JOURNAL OF GEOPHYSICALRESEARCH, VOL. 103,NO. Ell, PAGES25,851-25,864, OCTOBER 25, 1998 Mapping the Martian polar ice caps' Applications of terrestrial optical remote sensing methods Anne W. Nolin National Snow and Ice Data Center, Universityof Colorado,Boulder Abstract. With improvementsin bothinstrumentation and algorithms,methods formapping terrestrial snow cover using optical remote sensing data have progressed significantlyover the past decade. Multispectral data can now be used to determine notonly the presence or absenceof snowbut the fraction of snowcover in a pixel. Radiativetransfer models have been used to quantifythe nonlinearrelationship betweensurface reflectance and grainsize thereby providing the basisfor mapping snowgrain size from surface reflectance images. Model-derived characterization of the bidirectionalreflectance distribution function provides the meansfor converting measuredbidirectional reflectance to directionM-hemisphericMalbedo. In recent work,this approach has allowed climatologists to examine the large scale seasonal variabilityof albedoon the Greenlandice sheet. This seasonal albedo variability resultsfrom increasesin snowgrain size and exposureof the underlyingice cap •s the se•sonMsnow cover •bl•tes •w•y. With the currentM•rs GlobM Surveyor and future missionsto Mars, it will soonbe possibleto apply someof these terrestrialmapping methods to learnmore about Martian ice properties,extent, andvariability. Distinct differences exist between Mars and Earth ice mapping conditions,including surface temperature, ice type,ice-minerM mixtures, and atmosphericproperties, so a directapplication of terrestrialsnow and ice mapping methodsmay not be possible.However, expertise in mappingand interpreting terrestrialsnow and ice will contributeto the inventoryof techniquesfor mapping planetaryices. Furthermore, adaptation ofterrestrial methods will provide a basis for comparisonof terrestrialand planetary cryospheric components. 1. ComparisonBetween Ices on Earth temperatureregimes, ice on Earth exists in a statethat is closeto the meltingpoint. Seasonalinsolation dif- and Mars ferences lead to substantial variability of snow cover, In comparativeplanetology, we often look at similar- seaice, and ice sheet conditions extent over the course ities betweenEarth and Mars. One of the most striking of a year. OverEarth's northern hemisphere, seasonal featuresof both planets,with criticallinkages to cli- extremes of snow and ice cover range from a January mate changeand hydrology,is the existenceof large maximumof about23% to an Augustminimum of 3% icecaps in their polarregions. Earth's cryosphere can, [Barry,1985], with significant regional and interannual in someways, serve as an analogfor the Martianice. variabilityin mostmonths [Robinson and Dewey, 1990]. Comparedwith Earth,we have only a limitedset of re- It is this combinationof spectral,spatial, and temporal motesensing observations for the Martian polar caps. variabilitythat makessnow and ice the mostdynamic Future Mars missionsand continuedadvancements in of Earth's surface cover types. instrumentationand algorithmswill enhanceour abili- Althoughsurface temperatures are typicallycolder tiesto compareand understand terrestrial and Martian than thosefound on Earth, the polar regionsof Mars polar regions. still experiencechanges in frost-coveredarea, surface While we know terrestrialsnow to be highly reflecting albedoand sublimation rates [Kieffer e! al., 1976;Farmer in thevisible, part to the spectrum, in the neaf-infrared et al., 1976;Paige and Ingersoll,1985; Haberle and wavelengthsits albedoranges from about0.8 to near Jakosky,1990]. On Mars, the extentof ice is less zero(see Fig. 1). Unlikemost other surface cover well-knownand may extend into the lower latitudes types,snow and ice haveextreme spectral variability asground ice [Paige, 1992; Mellon and Jakosky, 1995]. overa spanof only 2/•m. In comparisonto Martian Mellon and Jakosky,[1995] showed that groundice is stableon Mars at latitudesgreater than 600 and that icemay be presentnear the surface.Thermal models Copyright1998 by the AmericanGeophysical Union. and observationsof polygonalfeatures also show that Paper number 98JE02082. groundice may be presentat lowerlatitudes during 0148-0227/ 98 / 98JE-02082,09. O0 periodsof highobliquity of the Martianorbit [Mel- 25,851 25,852 NOLIN: MAPPING THE MARTIAN POLAR ICE CAPS i r = 50gm 200gm 500 gm 1000gm 0.5 I R (oo)at 60ø illuminationangle 1 0.5 1 1.5 2 2.5 wavelength,gm Figure 1. Modeled directional-hemisphericalspectral reflectances of snowfor different optically equivalent grain radii. lonet al., 1997]. Studiesof terrestrial permafrostmay the layered terrains and unknown quantity of dust in shedlight on the dynamicsand distributionof Martian the north polar cap [Kieffer, 1990]. Clark and McCord, ground ice. [1982]modeled the reflectancespectrum of the polar cap As with Earth's fresh water, most Martian water re- surfaceusing a linear mixture of 60% medium-grained sidesin the polar regionsin the form of ice caps. While water frost and 40% clay minerals. They estimatedthat both the northern and southern Mars polar caps are the permanent cap could contain 10-40% by weight of bright comparedwith non-icecovered regions, the com- mineral particulates. The seasonalcycle of CO2 depo- positions of the ice caps differ. The more extensive sition is also affected by Martian dust. Atmospheric north polar cap consists of H20 ice and the smaller dust is removed as CO2 condenseson dust particles south polar cap is thought to be mostly or entirely CO2 and precipitates onto the ice cap surface. These dust ice [Kieffer et al., 1976;Farmer et al., 1976]. Modeling particles remain in the seasonalsurface deposits and and observational evidence support the idea of little, if are thought to control both surface albedo and subli- any, CO2 in the northern hemispherepermanent ice cap mation rates [Pollack et al., 1990]. This is a different [Mellon,1996; Clark and McCord,1982]. case than for Earth's polar caps that are nearly pure Polar and ground ice are thought to be important ice with generallyless than 0.5 ng/g elementalcarbon componentsof the Martian hydrologiccycle where sub- [Warren and Clarke,1990] and lessthan 26 ng/g dust limation processesdominate the ice-atmosphereinterac- [Kumai, 1976; Royer et al., 1983]. Changesin surface tions [Haberleand Jakosky,1990, Mellon and Jakosky, albedo and energy balance for terrestrial ice sheetsare 1995]. Measurementsof Martian atmosphericwater va- primarily driven by changesin surface snow grain size por showlarge changes in water vaporamounts through [Nolin and Stroeve,1997]. Thus comparisonsbetween the courseof a year [Jakoskyand Farmer, 1982] and Earth and Martian ice-atmosphereinteractions are not that the north polar cap is a likely source[Haberle and straightforward. Jakosky,1990]. A significant differencebetween Martian and terres- 2. Some Relevant Martian Ice Mapping trial ices is the amount of light absorbingparticulates in the Martian polar ice caps and seasonalfrost de- Questions posits. Dust is a significantcomponent of Martian ice, What is similar between Earth and Mars is that their with roughly equal parts of dust and water present in extensiveice capsappear •o record,respond to, andper- NOLIN: MAPPING THE MARTIAN POLAR ICE CAPS 25,853 turb climate change.On Earth, polar ice coringefforts 3. Terrestrial Snow Mapping Using have generateda climate time seriesextending back Optical Remote Sensing over 110,000 years. These data document changesin Earth climate driven by periodic variability in Earth- In this section,selected snow/ice mapping methods Sun geometry[Johnsen et al., 1997; Yang et al., 1997] are described. These optical remote sensingmethods as well as other, shorter-termfluctuations [Barlow et addressmapping snow/ice extent, grainsize and albedo al., 1993; White et al., 1997]. Other ongoingpolar and thus are considered candidates for adaptation to climatological investigationsare examining changesin Martian ice cap mappingendeavors. It shouldbe noted ice sheet elevation, ice flow, calving rates, surface en- that passivemicrowave and radar techniquesare widely ergy balance,and snowaccumulation rates over the ice used to map snow and ice on Earth. However,because sheets. These short-timescale variations, involving ice- such instrumentation has not been used in Mars map- Ocean-atmosphereinteractions, are not well-quantified.ping, and are not currentlyplanned, the focusof this The salient questions on Mars are in some ways article is on the use of optical remote sensingmethods. more exploratory in nature. For instance, although the As background,a brief overviewof the optical prop- Mariner 9 and Viking Orbiter 2 have provided imagesof ertiesof snowis given. However,for more detail on this Mars' polar regions,the exact compositionof eachpolar subject, the reader is referred to the more comprehen- cap remains somewhat obscured. Changesin ice extent sivework of Warren, [1982]. have been difficult to quantify becauseof lack of multi- spectral image data. Mapping ice extent and polar lay- 3.1. Optical Properties of Snow ered depositsis key to understandinglong term changes It is well known that the optical properties of ice can in Martian climate that are thought to be driven by be directly related to snowpackphysical properties such quasi-periodicorbital variations[Howard et al., 1982]. as grain size and albedo. An aggregationof ice particles On shorter timescales,the spatial extent and tempo- will reflect nearly