DETERMINATION OF THE WATER CONTENT OF THE OLYMPIA UNDAE FORMATION. W.C. Feldman1, R.C. Elphic1, S. Maurice2, D.J. Lawrence1, M.T. Mellon3, J.J. Hagerty1, T.H. Prettyman1, 1Los Alamos National Laboratory, Los Alamos, New Mexico, USA ([email protected]), 2Centre d’Etude Spatiale des Rayonnements, Toulouse, France. 3Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, Colorado, USA.

Introduction: A map of the water content of without including any water of hydration of the surface soils near the north pole of Mars is very CaSO4 that OMEGA observes. important because this region controls the pre- References: [1] Langevin et al., Nature, 37, sent climate of Mars and also provides an essen- 1585-1586, 2006. [2] Elphic, R. C. et al., 36th tial testing ground for theories of ground-ice LPSC, #2297. [3] Prettyman et al., 36th LPSC, stability, atmospheric water-vapor exchange, and #1384, 2005. the possible existence of a near-surface water /ice table. Visible images of Olympia Undae show that it has a crescent shape spanning 120o to 240o east longitude at about 80o latitude and is covered by relatively dark sand dunes without any water-ice cover. Recent results from the OMEGA Infrared Spectrometer aboard Mars Express [1] showed that the eastern edge of this crescent is covered by gypsum or bassanite. The Olympia Undae Formation is relatively small (~200 x 800 km) and therefore cannot be well characterized by the Mars Odyssey Neutron Spectrometer (NS), which has an intrinsic resolution of 600 km. However, a spatially deconvolved version of the NS data may attain a resolution of 250 km. We present here our first attempt to generate a deconvolved map of water content of the Olympia Undae Formation. Deconvolution Technique: The de- convolution technique used here is very similar to Jannson’s method [2,3], Ik+1 = Ik + r (O - p⊗Ik), where Ik+1 is the current estimate of the restored image, Ik is the previous estimate, r is a relaxation function, O is the original smoothed image, p is the total effective point spread function (equivalent to the Gaussian smoothed NS response function), and ⊗ denotes a convolution operation. Results: The original map of epithermal neutron counting rates is shown in Fig. 1a, and that of the deconvolved rates is shown in Fig. 1b. These rates were used to estimate lower bounds to the water-equivalent hydrogen (WEH) abundance, shown in Fig. 1c. Inspection shows that the minimum WEH abundance is close to 10% by mass and is centered between 180o and 240o at about 80o latitude. This mass fraction is expected if the sand grains are close-packed spheres with a pore volume of about 30%,