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Planetary and Space Science 55 (2007) 333–342

www.elsevier.com/locate/pss

Water vapor mapping on Mars using OMEGA/Mars Express

R. Melchiorria, T. Encrenaza,�, T. Foucheta, P. Drossarta, E. Lelloucha, B. Gondetb,J.-P. Bibringb, Y. Langevinb, B. Schmittc, D. Titovd, N. Ignatieve

aLESIA, Observatoire de Paris, FrancebIAS, Orsay, France

cLPG, Grenoble, FrancedMPI, Lindau, GermanyeIKI, Moscow, Russia

Accepted 30 May 2006

Available online 1 September 2006

Abstract

A systematic mapping of water vapor on Mars has been achieved using the imaging spectrometer OMEGA aboard the Mars Express

spacecraft, using the depth of the 2.6 mm (n1, n3) band of H2O. We report results obtained during two periods: (1) Ls ¼ 330–401

(January–June 2004), before and after the equinox, and (2) Ls ¼ 90–1251, which correspond to early northern summer. At low latitude,

our results are globally consistent with previous measurements from ground-based and space (MAWD/Viking and TES/MGS)

observations. However, at early northern summer and at high northern latitude (70–80 1N), the water vapor abundances, which we

retrieved, appear to be weaker than MAWD and TES results. At the time of water sublimation during early northern summer, there is a

maximum of water vapor content at latitudes 75–801N and longitudes 210–241E. This region is not far from the area where OMEGA

identified a high abundance of calcium-rich sulfates, most likely gypsum. Our data provide the first high-resolution map of the martian

water vapor content above the northern polar cap.

r 2006 Elsevier Ltd. All rights reserved.

Keywords: Mars; Mars atmosphere; Infrared spectroscopy

1. Introduction

The water vapor cycle on Mars is known to be verycomplex, and is a precious indicator of the planet’s climaticvariations. It has been regularly monitored over severaldecades, using ground-based observations and spaceexperiments, in order to better understand the seasonaland interannual variations of water vapor on Mars.Basically, two types of spectroscopic data have been used:the near-infrared H2O bands in the reflected part of thespectrum, and the rotational water lines, beyond 20 mm, inthe thermal infrared. The first global thermal mapping ofH2O was performed by the IRIS interferometer aboardMariner 9 (Conrath et al., 1973). It was followed by thevery complete survey achieved at 1.38 mm by the MAWD

e front matter r 2006 Elsevier Ltd. All rights reserved.

s.2006.05.040

ing author. Tel.: +331 45 07 76 91; fax: +33 1 45 07 28 06.

ess: therese.encrenaz@obspm.fr (T. Encrenaz).

(Mars Atmosphere Water Detection) experiment aboardthe Viking orbiters; these results were used for decades as areference (Farmer et al., 1977; Jakosky and Farmer, 1982;Fedorova et al., 2004). In parallel, ground-based monitor-ing of Martian water vapor was achieved, taking advantageof the Earth–Mars water line Doppler shift or usingisotopic HDO lines. Observations were performed in thenear infrared range (Sprague et al., 1996, 2003), in themillimeter range (Encrenaz et al., 1991, 1995, 2001) and inthe radio range (Clancy et al., 1992, 1996). In addition, thewater vapor abundance was also derived from the H2Obands at 2.6 mm (n1, n3) and 6.2 mm (n2) using the Short-Wavelength Spectrograph (SWS) instrument aboard theInfrared Space Observatory (ISO) Earth-orbiting satellite(Lellouch et al., 2000). Local measurements were obtainedat the same time (June–August 1997) by the MarsPathfinder lander (Titov et al., 1999). More recently, theTES instrument aboard the Mars Global Surveyor missionhas achieved a complete coverage of the water vapor

ARTICLE IN PRESSR. Melchiorri et al. / Planetary and Space Science 55 (2007) 333–342334

column density, using the rotational H2O transitions at20–35 mm (Smith, 2002, 2004).

All these data sets have shown evidence for large spatialand temporal variations of the Martian water vaporabundance, with a pronounced maximum, as high as80 pr-mm, at high northern latitudes at the time of earlynorthern summer (Ls ¼ 90–1201), while minima in thesouthern hemisphere at the same time are as low as 1–3pr-mm. There is a noticeable asymmetry between thenorthern and southern hemispheres, as the maximum athigh southern latitudes, at the time of early southernsummer, is only about 30 pr-mm. This asymmetry has beenexplained by the eccentricity of Mars which is such thatnorthern summer takes place near aphelion, leading to atemperature difference of about 20K between maximumtemperatures at high latitudes during northern and south-ern summers (Clancy et al., 1996). Global climatic models(Forget et al., 1999; Montmessin et al., 2004) are able toaccount for the general behavior of the seasonal water cycleas a result of seasonal cycle of Mars, due to the periodiccondensation of CO2 and H2O at the polar caps. However,the amplitude of the asymmetry is not fully reproduced,and the exact abundance of the water maximum at earlynorthern summer is not well constrained by climaticmodels (Forget et al., 1999).

The OMEGA imaging spectrometer aboard the MarsExpress spacecraft offers a new opportunity to monitor thewater vapor abundance on Mars, through the analysis ofthe (n1, n3) band at 2.6 mm. This band, in the wing of thestrong (n1+n3) CO2 band, centered at 2.75 mm, is located inthe reflected solar part of the Martian spectrum. Thus, ithas the advantage of being, to first order, independent ofthe temperature profile. In contrast, the analysis of spectralsignatures at higher wavelengths (n2 at 6.2 mm, rotationallines beyond 20 mm), formed in the thermal regime, requiresa good knowledge of the thermal profile. We note,however, that the use of the reflected solar componentleads to some uncertainty due to scattering by martianaerosols; this effect is discussed in more detail in Section 3.1.Another advantage of the reflected sunlight analysis is thatit allows to retrieve the water vapor content everywhere onthe Martian disk, even over the polar caps where thesurface temperature is too low for the thermal flux to bedetectable.

In this paper, we present the water vapor mappingobtained during the first year (2004) of the Mars Expressoperation. Section 2 presents the instrument and theobservations. Section 3 describes the method used, andthe atmospheric modelling. Section 4 shows the resultsobtained (1) before and after the north hemisphere springequinox (Ls ¼ 330–401) when the water content is expectedto be rather low and more or less uniform, and (2) at highnorthern latitudes during early northern summer, when thewater vapor content is maximum, due to the sublimation ofthe northern ice cap (the whole set of maps are Eastlongitude oriented). Preliminary results of this study havebeen published in Encrenaz et al. (2005).

2. The observations

The Mars Express spacecraft (Chicarro et al., 2004) waslaunched by ESA on June 2, 2003, and has been operatingin orbit around Mars since January 2004. The orbit isalmost polar and highly elliptical, with a period close to 7 h(6.72 h for the 100 first days, and 7.58 h after). As a result, agiven location observed by Mars Express on a given orbitwas again observed, with a slight shift, 11 orbits later (or 13orbits during the first 100 days). The periapsis wasprocessing in latitude by about 201 per month. The latitudeof the periapsis started from about 01 at the beginning ofthe mission, decreased down to about 80S in June 2004(orbit 500) then increased again regularly to reach amaximum latitude of about 8 1N in March 2005 (orbit1500), and so on.The OMEGA instrument (Bibring et al., 2004) is an

imaging spectrometer operating in the visible and near-infrared range, from 0.35 to 5.1 mm, with a spectralresolution of 7 nm below 1 mm, 14mm in the 1.0–2.5 mmrange, and 20 nm above 2.5 mm. With an instantaneousfield of view (IFOV) of 1.2mrad, its spatial resolution atthe surface of Mars ranges from about 300m (close toperiapsis) up to 4.8 km from an altitude of 4000 km. Inorder to have a continuous coverage during an orbit, thelongitudinal width varies from 16 to 128 IFOV, dependingon the distance and on the speed of the instrument from thesurface. For this reason each orbit is divided in severalsessions; namely an orbit is defined by a code xxxx-y wherex is the number of the orbit since the beginning of themission and y is the session associated (sessions do notdefine a pixel width). The present analysis is based on thedata of the IR-short-wavelength channel of OMEGA,which extends from 1.0 to 2.7 mm.The data presented here cover two periods:

�

Orbits 6–520 (January 2003–June 2003), Ls ¼ 330–401.These data cover mostly the mid-latitude regions, at aseason where the water vapor content is expected to below (Jakosky and Farmer, 1982; Smith, 2002, 2004),with little temporal evolution. � Orbits 900–1150 (October 2003–January 2004),

Ls ¼ 90–1251. These data cover the high northernlatitudes at the time of northern spring, when the watervapor content is expected to be at the maximum due tothe sublimation of the water ice cap.

Fig. 1 shows two raw OMEGA spectra between 1.0 and2.7 mm, recorded during Orbit 0037_3 , at the summit ofOlympus Mons (lat. ¼ 301N, long. 2271E) and at the footof the volcano (lat. ¼ 171N, long. 2271E). Each spectrum isthe average of 352 individual spectra, corresponding to 11pixels along the latitude (spacecraft motion) and 32 pixelsalong the longitude (for a surface of 0.81 lon.� 0.11 lat.).All OMEGA data were flat-fielded, corrected from non-linearity, dark current and bad pixels, and calibrated usinga standard procedure.

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Fig. 1. Examples of two averaged I/F spectra, at the summit ((lat ¼ 171N,

long ¼ 227E, red curve) and at the foot (lat ¼ 301N, long ¼ 2271E, black

curve) of Olympus Mons (Orbit 0037_3). Each spectrum is the average of

352 individual spectra corresponding to 11 pixels along the latitude

(spacecraft motion) and 32 pixels along the longitude. The main spectral

features are atmospheric features, which are much weaker at the top of the

volcano (red spectrum) than at the bottom (black spectrum).’’

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Fig. 2. Ratio of the two spectra shown in Fig. 1. Color curves: synthetic

ratios calculated with averaged surface pressures of 7.5mbar at the foot

and 1.1mbar at the summit. A constant mixing ratio is assumed for

Q(H2O): 150 ppm (red), 300 ppm (green) and 600 ppm (blue). The best fit is

achieved for Q(H2O) ¼ 150 ppm.

R. Melchiorri et al. / Planetary and Space Science 55 (2007) 333–342 335

In Fig. 1, all differences between the spectra at thesummit and at the foot of Olympus Mons are due to thedifferent intensities of atmospheric features (Bibring et al.,1991). Indeed, the mineralogy is basically the same at bothplaces, and the only difference is the atmospheric pathbetween the spacecraft and the surface: the altitude at thesummit is more than 20 km above the reference level whileour foot spectrum is about 1 km below it. The shape of

atmospheric features is better illustrated in Fig. 2, whichshows the ratio of the two previous spectra (Foot/Summit).As the atmospheric path is by far longer for the footspectra than that at the summit, the atmospheric signaturesappear in absorption in the ratio.In order to analyze these features, we have divided all

individual spectra by the averaged summit spectrum shownin Fig. 1. This allows us to eliminate the uncertaintyassociated with the OMEGA instrumental transfer func-tion and to obtain a good fit of observed atmosphericbands with modelled spectra. Because of the great stabilityof the instrumental parameters since the beginning of theoperations, this method can be applied on the whole dataset (at least up to orbit 1500), although it covers a widetemporal range (Mustard et al., 2005).

3. The radiative transfer model and the analysis of the

2.6 lm water band

3.1. Atmospheric and spectroscopic parameters

Comparing the ratio of observed spectra with syntheticcalculations first requires a proper modelling of thereference spectrum corresponding to the summit ofOlympus Mons. Using the European Martian ClimateDatabase (EMCD) developed at Oxford and at theLaboratoire de Meteorologie Dynamique (LMD) in Paris(Forget et al., 1999), we have chosen the atmosphericparameters corresponding to this location, for the seasonalperiod (Ls ¼ 3371) and the local time (13:00) correspond-ing to the OMEGA observation of Olympus Mons. Weused a surface pressure of 1.1mbar and a surfacetemperature of 275K. The atmospheric temperaturedecreased from 190K at the surface down to 150K atP ¼ 0.01mbar (z ¼ 45 km) and staid constant above thisaltitude level. Note that there is a strong temperaturecontrast (85K) between the surface temperature atOlympus summit and the atmospheric temperature atz ¼ 0 just above it. At the foot of Olympus Mons, weassumed a surface temperature of 280K and a temperatureprofile ranging from 250K at P ¼ 7.5mbar to 200K at1.6mbar and 150K at P ¼ 0.01mbar. In this case, thetemperature contrast between the surface and the firstatmospheric layer is 25K. We kept the surface pressure atthe Olympus foot as a free parameter to be determined bythe best fit of the CO2 bands.Radiative transfer calculations over the 1.0–2.7 mm range

were performed using a line-by-line code including thespectral signatures of CO2, H2O and CO. Spectroscopicparameters were taken from the GEISA data bank(Jacquinet-Husson et al., 1999). Line-broadening para-meters for CO2–CO2, H2O–CO2 and CO–CO2 collisionswere taken from Pollack et al. (1993) and referencestherein. We used a frequency step of 0.01 cm�1, which wasfound to be small enough for the spectral signatures to beaccurately reproduced. The results were convolved with theOMEGA spectral instrumental function.

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Fig. 3. (a) Curves of growth of the 2.6 mm H2O band, calculated for

different values of the surface pressure. From right to left : Ps ¼ 1.4, 4.0,

7.5 and 10.0mbar. The abscissa is the product of the surface pressure by

the H2O mixing ratio, i.e. the H2O partial pressure at the surface. The

ordinate is the depth of the water band: D ¼ 1�[2� I(2.60mm)/

(I(2.55 mm)+I(2.65 mm))]. In absence of saturation, this quantity is

proportional to the H2O column density. (b) The water vapor column

density as a function of the H2O mixing ratio, for different values of the

surface pressure, calculated for a cold temperature profile, for which the

saturation effect is maximum. From right to left : Ps ¼ 6.5, 7.5, 8.5, 9.5,

10.5mbar.

R. Melchiorri et al. / Planetary and Space Science 55 (2007) 333–342336

As mentioned in the introduction, our calculations donot include scattering by aerosols; indeed, implementingthis procedure in our calculations would be, in terms ofcomputer time, incompatible with the automatic methoddeveloped in the present study. Scattering by dust mayhave two opposite effects on our results. On one hand,multiple scattering can enhance the path length and thuslead to an overestimate of the gaseous column density. Onthe other hand, a layer of aerosols may reflect the solarphotons above the surface, and thus reduce the observedcolumn density. The latter effect is sometimes observed inthe core of the strong CO2 band at 2.0 mm. However, it canbe seen that, even in this case, the depth of the weak CO2

bands (at 1.44 mm in particular) is unaffected. We can thusconclude that the effect of dust scattering is, to first order,negligible in the case of the 2.6 mm H2O band, which has amaximum depth of a few percent.

3.2. Modelling the reference spectrum

We have first determined the surface pressure corre-sponding to our Olympus foot spectrum by fitting the CO2

bands, which shows a series of bands of differentintensities. The strongest one appears at 2.75 mm (n1+n3),then the next one (2n1+n3) is visible around 2.0 mm, andweaker ones are present at 1.6 mm (3n1+n2) and 1.44 mm(3n3). It can be seen that a very good fit is obtained over thewhole spectral range when a surface pressure of 7.5mbar isused at the Olympus foot, which is consistent with theGCM predictions, based upon an interpolation of theMOLA data (�7.3mbar).

Fig. 2 also shows 3 synthetic models corresponding to 3different values of the H2O mixing ratio, assumed to beconstant with altitude over the flank of Olympus Mons(Q(H2O) ¼ 150 ppm, 300 and 600 ppm). Water vaporsignatures are present at 1.38 mm (n1+n3) and 2.6 mm(n1, n3), in the wing of the strong 2.7 mm CO2 band. Wehave selected the 2.6 mm wavelength for our analysis (onthe slope of the 2.7 mm band), as it is by far the strongestsignature, and also because the 2.6 mm range is free ofmineralogical signatures. Fig. 2 shows that the best fit ofthe water band is obtained with an H2O mixing ratio of150 ppm. We have thus assumed this value for modellingour reference spectrum of Olympus summit; the corre-sponding H2O column density is 1.6 pr-mm. However,it should be mentioned that there is some uncertaintyassociated with the hypothesis of a constant H2O mixingratio along the flanks of Olympus Mons. We choosethis assumption as the simplest one. However, if thewater-mixing ratio at the top of Olympus was twice itsvalue at the foot, then the water column density would be3.2 pr-mm, while it could be as low as zero if the water vaporwas significantly depleted at the summit. Assuming that theH2O mixing ratio at Olympus summit is not enhanced bymore than a factor 2, we infer that the H2O column densityat the summit is 272 pr-mm. The corresponding error, onall measurements, associated to this uncertainty, is 2 pr-mm.

3.3. Analysis of the 2.6 mm H2O band

Synthetic calculations show that the depth of the 2.6 mmwater band depends not only upon the H2O columndensity, but also upon the surface pressure while it is, overa wide range, independent upon the temperature profile.Then, we have built a curve of growths of the 2.6 mm H2Oband depth as a function of the H2O surface mixing ratioQ, for a given set of surface pressures Ps. These curves ofgrowth are shown in Fig. 3a for a set of 4 surface pressures(1.4, 4.0, 7.5 and 10.0mbar). We have checked that thesecurves of growth are independent of the temperature over alarge range of atmospheric profiles, as the measured H2Oband depth varies by a few percent at most.

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Fig. 5. H2O column density recorded over Olympus Mons (Orbit 0037_3,

Ls ¼ 3371).

R. Melchiorri et al. / Planetary and Space Science 55 (2007) 333–342 337

We note that in absence of saturation, the product Q.Ps(partial pressure of H2O at the surface), shown in abscissa,is directly proportional to the H2O column density.Calculations show that this applies for water partialpressures typically lower than 0.002mbar, which approxi-mately corresponds to a water vapor column density of20 pr-mm. This is illustrated in Fig. 3b, which shows theH2O column density as a function of the H2O surfacemixing ratio for a given set of surface pressures, calculatedfor a cold temperature profile, for which the saturationeffect is maximum. This profile corresponds to the atmo-spheric conditions of early northern summer at highnorthern latitudes (Ts ¼ 250K, T(z ¼ 0) ¼ 220K). As thewater content increases, saturation occurs at lower andlower altitudes; in this case, saturation takes place directlyat the surface for H2O mixing ratios higher than 3.5 10-3;the corresponding water vapor column density is then140 pr-mm.

It should be mentioned that the water vapor columndensity is more appropriate than the H2O surface-mixingratio to present our results because, to first order, it is thephysical quantity inferred from our analysis. This para-meter is thus used in the following sections.

In order to determine the surface pressure, we firstconsidered the measurement of the CO2 band depth. Inparticular, the weak 1.44 mm band shows an excellentlinearity up to surface pressures of 10mbar. This method,however, has a severe limitation associated to the presenceof water ice signature in some OMEGA spectra, especiallyover the polar caps (Langevin et al., 2005a). An example ofsuch spectra is shown in Fig. 4. In the presence of water ice,strong signatures appear around 1.5 and 2.0 mm so that the1.44 mm CO2 band is no more detectable. The water icesignature appears over the polar caps, but also at locationswhere water ice clouds are present. For this reason, we

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Fig. 4. Examples of reflectance factors in the northern polar region, with

and without the presence of water ice. Data are taken from orbit 902_1.

Black line : lat. above 81N; red line : latitude below 801N. The black

spectrum shows two strong and broad bands at 1.5 and 2.0mm,

characteristic of water ice. The red spectrum, taken at lower latitudes,

does not show the water ice signatures.

have chosen to use the surface pressure predictions givenby the EMCD on the basis of the interpolation of theMOLA data. We have checked that, in the spectra wherethe water ice signature is absent, the 1.44 mm CO2 band iswell fitted using the EMCD values of the surface pressure.Fig. 5 shows the inferred H2O column density for orbit

0037_3 (Ls ¼ 3371), which was used as reference. Themaximum H2O column density, north of Olympus summit,is about 15 pr-mm, in good agreement with previousdeterminations (Smith, 2002, 2004). In this case, saturationis negligible and the H2O column density is directly relatedto the H2O partial pressure at the surface.

4. Results

4.1. The equinox period (Ls ¼ 330–401)

Between Ls ¼ 3301 and 401, i.e. before and after north-ern spring equinox, the water vapor content is expected tobe rather homogeneous over the Ls range, with latitudevariations ranging from about 3 to 15 pr-mm (Smith, 2002,2004). Fig. 6 shows the H2O mixing ratio Q and the H2Ocolumn density as a function of latitude and longitude. Wenote that the water column density mostly reflects thetopography, and that water saturation is again negligible.The sampling in latitude and longitude will be hopefullycompleted at the end of the extended Mars Expressmission.

4.2. The water sublimation during early northern summer

(Ls ¼ 93–1261)

Figs. 7–12 show the water vapor column density at highnorthern latitudes for 6 time steps (Ls ¼ 93–1001,101–1051, 106–1101, 111–1151, 116–1201, 121–1261). Thegeometrical configuration of Mars Express orbits allowedus to obtain a complete coverage of the northern polar cap

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Fig. 7. H2O column density at high northern latitudes for Ls ¼ 93–1001.

The maximum column density is 70 pr-mm.

Fig. 8. H2O column density at high northern latitudes for Ls ¼ 101–1051.

The maximum column density is 70 pr-mm.

Fig. 6. A map of the H2O column density for Ls ¼ 330–401 (orbits 6–520),

as a function of latitude and longitude.

Fig. 9. H2O column density at high northern latitudes for Ls ¼ 106–1101.

The maximum column density is 70 pr-mm.

Fig. 10. H2O column density at high northern latitudes for

Ls ¼ 111–1151. The maximum column density is 70 pr-mm.

Fig. 11. H2O column density at high northern latitudes for

Ls ¼ 116–1201. The maximum column density is 70 pr-mm.

R. Melchiorri et al. / Planetary and Space Science 55 (2007) 333–342338

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Fig. 12. H2O column density at high northern latitudes for

Ls ¼ 121–1261. The maximum column density is 70 pr-mm.

Fig. 13. Altimetry map of the northern polar cap (MOLA 32 data).

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Fig. 14. Laboratory spectrum of H2O snow for different grain size (from

top to bottom, 10, 100 and 1000mm). The figure is taken from Schmitt

et al. (2003).

R. Melchiorri et al. / Planetary and Space Science 55 (2007) 333–342 339

over this time sequence. It can be seen that the water vaporcontent increases at the beginning of the sequence andremains approximately constant over the rangeLs ¼ 100–1151. It may show a second increase forLs41201, but the spatial coverage is no more sufficientfor a conclusive statement.

We note that the OMEGA data provide for the first timethe distribution of the water vapor content above the polarcap, as this region is not observable in the thermal infrared,because of its low surface temperature. In addition, thehigh spatial resolution provided by the OMEGA dataallows for the first time to correlate the atmospheric watercontent with the fine structure of the north polar cap, asshown by the MOLA (version 1/32) data (Fig. 13). It canbe seen that enhancements of the water vapor columndensity are closely correlated to altitude changes in thepolar cap. In addition, enhanced water abundance isclearly visible at the edge of the polar cap, at latitudes75–801N, especially at longitudes 210–2401E. We note thatthis region is not far from the dark longitudinal dunes ofOlympus Planitia where OMEGA detected a large contentof carbon-rich sulfate (80–851N), most likely gypsum(Langevin et al., 2005a, b). However, the two areas donot coincide exactly, so there is no clear evidence for acorrelation between the presence of doubly hydratedsulfate (CaSO4, 2H2O) at the surface and an increasedamount of atmospheric water vapor. We note that thevariability of the water vapor detection from differentorbits covering this region proves that the 2.5 mm sulfateband does not influence our measurements.

An important comment has to be made about Fig. 7(Ls ¼ 93–1001). It can be seen that the central blue region(corresponding to a minimum value of the water vaporcontent) seems to be correlated with the maps of smallgrains of H2O ice at the surface, obtained by OMEGA forLs ¼ 931, (Langevin et al., 2005b, Fig. 2), This suggeststhat, in this case, our determination of H2O is most likely

contaminated by the spectrum of these small icy grains,which have a typical size of about 10–100 mm. As shown inFig. 14, the laboratory spectrum of small icy grains(Schmitt et al., 2003) exhibits a small local maximumexactly at 2.6 mm (see also Langevin et al., 2005b, Fig. 3).As a result, the apparent depth of the water vapor band,measured with respect to this continuum, tends to beunderestimated and the inferred H2O column density isunderestimated accordingly. It can be seen from Fig. 14that the ratio [2� I(2.60 mm)/I(2.55 mm)+I(2.65 mm)] isabout 2% for 10-mm grains and 6% for 100-mm grains.In the ‘‘snake-like’’ blue region shown in Fig. 7 wherethe inferred water vapor content is minimum (about5–10 pr-mm), the measured H2O line depth is about 2–3%(Fig. 3a). Correcting this value with the contribution due tothe H2O ice signature would lead to an enrichment of theH2O water vapor up to about 30–50 pr-mm, As a result,

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there is no more evidence for a decrease of the water vaporcontent above the pole. This is also confirmed in Fig. 8-12,which show a progressive decrease of the dark-blue ‘‘snake-like’’ feature and a progressive increase of the H2O columndensity over the poles as the small icy grains evaporate(Langevin et al., 2005b). In conclusion, we interpret theapparent decrease of the H2O column density over the poleas due to the presence of small icy grains, which evaporateprogressively, and we see no evidence for a decrease of thewater vapor content over the pole with respect to thesurrounding areas.

5. Discussion

Before comparing our results to previous water vapordata sets, we need to assess the uncertainty associated toour measurements.

5.1. Uncertainty analysis

There are several sources of uncertainties in our analysis:signal-to-noise ratio of the data, non-linearity of thedetector response, water content in the reference spectrum,modelling uncertainty associated to the atmosphericparameters.

5.1.1. Intrinsic S/N in an individual spectrum

The 1�s noise level of an individual spectrum isestimated to 2 DN, while the continuum level, at 2.6 mm,ranges from about 8000 DN for the brightest spectra,recorded under maximum insolation (Fig. 1) down toabout ten times less for regions showing the water icesignature (Fig. 2). The 1�s S/N per individual spectrumthus ranges from about 400 to 4000 in the 2.6 mmcontinuum, inside and outside the polar cap, respectively.As all spectra are ratioed to a given reference, the 1�s S/Nper individual spectrum ranges from about 300 to 3000.The depth of the 2.6 mm H2O band is typically 1–6%, sothat the associated uncertainty ranges between 1% and60% per spectrum. Figs. 7–12 each correspond to anintegration of about 20–40 orbits, of 5–6 sequences each,with 64–128 longitudinal pixels and more than 200 pixelsalong the latitude axis for each orbital sequence. Eachindividual pixel in Figs. 7–12 is the average of about tenOMEGA pixels, and the associated S/N uncertainty on theline depth thus varies from 0.3% to 20%; the correspond-ing uncertainty on the water column density is less than30%. We believe that this number is conservative, becauseit corresponds to the combination of all unfavorableconditions. In fact, at high northern latitudes, thecontinuum level is lower because of the water ice signature,but the spatial coverage is better as most of the orbitsoverlap, so that both effects compensate. In order to takeinto account the overlap of the orbits the OMEGA spatialresolution has been reduced to a grid of 0.11� 0.11(OMEGA maximum resolution is �0.011).

5.1.2. Non-linearity of the detector response

It has been shown that the response of the detector,if plotted as a function of the received signal, exhibits anoscillation with amplitude of about a percent. The effect isstronger when the continuum is high. The non-linearity ofthe detector response is expected to limit the S/N of ourdata in the bright regions, but should not affect the polarcaps areas.

5.1.3. Water content in the reference spectrum

It has been mentioned above that the uncertainty aboutthe water column density above Olympus Mons isestimated to be �2 pr-mm. Thus, this source of uncertaintyis dominant for the data set acquired at Ls ¼ 330–401(Fig. 6), where the water vapor column density is typically10 pr-mm. On the other hand, this source of error is only afew percent for the maps obtained at Ls ¼ 93–1261, and isthen a minor contributor to the total uncertainty.

5.1.4. Uncertainties associated to atmospheric modelling

The main parameter in the retrieval of the water vaporcolumn density is the surface pressure. The Ps valuesinferred from the GCM on the basis of the MOLA datainterpolation are believed to be accurate within a fewpercent (Forget et al., 1999). However, the spatialresolution of our surface pressure map has been chosenin order to take into account one pixel any 10 over thelongitude and 1 pixel over 40 over the latitude, creating agrid of �0.31� 0.31 which strongly depends on theobservation, this choice does not influence our calculationsbecause of the OMEGA grid of 0.11 already mentioned.With regard to the temperature dependence, as mentionedabove, calculations show that the measured depth of the2.6 mm H2O band vary by less than 3% when thetemperature profile varies over the whole range of expectedseasonal variations. The associated uncertainty on the H2Owater content is about twice this value.In summary, taking into account all possible sources of

errors, we estimate that our error bar on the H2O columndensity is about 35% for the Ls ¼ 330–401 period, and isless than 30% for the Ls ¼ 93–1261 period. We take thisvalue as our 1�s error bar for the whole OMEGA data set.Obviously some areas have a lower error bar, eitherbecause they correspond to a strong continuum at 2.6 mm(absence of water ice) or because they correspond tomultiple orbit overlaps. The quality of the OMEGA data isalso illustrated by the repeatability of the water maps inFigs. 8–10.

5.2. Comparison with previous data sets

Two global data sets of the water vapor column density,over a whole seasonal cycle, have been obtained by spacemissions: the MAWD-Viking data (Jakosky and Farmer,1982) and the TES-MGS data (Smith, 2002, 2004). Tables 1and 2 show a comparison of our results, integrated over the

ARTICLE IN PRESS

Table 1

Comparison of water vapour data sets, Ls ¼ 330–401

Latitude MAWD-Viking TES-MGS This work

501N 5�10 5�10 5�10

301N 10 10–15 10�15

0 10 10 10�15

301S 5�10 5�10 10

601S 1�5 5 5�10

801S o3 o5

Table 2

Comparison of water vapour data sets, Ls ¼ 93–1261

Latitude MAWD-

Viking

TES-MGS This work

701N 70 60 30–50

801N 80 75 40–70

901N 30 (without the

granular icy region)

R. Melchiorri et al. / Planetary and Space Science 55 (2007) 333–342 341

longitude, with these two data sets, for the two periodsdiscussed in this paper.

As shown in Figs. 5–13, our data show strong long-itudinal variations. This is especially true in Fig. 13 for theregion of enhanced water vapor content corresponding toOlympia Planitia, but it is also clearly visible in Fig. 6b.In spite of this limitation, Tables 1 and 2 show that, whilethe results are in good general agreement for the equinoxregion, our results are significantly lower than previousanalyses in the case of the northern polar cap sublimation.It should be mentioned, however, that, while the first TESresults indicated maxima of 60–75 pr-mm (Smith, 2002), inthe latest TES study (Smith, 2004) no absolute value isgiven for the maximum water content at Ls ¼ 100–1201;the only indication given on the map is that the maximumvalue is higher than 40 pr-mm, in good agreement with ourresults. Finally, we also note that our last map, whichunfortunately shows an incomplete spatial coverage, seemsto indicate a local water vapor enhancement with valuesabove 60 pr-mm.

In conclusion, the present analysis of the OMEGA dataset shows that a retrieval of the water vapor content in theatmosphere is possible. However, above the north pole, aprecise retrieval of the water vapor content is not possiblebecause of the presence of small-size grains at the surface.

The main conclusions of this paper can be summarizedas follows:

�

High-resolution water vapor maps above of the north-ern polar region have been retrieved for the first time, atthe time of water vapor sublimation. However, abovethe pole itself, no precise measurement is possible due tothe presence of small icy grains.

�

An enhancement of the water vapor content appears atlatitudes 75–801N and longitude 210–2401E. We notethat this region is not far from the area where carbon-rich hydrated sulfates have been identified in largeamounts, however there is no evidence for a clearcorrelation. � The water column densities derived at high northern

latitude during early northern summer appear lowerthan the results inferred from previous studies.

In the future, this analysis will be extended to the wholeOMEGA data set and will hopefully cover the wholeseasonal water cycle at the end of the Mars Expressextended mission.

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