Mars Odyssey Gamma-Ray Spectrometer

2001 Mars Odyssey GRSRDS 1

HEND Workshop 2002May 20th – 22nd 2002

Mars Odyssey Gamma-Ray Spectrometer

Richard StarrNASA/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• 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



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).



Ge vs. NaI



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.



GRS Coverage



Cruise Spectrum



Background Lines

8104

2

3

4

5678

105

2

3

4

5678

106

2

3

4

Cou

nts p

er C

hann

el

1400keV12001000800600400200Energy (keV)

#075mGe

76Ge(p,p'n)132.68 #1

46mSc136.00

#265Ga(spal)

~153.60

#371As

72Ge(p,2n)181.52

#467Ga

70Ge(p,a)190.39

#571mGe

72Ge(p,p'n)194.43

#6212Pb

235.75

#744mSc(spal)

269.56

#8214Pb

294.18

#967Ga

70Ge(p,a)308.91

#10228Ac

337.10

#11214Pb

350.56

#1243K43Sc

372.29

#1367Ga

70Ge(p,a)402.69

#1423Na

69mZn(n,ng);spal

438.87

#1523Mg

450.71

#1624mNa472.24

#177Be

(spal)477.17

#18annih

511.00

#1969Ge

70Ge(p,p'n)584.02

#2074Ge74As

(SAW)~598.00

#21214Bi

608.71

#2243K

617.39

#2663Cu

670.77

#2772Ge

72Ge(n,n')~693.00

#2810B

719.02

#2958Co(spal)811.24

#3058Co(spal)

817.97

#3172Ge

(SAW)~835.50

#3227Al(843.8)

27Mg(844.01)56Fe(846.7)

~843.60

#3369Ge

70Ge(p,p'n)882.65

#3446Ti46Sc

889.55

#35228Ac

910.98

#3763Cu

962.50

#38228Ac

969.26

#3925Mg

974.81

#4048Ti48V48Sc

(n,ng)983.72

#4227Al

1014.65

#4345Ar

1021.10

#4448Sc

1038.20

#4566Ga

70Ge(n,na)1048.95

#4668Ga

72Ge(p,na)1087.00

#4769Ge

1107.00

#4869Ge

70Ge(p,p'n)1117.70

#5065Zn

(Spal)1124.50

#5144Sc

(Spal)1157.00

#5260Co

1173.12

#5422Na22Ne

1274.44

#5548Ti48V48Sc

1312.29

#5660Co

1332.94

#5769Ge

1346.68

#5824Mg

Al24Na

(n,ng)1368.58

#5952mMn(Spal)

~1434.30

#6040K

1460.78

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.



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



Detector Configuration

Mars OdysseyGRS Detector



Line Shape and Trapping

100

1000

10000

1300 1310 1320 1330 1340 1350

Coaxial n-Type NC

Cou

nts

[1]

Energy [keV] MPC MainzLinie n-Typ Detektor

Inside: n-contact Outside: p-contact

Germanium crystal

Hole current



Radiation Damage and Detector Annealing



Comparison of Cruise to Mars Orbit



Orbital Spectrum – High Energy



Orbital Spectrum – Low Energy



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.



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.

Mars Odyssey Gamma-Ray Spectrometer

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