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Mars Odyssey Gamma-Ray Spectrometer

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

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Mars Odyssey Gamma-Ray Spectrometer

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

  2. 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

  3. 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 (<130 K).

  4. Ge vs. NaI

  5. 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.

  6. GRS Coverage

  7. Cruise Spectrum

  8. 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.

  9. 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      F>10 MeV    F>30 MeV    F>60 MeV       4/11/01     2.4E+8      3.3E+7      6.0E+6       4/15/01     4.5E+8      1.5E+8      7.0E+7       4/18/01     1.7E+8      4.8E+7      1.8E+7       5/08/01     2.5E+7      1.3E+6      2.5E+5       5/20/01     5.0E+6      1.8E+6      8.0E+5       6/15/01     1.9E+7      1.7E+6      5.0E+5       8/16/01     2.8E+8      9.8E+7      3.1E+7       9/25/01     7.4E+9      1.2E+8      1.9E+8      10/02/01     9.8E+8      6.5E+7      3.6E+6      10/19/01     1.2E+7      2.2E+6      4.0E+5      10/22/01     1.4E+7      4.5E+6      1.5E+6      11/05/01     1.5E+10     3.0E+9      6.0E+8      11/23/01     8.1E+9      8.0E+8      7.0E+7      12/16/01     3.6E+8      9.0E+7      2.4E+7      12/31/01     2.7E+8      1.5E+7      9.0E+5       1/11/02     1.4E+8      6.0E+6      3.0E+5

  10. Detector Configuration Mars Odyssey GRS Detector

  11. Line Shape and Trapping Hole current Inside: n-contact Outside: p-contact Germanium crystal

  12. Radiation Damage and Detector Annealing

  13. Comparison of Cruise to Mars Orbit

  14. Orbital Spectrum – High Energy

  15. Orbital Spectrum – Low Energy

  16. Why do we believe it’s 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.

  17. 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 (>2 years) many of these elements will be mapped with a spatial resolution of order a few hundred kilometers.

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