2000) used 45 end-members
  representing igneous, sedimentary and metamorphic
  minerals (rms 0.00018, 0.0009)
• Hamilton et al. (2001) used a narrower range of
  29 mineral spectra of unweathered basalts and
  andesites (rms 0.0026, 0.0014)
• (rms average error over the entire spectrum)
• Bandfield, J.L., Hamilton, V.E., Christensen P.R.
  (2000) A global view of Martian surface
  composition from NGS-TES. Science 287,
  1626-1630.
• Hamilton V.E., Wyatt M.B., McSween H.Y.,
  Christensen P.R. (2001) Analysis of terrestrial
  and Martian volcanic compositions using thermal
  emission spectroscopy 2. Application to Martian
  surface spectra from the Mars global surveyor
  thermal emission spectrometer. J. Geophys.
  Research, 106, 14,733-14,746..

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Two distinct surface types found on Mars

• Type 1 - Similar to Basalt Type 2
  Andesite?
• ( lt 52 wt SiO2) (
  52-63 wt SiO2)
• Mostly in southern highlands Mostly in
  northern lowlands
• 
  (similar to Pathfinder results)
• (note the larger percentage of high silica glass
  is the main diff.)

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A note on Methods

• TES data cover 1650-200 cm-1, spectral fitting
  constrained to 1,280-400 cm-1
• CO2 exclusion region (Bandfield et al 2000)
  540-800 cm-1
• Atmosphere correction excludes high wave number
  region because of numerous water vapor and minor
  CO2 features
• 400-200 cm-1 range excluded due to residual
  atmospheric water vapour rotational bands in
  Martian surface spectra and ASU mineral library
  spectra.
• Range is restricted so that algorithm wont
  attempt to fit water vapor features instead of
  surface mineralogy.

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MGS TES Basalt Map
Basalt (Type 1 spectra) concentrated in Southern
Highlands
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MGS TES Andesite Map
Andesite (type 2 spectra) appears concentrated in
Northern Lowlands, but also intermixed with
basalt in Southern Highlands.
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Mars Andesite
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Mars Basalt
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Mars Hematite
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Ways to form Andesite (on Earth)

• Converging plate margins (tectonics!) Water in
  descending oceanic crust promotes melting of
  mantle wedge above it
  
  --- or ----
• Fractional crystallization of basaltic magma
  (settling out of crystals with less SiO2).
• Depending on starting comp., 60-90 of original
  basalt magma must crystallize to give andesite.
• The andesite should be intimately associated with
  the basalt, which it is not in the northern
  hemisphere.

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Compare to possible global ocean
Wyatt and McSween (2002, Nature) noticed. The
surface interpreted as andesite (red, Surface
Type 2) is concentrated in the northern
hemisphere in a large, low region previously
interpreted as an ancient ocean on Mars. The
white line outlines the location of the possible
shoreline. This poses a new question maybe
aqueous alteration?
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Redo deconvolution without high-silica glass
phase.

• Wyatt and McSween (2002, Nature)
• Without using any silica glass, they still
  produced a good fit for basalt for Surface Type 1
  
• Rms fits
• Bandfield et al. 0.0018
• Hamilton et al. 0.0026
• Wyatt and McSween 0.0018
• The low r.m.s. values and similar modeled
  mineral abundances derived from different end
  member sets indicate these modes accurately
  reflect the Surface Type 1 composition.

Surface Type 1
Wyatt, M.B. and McSween, H.Y. (2002) Spectral
evidence for weathered basalt as an alternative
to andesite in the northern lowlands of Mars,
Nature, 417, 263-266.
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Redo deconvolution without high-silica glass
phase.

• Wyatt and McSween (2002, Nature)
• For Surface Type 2, clays replaced high-silica
  glass in the modeled compositions.
• This can be interpreted as weathered basalt
  instead of andesite.
• Rms fits
• Bandfield et al. 0.0009
• Hamilton et al. 0.0023
• Wyatt and McSween 0.0014

Surface Type 2
Wyatt, M.B., McSween, H.Y. (2002) Spectral
evidence for weathered basalt as an alternative
to andesite in the northern lowlands of Mars,
Nature, 417, 263-266.
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Spectra of clays vs. high silica glass

• The main absorption feature to distinguish clays
  from high-silica glass is in 500-550 cm-1
  wavenumber range.
• But, CO2 atmosphere of Mars is opaque in this
  region.
• Otherwise they are similar in overall shape and
  positions of spectral features.
• Clays are Fe-smectite and Ca-montmorillonite.

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Compare Mars to Columbia River Basalt (CRB)

• Wyatt and McSween obtained spectra of fresh-cut
  and weathered surfaces of CRB.

Light colored fresh basalt Darker weathered
basalt
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Compare Mars to Columbia River Basalt (CRB)

• Added a blackbody component to account for
  band-depth particle-size effects. (TES sand
  sized particles)
• When deconvolved weathered CRB with Bandfield et
  al. and Hamilton et al. mineral sets, got high
  plag, pyroxene and silica glass (which is not
  present in CRB)
• Using clays, WMcS modeled plag alteration
  minerals lesser pyroxene (best agreement to
  actual composition)

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Implications of Wyatt and McSween modeled mineral
content

• Maybe there is high-SiO2 glass. But this could
  also represent an amorphous high-silica
  alteration product spectrally similar to high
  silica glass not Andesite.

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Implications of Wyatt and McSween modeled mineral
content

• Maybe there is high-SiO2 glass. But this could
  also represent an amorphous high-silica
  alteration product spectrally similar to high
  silica glass not Andesite.
• Or, maybe there are alteration products present
  (Clays)

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Implications of Wyatt and McSween modeled mineral
content

• Maybe there is high-SiO2 glass. But this could
  also represent an amorphous high-silica
  alteration product spectrally similar to high
  silica glass not Andesite.
• Or, maybe there are alteration products present
  (Clays)
• But, in any case --- the Martian northern lowland
  plains materials are basalts weathered under
  submarine conditions and/or sediments derived
  from weathered basalts and deposited in the
  northern basin.

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Infrared stealthy surfaces Why TES and THEMIS
may miss substantial mineral deposits on Mars
Kirkland et al., 2003

• Optically rough surfaces can remain undetected
  (including regional mineral deposits and rock
  outcrops at 100 exposure)
• Rough materials are called Infrared Stealthy
• Claimed TES mineral results are based on washed,
  pure minerals, large samples and smooth surfaces.
  

Kirkland L.E., Herr K.C., Adams P.M. (2003)
Infrared stealthy surfaces Why TES and THEMIS
may miss some substantial mineral deposits on
Mars and implications for remote sensing of
planetary surfaces, J. Geophys. Research, 108,
Dec 2003, pp., 11-1.
(Themis Thermal Emission Imaging System on Mars
2001 Odyssey Orbiter, also ancient Greek goddess
of justice)
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Spectral contrast reduction caused by texture

• Comparing spectral contrast variation of
• Calcite hand sample
• Calcrete hand sample (intensely lithified)
• Airborne spectrum of region covered with calcrete
  boulders
• Field spectrometer of same target region.
• Close-up of 11 ?m region
• So, texture can impact the spectral signature of
  all geological classes.

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Three cases of spectral misbehavior impact Mars
mineral interpretations

• Case 1 Minerals that are actually there are
  undetected.
• Case 2 Abundance variations and surface texture
  variations have same effect on spectrum.
  Increased smoothness looks same as increased
  abundance.
• Case 3 Physical effects actually alter spectral
  band shape.
• So, roughness at the grain scale can determine
  whether or not a mineral is detectable.

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Future work

• Gamma ray spectrometer on board Mars Odyssey
  spacecraft can measure actual elemental
  abundances (instead of controversial modal
  mineralogy)
• If type 1 and 2 differ appreciably in Si, then it
  is more likely that they are basalt and andesite.
• Mini-TES results from Spirit and Opportunity.
• 2008 mission to northern plains of Mars.
• Need more modeling to distinguish between clays
  and high-silica alteration coatings on basalt
  which are spectrally similar to andesite.
• Develop theoretical models describing how global
  weathering could occur on Mars which is
  consistent with the small amount of carbonates
  that are found.

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SNC Meteorites and TES

• Another problem with the TES spectra - does not
  find any area on Mars that matches the SNC
  meteorites - calling into question how good a job
  it is doing identifying the proper minerals
• But since they seem to have been ejected by just
  a few giant impacts, maybe they just happen to
  come from a few unusual areas that havent been
  imaged, or that do not cover a large enough area
  to be represented by TES. - Could also be from
  deeper in the crust. ALH 840001 is thought to
  be deep.

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Pathfinder and Andesite

• Did Pathfinder (1997) really find andesite at its
  landing site, i.e. that TES confirmed the
  finding of andesite on Mars?
• A design difference between flight APXS and lab
  APXS resulted in 14 greater alpha intensity in
  flight instrument.
• Preliminary results overestimated the light
  elements and underestimated the heavy elements.
• Also had inconsistencies in the reference library
  and mixed reference library with polished and
  rough surfaces.
• This calls into question the original
  interpretation that Barnacle Bill was
  andesitic.

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References

• Bandfield, J.L., Hamilton, V.E., Christensen P.R.
  (2000) A global view of Martian surface
  composition from NGS-TES. Science 287,
  1626-1630.
• Bandfield, J.L., Christensen P.R., Smith, M.D.
  (2000) Spectral data set factor analysis and
  end-member recovery Application to analysis of
  Martian atmospheric particulates. J. Geophys.
  Research, 104, 9573-9588
• Christensen P.R., Bandfield, J.L., Smith, M.D.,
  Hamilton V.E., Clark R.N.. (2000) Identification
  of a basaltic component on the Martian surface
  from thermal emission spectrometer data. J.
  Geophys. Research, 105, 9609-9621.
• Hamilton V.E., Wyatt M.B., McSween H.Y.,
  Christensen P.R. (2001) Analysis of terrestrial
  and Martian volcanic compositions using thermal
  emission spectroscopy 2. Application to Martian
  surface spectra from the Mars global surveyor
  thermal emission spectrometer. J. Geophys.
  Research, 106, 14,733-14,746..
• Kirkland L.E., Herr K.C., Adams P.M. (2003)
  Infrared stealthy surfaces Why TES and THEMIS
  may miss some substantial mineral deposits on
  Mars and implications for remote sensing of
  planetary surfaces, J. Geophys. Research, 108,
  Dec 2003, pp., 11-1.
• Minitti M.E., Rutherford M.J., Weitz C.M. (2001)
  Spectra of Martian andesitic materials, LPSC
  XXXII, Abstract 1976.
• Ruff S.W., (2003) Basaltic andesite or weathered
  basalt A new assessment, Sixth International
  Conference on Mars (abstract 3258)
• Smith, M.D., Bandfield, J.L., Christensen, P.R.
  (2000) Separation of atmospheric and surface
  spectral features in Mars Global Surveyor Thermal
  Emission Spectrometer (TES) spectra. J. Geophys.
  Research, 104, 9589-9607
• Wyatt, M.B., McSween, H.Y. (2001) An alternative
  hypothesis for basalt and andesite on mars
  Global surface compositions from MGS-TES. 64th
  Annual Meteoritical Society Meeting, Abstract
  5392.
• Wyatt, M.B., McSween, H.Y. (2002) Spectral
  evidence for weathered basalt as an alternative
  to andesite in the northern lowlands of Mars,
  Nature, 417, 263-266.

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What is a Michelson Interferometer?
D m?/2

• The Michelson interferometer produces
  interference fringes by splitting a beam of
  monochromatic light so that one beam strikes a
  fixed mirror and the other a movable mirror. When
  the reflected beams are brought back together, an
  interference pattern results.

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Why the dust spectrum does not contain surface
spectral characteristics

• Close similarity between derived dust spectrum
  and spectra acquired with high dust opacity
  and/or high emission angles.
• Good agreement between the derived atmospheric
  dust spectrum and TES limb spectra that view only
  the atmosphere
• Consistent surface spectra derived using the dust
  spectrum for atmospheric dust opacities that
  varied by a factor of 5-10
• Consistent atmospheric dust spectra in pairs of
  day/night observations in which the dust is
  alternatively viewed in transmission and emission.