NASA Technical Reports Server (NTRS) 20140005705: Vertical Distribution of Dust and Water Ice Aerosols from CRISM Limb-geometry Observations

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JOURNALOFGEOPHYSICALRESEARCH:PLANETS,VOL.118,1–14,doi:10.1002/jgre.20047,2013
Vertical distribution of dust and water ice aerosols from CRISM
limb-geometry observations
Michael D. Smith,1 Michael J. Wolff,2 R. Todd Clancy,2 Armin Kleinböhl,3 and
Scott L. Murchie4
Received31May2012;revised14November2012;accepted18December2012.
[1] Near-infraredspectratakeninalimb-viewinggeometrybytheCompactReconnaissance
ImagingSpectrometerforMars(CRISM)onboardtheMarsReconnaissanceOrbiterprovide
ausefultoolforprobingatmosphericstructure.Specifically,theobservedradianceasa
functionofwavelengthandheightabovethelimbenablestheverticaldistributionofbothdust
andwatericeaerosolstoberetrieved.MorethanadozensetsofCRISMlimbobservations
havebeentakensofarprovidingpole-to-polecrosssections,spanningmorethanafull
Martianyear.Radiativetransfermodelingisusedtomodeltheobservationstakinginto
accountmultiplescatteringfromaerosolsandthesphericalgeometryofthelimb
observations.Bothdustandwatericeverticalprofilesoftenshowasignificantvertical
structurefornearlyallseasonsandlatitudesthatisnotconsistentwiththewell-mixedor
Conrath-vassumptionsthathaveoftenbeenusedinthepastfordescribingaerosolvertical
profilesforretrievalandmodelingpurposes.Significantvariationsareseenintheretrieved
verticalprofilesofdustandwatericeaerosolasafunctionofseason.Dusttypicallyextendsto
higheraltitudes(~40–50km)duringtheperihelionseasonthanduringtheaphelionseason
(<20km),andtheHellasregionconsistentlyshowsmoredustmixedtohigheraltitudesthan
otherlocations.Detachedwatericecloudsarecommon,andwatericeaerosolsareobserved
tocapthedustlayerinallseasons.
Citation: Smith,M.D.,M.J.Wolff,R.T.Clancy,A.Kleinbo¨hl,andS.L.Murchie(2013),Verticaldistributionofdust
andwatericeaerosolsfromCRISMlimb-geometryobservations,J.Geophys.Res.Planets,118,doi:10.1002/jgre.20047.
1. Introduction mostly imposed because nadir-viewing observations provide
limitedinformationaboutverticalstructure.Thesestudieshave
[2] TheverticaldistributionofaerosolsintheMartianatmo- produced a very useful first look at the climatology of Mars
sphere is of great interest because it plays a large role in the
aerosols,butithasalsobeenknownforsometimethat,atleast
vertical distribution of solar energy in the atmosphere and
undersomeconditions,dustandespeciallywatericeaerosols
therefore controls heating rates throughout the atmosphere
are not well mixed, and recent work (e.g., [Guzewich et al.,
[Gierasch and Goody, 1972]. The vertical distribution of
2011; Richardson et al., 2011; Rothchild et al., 2011]) has
aerosolsisalsosensitivetomanyphysicalprocesses,including demonstrated that using a more realistic vertical profile for
dynamics, transport, condensation microphysics, surface- aerosol instead of the traditional simplified expressions leads
atmosphereinteraction,andphotochemistry,andsoitprovides
to notable improvements between the modeled and the ob-
sensitivetestsandconstraintsforawidevarietyofmodels.
servedthermalstructure.
[3] Inthepast,anumberofretrievalalgorithms(e.g.,[Smith, [4] Observations taken at a geometry viewing the limb
2004, 2009])haveusedthesimplifyingassumptionthat dust
enabletheverticaldistributionofaerosolstobedetermined,
is well mixed with the background atmosphere. Although
butuntilrecently,suchretrievalshavebeenrelativelyfewin
partially a computational necessity, this assumption was
number.ImagesfromorbitoftheMartianlimbbyMariner9
[Anderson and Leovy, 1978], Viking [Jaquin et al., 1986]
and the Mars Global Surveyor (MGS) MOC [Cantor,
1NASAGoddardSpaceFlightCenter,Greenbelt,Maryland,USA.
2SpaceScienceInstitute,Boulder,Colorado,USA. 2007] cameras show that the haze associated with dust
3JetPropulsionLaboratory,Pasadena,California,USA. extends much higher above the surface during dust storms
4Applied Physics Laboratory, The Johns Hopkins University, Laurel, than when the atmosphere is less dusty. Observations of
Maryland,USA.
theMartianlimbtakeninasolaroccultationgeometrytaken
Correspondingauthor:M.D.Smith,NASAGoddardSpaceFlightCenter, by Phobos [Chassefière et al., 1992] and by the Mars
Greenbelt,MD20771,USA.([email protected]) ExpressSPICAMinstrument[Montmessinetal.,2006;Rannou
et al., 2006] show detached layers of water ice clouds
©2013.AmericanGeophysicalUnion.AllRightsReserved.
2169-9097/13/10.1002/jgre.20047
1
SMITHETAL.:CRISMLIMBAEROSOLS
superimposed on a background dust haze layer that varies [10] Observationsofthelimbarenotanominalobservation
indepthwithseason. forCRISMbecausetherangeofgimbalmotiondoesnotallow
[5] Inthepastfewyears,amorecompletecharacterization thelimbtobeobservedinthenominalspacecraftnadir-pointing
oftheverticaldistributionofaerosolshasbeencompiledusing orientation.However,asaspecialobservation,thespacecraftis
limb-geometry observations by MGS Thermal Emission occasionallyorientedsothatCRISMis abletoobserveeither
Spectrometer (TES) [McConnochieandSmith,2008; Clancy the forward or aft limb. During each opportunity for CRISM
et al., 2010] and Mars Reconnaissance Orbiter (MRO) Mars limbobservations,theinstrumentiscommandedtorepeatedly
Climate Sounder (MCS) [McCleese et al., 2007, 2010; scan upward and downward across the limb. Each limb
Kleinböhletal.,2009,2011;Heavensetal.,2011].Theretrie- scan thus consists of a spectral image cube with one spatial
vals from these observations have revealed the vertical dis- dimension parallel to the limb and one spatial dimension
tribution of aerosols over the course of the Martian year, perpendicular to the limb. The limb observations are taken in
includingduringgreatduststorms.Findingsshowthatdetached amodethataverages10pixelsineachspatialdirection,which
water ice clouds are common and that there are often signific- providesaspatialsamplingatthelimbtangentpointofroughly
ant departures of dust vertical distribution that are well mixed 800m.Wefurtheraveragethecentral40pixelsinthedirection
duringthenorthernspringandsummerseasonsinthetropics. parallel to the limb to increase signal-to-noise ratio with
[6] Near-infrared spectra returned by the MRO Compact minimalimpactonverticalresolution.Eachlimbscanincludes
Reconnaissance Imaging Spectrometer for Mars (CRISM) coveragefrombelowthelimbtoatangentpointatleast100km
[Murchie et al., 2007, 2009] taken in the limb-viewing abovethelimb.Limbobservationsarealwaystakeninthefull
geometryprovideanothergoodsourceofinformationabout hyperspectralmodeoftheCRISMinstrument.
theverticaldistributionofdustandwatericeaerosols.These [11] Allofthelimbscanstakenduringoneparticularspecial
observations,takeneveryfewmonths,serveasanimportant observation opportunity form a limb set. Each limbset nomi-
complement to the aerosol profiling provided by the MRO nallycontainsabout50scansacrossthelimbacquiredpoleto
MCS instrument by allowing independent validation and poleduringtwoorbitsatroughly100(cid:2)Wand290(cid:2)Wlongitude,
givingadditionalinformationonparticlephysicalandscattering overtheTharsisandSyrtisMajor/Hellasregions,respectively.
properties through multi-wavelength studies. CRISM limb At each longitude, limb scans are spaced roughly 10(cid:2) in
observations have also been used to study other atmospheric latitude.Figure1showsamapgivingthelatitudeandlongitude
phenomenasuchasO airglow[Clancyetal.,2012]. locations of a typical set of CRISM limb observations.
2
[7] In this paper, we present the results of retrievals of Beginning with the most recent limb set taken in April 2012,
the vertical distribution of dust and water ice aerosols in the athirdorbitwithanequatorcrossingat0(cid:2)longitudewasadded
Martian atmosphere using limb-geometry spectra taken by tothenominaltwoorbitsshowninFigure1.
the CRISM instrument over more than one full Martian [12] ThefirstCRISMlimbsetwastakeninJuly2009,and
year. In section 2, we discuss the CRISM instrument and additional limbsetshavebeen addedsemi-regularly roughly
limb-viewing data set. Details of the retrieval algorithm are every 2 to 4months. As of this writing, a total of 15 limb
given in section 3. In section 4, we present the results of the setshavebeenobtained,spanningmorethanoneMartianyear
retrievals,and those resultsare discussed and compared with (Table 1). Figure 2 gives a graphical representation of the
otherobservationsinsection5. seasonalcoverage.Allseasonsarerepresentedalthoughthere
are fewer limb scans in southern spring/northern fall. An
additional CRISM limb set taken in August 2010, or Mars
2. Data Set year(MY)30,solarlongitude(L)=137(cid:2),isnotusedinthis
s
studybecauseaproblemwiththeCRISMcoolersduringthe
[8] The MRO arrived at Mars in March 2006, completed
observationresultedinlossofmostuseabledatainthenear-
aerobraking in August 2006, and began its primary science
infraredportionoftheCRISMspectralcoverage(>1000nm).
phase in November 2006; it continues to operate in its
extended mission at the time of this writing. MRO operates [13] At the near-infrared wavelengths used in this study,
there is negligible contribution from thermal radiation, so
from a Sun-synchronous, near-polar, near-circular (~300km
werelyonscatteredsolarlightforsignalfromaerosols.This
altitude) orbit with a mean local solar time of about 3:00A.
means that we cannot view the winter polar regions or at
M./P.M.[ZurekandSmrekar,2007].
90
2.1. CRISM Instrument andLimbObservations
60
[9] CRISM is a hyperspectral imager with a nadir e 30
3sp6a2t–ia3l9r2e0sonlmutiwonithofa15sp–e1c9trmal/psixamelpalinndgaosfp6ec.5tr5anlmranagnedoaf atitud 0
L-30
spectral resolution in the near-infrared (~2000nm) of about
10–15nm [Murchie et al., 2007, 2009]. A gimbal allows -60
-90
off-nadir pointing in the fore or aft direction along track,
180 90 0 270 180
and spacecraft roll allows off-nadir pointing in the across- West Longitude
track direction. There are two detector arrays each with
640 spatial pixels across track and 480 spectral pixels. Figure 1. The latitude and longitude locations (small red
One detector covers visible to near-infrared wavelengths squares)ofCRISMlimbscansforalimbsettakeninJuly2009
(362–1056nm),whiletheothercoversinfraredwavelengths (MY 29, L =301(cid:2)). A typical limb set contains pole-to-pole
s
(1001–3920nm). Hyperspectral images are built up using a coverage at two longitudes at approximately 100(cid:2)W and
combination oforbitalmotionandgimbalmovements. 290(cid:2)W.OtherCRISMlimbsetshavesimilardistribution.
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SMITHETAL.:CRISMLIMBAEROSOLS
Table1. DatesofCRISMLimbSets
0.2
EarthDate MarsDateandSeason
10–11July2009 MY29,L =301(cid:2)
1722086A––M12p19railyFA2e2p0b0r1ri1lu00a2r0y120010 MMMMYYYY33330000,,,,LLLLsssss====57890446(cid:2)(cid:2)(cid:2)(cid:2) served I/F 0.01.51
22–24August2010 MY30,Ls=137(cid:2) Ob
17October2010 MY30,Ls=166(cid:2) 0.05
5–6December2010 MY30,L =193(cid:2)
s
31Marchto1April2011 MY30,L =265(cid:2)
s
14–15May2011 MY30,Ls=293(cid:2) 0
28June2011 MY30,L =319(cid:2)
22–23August2011 MY30,Ls=349(cid:2) 1000 2000 3000 4000
s
13September2011 MY31,L =0(cid:2) Wavelength (nm)
s
10–11December2001 MY31,L =41(cid:2)
s
24–26April2012 MY31,L =102(cid:2) Figure 3. A typical CRISM limb scan from the limb
s
set taken in July 2009 (MY 29, L =301(cid:2), 19(cid:2)S latitude,
s
290(cid:2)W longitude). Color indicates tangent height above the
surface,frompurple(0km)tored(50km).Theblackvertical
lines indicate the 10 wavelengths used to retrieve aerosol
verticalprofiles.
L = 90°
s
nonzerovalueatwavelengthsgreaterthan3400nmbecause
ofunremovedthermalemissioninsidetheinstrument.
MY 29
MY 30 3. Retrieval Algorithm
MY 31
L =180° L = 0° [15] The variation of the observed continuum signal as a
s s functionofwavelengthandtangentheightisusedtoretrieve
theverticalprofilesofdustandwatericeaerosols.Dustand
watericearedistinguishedbythedifferenceoftheirspectral
variations of extinction and scattering properties using a
set of 10 spectral channels that avoid the gas absorptions
and are widely spaced across the CRISM spectrum (black
verticallinesinFigure3).
L = 270°
s [16] Radiative transfer modeling of CRISM spectra has
been used to retrieve the vertical distribution of dust and
Figure 2. The seasonal dates of all CRISM limb sets. The
watericeaerosols.Thebasicideaistovarythetwoprofiles
observationsspanmorethanoneMartianyear,andallseasons
until a best fit is achieved between the computed and the
arerepresented.
observed spectrum as a function of tangent height at the 10
selected wavelengths. This process is explained in more
night. For adequate solar illumination and signal-to-noise
detailbelow.
ratio, we restrict our retrievals to limb scans with a solar
incidenceangleof85(cid:2) orless.
3.1. Radiative Transfer andRetrievalProcess
[17] The near-infrared light observed by CRISM comes
2.2. Typical CRISMLimb SetSpectra from aerosol scattering of sunlight into the line of sight of
[14] A typical limb scan is shown in Figure 3 (MY 29, theinstrument.Thelightcaneitherbescattereddirectlyfrom
L =301(cid:2), 19(cid:2)S latitude). An average of 40 10X-binned the solar beam or from sunlight reflected off the surface.
s
pixels parallel to the limb is included in each spectrum, Therefore, the observed signal is sensitive to both aerosol
andforclarity,onlyeverytenthaveragedspectrum(perpen- optical depth (and its vertical distribution) and surface
dicular to the limb) is shown. The spectra with the lowest albedo.Atthesewavelengths,thermalradiationisnegligible,
tangent height (purple) are essentially the same as the and the observed signal does not depend on atmospheric or
nadir-geometry spectra, except for stronger gas absorptions surfacetemperature.
due to the longer atmospheric path length. As the tangent [18] Because aerosol scattering is dominant, full multiple
height increases (blue, green, yellow, red), the observed scatteringmustbeincludedintheradiativetransfermodeling.
signalgraduallyfallstozero.Theoverall“continuum”level In addition, the limb-viewing geometry requires that the
isproduced by the scatteringof sunlight from aerosols,and sphericalgeometryinherentintheobservationsalsobeexplicitly
numerous absorptions from atmospheric gases (primarily treated. However, a fully spherical radiative transfer code
CO ) are clearly visible. In this case, the continuum level withmultiplescatteringisextremelycomputationallyexpen-
2
falls monotonically to zero as tangent height increases, but sive. As a result, we choose a middle ground that combines
insomecases,stronglayeringofaerosolscausesanincrease efficiency and an approximate, though accurate, treatment of
in observed signal with height. The continua fall to a thesphericalgeometrythatallowsforrelativelyrapidretrievals.
3
SMITHETAL.:CRISMLIMBAEROSOLS
Thecodehasbeenextensivelytestedandvalidatedagainstan
“exact” Monte Carlo code [Whitney et al., 1999; Wolff et al.,
2006]andfoundtobeaccuratewithinafewpercentoverawide
rangeofconditionsandviewinggeometries.
[19] The forward radiative transfer model uses the discrete
ordinatesmethodtotreatmultiplescattering[e.g.,Goodyand Figure5. Inthepseudo-sphericalapproximation,(left)limb
Yung, 1989; Thomas and Stamnes, 1999]. The atmosphere is viewinginthesphericalgeometry(redline)isapproximated
divided into vertical layers, and the number of radiation using (right) a curved path in a plane-parallel geometry.
“streams”canbesetashighasnecessarytoaccuratelymodel The emission angle at each layer boundary in the spherical
the angular dependenceof scattering. We use 50 atmospheric geometry is accurately computed and used to compute the
layers,eachwitha0.2scaleheight(~2km)thickness,and64 equivalentcurvedpath.
streams(32pairs)todescribetheradiationfield.Atmospheric
statevariables(temperature,gasabundance,aerosolabundance, angleforthepathattheboundaryofeachlayer.Thisisoften
aerosol scattering properties, etc.) are specified separately for called the “pseudo-spherical approximation” [e.g., Spurr,
eachverticallayer. 2002;ThomasandStamnes,1999]andisatleasttwoorders
[20] Aerosol size is taken from careful modeling of the ofmagnitudefasterthanatypicalMonteCarlocomputation.
spectraldependenceofdustabsorptioninTESspectra[Wolff Figure6showsacomparisonofourpseudo-sphericalmodel
and Clancy, 2003] and from multiwavelength comparisons against an exact Monte Carlo code. The pseudo-spherical
of TES, mini-TES, and CRISM spectra [Clancy et al., model is generally accurate to within a few percent over a
2003; Wolff et al., 2006, 2009]. Based on these analyses, widerangeofconditions.
wehavechosenasingleparticlesizedistributionthatisused [22] The retrieval algorithm finds the model parameters
for all the retrievals, with effective radius r =1.5mm that provide the best fit in a chi-squared sense between the
eff
for dust and r =2.0mm for water ice aerosols. Aerosol observed data (radiance at 10 wavelengths as a function of
eff
scattering properties are taken from a detailed modeling of height above the limb) and the radiance computed from
CRISM emission phase function spectra, where the same the forward radiative transfer model. The minimization of
spotonthesurfaceisobservedatanumberofdifferentemis- chi-square is accomplished using the nonlinear Levenberg-
sion angles as the spacecraft flies overhead [Wolff et al., MarquardtroutinefoundinPressetal.[1992].Inourexperience,
2009]. Figure 4 shows the extinction efficiency and single- we have found this routine to be reasonably fast and
scatteringalbedoasafunctionofwavelengthintheCRISM extremelyrobust.
spectral range for this choice of dust and water ice and the [23] The quantities retrieved are the vertical profiles of
different spectral character of the two aerosols over the dustandwatericeaerosolsandtheeffectiveLambertalbedo
wavelengths chosenforthe retrieval (Figure 3). of the surface at the 10 wavelengths used. The dust and
[21] The spherical geometry is treated by computing the water ice aerosol vertical profiles are each specified just
diffuse radiation field in a standard plane-parallel geometry above the surface and then every 0.4 pressure scale heights
using the discrete ordinates approach but then integrating between 0.2 and 6.6 scale heights above the surface. This
the source functions along the equivalent curved path typically gives coverage up to at least 60km above the
throughthelayers.AsillustratedinFigure5,thecurvedpath surface with a vertical resolution <5km. Together with the
isdefinedbycomputingthecorrectlimb-geometryemission 10 surface albedos, there are 46 parameters retrieved. All
1.2
80
m) Monte Carlo
1 e (k60 Pseudo-Spherical
c
0 a
ϖand 0.8 e Surf40 τ = 2.0
Q ext 0.6 Dust extinction, Qext Abov τ = 0.7
0.4 Dust single-scattering,ϖ0 ght 20
Ice extinction, Qext ei
Ice single-scattering, π H
0
0.2 0
1000 2000 3000 4000 0 0.05 0.1 0.15 0.2
Wavelength (nm) Computed Radiance (I/F)
Figure 4. The extinction coefficient, Q , and single- Figure 6. A comparison of computed radiance at a wave-
ext
scattering albedo, o(cid:2) , for dust and water ice aerosols used length of 1mm in the limb geometry between the “exact”
0
in this retrieval. The extinction coefficient has been scaled Monte Carlo results (black lines) and the pseudo-spherical
tounityatthereferencewavelengthof2219nm.Thesescat- approximation (red lines) for two different column optical
teringpropertiesarebasedontheanalysisofTESandCRISM depthsofdustaerosol,t.Thepseudo-sphericalapproximation
observations [Wolff and Clancy, 2003; Clancy et al., 2003; isaccuratetowithinafewpercentoverawiderangeofcondi-
Wolff et al., 2009] and are representative of a mean particle tions and is two orders of magnitude faster than the Monte
sizeof1.5mmfordustand2.0mmforwatericeaerosols. Carlocalculation.
4
SMITHETAL.:CRISMLIMBAEROSOLS
observations with tangent heights between 5 and 60km are 60 60
included in the retrieval, providing approximately 500 total Columnτ = 0.4 Columnτ = 1.8
observationsoverthe10wavelengths.Theviewinggeometry, m) 50 m) 50
k k
includingtheincidenceangle,emissionangle,phaseangle,and e ( e (
distance between Mars and the Sun, is taken from spacecraft ac 40 ac 40
recordsandisassumedtobewellknown.Thesolarspectrum urf urf
S S
usedisthatadoptedbytheCRISMteam,whichistakenfrom ve 30 ve 30
the terrestrial MODTRAN atmospheric radiation code [Berk bo bo
A A
etal.,1998]. ht 20 ht 20
g g
ei ei
3.2. Output Quantities H H
10 10
[24] Thefinaloutputfromtheretrievalprocessare18point
verticalprofilesfordustandwatericeaerosolopacityandthe
0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1
equivalent Lambert albedo at each of the 10 wavelengths Relative Sensitivity Relative Sensitivity
used in the retrieval. These quantities are output to a file
alongwithadditionaldatadescribingthequalityofthefitfor Figure 7. Therelativesensitivityofobservedsignaltoaero-
eachparameter. solmixingratioasafunctionofheightfortypical(left)low-dust
[25] Theaerosolopacities aregiveninterms of anaerosol and(right) high-dustloadingcases.Eachcurverepresents the
“mixingratio”thatexpressestheopticaldepthofdustorwater computedchangeinobservedsignalforanincrementalchange
ice across a model layer in terms of the atmospheric mass industmixingratioinasinglemodellayer.Theverticalrange
in that layer. This effectively divides out the exponential overwhichtheretrievalissensitiveisshownbytheamplitude
variation of atmospheric density with height and allows the of the curves. There is no sensitivity to near-surface aerosol
vertical structure of aerosols to be seen more easily. In this whencolumnopticaldepthishigh.
representation,thetotalcolumnopticaldepth,t,canbefound
by summingthe aerosol mixingratioineachlayer weighted vertical range (25km or higher in the example shown with
bythemassinthelayer: columnopticaldepthgreaterthanunity).Foreachretrieval,
thesensitivityfunctionsarecomputedandusedtodetermine
lXayers the vertical range over which there is sensitivity to the
tðlÞ¼ niQextðlÞΔpi=psurf; (1) aerosol profile, and only those values are kept and reported
i¼1 here. The width and overlap of these curves also allow for
wherethesumisperformedoveralllevels,n istheaerosol anestimateoftheverticalresolutionoftheretrievedprofiles,
i
(dust or water ice) mixing ratio, and Q is the extinction whichweestimate as5km.
ext
coefficient, which is a function of wavelength, l. The ratio [27] Figure 8 shows the best-fit computed radiance to the
of the difference in atmospheric pressure across the model same observed CRISM limb scan as shown previously in
layer, Δp, to the surface pressure, p , is equivalent to the Figure 3. The quality (root-mean-square (rms) difference
i surf
fraction of the column mass contained within a particular between observed and best fit radiance) of this fit is good.
layer. Surface pressure is retrieved from each CRISM limb Theretrievedverticalprofilesfordustandwatericeaerosols
scan in a separate step after the aerosol retrieval using the showthatdustisnearlywellmixedbelowabout25kmand
CO band near 2000nm. There is a separate mixing ratio, fallsoff rapidlyabovethatlevel.Adiscretewatericecloud
2
n, for each aerosol, dust and water ice. In the simple case
i
of a well-mixed aerosol, the mixing ratio is constant for all
levels,i,andthecolumnopticaldepthissimplytheproduct 60 60
ofthemixingratioandthewavelength-dependentextinction Observed Dust
coefficient. In this paper, we reference all mixing ratio m) 50 Best Fit m)50 Water Ice
k k
values for both dust and water ice to a wavelength of e ( e (
2219nm, which is one of the wavelengths used in the ac 40 ac40
retrieval near the center of the spectral range. Equivalently, urf urf
S S
we have scaled the extinction coefficient for both dust and ve 30 ve 30
watericetobeunityat2219nm,asshowninFigure4. bo bo
A A
[26] At a tangent height where the line-of-sight aerosol ht 20 ht 20
optical depth becomes significantly greater than unity, the g g
ei ei
retrieval becomes insensitive to variations in the aerosol H H
10 10
vertical distribution below that level. Figure 7 shows the
relative sensitivity of observed signal, I/F, to a change in
0 0.05 0.1 0.15 0.2 0 0.2 0.4 0.6
aerosolmixingratioasafunctionofheight.Eachlineshows
I/F Aerosol Mixing Ratio
the response to adding an increment of dust aerosol to a
single layer in the model (the results are similar for water Figure8. (left)Anexampleofamodelfit(redline)tothe
iceaerosol).Fortherelativelyclearcasewithcolumnoptical observations(blackline)forasinglelimbscan(sameshown
depth 0.4, there is sensitivity to changes in the aerosol as in Figure 3; MY 29, L =301(cid:2), 19(cid:2)S latitude, 290(cid:2)W
s
vertical profile down to approximately 10km above the longitude).Eachcurverepresentsoneofthetenwavelengths
surface. However, at times with high dust or water ice usedintheretrieval.(right)Thecorrespondingretrieveddust
optical depth, the retrieval is valid over a more restricted andwaterice aerosolverticalprofiles.
5
SMITHETAL.:CRISMLIMBAEROSOLS
formsinalayercenteredataheightofabout38km,capping differencebetweenobservedandbest-fitsolutions.Figure9
thedustdistribution.Althoughthewatericecloudhasmuch showsthisestimate,whichisthesamefordustandwaterice
higher peak mixing ratio than the dust, it is located much mixing ratio at 2219nm. The retrieval has best sensitivity
higher above the surface where there is less atmospheric (and lowest uncertainty) between an altitude of about 15
mass and is confined to about a scale height in thickness and 30km, and uncertainties increase at the highest and
sothatthetotalintegratedcolumnopticaldepthofthewater lowest altitudes. An uncertainty of 0.05 in the mixing ratio
ice cloud is 0.35 compared to the integrated dust optical corresponds to a fractional uncertainty of roughly 10% for
depthof0.38(extrapolatingthelowestretrieveddustmixing prominent clouds.
ratiotothesurface).
3.3. Uncertainties 4. Results
[28] Because of the large amount of spatial averaging [32] Here we present the results of retrievals of dust and
parallel to the limb, the formal uncertainties calculated by watericeaerosolverticalprofilesfromtheCRISMlimbsets.
propagation of instrument noise are relatively small except All of the limb scans for aparticular limb set are combined
near the top of the retrieval domain where the observed to show the results in cross-section form as a function of
signal goes tozero. The height at which the observedsignal latitudeandheightabovethesurface.Onlythoselimbscans
becomes small depends on a number of factors including takenduringdaylightwithasolarincidenceangle<85(cid:2) are
season,illuminationgeometry,andtheamountanddistribution shown.Allretrievalresultsareshownintermsofthemixing
ofaerosols,butinmostcases,thereisappreciablesignaltoat ratiodescribedearlieratareferencewavelengthof2219nm.
least40–50km.Alsoofrelevancearetheuncertaintiesrelated
to systematic errors in the assumptions and approximations 4.1. Typical AerosolCross Sections
utoseedvailnuathteearnedtriaerveablepsrtoecsetsims.aTtehdesbeyunnucmeretariicnatileesxapreeridmifefinct.ult [33] TheretrievedresultsfromJuly2009(MY29,Ls=301(cid:2))
shown in Figure 10 reveal a moderately dusty atmosphere
[29] Model-related assumptions, such as the number of withcolumn-integrateddustopticaldepthsof0.3–0.5atlow
vertical levels used to model the atmosphere, the number
latitudes. Significant vertical structure is apparent at nearly
of streams used in the discrete ordinates, and the number
all latitudes. The “top” of the dust distribution is at about
of terms used in the Legendre polynomial expansion of 30–40kmabovethesurfacebetween50(cid:2)Sand30(cid:2)Nlatitude
the aerosol scattering phase function were tested by (for
but drops to about 25km over the summer (southern) high
example) doublingthenumberoflevels,streams,ortermsin
latitudes and to about 10–15km over the winter (northern)
the expansion. All of these resulted in changes in retrieved
mid and high latitudes. The mixing ratio of dust appears to
abundances of less than 2%. Use of the pseudo-spherical
be somewhat depressed below the main low-latitude water
approximation for the limb scattering also results in small
ice clouds. The layers of dust above the main haze in the
errors(recallFigure6).
north that are associated with high water ice clouds may be
[30] Perhapsthelargestsourceofuncertaintyistheassump- artifacts indicative of an ice particle size different than the
tionofasinglesetofscatteringpropertiesforthedustaerosols
assumedr =2.0mmatthoselocations.
anda(different)singlesetofscatteringpropertiesforthewater eff
[34] Watericeaerosolsarealmostcompletelyconfinedto
ice aerosols for all seasons and locations. This implies a
layers at high altitudes, often capping the dust layer as also
uniform composition and particle size distribution for each
observed by Kleinböhl et al. [2009]. A low-latitude water
ofthetwoaerosoltypes.Whilepreviousstudieshaveshown
variationsinparticlesizewithseasonandlocation[e.g.,Wolff
and Clancy, 2003; Clancy et al., 2003; Wolff et al., 2006,
2009],numericalexperimentsshowthatthesevariationsserve
primarily to alter the relative amplitude (or mixing ratio) of 60
features through changes in the extinction coefficient as a
functionofwavelengthandthattheydonotchangetheoverall m)
characterofaerosolverticalprofiles.Whenaparticlesizefora e (k
given aerosol (dust or water ice) is used that does not fit ac 40
the observations well, an artifact can arise in the retrieved urf
S
abundance of the other aerosol. For example, using a water e
ice particle size that is “wrong” such that it does not fit the ov
b
opbrosfierlveaatitonthsewleolclactainoninstroofdupcroemaninaerntitfawctaitnerthieceretcrlioeuvdeds.dOusnt ght A 20
ei
thebasisofnumericalexperimentsusingarangeofeffective H
particle sizes for dust and water ice aerosols, we found that
using an effective particle radius of r =1.5mm for dust
eff
and r =2.0mm for water ice aerosols gave the best fits 0
eff
(lowest residual between computed and observed radiance)
0 0.05 0.1 0.15 0.2
andfewestartifactsinaglobalsense. Mixing Ratio Uncertainty
[31] Anoveralluncertaintyintheretrieveddustandwater
icemixingratiowasestimatednumericallyastheincrement Figure9. Thetotalestimated uncertainty inretrieveddust
in mixing ratio that produces a corresponding difference in andwaterice aerosol mixing ratioat 2219nm as afunction
computedradiancethatisequaltotheaveragermsradiance ofheightabove thesurface.
6
SMITHETAL.:CRISMLIMBAEROSOLS
DUST: MY 29, L = 301°, ~100° W Long. DUST: MY 29, L = 301°, ~290° W Long.
m) 60 s 0.9 m) 60 s
k k
e ( e (
c o c
e Surfa 40 xing Rati 0.6 e Surfa 40
v Mi v
Abo 20 ust 0.3 Abo 20
ht D ht
g g
Hei 0-90 -60 -30 0 30 60 90 0 Hei 0-90 -60 -30 0 30 60 90
Latitude Latitude
ICE: MY 29, L = 301°, ~100° W Long. ICE: MY 29, L = 301°, ~290° W Long.
m) 60 s 1.5 m) 60 s
urface (k 40 Ratio 1.0 urface (k 40
S g S
Height Above 200-90 -60 -30 0 30 60 90 Ice Mixin 00.5 Height Above 200-90 -60 -30 0 30 60 90
Latitude Latitude
Figure 10. Retrieved cross sections of (top) dust and (bottom) water ice for the limb set taken in July
2009 (MY 29, L =301(cid:2)). (left) Retrievals from the orbit near 100(cid:2)W longitude. (right) Retrievals from
s
theorbitnear290(cid:2)Wlongitude.
ice cloud between 30 and 40km above the surface was [35] Figure 11 shows the retrieved results from the most
present between 30(cid:2)S and 20(cid:2)N latitude, while a separate recent limbsettakenin April 2012(MY 31,L =102(cid:2)),the
s
group of clouds was present at 40–55km above the surface opposite season as shown in Figure 10. At this time, there
at mid-northern latitudes. A single isolated cloud at 45km was much less dust, but there was a prominent water ice
altitude was observed near the south pole in one orbit, cloud at low latitudes. The small amount of dust that is
butnotintheother. observed was confined below 20km at all latitudes with
DUST: MY 31, L = 102°, ~100° W Long. DUST: MY 31, L = 102°, ~290° W Long.
m) 60 s 0.9 m) 60 s
k k
e ( e (
Surfac 40 g Ratio 0.6 Surfac 40
e xin e
Abov 20 ust Mi 0.3 Abov 20
ht D ht
g g
Hei 0 0 Hei 0
-90 -60 -30 0 30 60 90 -90 -60 -30 0 30 60 90
Latitude Latitude
ICE: MY 31, L = 102°, ~100° W Long. ICE: MY 31, L = 102°, ~290° W Long.
m) 60 s 1.5 m) 60 s
k k
e ( e (
urfac 40 Ratio 1.0 urfac 40
S g S
Above 20 ce Mixin 0.5 Above 20
ht I ht
g g
Hei 0 0 Hei 0
-90 -60 -30 0 30 60 90 -90 -60 -30 0 30 60 90
Latitude Latitude
Figure11. Retrievedcross sectionsof(top)dustand(bottom)watericeforthelimbsettakeninApril
2011(MY31,L =102(cid:2)).(left)Retrievalsfromtheorbitnear100(cid:2)Wlongitude.(right)Retrievalsfromthe
s
orbitnear290(cid:2)Wlongitude.
7
SMITHETAL.:CRISMLIMBAEROSOLS
highestopacitynearthesummer(north)pole.Awell-formed where CRISM cannot observe because of lack of sunlight.
low-latitude water ice cloud extended from 20(cid:2)S to 30(cid:2)N At the equinoxes, there is some indication of low-altitude
latitudeandhadaverticaldepthofabout10km.Theheight cloudsatthe mostpoleward latitudes.
ofthecloudvariedsystematicallywiththelatitude,withthe [40] Water ice clouds show more variation between the
cloud located between 20 and 30km at 20(cid:2)S latitude and two longitudes observed than does dust. In general, the
gradually becoming highertothenorth reaching analtitude watericecloudsretrievedfromtheorbitat100(cid:2)Wlongitude
of 35–45km at its northernmost extent at 30(cid:2)N latitude. (Tharsis) are systematically lower by 5–10km and appear
Anisolatedwatericecloudabovethesummer(north)polar more disorganized with greater vertical structure than the
regionat80(cid:2)Nlatitudeand35kmaltitudemirrorsthesimilar clouds retrieved from the orbit at 290(cid:2)W longitude (Syrtis
isolated water ice cloud observed over the summer (south) Major and Hellas). There also tend to be more clouds with
poleinJuly2009(Figure 10). somewhat higher column optical depth in the Tharsis orbit
[36] The two longitudes observed for each CRISM limb (see Figures 10 and 11 for examples). In the one limb set
set allow for a rough comparison of longitude variability, (MY31,L =102(cid:2))thatcontainsathirdorbitat0(cid:2)longitude
s
which is enhanced by the strong topographic differences (MeridianiPlanum),thecloudsintheMeridianiorbitappear
between the two observed longitudes (Tharsis at 100(cid:2)W more like those in the Syrtis/Hellas orbit than those in the
longitude vs. Syrtis Major and Hellas at 290(cid:2)W longitude), Tharsis orbit.
and indeed, although the overall distribution between
the two longitudes is often similar, retrieved aerosol cross 4.3. Repeatability
sections do show differences in the vertical structure that [41] AsCRISMworksthroughitssecondMartianyearof
are often associated with topography. In the case from limbobservations,wecannowbegintoexaminetherepeat-
MY29,L =301(cid:2),thereisasignificantenhancementofdust abilityoftheaerosolcrosssectionsfromoneMartianyearto
s
over Hellas basin (30(cid:2)S–50(cid:2)S latitude, 290(cid:2)W longitude), thenext.Figures14and15showtwoexamplesatdifferent
with dust also extending to a higher altitude above the seasons,withbothcasesshowingthelongitudenear290(cid:2)W.
surface. In the case from MY 31, L =102(cid:2), the differences ThelateperihelionseasonisshowninFigure14,whenthere
s
are more minor, confined primarily to the structure of the is still a relatively large amount of dust in the atmosphere
low-latitude watericecloud. (although neither MY 29 nor MY 30 was a year with a
planet-encircling dust storm). There are differences in the
4.2. Variation WithSeason details, but the overall pattern for both dust and water ice
[37] A sampling of the seasonal dependence of dust and is similar. In both MY 29 and MY 30, dust extended to
watericeaerosolcrosssectionsretrievedfromCRISMlimb about 35–40km in height between 50(cid:2)S and 30(cid:2)N latitude
setsisshowninFigures12and13.Asexpected[e.g.,Smith, with an abrupt drop to 25–30km poleward of 60(cid:2)S, and in
2008, and references therein], there is significantly more bothyears,therewasasignificantenhancementofdustover
dust overall during the perihelion season (L =180–360(cid:2)) Hellas with the top of the dust layer extending to nearly
s
than during the aphelion season (L =0–180(cid:2)). The dust 50km. A water ice cloud capped the dust at a 35–45km
s
typically extends to a height of about 40km above the altitude between 20(cid:2)S and 20(cid:2)N latitude in both Martian
surface when significant dust is present and is confined years with disorganized higher altitude clouds poleward of
within10–20kmofthesurfacewhenthecolumn-integrated 30(cid:2)N. The isolated cloud near the south pole observed in
opticaldepthofdustislow(aphelionseasonandwinterhigh MY29wasnot presentinMY 30.
latitudes). Although all seasons show significant departures [42] Aphelion season cross sections (Figure 15) taken one
from well-mixed profiles, the equinoctal seasons tend to Martianyearapartalsoshowclosecorrespondence.Thereislit-
show the most structure in the vertical distribution of dust tledustobservedonthisseason(L (cid:3)100(cid:2))ineitherMY30or
s
and appear to be transitional times between the “dusty” MY31,andwhatisobservedisconfinedbelowa20kmalti-
andmore “clear”seasons. tude and primarily in the northern hemisphere. The low-lati-
[38] In these cross sections that are all taken at 290(cid:2)W tudeaphelion-seasoncloudbeltdominatesthewatericeaero-
longitude, there is consistently enhanced dust over Hellas sol cross section for both years, with its characteristic “tilt”
(30(cid:2)S–50(cid:2)Slatitude),withdustextendingtohigheraltitudes towardahigheraltitudeatmorenortherlylatitudes.
and showing greater vertical structure than in neighboring [43] Both of these examples represent nominal cases. Of
latitudes.Thecrosssectionsat100(cid:2)Wlongitude(notshown) course,duringlargeduststorms,wewouldexpectsignificant
showsimilartrends,exceptwithouttheHellasenhancement variationsinthe retrieved cross sections. Withluck, a future
andtypicallyslightly lessdustoverall. CRISM limb set will catch an active planet-encircling dust
[39] Watericeclouds(Figure13)arepresentinallseasons storminprogress.
but are most extensive around the northern hemisphere
summer solstice, when they extend much closer to the 5. Discussion
surface and have column optical depths that can approach
unityat2219nm.Cloudsareoftenobservedtocapthemain 5.1. ComparisonWithMarsClimate Sounder
dust layer, but there are exceptions. Clouds are observed Observations
almostalwaysasdiscretelayers,typically5–10kmindepth, [44] The MCS instrument [McCleese et al., 2007] on
with base levels as low as 20km above the surface at low boardMROalsoretrievesverticalprofilesofdustandwater
latitudes during the northern hemisphere summer season iceaerosols[Kleinböhletal.,2009,2011;McCleeseetal.,2010].
and as high as 40–50km above the surface for isolated The MCS retrievals were performed on observations taken in
clouds at mid latitudes in all seasons. It is likely that water the thermal infrared (12–22mm), while the CRISM retrievals
icecloudsalsoformnearthesurfaceatwinterhighlatitudes use near-infrared (1.2–3.7mm) observations. Although exactly
8
SMITHETAL.:CRISMLIMBAEROSOLS
MY 30, L = 166° MY 30, L = 193°
s s
60 60
m) 40 m) 40
k k
ht ( ht (
g g
ei 20 ei 20
H H
0 0
-90 -60 -30 0 30 60 90 -90 -60 -30 0 30 60 90
Latitude Latitude
MY 30, L = 265° MY 30, L = 293°
s s
60 60
m) 40 m) 40
k k
ht ( ht (
g g
ei 20 ei 20
H H
0 0
-90 -60 -30 0 30 60 90 -90 -60 -30 0 30 60 90
Latitude Latitude
MY 31, L = 0° MY 31, L = 41°
s s
60 60
m) 40 m) 40
k k
ht ( ht (
g g
ei 20 ei 20
H H
0 0
-90 -60 -30 0 30 60 90 -90 -60 -30 0 30 60 90
Latitude Latitude
MY 31, L = 102°
s
60
0 0.3 0.6 0.9
m) 40
k
ht ( Dust Mixing Ratio
g
ei 20
H
0
-90 -60 -30 0 30 60 90
Latitude
Figure 12. Cross sections showing the seasonal variation of dust vertical and latitudinal distributions.
Allretrievals are fromorbitsnear290(cid:2)Wlongitude.
simultaneousobservationsbetweenMCSandCRISMarenot of comparison. Overall, the two instruments produce similar
possible because the MRO spacecraft must be pitched away results.Althoughtherewasnotmuchdustintheatmosphere
from its normal nadir orientation for CRISM to observe the atthistime,bothinstrumentsshowedanenhancementindust
limb, it is still possible and instructive to compare nearly over Hellas (30(cid:2)S–50(cid:2)S latitude) with the dust extending
simultaneous observations. In general, MCS observes the tohigheraltitudesabovethesurfaceinthatregion.Thecorre-
forward limb while CRISM observesthe aftlimbduringthe spondencebetweentheretrievedwatericecloudsisevenmore
sameorbit,suchthatbothinstrumentsobserveasimilar part striking.Inthesouth,thereisalow-levelenhancementbelow
oftheatmospherewithinabout15min. 20km and poleward of 50(cid:2)S latitude and isolated clouds at
[45] Figure16showsonesuchcomparisonfortheCRISM 35km,40(cid:2)Sandat48km,50(cid:2)S.Ahighercloudnear60km,
limb set taken in October 2010 (MY 30, L =166(cid:2)) for the 40(cid:2)SlatitudeobservedbyMCSisbeyondtheretrievalrange
s
orbit near 290(cid:2)W longitude. Figure 16 (top) shows the dust of CRISM. In the north, a line of clouds is observed by
(left) and water ice (right) cross sections retrieved from both instruments that begins about 30km above the surface
CRISM,whilethebottomshowsthedustandwatericecross at 35(cid:2)N latitude and extends upward and poleward to
sections retrieved from MCS. The mixing ratios have been about45kmaltitudeat50(cid:2)N.Polewardofthoseclouds,both
scaledtoacommonwavelengthreference(2219nm)forease instruments observe a general weak enhancement in water
9
SMITHETAL.:CRISMLIMBAEROSOLS
MY 30, L = 166° MY 30, L = 193°
s s
60 60
m) 40 m) 40
k k
ht ( ht (
g g
ei 20 ei 20
H H
0 0
-90 -60 -30 0 30 60 90 -90 -60 -30 0 30 60 90
Latitude Latitude
MY 30, L = 265° MY 30, L = 293°
s s
60 60
m) 40 m) 40
k k
ht ( ht (
g g
ei 20 ei 20
H H
0 0
-90 -60 -30 0 30 60 90 -90 -60 -30 0 30 60 90
Latitude Latitude
MY 31, L = 0° MY 31, L = 41°
s s
60 60
m) 40 m) 40
k k
ht ( ht (
g g
ei 20 ei 20
H H
0 0
-90 -60 -30 0 30 60 90 -90 -60 -30 0 30 60 90
Latitude Latitude
MY 31, L = 102°
s
60
0 0.5 1.0 1.5
m) 40
k
ht ( Water Ice Mixing Ratio
g
ei 20
H
0
-90 -60 -30 0 30 60 90
Latitude
Figure13. Crosssectionsshowingtheseasonalvariationofwatericeverticalandlatitudinaldistributions.
Allretrievalsarefromorbitsnear290(cid:2)Wlongitude.
icemixingratiooverabroadrangeofaltitudes.Theisolated and contains a dust mixing ratio several times larger than
dust clouds at 35–50km at mid-northern and southern immediatelyaboveorbelowit,especiallyatitsnorthernmost
latitudes in the CRISM cross section are likely artifacts as extent.MoredetacheddustcloudsareapparentaboveHellas
theyalignwithstrongwatericeclouds. (35(cid:2)S–50(cid:2)Slatitude)inboththeCRISMandMCSretrievals.
[46] OneofthemoststrikingfeaturesoftheMCSretrievalsis The HADTM is present in some other CRISM retrievals
theso-calledhigh-altitudetropicaldustmaximum(HATDM), (e.g., the MY 31, L =0(cid:2) case shown in Figure 12 at 30(cid:2)N
s
whichisthelocalmaximainthedustmixingratiocommonly latitude and 30km altitude), but it is not prominent in many
observed over the tropics during the northern spring and CRISMretrievals.However,intheMCSdataset,theHADTM
summer seasons at altitudes of 15–25km [Heavens et al., appears much more frequently in retrievals using nighttime
2011]. Figure 17 shows a case where the HATDM was observations than in those using daytime observations
observedinsimultaneousobservationsbyCRISMandMCS [Heavensetal.,2011],andinthe(daytimeonly)simultaneous
(MY30,L =193(cid:2),orbitnear290(cid:2)Wlongitude).Inthiscase, MCS and CRISM observations, whenever the HADTM is
s
theHATDMshowsupasaclearmaximuminthedustmixing observedintheMCSretrievals,itisalsoobservedintheCRISM
ratiobetween20(cid:2)Nand45(cid:2)Nlatitudeataheightof25–30km retrievals. Comparisons of other CRISM limb sets with nearly
abovethesurface.Thedustlayerisnomorethan5kmthick simultaneousMCSretrievalsshowsimilarcorrespondence.
10