2001 Mars Odyssey page 1 W o r k s h o p H E N D Institute for Space Research, June , 2003 Model-dependent deconvolution of HEND neutron data by MCNPX code M. Litvak, I. Mitrofanov, Charyshnikov, V. Grinkov, A. Kozyrev, A. Sanin V. Tret’yakov Institute for Space Research D. Drake Santa Fe, USA

2001 Mars Odyssey page 2 W o r k s h o p H E N D Institute for Space Research, June , 2003 MODEL Atmosphere CO 2 deposit Dry regolith Wet regolith

2001 Mars Odyssey page 3 W o r k s h o p H E N D Institute for Space Research, June , 2003 MODEL PARAMETERS  Flux of CGR  Atmosphere thickness at different times  Chemical composition of atmosphere  Thickness of CO 2 frost  Presence of water (%) in CO 2 frost  Thickness of dry regolith  Chemical composition of regolith  Presence of water (%) in dry regolith  Presence of water (%) in wet regolith

2001 Mars Odyssey page 4 W o r k s h o p H E N D Institute for Space Research, June , 2003 “Chemical analyses of Martian soil and rocks obtained by the Pathfinder Alpha Proton X-ray spectrometer” Radiation Physics and Chemistry 61 (2001)191 –197 MODEL PARAMETERS Martian Soil

2001 Mars Odyssey page 5 W o r k s h o p H E N D Institute for Space Research, June , 2003 MODEL PARAMETERS Martian Atmosphere: Chemical composition and thickness Major gases: Carbon Dioxide (CO 2 ) % ; Nitrogen (N 2 ) - 2.7% Argon (Ar) - 1.6%; Oxygen (O 2 ) %; Carbon Monoxide (CO) % Thickness of atmosphere for given Mars region and season is taken from the Ames global climate model

2001 Mars Odyssey page 6 W o r k s h o p H E N D Institute for Space Research, June , 2003 HOW TO CALCULATE: GEOMETRY The layer produced the neutrons

2001 Mars Odyssey page 7 W o r k s h o p H E N D Institute for Space Research, June , 2003 HOW TO CALCULATE: DATA SELECTION  The Surface of Mars may be divided into the set of regions of interest.  The properties of surface should be homogenous inside selected region  The size of each region should be more than footprint.

2001 Mars Odyssey page 8 W o r k s h o p H E N D Institute for Space Research, June , Calculation of neutron flux for model of homogeneous sphere with some initial parameters 2. Convolution of model spectra with efficiency functions for different detectors to get model counts 3. Comparison of model count rate and real count rate of HEND sensors for this Martian area of interest and for the same seasons 4. Fitting of parameters of geometry, water percentage in model of homogeneous sphere h dry %H 2 0 dry %H 2 O wet h CO2 HOW TO CALCULATE: ALGORITHM

2001 Mars Odyssey page 9 W o r k s h o p H E N D Institute for Space Research, June , 2003 HOW TO CALCULATE: MINIMIZATION Search of model best fit parameters based on  2 minimization: Where:  C i is detector counts, i=1,5 (SD,MD,LD,SC1,SC2).  M i is model counts predicted by MCNPX for (SD,MD,LD,SC1,SC2).   i is statistical error for C i, M i SD, MD, LD are integral counts detected in proportional counters SC 1 is fast neutron counts in energy range ( MeV) registered in stilben SC 2 is fast neutron counts in energy range >2.5 MeV registered in stilben

2001 Mars Odyssey page 10 W o r k s h o p H E N D Institute for Space Research, June , 2003 BEST FIT PARAMETERS: CGR flux  The selection of HEND data accumulated above large region near Solis Planum. It is driest place on Mars where variation of neutron flux explained only by variations of CGR flux or by Solar activity.  The selection of different time intervals: Ls={ [330 o- 360 o ], [0 o - 30 o ],[30 o -60 o ], [60 o -90 o ], [90 o -120 o ], [120 o -150 o ]  Applying of the homogenous model to estimate flux of CGR: {Atmosphere +homogenous regolith layer with 2% of water}

2001 Mars Odyssey page 11 W o r k s h o p H E N D Institute for Space Research, June , 2003 BEST FIT PARAMETERS: FROST FREE AREAS  The selection of data for summer surface of Mars (season when CO 2 frost is absent)  First step:Applying of the homogeneous regolith model with one free parameter describing content of water in the regolith.  Second step: If homogeneous model was rejected two layers regolith model should be tested with 2% of water in upper layer and two free parameters: thickness of upper layer and water content in bottom layer.

2001 Mars Odyssey page 12 W o r k s h o p H E N D Institute for Space Research, June , 2003  The best fitting parameters describing summer surface are fixed for given region.  The data for given region are taken for different winter periods of time.  The real data are fitted by model of summer surface with added layer of CO 2 frost with variable thickness. BEST FIT PARAMETERS: CO 2 FROST THICKNESS