JeanLoup Bertaux and Franck Montmessin Service dAronomie du CNRS

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Jean-Loup Bertaux and Franck MontmessinService
dAéronomie du CNRS

• D/H RATIO ON MARS AND ITS IMPLICATIONS FOR RECENT
  WATER RESERVOIR EVOLUTION

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HDO on Mars

• D/H ratio is a key tracer for water history
• Ground-based NIR measurements
• Owen et al. (1980) (D/H)Mars (6?3)?SMOW
• Krasnopolsky et al. (1997) (D/H)Mars(5.5?2)?SMO
  W
• SMOW 1 for water in oceans HDO/H2O3.2 10-4

Simple minded approach assuming NO escape of D
atoms Assuming SMOW1 for early Mars and 20 m
equivalent water ice now on Mars (reservoirs
Perennial polar caps, polar layered
terrains) Early Mars had 120 m equivalent water
(Earth, 2.8 km)
but exchangeable reservoir ! (time scale ?)
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Fractionation Factor F

• Escape fluxes to space of D and H atoms
• Concentration just above ground

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Species and mechanisms

• Sublimation of HDO and H2O? atmosphere
• Photo-dissociation of HDO and H2O, formation of
  HD and H2, peak at gt 25 km of altitude
• upward diffusion and formation of H and D atoms
  at 80-100 km altitude
• Escape to space of H and D atoms.

• CH4 and CH3D ? No! (no cows on Mars)

Mechanisms of fractionation (change of D/H
ratio) Escape from Mars thermal escape,
non-thermal escape Photolysis of H2O and HDO
(PHIFE) Condensation/sublimation (HDO, H2O)
(VPIE, or CEFE
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EscapeThe exosphere (collisionless)
Trajectories of Hydrogen atoms in the exosphere
Hyperbolicescape, if Vgt Vesc
ballistic
Thermalized atmosphere
exobase (or critical level, 200 km)
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Non thermal escape of H

• Main productive reactions for H in the upper
  atmosphere (photo-chemical model, Krasnopolsky,
  2002)
• CO2 H2 ---- gt HCO2 H (1.05 108
  / cm2 s)
• HCO e ---- gt CO H (1.17 108 / cm2 s)

If the reaction occurs below the exobase ---- gt
thermalization of H If the reaction occurs above
the exobase ----gt excess energy of H (0.1-1
ev) Escape of H atom on hyperbolic
trajectory Other non-thermal scheme possible
(Leblanc and Chassefière)
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Lyman-? picture of Mars (HST, John Clarke)
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H escape from Mars

• From the analysis of SPICAM/MEX Lyman-? data
  (Chaufray et al, 2008)
• Cold population FJH0.5 108 atoms/cm2 s 0.1 m
  H2O/Gyr)
• Non Thermal, Hot population
• FH 2.5 108 atoms/cm2 s 0.4 m H2O/Gyr)

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The PHIFE effectPhoto-ionization Fractionation
Effect
Cheng et al., 1999
HDO is less dissociated than H2O factor3 ? will
change HD/H2
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Vapour Pressure Isotope Effect (CEFE)

• At thermodynamic equilibrium
  ps(H2O) gt ps(HDO)
• D/H ratio in ice higher than D/H in vapour
• Fractionation coefficient

At T190 K
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1D Mars Models

• Two 1D, steady-state studies (Fouchet Lellouch
  2000, Bertaux Montmessin 2001)
• Above condensation level D/H 0 and 80 of
  global D/H ratio
• Earth Stratospheric, polar D/H ? 50 of
  SMOW
• Paucity of D at high altitudes (low F)

HDO depletion rate vs. height
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Fractionation Factor F

• Escape fluxes to space of D and H atoms
• Concentration just above ground

Various estimates F0.32 Kass and Yung (1988)
F0.02 Krasnopolsky et al. 1998 (from D atoms HST
(Lyman-?)
F0.11 Krasnopolsky and Feldman, from H2 UV
emission (FUSE)
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The steady state solution
The time to reach the steady state depends on the
size of the atmospheric reservoir 105 year,lt
obliquity cycle period
?H?1.5 108 at/cm2 s Constant (10,000 yr)
Fractionnation Factor F (or R)
?Dk HDO0
Atmosphere 10 µm.pr
North Polar Cap ice
A(t0) Abulk
Abulk HDO/H2O
Ass(steady state) Abulk / F
The steady state ratio is independant of the size
of atmospheric reservoir
F0.11 (Feldman and Krasnopolsky, 2001) Ass ? 9
Abulk compatible with Abulk1.8 SMOW, Ass5 SMOW
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The time to reach the steady state depends on the
size of the atmospheric reservoir 105 year,lt
obliquity cycle period
?H?1.5 108 at/cm2 s constant
?Dk HDO0
Atmosphere 10 µm.pr
North Polar Cap ice
A(t0) Abulk
Abulk HDO/H2O
Ass(steady state) Abulk / F
CAVEAT The atmospheric reservoir is probably
recycled many times over aeons.
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Milkovich and Head, 2005
Fischer (2007)
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GCM Water Cycle with HDO

• LMD/GCM (Montmessin,Fouchet , Forget,2005)
• Cloud scheme
• Condensation, sedimentation
• Particle size predicted
• No regolith
• HDO source North polar cap
• HDO/H2O 1.7 10-3 ? D/H 5.6 x SMOW
• Five years of simulation

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H2O and HDO Maps
SOL
SOL
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HDO/H2O (wrt. SMOW)
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Vertical Variations of D/H
Ls 90

• Vertical confinement of HDO vapour
• Reduce the D-escape
• High D/H in the downwelling branch
• Trace ice transport
• Trace the dynamics

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Observations (Villanueva et al., 2007)

• NIRSPEC measurements

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• no need for venting of depleted HDO reservoir !

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Conclusions Perspectives

• HDO measurements show variations
• First 3D simulations of the HDO cycle GCM show
  moderate seasonal variations
• Strong D-depletion above the seasonal caps both
  in data and GCM (model has 5 SMOW in ice cap) no
  need for venting of depleted HDO reservoir
• Strong D-depletion above the hygropause
• D/H probes the dynamics (not yet measured)
• What is the HDO/H2O ratio in perennial caps?

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Conclusions Perspectives

• First 3D simulations of the HDO cycle
• Moderate seasonal variations
• Strong D-depletion above the seasonal caps
• Strong D-depletion above the hygropause
• D/H probes the dynamics
• Impacts on the D escape rate
• Magnitude of the planetary water loss

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Modelling the HDO Cycle in the Martian Atmosphere

• Montmessin F. (NASA/ARC) Fouchet T.
  (LESIA/OBSPM) Forget F. (LMD/IPSL)

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HDO on Mars

• D/H ratio is a key tracer for water history
• Ground-based NIR measurements
• Owen et al. (1980) (D/H)Mars (6?3)?SMOW
• Krasnopolsky et al. (1997) (D/H)Mars(5.5?2)?SMO
  W
• Differential escape of D and H (F0.02)
• Detail understanding of D-chemistry and
  D-transport phenomena

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Milkovich and Head, 2005
..these two factors favor accumulation and
storage of volatiles (water, dust) in the North
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Why bother to study D/H ratio?

• Long term evolution How much water at
  formation?
• Determine the  age  of ice?
• Reconstruct the climate of the past ? (mean
  temperature of atmosphere, core ice drilling
  inAntarctica and Groenland)

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D/H Seasonal Variations

• Small seasonal variations ?10
• Smaller than measurements error bars
• Global atmospheric D/H smaller than in the polar
  cap
• 5.3?SMOW vs. 5.6?SMOW prescribed in the cap
• Isotopic filtration above the summer cap

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HDO/H2O observations
-the smaller H2O µm.pr, the richer in HDO/H2O
Opposite to expectation! (preferential
condensation of HDO) or
Complete sublimation of previoulsy enriched
condensate
SMOW
-the larger H2O µm.pr, the poorer in HDO/H2O
Extrapolation to 65 gives SMOW1! (no
enrichment in polar cap?)
Mumma et al. (2003)