First International Conference on Mars Polar Science 3049.pdf

MAPPING THE MARTIAN POLAR ICE CAPS: APPLICATIONS OF TERRESTRIAL OPTICAL REMOTE SENSING METHODS. A. W. Nolin, Cooperative Institute for Research in Environmental Sciences, National Snow and Ice Data Center, Campus Box 449, University of Colorado, Boulder CO 80309-0449, USA ([email protected]).

Terrestrial Optical Remote Sensing of Snow sat TM) provide sufficient spectral resolution for sub- and Ice: With improvements in both instrumentation pixel mapping. Imaging spectrometer data would pro- and algorithms, methods for mapping terrestrial snow vide the ability to quantify mineral-ice mixtures and to cover using optical remote sensing data have pro- better characterize the Martian atmosphere. These are gressed significantly over the past decade. Multi- both needed for albedo determinations while only the spectral data can now be used to determine not only former is required for sub-pixel frost/ice mapping. the presence or absence of snow but the fraction of Perhaps the most significant terrestrial mapping ap- snow cover in a pixel [1,2]. Radiative transfer models plication is the potential use of the Mars Orbiter Laser have been used to quantify the non-linear relationship Altimeter (MOLA) to map grain size on the Martian between surface reflectance and grain size thereby polar caps. For dust concentrations less than about 1% providing the basis for mapping snow grain size from by weight, grain size remains the dominant surface surface reflectance images [3]. Because sub-pixel property that determines surface albedo. With its ex- mixtures of snow and other land cover types create ponential response to temperature, grain size is also erroneous estimates of snow grain size, the snow frac- the physical property with the highest sensitivity to tion information can be used in tandem with the grain changes in the thermodynamic state of the ice. size algorithm to limit its use to only those pixels that Changes in the energy balance of the surface frost have complete snow cover. Model-derived characteri- regions should be reflected in changing grain size zation of the bidirectional reflectance distribution even before a change in the total frost covered area function (BRDF) provides the means for converting would be detected. measured bidirectional reflectance to directional- Distinct differences exist between Mars and Earth hemispherical albedo. In recent work, this approach ice mapping conditions including surface temperature, has allowed climatologists to examine the large-scale ice type, ice-mineral mixtures and atmospheric prop- seasonal variability of albedo on the Greenland ice erties so a direct application of terrestrial snow and ice sheet [4,5]. This seasonal albedo variability results mapping methods may not be possible. However, ex- from increases in snow grain size and exposure of the pertise in mapping and interpreting terrestrial snow underlying ice cap as the seasonal snow cover ablates and ice will contribute to the inventory of techniques away. for mapping planetary ices. Furthermore, adaptation Martian Applications of Optical Remote Sens- of terrestrial methods will provide a basis for compari- ing Methods: With the current Mars Global Surveyor son of terrestrial and planetary cryospheric compo- and future missions to Mars, it will soon be possible to nents [6]. apply some of these terrestrial mapping methods to References: [1] Nolin A.W., Dozier J. and Mertes learn more about Martian polar caps. What is most L.A.K. (1993) Ann. Glaciol., 17, 121-124. [2] Rosen- needed for this purpose is multi-spectral optical im- thal W. and Dozier J. (1996) Water Resour. Res., 32, agery. The extent and variability of the ice caps and 115-130. [3] Nolin A.W. (1993) Rem. Sens. Environ., their seasonal CO2 frost covering can be mapped with 44, 232-238. [4] Nolin A.W. and Stroeve J. C. (1997), only a few spectral bands distributed through the visi- Ann. Glaciol., 25, 51-57. [5] Stroeve J.C., Nolin A.W. ble and near-infrared wavelengths. For instance, just a and Steffen K. (1997) Rem. Sens. Environ., 62, 262- few, well-situated bands (e.g. those used on the Land- 276. [6] Nolin A.W. (in press) JGR.