Some aspects ofthe Martian atmospheric variations seen from Mars Odyssey GRS C. d’Uston O. Gasnault

M-S3, Oct , 2010Mars atmosphere seasonal variations from MOd/GRS data; C. d'Uston & O. Gasnault Mars Odyssey GRS Events which are detected in the GRS are due to: Either  photons planet (from the planet, the instrument, the S/C, the Sun or the Galaxy) Many planetary  photons are produced by the interaction of cosmic rays with the materials of the surface. To reach the detector, all have to travel through the surface and the atmosphere Or energetic particles : p +, e - or n (also from the same various sources) Introduction : How to observe atmospheric variations from an orbiting GRS. Atmosphere (CO2,N,Ar)

M-S3, Oct , 2010Mars atmosphere seasonal variations from MOd/GRS data; C. d'Uston & O. Gasnault Mars Odyssey GRS 1 st factor of time variability : the incident cosmic radiation : It changes the fast neutron production and consequently all the neutron spectrum and the prompt -ray emissions of all elements with the exception of natural radioactive elements (K, Th, U) It also affects the production of radiogenic elements with delayed emissions It changes the high energy count-rate of the detector

M-S3, Oct , 2010Mars atmosphere seasonal variations from MOd/GRS data; C. d'Uston & O. Gasnault Mars Odyssey GRS 2 nd factor of time variability : the atmosphere the atmosphere : 1.Changes part of the neutron production, and consequently the  -ray emissions 2.Changes the transport from the solid surface to the orbit : Variations of the atmospheric thickness modify the attenuation : Count-rate = A x EXP(-  g ) + B With  attenuation coefficient for the atmosphere CO 2 And g the column density of the atmosphere

M-S3, Oct , 2010Mars atmosphere seasonal variations from MOd/GRS data; C. d'Uston & O. Gasnault Mars Odyssey GRS Then it is possible to monitor seasonal changes of the polar ice caps as a modification of the surface state by monitoring the H line intensity linked to the abundant underlying water content Fast, epithermal and thermal neutron count-rates of the atmosphere composition of some elements e.g. by analysing the Ar line intensity of the atmosphere thickness through K line temporal variations Continuum intensity temporal variations Monitoring of the Martian atmosphere with gamma-ray spectroscopy

6 Monitoring of the Martian atmosphere with gamma-ray spectroscopy Mars Odyssey has been monitoring the gamma- ray leakage from Mars between 2002 and The spectrum of these gamma-rays is made of a dominating continuum + lines (only 4%) : Primary objective: geochemical mapping of the surface using spectral lines: Maps of Fe, Cl, Si, H, K, Th, … JGR special issue (2008) Column density, spatial extent, and mass of the seasonal carbon dioxide frost on the poles of Mars as a function of time [Prettyman et al., 2004; Kelly et al., 2007, Litvak et al., 2005] in agreement with the General Circulation Models Factor of 6 enhancement of Ar measured over south polar latitudes (no similar strong enhancement over north-polar regions) [Sprague et al., 2004, 2007]

M-S3, Oct , 2010Mars atmosphere seasonal variations from MOd/GRS data; C. d'Uston & O. Gasnault Mars Odyssey GRS In a given region outside the polar cap areas, its intensity should be constant unless the transport properties through atmosphere change. S/C & instrument parasitic contribution may be estimated from pure CO2 covered South pole winter spectrum K line emission doesn’t depend on GCR flux. Concerning the atmosphere thickness variations

M-S3, Oct , 2010Mars atmosphere seasonal variations from MOd/GRS data; C. d'Uston & O. Gasnault Mars Odyssey GRS K line count-rate Transmission coefficient K line count rate after atmospheric correction (Forget’s MGCM) With  = cm2/g

M-S3, Oct , 2010Mars atmosphere seasonal variations from MOd/GRS data; C. d'Uston & O. Gasnault 9 Considering the findings The goal is try to monitor the atmospheric variations in different regions of Mars by measuring a significant parameter every season. For this, the K line intensity measured every season in well defined regions doesn’t achieve the necessary precision because of the low statistics. Then the idea is to look for a different criterion which achieves high enough statistical significance :  Analyse specific part of the continuum

M-S3, Oct , 2010Mars atmosphere seasonal variations from MOd/GRS data; C. d'Uston & O. Gasnault Mars Odyssey GRS Count-rates in wide energy bands are much larger than in the K line. Choice of 4 bands with no significant line : 0.2 – 0.3 MeV 1.83 – 1.93 MeV 2.8 – 3.5 MeV 7.8 – 9 MeV

M-S3, Oct , 2010Mars atmosphere seasonal variations from MOd/GRS data; C. d'Uston & O. Gasnault 11 Measurements for various bands in latitude South North

M-S3, Oct , 2010Mars atmosphere seasonal variations from MOd/GRS data; C. d'Uston & O. Gasnault 12 HELLAS ARGYRE LOWLANDS HIGHLANDS VOLCANOS 1° = 60 km

M-S3, Oct , 2010Mars atmosphere seasonal variations from MOd/GRS data; C. d'Uston & O. Gasnault Mars Odyssey GRS

M-S3, Oct , 2010Mars atmosphere seasonal variations from MOd/GRS data; C. d'Uston & O. Gasnault 14 Hellas as a function of time 45% variation due to comic-rays 6% fluctuation due to atmospheric breathing  Low-energy continuum (CR?) increase is fairly monotonous

M-S3, Oct , 2010Mars atmosphere seasonal variations from MOd/GRS data; C. d'Uston & O. Gasnault 15 To outline the seasonal variations, data were first normalized to a reference region (belly_band).

M-S3, Oct , 2010Mars atmosphere seasonal variations from MOd/GRS data; C. d'Uston & O. Gasnault 16

M-S3, Oct , 2010Mars atmosphere seasonal variations from MOd/GRS data; C. d'Uston & O. Gasnault 17 Relative variations in the South

M-S3, Oct , 2010Mars atmosphere seasonal variations from MOd/GRS data; C. d'Uston & O. Gasnault 18  Continuum anti correlated to surface pressure Hellas normalized to Belly Band – 1 year

M-S3, Oct , 2010Mars atmosphere seasonal variations from MOd/GRS data; C. d'Uston & O. Gasnault 19 Hellas normalized to Belly Band – all years  Features repeat year after year quite well

M-S3, Oct , 2010Mars atmosphere seasonal variations from MOd/GRS data; C. d'Uston & O. Gasnault 20 Saturation effects?

M-S3, Oct , 2010Mars atmosphere seasonal variations from MOd/GRS data; C. d'Uston & O. Gasnault 21 Argyre and Lowland show behavior similar to Hellas In the south In the north

M-S3, Oct , 2010Mars atmosphere seasonal variations from MOd/GRS data; C. d'Uston & O. Gasnault 22 Does not work as well in the highlands… Remark : Highlands located both in south, and north

M-S3, Oct , 2010Mars atmosphere seasonal variations from MOd/GRS data; C. d'Uston & O. Gasnault 23 HELLAS ARGYRE LOWLANDS HIGHLANDS VOLCANOS 1° = 60 km

M-S3, Oct , 2010Mars atmosphere seasonal variations from MOd/GRS data; C. d'Uston & O. Gasnault 24 Seasonal Breathing The variations of gamma-ray fluxes are very small but still significant. There is a fair correlation with General Circulation Models. Relative variations in the northern lowlands are opposite to those in the southern highlands. In first approximation, the thicker is the atmosphere, the more variable the GRS count-rate is. We might also see some longitudinal or regional variations.

M-S3, Oct , 2010Mars atmosphere seasonal variations from MOd/GRS data; C. d'Uston & O. Gasnault 25 Conclusion Gamma-ray continua are not as rich in information as the lines, which give absolute abundances of individual chemical elements present in the soil. However the gamma-ray spectra are dominated (at low energy) by the continuum that comes from the planet surface. On Mars, we use this data to infer atmospheric column density variations with space and time. Gamma-ray photons have a low probability to be absorbed by such a thin atmosphere and therefore the observed variations are small, but still significant because the counting rate is high. On going effort: Day vs. night Include epochs 2 & 3 Mapping Time/frequency analysis

M-S3, Oct , 2010Mars atmosphere seasonal variations from MOd/GRS data; C. d'Uston & O. Gasnault 26

M-S3, Oct , 2010Mars atmosphere seasonal variations from MOd/GRS data; C. d'Uston & O. Gasnault 27

M-S3, Oct , 2010Mars atmosphere seasonal variations from MOd/GRS data; C. d'Uston & O. Gasnault 28

M-S3, Oct , 2010Mars atmosphere seasonal variations from MOd/GRS data; C. d'Uston & O. Gasnault 29

M-S3, Oct , 2010Mars atmosphere seasonal variations from MOd/GRS data; C. d'Uston & O. Gasnault 30 We are also trying various bands…

M-S3, Oct , 2010Mars atmosphere seasonal variations from MOd/GRS data; C. d'Uston & O. Gasnault Mars Odyssey GRS Main Components of the GRS continuum

M-S3, Oct , 2010Mars atmosphere seasonal variations from MOd/GRS data; C. d'Uston & O. Gasnault 32 Atmospheric study Gamma-derived spectra (until June 2009), channels 300 to 480 ( keV) Compare variations in the continuum to the General Circulation Model lowlands highlands Hellas Argyre belly band

M-S3, Oct , 2010Mars atmosphere seasonal variations from MOd/GRS data; C. d'Uston & O. Gasnault 33

M-S3, Oct , 2010Mars atmosphere seasonal variations from MOd/GRS data; C. d'Uston & O. Gasnault 34

M-S3, Oct , 2010Mars atmosphere seasonal variations from MOd/GRS data; C. d'Uston & O. Gasnault 35

M-S3, Oct , 2010Mars atmosphere seasonal variations from MOd/GRS data; C. d'Uston & O. Gasnault 36

M-S3, Oct , 2010Mars atmosphere seasonal variations from MOd/GRS data; C. d'Uston & O. Gasnault 37

M-S3, Oct , 2010Mars atmosphere seasonal variations from MOd/GRS data; C. d'Uston & O. Gasnault 38

M-S3, Oct , 2010Mars atmosphere seasonal variations from MOd/GRS data; C. d'Uston & O. Gasnault 39

M-S3, Oct , 2010Mars atmosphere seasonal variations from MOd/GRS data; C. d'Uston & O. Gasnault 40 MCNPX MPF compositio n with some REE (0.6ppm Th and chondritic ratios) and 3% H2O GCR spectrum arbitrary intensity Atmospheric composition and profile Neutron flux distribution in the surface as a function of energy and depth as a function of atmospheric thickness

M-S3, Oct , 2010Mars atmosphere seasonal variations from MOd/GRS data; C. d'Uston & O. Gasnault 41 Integration over depth, and transport to surface Gamma production cross sections Neutron flux distribution F(E, depth, thickness) Mass attenuation coefficients Surface fluxes of gamma lines at various emission angles for various atmospheric thicknesses K-tables computed only once K, Th, and U emission at the surface are independent of the neutrons & atmosphere

M-S3, Oct , 2010Mars atmosphere seasonal variations from MOd/GRS data; C. d'Uston & O. Gasnault 42 Integration over Martian surface within a circle of 17° arc radius 99% of the signal Measurement conditions in each 0.5°x0.5° bin (S/C orbital position, bin area) Surface fluxes of gamma lines at various emission angles for various atmospheric thicknesses GS detection efficiency matrix Mass attenuation coefficients Atmospheric GCM MOLA elevation model Computed counts for the various gamma-ray lines (Si, Fe, H, Cl…) for each s spectrum CELL-tables function of local time