Martian water vapor: Mars Express PFSLW observations

1
Martian water vapor Mars Express PFS/LW
observations

• T. Fouchet, E. Lellouch, T. Encrenaz
• Observatoire de Paris
• F. Forget, F. Montmessin
• Institut Pierre Simon Laplace
• N.I. Ignatiev, D. Titov, M. Tschimmel
• Max Planck Institut for Solar System Research
• V. Maturilli, V. Formisano, M. Giuranna
• IFSI/CNR

2
Mars Express Objectives

• Study the inter-annual variations
• A steady state cycle?
• Study the water vapor as a function of
• Altitude ? vertical profile
• Latitude, longitude ? dynamical control of water
  vapor transport
• Local time ? Subsurface/atmosphere water exchange
• Contribute to better understand
• the surface-atmosphere exchanges
• the nature and variability of the water sources
  and sinks

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Planetary Fourier Spectrometer

• Characteristics
• channels LW (250-1700 cm-1) et SW (1700-8200
  cm-1)
• ?? 1.4 cm-1
• FOV 2.8 (LW) et 1.6 (SW) 7 et 12 km at z
  250 km
• Observations in nadir
• LW suited for water retrieval
• Large spectral coverage ? Simultaneous
  measurement of the temperature field (CO2
  inversion at 15 µm) and water (rotational lines)
• LW Spectra high S/N (up to 1200 cm-1), no
  calibration problem

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Water vapor retrieval

• Uniform continuum, few mineralogical bands
• Dust scattering negligible
• Large temperature contrast between the surface
  and the atmosphere 40K
• Uniform vertical profile up to the saturation
  altitude column density retrieval
• Random uncertainties 1-2 pr.-?m

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CO2-broadening

• Largest source of systematic uncertainty
• Water column proportional to the CO2-broadening
  coefficient
• Smith (2002, 2004) adopted a coefficient of 1.5 ?
  ?(N2)
• We used the full description of Gamache et al.
  (1995)
• 15 less water than with M. Smith assumption

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PFS-LW water cycle
MGS/TES (Smith, 2002)
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PFS-LW/TES Comparison (1)

• Inter-annual variations?
• Concomitant TES and PFS/LW observations up to 31
  august 2004.
• Averages over a grid of 2?2 (latitude ? Ls)
• TES retrieves systematically 1.5 times more water
  than PFS/LW
• Local time effect? (TES 1-3pm, PFS 8am -3pm)
• Ground-based observations give a column density
  smaller than TES (Sprague et al.)
• Small local time variations

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PFS-LW/TES Comparison (2)

• We instead suggest that the TES water columns are
  biased towards high values
• M. Smith reanalysis
• 6.25 cm-1 gives lower values than 12.5 cm-1 all
  lines give same water abundance
• _at_12.5 cm-1 , different lines give different
  abundance
• 250 and 280 cm-1 lines give 25 less better
  repeatability

6.25 cm-1
12.5 cm-1
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PFS-LW/TES Comparison (3)

• Add a 15 more downward revision for the
  CO2-broadening coefficient
• With both corrections, the MGS/TES and PFS/MEX
  database are in good agreement

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New CO2-broadening coefficient

• Increase the water vapor abundance by 10
  compared to Fouchet et al. (2007)
• Indistinguishable from the revised TES dataset

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Mid latitude spatial variations

• Water maxima over Arabia and Tharsis
• Observed by TES/MGS and all the other MeX
  instruments
• Related to the H2O subsurface content?

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Mid latitude spatial variations

• LMD/MGCM simulations
• Qualitatively reproduce the zonal distribution
• Quantitatively 20 for the model, factor of 2
  for observations
• Horizontal convergence forced by mid-latitude
  steady waves
• Vertical divergence
• Condensation, sedimentation, sublimation that
  locally enriched the atmosphere in water vapor
• A surface water may not be needed

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Mid latitude spatial variations

• LMD/MGCM simulations
• Qualitatively reproduce the zonal distribution
• Quantitatively 20 for the model, factor of 2
  for observations
• Horizontal convergence forced by mid-latitude
  steady waves
• Vertical divergence
• Condensation, sedimentation, sublimation that
  locally enriched the atmosphere in water vapor
• A surface water may not be needed

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Vertical Distribution (1)

• It was hoped that PFS could measure the vertical
  distribution by combining thermal and near IR
• Too low S/N in NIR
• Retrieve the water abundance on the volcanoes
  flank
• ISM/Phobos suggested a possible rise of the
  mixing ration with altitude.
• Possible subsurface-atmosphere exchange
• Possible outgassing

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Water vapor above the volcanoes
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Vertical Distribution (2)

• Correlation of water vapor column density with
  surface pressure
• Increasing water column with surface pressure
• But not a proportional relationship
• Need for a layer of 3-4 pr.-?m independently of
  the surface pressure

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Vertical Distribution (3)

• Anti-correlation between the normalized water
  column density and surface pressure
• Quantified with rank or Spearman correlation
• Water vertical distribution is non-uniform
• But no information on the real vertical profile

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Condensation Effect ?

• Water enrichment by water condensation,
  sedimentation and sublimation
• A layer with a large mixing ration below the
  cloud layer
• Extracted from the LMD/MGCM the predictions
  corresponding to PFS measurements
• Display the normalized column density as function
  of surface pressure
• Something missing to the model
• To be link with the low longitudinal variations

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Subsurface/atmosphere exchange?

• Not accounted for in the LMD/MGCM
• A study by Böttger et al. (2004)
• Adsorption/desorption of 10 of the water column
  in a diurnal cycle
• Water concentrated in the boundary layer
• Prediction of 1-2 pr.-?m rather than 3-4 as
  observed
• Local time variations with Mars Express
• Local time and season are extremely correlated

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Conclusions

• A drier water cycle
• PFS, SPICAM and OMEGA give water abundance lower
  than old MGS/TES
• Consistent with revised TES climatology
• High stability of the Martian water cycle
• PFS GCMs simulations
• A process missing to fully reproduce the
  observations
• A subsurface water source?
• Stronger enrichment due to cloud formation?
• Optimized the GCM for new conditions
  implications for O-chemistry and past climate
  simulations

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Water cycle

• Water vapor sublimes from polar caps
• Seasonal asymmetry in meridional transport

TES/MGS observations, Smith (2004)
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Temperature profile inversion
CO2
Information content
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Polar cap recession

• In winter, seasonal cap deposition (CO2 and H2O)
  at mid-latitudes
• In spring, as the CO2 frost recedes, water ice is
  left behind
• Hot air masses moving poleward carry recently
  sublimed water vapor
• Water ice once again forms at higher latitudes

OMEGA
Latitude