Icarus 196 (2008) 422–432 www.elsevier.com/locate/icarus

Hydrogen content of sand dunes within Olympia Undae

W.C. Feldman a,∗, M.C. Bourke a,b, R.C. Elphic c,S.Mauriced, J. Bandfield e, T.H. Prettyman c, B. Diez d,D.J.Lawrencec

a Planetary Science Institute, 1700 E. Fort , Suite 106, Tucson, AZ 85719, USA b Oxford University Centre for the Environment, University of Oxford, Oxford, OX13QY, UK c Los Alamos National Laboratory, Mail Stop D466, Los Alamos, NM 87545, USA d Centre d’Etude Spatiale des Rayonnements, 9 av. Colonel Roche, 31500, Toulouse, France e Arizona State University, School of Earth and Space Exploration, Tempe, AZ 85287-6305, USA Received 13 April 2007; revised 20 July 2007 Available online 15 December 2007

Abstract Neutron currents measured using the Odyssey Neutron Spectrometer, seasonally varying temperatures measured using the Thermal Emis- sion Spectrometer, and visible images measured using the High Resolution Imaging Science Experiment (HiRISE) are studied to determine the water content and stratigraphy of Olympia Undae. Both the neutron and thermal infrared data are best represented by a two-layered model having a water–ice equivalent hydrogen content of 30 ± 5% in a lower semi-infinite layer, buried beneath a relatively desiccated upper layer that is 9 ± 6g/cm2 thick (about 6 cm depth at a density of 1.5 g/cm3). A model that is consistent with all three data sets is that the dunes contain a top layer that is relatively mobile, which overlays a niveo-aeolian lower layer. The geomorphology shown by the HiRISE images suggests that the bottom layer may be cemented in place and therefore relatively immobile. © 2007 Elsevier Inc. All rights reserved.

Keywords: Mars, polar geology; Mars, polar caps; Ices

1. Introduction Saunders and Blewett, 1987) that are not cemented in bulk. Alternatively, the erg is thought to be predominantly com- The most extensive sand dune deposit on Mars encircles posed of sand sized andesite or weathered- fragments the north-polar residual water–ice deposit. It has its largest (Edgett and Christensen, 1991; Bandfield, 2002; Edgett et al., contiguous areal extent between 155◦ and 230◦ E and 78◦ 2003). In addition, the east end of Olympia Undae has a strong and 83◦ N, which has been recently renamed Olympia Un- gypsum signature in OMEGA (Observatoire pour la Miner- dae (Tanaka et al., 2005). Although the dunes have been stud- alogie, l’Eau, les Glaces et l’Ativité) data (Langevin et al., ied extensively using visible and infrared imaging data of the 2005). The highest concentration of gypsum is located up- Viking orbiters (e.g., Cutts et al., 1976; Tsoar et al., 1979; wind of the dune field, extends across the dunes, decreasing Ward and Doyle, 1983; Thomas, 1987; Lancaster and , in concentration downwind, suggesting deposition via an ae- 1990), many fundamental issues regarding the origin, evolution, olian dust plume. Alternate sources of gypsum signature de- and internal structure of the dunes remain unknown. For ex- tected in the dunefield are thought to be from the polar-layered ample, the apparent thermal inertia of the dunes is modest to deposits’ basal unit, exposed between the dunes and along low (Paige et al., 1994) suggesting that they are made up of the ice cap edge. However, this source is not supported in irregularly-shaped cemented dust and sand fragments (Greeley, the OMEGA or CRISM (Compact Reconnaissance Imaging 1986) or mixtures of ice and silicate dust (Saunders et al., 1985; Spectrometer for Mars) data analysis (Langevin et al., 2005; Roach et al., 2007). * Corresponding author. All the foregoing models suggest that the surface sediment E-mail address: [email protected] (W.C. Feldman). of these dunes should be mobile, and recent findings indicate

0019-1035/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.icarus.2007.08.044 Hydrogen content of sand dunes 423 significant sediment transport is possible from small dunes in the north polar region (Bourke et al., 2008a). However, it does appear that most polar dunes are relatively immobile and may be stabilized by surface crusting (Schatz et al., 2006) or ce- mented by niveo-aeolian deposits (Bourke, 2004). In order to help resolve some of these issues, we report here on the results of a study of the hydrogen content of the north-polar sand dunes within Olympia Undae. Thermal and epithermal neutron currents measured using the Mars Odyssey Neutron Spectrometer (MONS) are analyzed for this purpose. These data yield an estimate of the water-equivalent hydrogen (WEH) mass fraction in near surface material, which can pro- vide limits on the degree of ice content given an assumption of dune mineralogy and porosity. Although the Olympia Undae Formation is large (∼300 × 800 km, which corresponds to the central contiguous portion of the dune fields in the geologic map of Tanaka, 2005)it Fig. 1. Geometry of the two-layer model of surface used to analyze is smaller than the MONS instrument footprint. It can there- thermal and epithermal neutron currents measured using MONS. fore not be well characterized by the MONS, which has an intrinsic Full Width at Half Maximum spatial resolution of rents), all measured data have been corrected for a constant about 600 km. However, a spatially deconvolved version of the 16 g/cm2 atmosphere, the water mass fraction of the upper MONS data may attain a resolution of 300 km. We present layer is chosen to be 0.01, and the elemental composition of here our preliminary deconvolved maps of the water content both soil layers is chosen to be an average of that measured and stratigraphy of Olympia Undae for this purpose and inter- at the Viking 1, Pathfinder, and Mars Exploration rover (at pret them in terms of potential water–ice content and layering. both Meridiani and ) landing sites (see Diez et al., 2007; The resultant layering is checked using an analysis of the sea- Feldman et al., 2007 and references therein). sonal variation in temperature of the dune field using the Ther- The first deconvolution technique used here is simi- mal Emission Spectrometer (TES) (Christensen et al., 2001; lar to the van Cittert deconvolution method (Van Cittert, 1931; Bandfield, 2007). Jansson, 1997), Ik+1 = Ik + r(O − p ⊗ Ik), where Ik+1 is the current estimate of the restored image, Ik is the previous es- 2. Neutron data reduction procedure timate, r is a relaxation function, O is the original smoothed image, p is the total effective point spread function (equiva- Thermal and epithermal neutron currents measured using lent to the Gaussian smoothed MONS response function) and MONS between February 2002 and December 2006 were an- ⊗ denotes a convolution operation. The iteration procedure was alyzed in multiple ways to determine limits on the WEH terminated just before the water content of the residual po- mass fraction of Olympia Undae dune material and the po- lar cap exceeded 100%. Whereas the epithermal neutron map tential layering of these deposits. Results returned using two was deconvolved using the response function of the downward- different data-reduction algorithms (Prettyman et al., 2004; looking prism of MONS by itself (see Feldman et al., 2002 Maurice et al., 2007) were inter-compared and then averaged and Boynton et al., 2004 for a description of the instrument), in order to provide an estimate of systematic errors result- the thermal neutron map was deconvolved in two steps. The ing from this part of the analysis procedure. Next, we im- proved the spatial resolution of resultant WEH maps using first step involved deconvolving the response functions of the two different deconvolution techniques in order to place lim- forward- and backward-looking prisms separately. The second its on the modifications of the counting-rate data as measured, step reconstructed the deconvolved thermal neutron map by tak- that are produced by application of these procedures. Our fi- ing the difference between the separately deconvolved forward nal products consist of lower bounds of the water content of and backward facing prism counting rates. these dunes by applying: (1) a single-layered model of sur- The second technique used the Pixon method developed by face soils using the measured epithermal neutron currents alone, Puetter (1995) and explored by Lawrence et al. (2007) using and (2) a two-layered model using both the measured ther- the Th map of the Moon measured using the Lunar Prospector mal and epithermal neutron currents. As shown in Fig. 1, Gamma-Ray Spectrometer. In contrast to the Jansson method the single-layered model is a special case of the two-layered that uses a fixed grid of measurements, the Pixon technique is model. Whereas the burial depth, D, of a water-rich permafrost a spatially adaptive, image restriction method. This method re- layer is chosen to be zero in the single layered model, it is turns the smoothest possible image that is consistent with the nonzero in the two-layered model. In both models, the forma- uncertainties in the data. This goal is reached by varying its tion is assumed to extend uniformly to the edge of the MONS mesh size to match the information content of an image rela- field of view. In order to constrain this model using only two tive to its noise uncertainties (Puetter, 1995) and is applied to measured quantities (the thermal and epithermal neutron cur- the neutron counting rates in all three prisms separately. 424 W.C. Feldman et al. / Icarus 196 (2008) 422–432

3. Results layered model amounts to WEH(min) = 0.272±0.025 at 81◦ N and −171◦ E. In order to place this position in perspective, it is located Stereographic projections of the WEH content of surface by an ‘X’ in the geologic (Tanaka et al., 2005) and elevation layers at high northern latitudes are shown in Fig. 2. Single- (Zuber et al., 1998) maps of northern high latitudes, shown in layered WEH mass fractions superimposed on a shaded relief Figs. 3a and 3b, respectively. This minimum is seen to be near map measured using Mars Orbiter Laser Altimeter (MOLA) the azimuthal center of the Olympia Undae formation (Fig. 3a) ( et al., 2001) poleward of 45◦ NareshowninFig. 2a. but offset towards its equatorial edge in latitude. It is also seen Inspection shows a water–ice residual cap that is slightly offset ◦ to be on the sloping margin of the sand sea, slanted towards the from the north pole toward 0 longitude. A secondary maxi- equator, shown in Fig. 3b. mum in WEH covers the Scandia Colles formation (Tanaka et Fig. 2b gives the WEH mass fraction of the semi-infinite ◦ − ◦ al., 2005) centered at about 70 N and 130 E. At our spatial lower layer, Wdn, using a two-layered model (Diez et al., 2007; resolution, this formation is connected to an arc of WEH-rich ◦ Feldman et al., 2007). The top layer is assumed to have a WEH terrain that extends at constant latitude to about 150 E. Be- mass fraction of 0.01. Although both single-layer and two-layer tween this arc and the residual water–ice cap is terrain that has a models give the same overall lateral structure of the WEH de- relative minimum in WEH content co-located with the Olympia posits, the two-layer model gives a slightly higher minimum Undae formation. The minimum WEH mass fraction in the one- WEH mass fraction, Wdn(min) = 0.35 ± 0.03. Fig. 2c gives the burial depth of the bottom, WEH-rich layer, in g/cm2. Inspection shows a ring centered at 60◦ N of appar- ent maximum burial depth (Feldman et al., 2007) that encircles a surface deposit of water ice that covers the north pole. The minimum WEH deposit that covers the sand dune formation in Olympia Undae corresponds to 7 ± 3g/cm2, which is at 4.7 cm depth if the density is 1.5 g/cm3. The results of the Pixon deconvolution procedure are shown in Fig. 4. Meridional transects of measured epithermal neutron counting rates and the WEH content derived from the one-layer model through the center of the deposit at −171◦ E, are shown in Fig. 4a. A relative maximum counting rate occurs at about 80◦ N, which corresponds to a minimum WEH mass fraction of 0.20. Maps showing the deconvolved spatial distribution of counting rates and WEH abundances are shown in Figs. 4b and 4c, respectively. Both these maps and the transects can only be considered approximate at this time because a study of the residual counting rates between the measured rates and the con- volved model rates do not appear to be random [they appear to contain clumps, see Lawrence et al. (2007) for a discussion of residuals in the context of spatial deconvolution]. Although we do not know what the origin of this result is, we note that the difference in minimum WEH abundance derived from the two deconvolution techniques (Jansson and Pixon) amount to 0.07 with an average value of 0.235. Quantitative results of all determinations of WEH mass frac- tions and burial depths at the spatial minimum located at 81◦ N and −171◦ E are collected in Table 1. Also included is an es- timate of the open pore volume, PV, between soil grains that would be needed to hold the minimum water mass fractions of WEH or Wdn. The density of interstitial water ice, ρI , was as- 3 sumedtobe0.92g/cm and that of the sand grains, ρs ,was 2.65 g/cm3 giving   PV = (ρsWdn)/ ρI + (ρs − ρi)Wdn .

◦ 4. Discussion Fig. 2. Maps of the water-equivalent hydrogen content of Mars north of +45 latitude. The top panel shows the WEH mass fraction using a one-layered model, and the middle panel shows the WEH mass fraction of the lower layer Our best fitting neutron model of Olympia Undae requires at in a two-layered model. The lowest panel gives the burial depth of the lower the least, a two-layered structure in order to fit both the mea- layer in g/cm2. sured thermal and epithermal neutron currents. The topmost Hydrogen content of sand dunes 425

◦ ◦ Fig. 3. A geologic map of Mars north of 75 (a: Tanaka, 2005) and a map of the elevation north of 75 (b: Zuber et al., 1998). The X in both panels gives the location of minimum WEH content of surface soils. layer is relatively desiccated, chosen here to contain 0.01 by 2003; Armstrong et al., 2005; Fergason et al., 2006a, 2006b; mass of water-equivalent hydrogen, has an average thickness of Bandfield, 2007), the seasonal TES temperatures were fit using 7 ± 3g/cm2, and overlays a semi-infinite lower layer that con- a nonlinear least-squares fitting routine (model parameters are ◦ ◦ tains 0.295 ± 0.05 by mass WEH. listed in Table 2). The data were fit between Ls of 85 and 170 The seasonal variation in surface temperatures measured by (avoiding seasonal CO2 frost) allowing top layer thermal iner- the TES spectrometer are also consistent with a layered sur- tia and depth to a water–ice rich high inertia layer to vary as face. Ice-cemented materials will have a higher thermal inertia free parameters. The best-fit model determined a top layer ther- than poorly consolidated sand or soil. At high latitudes, both mal inertia of 280 J m−2 K−1 s−1/2 with the ice-bearing layer at the diurnal and seasonal temperature variations are prominent. 7.5 cm depth (Fig. 5). The fit is considerably worse (RMS er- For example, Titus et al. (2003) and Armstrong et al. (2005) rors of 6.4 K versus 2.8 K) with the absence of a high inertia used these combined cycles apparent in seasonal day/night tem- layer at depth. The determination of depth is not precise, how- perature data to distinguish layered and nonlayered materials. ever. Top layer inertias and water ice depths can be changed Bandfield (2007) also used seasonal temperatures to predict wa- to 200 J m−2 K−1 s−1/2 and 3 cm or 340 J m−2 K−1 s−1/2 ter ice depths, but without the use of the daytime (low angles of and 11.6 cm without significantly increasing the RMS error solar incidence) temperatures. The daytime temperatures were (<0.5 K; see Fig. 5). This range is well within the water ice ta- not used because of the dominant influence of slopes, surface ble depth predicted by the MONS determination. Previous work albedo, and atmospheric properties and their associated uncer- assuming a nonlayered surface and using Viking Infrared Ther- tainties on the derivation of surface thermophysical properties. mal Mapper (IRTM) data determined thermal inertia values of This analysis relies on the variable seasonal influence on night- 200 to 350 J m−2 K−1 s−1/2 in the Olympia Undae region near time surface temperatures during the frost-free summer season 80◦ N, −170◦ E, consistent with the top layer inertia results to predict water ice burial depths. shown here (Paige et al., 1994). TES spectrometer-derived surface temperatures acquired at In addition, the model assumes a level surface, when in fact a local time of 0200–0300 from 79.5◦ to 80.5◦ N and −165◦ the dune surfaces have high slopes of a variety of azimuths, ◦ ◦ to −175 E were averaged into 4.5 bins of Ls . The data in- adding a potential surface temperature bias. To gain an estimate cluded all three martian northern summers when the TES spec- of the upper magnitude of these effects, the modeled radiance of trometer was operating and emission angles were restricted equal proportions of north//east/west oriented 30◦ slopes to less than 20◦. Using a two-layered thermal model (devel- were averaged and converted to apparent brightness tempera- oped and provided by H.H. Kieffer and used in Titus et al., tures at 25 µm (approximately the same wavelength used for 426 W.C. Feldman et al. / Icarus 196 (2008) 422–432

Fig. 4. (a) Meridional transects of measured epithermal neutron counting rates and the WEH content derived from the one-layer model through the centerofthe ◦ deposit at −171 E. Maps showing the deconvolved spatial distribution of counting rates (b) and WEH abundances (c). The slanted dashed line in (b) gives the location of the transect.

Table 1 may have little effect on the precision of the surface tem- One- and two-layered model parameters for the water content of the sand-dune perature modeling, they almost certainly influence the local ◦ − ◦ formation within Olympia Undae at 81 Nand 171 E depth of the water ice table (e.g., Sizemore and Mellon, 2006; Model Parameter Value Aharonson and Schorghofer, 2006). South facing slopes receive One layer, as measured WEH mass fraction 0.272 ± 0.025 the greatest amount of annual insolation and north facing slopes ± One layer, deconvolved WEH mass fraction 0.235 0.035 receive the least, resulting in relatively deep and shallow water Two layer, as measured WEH lower layer 0.35 ± 0.030 Two layer, as measured Burial depth 7 ± 3g/cm2 ice stability depths, respectively. It should be assumed that the Two layer, deconvolved WEH lower layer 0.295 ± 0.05 modeled water ice table depths represent an intermediate value Two layer, deconvolved Burial depth 6 ± 3g/cm2 within a heterogeneous region. One layer, as measured Pore volume 51.5 ± 3.5% In support for our conclusion that a single-layered model is Two layer, deconvolved Pore volume 54.5 ± 5.7% not sufficiently complex to fit the neutron and thermal data, newly acquired images of the sand dunes within Olympia TES data surface temperature determinations). This distribu- Undae using High Resolution Imaging Science Experiment tion of slopes resulted in a slightly shallower water ice (6.5 (HiRISE) show a rather complicated geomorphology, as shown versus 7.5 cm) and lower top layer thermal inertia (240 ver- in Fig. 6. The dunes form transverse linear arrays separated by sus 280 J m−2 K−1 s−1/2) in the best fit model. While slopes apparently flat basement material, which in patches has a rel- Hydrogen content of sand dunes 427

Table 2 in polar deserts on Earth and consist of inter-bedded sand and Thermal model parameters used for fitting TES temperature data ice that are often indurated and resistant to erosion. On Earth, Layer properties niveo-aeolian deposition occurs due to the burial of precipitated − Density of top layer 1650 kg m 3 snow and frost layers by rapidly aggrading sand. Alternatively, − Density of bottom layer 2018 kg m 3 − − snow can be transported simultaneously with sedimentary parti- Specific heat of bottom layer 1040 J kg 1 K 1 − − cles and deposited on the dune. For martian aeolian landforms, Specific heat of top layer 837 J kg 1 K 1 − − the definition has been extended to include diffusion of water- Thermal conductivity of bottom layer 2.5 W m 1 K 1 vapor molecules from the atmosphere to fill sub-surface pore Model layers and iterative properties volumes (Bourke, 2004). Coupled with the occurrence of niveo- First layer thickness (skin depth) 0.20 aeolian deposits is the potential for melting (perhaps during Succeeding layer thickness ratio 1.15 Number of layers 29 previous high obliquity cycles), evaporation and sublimation Model iterations before output (80 iter./yr) 160 of volatiles that are located close to the surface. This phase change should be associated with morphological changes that Surface properties Surface albedo 0.11 are visible on the surface of the dunes. Indeed, we have ob- Surface emissivity 0.98 served geomorphic features on the dune and interdune surfaces Visible albedo of CO2 frost 0.65 that suggest subsurface volatiles are, or have been present. Thermal infrared emissivity of CO2 frost 1.0 For example, circular depressions are located periodically Dust properties along the dune crest (Fig. 8b). These are between 5 and 7 m Dust opacity (visible wavelengths) 0.2 in diameter and have estimated depths (using shadow length) Visible to 9 µm wavelength opacity ratio 2.0 of 0.7 m. Additional elongated depressions (Fig. 8c), also lo- Henyey–Greenstein asymmetry factor 0.50 cated along the dune crest, have rounded end planforms and Single scattering albedo 0.90 ◦ Twilight extension angle 1.0 are proposed to form by the coalescence of individual circu- lar depressions. These forms occur frequently along the dune crests and modify significant lengths of dune crest morphol- atively high albedo that reveals patterned ground beneath. If ogy (Fig. 8a). These features may be similar to sinkholes found these patches are not surface deposits of water ice, they must on polar desert dunes on Earth (Koster and Dijkmans, 1988). only contain a thin veneer of bright dust so that the patterned Terrestrial dune sinkholes are formed by the loss in volume of ground beneath shows clearly through. underlying volatiles. These features are found elsewhere in the These patches are nestled into the corners of rectilinear pat- martian polar deserts. Their position along the dune crest and terned dunes with relatively flat interdune areas. The larger not elsewhere may be linked to larger diurnal insolation re- dunes are aligned roughly east–west and are separated by broad ceipts at that location on the dune. These pits, then, identify interdune areas that have smaller narrower dunes arranged or- the locations of denivation, but do not necessarily map out the thogonal to the larger dunes (Fig. 6). A simplified schematic potentially wider distribution of sub-surface volatiles. representation of a cross section through the larger dunes is Sinuous, narrow, rectilinear and branching depressions, shown in Fig. 7. Measurements of the spatial structure of a measuring 25 to 70 m long and <1 m wide, are observed subset of this dune field were made from a HiRISE image to on the lee face of the larger dunes and on both flank slopes provide an estimate of its spatial dimensions. These dimensions of the smaller orthogonal dunes (Fig. 9). They truncate the are presented in Table 3. ubiquitous small-scale transverse ripple features (λ = 2.5m, Using the dimensions in Table 3 along with the dimensions with estimated heights of <0.5 m) on the dune surface. These of a representative sample of the bright patches (which are features suggest cohesion of the surface sediment and the oper- not given in Table 3), we estimate that the total area of the ation of tensional stresses. They are similar to tensional cracks bright patches clearly visible in the HiRISE image approxi- reported on dune surfaces in polar deserts. On Earth, the cohe- mates 0.8%. Assuming that this estimate is representative of sion is a function of moisture content (from snowmelt) and the the entire sand sea, we believe that the relative area covered by source of tensional stress is a loss of subsurface volatile volume these patches is so small that even if they are composed of 100% (Koster and Dijkmans, 1988). On Mars, cohesion of the surface water ice, they would not contribute significantly to the mea- sediment may be provided by crusting (Sullivan et al., 2004; sured thermal and epithermal neutron currents. Nevertheless, Richter et al., 2006). These cracks, along with the absence of the fact that a significant fraction (0.13) of the entire formation active dry grain flow on the dune avalanche face, suggest that is relatively flat, of variable sediment cover thickness, and the sand transport has not been active recently. High-albedo sur- remainder consists of high dune morphology (50–75 m), could faces exposed in the interdune area have a number of associated be reason enough to suspect that a simple two-layer model that morphologies. High albedo arcuate ridges trending in the direc- extends to the horizon in the MONS field of view (represented tion of the larger dunes may be indurated remnants of barchan in Fig. 1) should not be able to represent reality. dune strata that remained in situ as the dunes migrated down- A closer look at a HiRISE image for the dunes in Olympia wind during an earlier dune-building phase. Similar features Undae (PSP_001736_2605) suggests the presence of niveo- are reported from the permafrosted dunes in Valley aeolian deposits. These are mixed deposits of wind-driven Antarctica (Morris et al., 1972), and from the chemically in- snow, sand, and dust (Cailleux, 1978). Such deposits are found durated dunes of White Sands National Park, New Mexico 428 W.C. Feldman et al. / Icarus 196 (2008) 422–432

◦ ◦ Fig. 5. TES-derived and model seasonal surface temperatures at 80 Nand−170 E. (a) TES measured temperatures (solid) compared with modeled temperatures with a top layer thermal inertia of 280 (MKS units) and ice at 7.5 cm depth (dashed). Dotted lines give an estimate of error in the TES measurements based on ±3K at 150 K. (b) TES-measured temperatures (solid) compared with a variety of modeled temperatures; top layer inertia of 280 (MKS units) with ice at 7.5 cm depth (short dash); top layer inertia of 280 (MKS) with no ice present (long dash); top layer inertia of 200 (MKS) with ice at 3 cm depth (dotted); top layer inertia of 340 with ice at 11.5 cm depth (dash–dot).

(McKee, 1966). However the occurrence of these arcuate strata albedo to the adjacent aeolian sand. This morphology is sug- in Olympia Undae with polygonal terrain (Fig. 10b) combined gestive of permafrost and cryoturbation processes (Mangold, with the modeled stability of ground ice at these northern lati- 2005). tudes, suggests that they are more likely to be exposed indurated Adjacent to the polygonal pattern is a patch of amorphous permafrost layers rather than geochemically cemented strata. sand (Fig. 10b). This is striking because of the ubiquitous orga- The polygonal pattern of the high-albedo interdune strata ob- nization of the adjacent sand into ripple features and polygon served in HiRISE images (Roach et al., 2007) suggests to us the ridges. The patch forms an irregular boundary with the ripples presence of volatiles. In the lower part of Fig. 10b the polygons but does not appear to overlie them. We suggest that this mor- are bound by raised ridges, composed of sediment similar in phology combined with its location suggests a loss of volatiles Hydrogen content of sand dunes 429

Table 3 Summary of Olympia Undae morphometrics Parameter Value

Large dune wavelength λL (m) 470 Large dune width WL (m) 375 a ◦ ◦ Large dune height HL (m) (33 and 25 )75 (100–75) Area of substrate exposure (% of total area) 0.8 Small dune wavelength λS (m) 230 Small dune width WS (m) 80 Fractional area of large dune f1 = (WL/λL) 0.8 Fractional area of small dune f2 = (Ws/λS ) 0.35 Fractional area of interdune (1 − f1)(1 − f2) 0.13 Notes. Dune morphometry at the sample site in Olympia Undae. However the presence of small ripples on the lee slope suggest that the dune flank does not ◦ lie at the angle of repose (assumed to be 33 ). A lower angle is consistent with dune inactivity. We have therefore estimated dune height assuming an alterna- ◦ tive slope of 25 . This lower estimate is likely more appropriate for dunes in this study area. a Dune height was estimated using the slip face length method outlined in Bourke et al. (2006).

westward across the dunefield (Langevin et al., 2005). Subse- quent analysis using CRISM data found a relatively stronger gypsum signature along the dune crests than in the inter-dune areas. This signature may be a result of a coarser grain size along the crest, cemented grains, or higher concentrations of gypsum (Roach et al., 2007). If this deposit (or in fact any other contaminant deposit that consists of hydratable minerals) has intercalated the sand grains then this would be reason by itself why a simple two-layered model should not apply. However, gypsum may not indicate a problem for the two-layer model because the material visible in CRISM data is confined to a thin mantle on top of the crests of the dunes and does not ap- ◦ ◦ Fig. 6. HiRISE image TRA_000861_2580, 78.0 N, 303.9 E, image width is pear to be a significant contaminant of the macroscopic dune 6.4 km. formation on a scale of the neutron field of view (Roach et al., 2007). However, its distribution at depth is presently un- known. Nevertheless, if the sand grains are composed of sediments containing other hydratable minerals, then our estimated open pore volume will be lower. However, if the sand-composition model of mixtures of ice and silicate dust (Saunders et al., 1986; Saunders and Blewett, 1987) is adopted, and an open pore vol- ume of sand before ice cementation is chosen to be 50%, then Fig. 7. A simplified schematic representation of a cross section through the our estimated WEH content of a single semi-infinite sand de- large array of sand dunes in Olympae Undae. These dunes form a linear array posit could fit the current paradigm of these sand dunes. For that can be characterized by a wavelength, λ, a height, H , a total width of the example, our results are consistent with a fully ice-filled pore dune, W , and the inter-dune width, λ − W . volume at depth covered over by a relatively desiccated, loose sand cover. Such a structure would then have a relatively low resulting in the collapse of polygon ridges (or indeed ripples) at thermal inertia, as determined using Viking Orbiter (Paige et this location. This morphology therefore suggests that the en- al., 1994) and TES data, yet be relatively immobile because it richment and loss of volatiles from the surface sediment is an would be fully cemented at depth. This suggests that frozen wa- active process on Mars and one that affects surface morphol- ter (ice or snow) is present in the polar sand dunes on Mars and ogy. supports the assertion that some dunes in the north polar re- Another potential component of the composition of Olympia gion are composed of niveo-aeolian deposits (Bourke, 2004). Undae sand dunes is gypsum. The OMEGA experiment aboard The emplacement mechanism of these deposits is still under observed a concentration of gypsum at the east- discussion but may be diffusion, precipitation or a combination ern margin of Olympia Undae with decreasing concentrations of both. 430 W.C. Feldman et al. / Icarus 196 (2008) 422–432

Fig. 9. Section of dune lee slope from HiRISE image PSP_001736_2605. The Sun is shining from the left. This figure illustrates the occurrence of cracks, hypothesized to be tensional cracks formed on polar dunes on Earth by a com- bination of cohesion and loss of volatile volume in the subsurface. Fig. 8. Section of dune crest from HiRise image PSP_001736_2605. The Sun is shining from the left. (a) Black arrows indicate locations where the dune crest has been altered by hypothesized sublimation pit formation. (b) Two individual pits located on the crest. (c) An elongated depression 5-m wide and 27-m long lays a niveo-aeolian lower layer. The geomorphology shown formed by coalescing of several pits along a dune crest. by the HiRISE images suggests that the bottom layer may be cemented in place and therefore relatively immobile although 5. Summary and conclusions permafrosted dunes in Antarctica have been shown to migrate in recent times (Bourke et al., 2008b). The composition of the Thermal and epithermal neutron currents measured using lower layer may be: (a) an indurated, hydrated mineral such MONS, temperatures measured using TES, and geomorphic as gypsum, (b) pore-volume water ice emplaced by vapor dif- observations from HiRISE, were analyzed to determine the fusion, or (c) multiple layers of water ice and dust delivered likelihood, average content, and stratigraphy of water ice within as snow combined with sand during previous obliquity cy- the sand dunes in Olympia Undae. The WEH mass fraction cles. of surface soils is found to be a relative minimum at 81◦ N and −171◦ E. Both the neutron and thermal infrared data are Acknowledgments best represented by a two-layered model having a water–ice equivalent hydrogen mass fraction of 0.295 ±0.05 in a lower semi-infinite layer, buried beneath an upper layer having 0.01 We thank A.E. McEwen and the HiRISE team for making WEH mass fraction that is 9 ± 6g/cm2 thick (about 6 cm depth the HiRISE images available for this study. This work was sup- at a density of 1.5 g/cm3). ported in part by NASA’s Mars Odyssey program and MDAP A model that is consistent with all three data sets is that NNG04GJ92G (MB). It was carried out under the auspices of the dunes contain a relatively desiccated top layer, which over- the Planetary Science Institute PSI Publication #420. Hydrogen content of sand dunes 431

Fig. 10. Characteristics of the interdune area from HiRISE image PSP_001736_2605. The Sun is shining from the left. The top images shows narrow arcuate ridges (black arrows) that may be indurated stratigraphy of small transverse dunes. The locations of sub-panels (a) and (b) are indicated by black boxes. (a) Apparently indurated transverse aeolian ridges formed of high albedo material overlain by darker rippled sediment. (b) Bottom of image: Polygonal terrain with dark circumferal ridges. White arrow: Amorphous dark sediment amongst ripples and polygonal terrain.

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