Mars Odyssey Gamma-Ray Spectrometer

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Mars Odyssey Gamma-Ray Spectrometer
Richard Starr NASA/GSFC Catholic University and
the GRS team
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Mars Odyssey GRS Timeline

• 2001 April 07 Launch
• 2001 June 7 day warm anneal (42 C)
• 2001 June 27 Begin cruise data collection
• 2001 August 30 End cruise data collection
• 2001 October 23 Mars orbit insertion
• 2002 February 09 Begin mapping phase
• 2002 March 10 day warm anneal (52 C)
• 2002 March 26 Resume mapping
• 2002 May 10 day hot anneal (73 C)
• 2002 May 21 Resume mapping
• 2002 June 04 Boom deployment

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Gamma-Ray Spectrometer
The Mars Odyssey gamma-ray spectrometer is a 67
mm diameter 67 mm long, high-purity, n-type Ge
crystal that is encapsulated in a sealed titanium
canister. The detector is passively cooled to
cryogenic temperatures (lt130 K).
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Ge vs. NaI
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GRS Accumulation Times
The gamma-ray signal comes from the upper 20 to
30 cm of soil. Thermal and epithermal neutrons
are sensitive to composition about a factor of 2
or 3 deeper than gamma rays.
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GRS Coverage
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Cruise Spectrum
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Background Lines
Over 100 background lines have been identified.
The intensity of many will be reduced after boom
deployment. Others, resulting from detector
materials like Ge and Ti, will not be affected.
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Solar Proton Events During MO Cruise
   Event-Integrated Fluences for Solar Particle
Events since 7 April 2001     (Fluences, F, are
omnidirectional - 4-pi - protons/cm2)       
Date      Fgt10 MeV    Fgt30 MeV    Fgt60 MeV      
4/11/01     2.4E8      3.3E7      6.0E6      
4/15/01     4.5E8      1.5E8      7.0E7      
4/18/01     1.7E8      4.8E7      1.8E7      
5/08/01     2.5E7      1.3E6      2.5E5      
5/20/01     5.0E6      1.8E6      8.0E5      
6/15/01     1.9E7      1.7E6      5.0E5      
8/16/01     2.8E8      9.8E7      3.1E7      
9/25/01     7.4E9      1.2E8      1.9E8     
10/02/01     9.8E8      6.5E7      3.6E6     
10/19/01     1.2E7      2.2E6      4.0E5     
10/22/01     1.4E7      4.5E6      1.5E6     
11/05/01     1.5E10     3.0E9      6.0E8     
11/23/01     8.1E9      8.0E8      7.0E7     
12/16/01     3.6E8      9.0E7      2.4E7     
12/31/01     2.7E8      1.5E7      9.0E5      
1/11/02     1.4E8      6.0E6      3.0E5
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Detector Configuration
Mars Odyssey GRS Detector
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Line Shape and Trapping
Hole current
Inside n-contact
Outside p-contact
Germanium crystal
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Radiation Damage and Detector Annealing
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Comparison of Cruise to Mars Orbit
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Orbital Spectrum High Energy
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Orbital Spectrum Low Energy
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Why do we believe its H20?

• Hydrogen can combine with many elements, such as
  sulfur to form H2S, or metals to form hydrides,
  but these compounds are not likely to be stable
  given the highly oxidizing conditions on Mars.
• Many theoretical studies have predicted the
  regions where water ice should be
  thermodynamically stable on Mars.
• Farmer and Doms (1979) conclude that ground ice
  should be stable in the regolith where
  temperatures never exceed 200 K.
• 10 cm depth at 80 latitude
• 100 cm depth at 50 latitude
• Mellon and Jakosky (1993) model water ice
  stability at various depths below the surface
  versus latitude.

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Summary

• The Mars Odyssey gamma-ray and neutron
  spectrometers have identified a significant water
  ice component south of -60 latitude.
• The ice is not uniformly distributed within the
  soil but is buried under an ice-poor layer.
• North of 60 latitude there is a thick seasonal
  CO2 cap that is opaque to gamma rays.
• We are detecting many gamma-ray lines from
  elements on the surface of Mars, in addition to
  H, that are of geochemical significance Th, U,
  K, O, Si, Mg, Cl, Fe
• Over the life of the mission (gt2 years) many of
  these elements will be mapped with a spatial
  resolution of order a few hundred kilometers.