PowerPoint Presentation by WV05FP

VIEWS: 0 PAGES: 37

									                                           Andesites on
                                             Mars?



                                             Dianne Taylor
                                              Winter, 2004
                                           “The Mars Debates”

Mars Science A Surveyor's Chronicles.htm
                                              Andesites on
                                                Mars?

                           Weathered basalt   I think not.

                                                Dianne Taylor
                                                 Winter, 2004
                                              “The Mars Debates”

Mars Science A Surveyor's Chronicles.htm
What is TES? Thermal Emission Spectroscopy
•    The TES Instrument has 3 parts:
    1.   Michelson Interferometer/spectrometer
    2.   Bolometric (broadband) thermal radiance (4.5 to ~100 µm) channel
    3.   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)
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.
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.
Atmospheric correction Models
              Two atmospheric correction
                 models:
                   1. Radiative transfer model
                   2. 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)
         Radiance and Emissivity
            70 F   • 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
Quartz                terms of radiance – Watts per
                      unit of area
                    • The radiance from a mineral
                      at one temperature will be
                      different from that at another
                      temperature
                     (From ASU Thermal Emission Spectroscopy
                     website)
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 <1 are wavelengths
                 where molecules absorb
                 energy
               • For quartz, SiO2 molecules
                 are responsible for absorption.
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.
       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
                  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..
       Two distinct surface types found on Mars




Type 1 - Similar to Basalt            Type 2 – Andesite?
( < 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.)
                         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 won’t attempt to fit water vapor features
   instead of surface mineralogy.
          MGS TES Basalt Map




Basalt (Type 1 spectra) concentrated in Southern Highlands
        MGS TES Andesite Map




Andesite (type 2 spectra) appears concentrated in Northern
Lowlands, but also intermixed with basalt in Southern Highlands.
Mars Andesite
Mars Basalt
Mars Hematite
      Ways to form Andesite (on Earth)
1. Converging plate margins (tectonics!): Water
   in descending oceanic crust promotes melting
   of mantle wedge above it
                       --- or ----
2. 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.
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?
      Redo deconvolution without high-silica glass phase.
           Surface Type 1                       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.”

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.
       Redo deconvolution without high-silica glass phase.
          Surface Type 2                         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



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.
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.
   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
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, W&McS modeled
                          plag + alteration minerals +
                          lesser pyroxene (best agreement
                          to actual composition)
 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.
 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)
 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.
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)
Spectral contrast reduction – caused by texture
                      Comparing spectral contrast variation
                         of:
                         1. Calcite hand sample
                         2. Calcrete hand sample
                             (intensely lithified)
                         3. Airborne spectrum of region
                             covered with calcrete
                             boulders
                         4. Field spectrometer of same
                             target region.
                      Close-up of 11 m region

                      So, texture can impact the spectral
                          signature of all geological classes.
 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.
                       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.
        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 haven’t 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.
         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.
                                              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.
 What is a Michelson Interferometer?
                                         D = mλ/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.
Why the dust spectrum does not contain
   surface spectral characteristics
1.   Close similarity between derived dust spectrum and
     spectra acquired with high dust opacity and/or high
     emission angles.
2.   Good agreement between the derived atmospheric dust
     spectrum and TES limb spectra that view only the
     atmosphere
3.   Consistent surface spectra derived using the dust
     spectrum for atmospheric dust opacities that varied by a
     factor of 5-10
4.   Consistent atmospheric dust spectra in pairs of day/night
     observations in which the dust is alternatively viewed in
     transmission and emission.

								
To top