Recent high-latitude icy mantle in the northern plains of Mars: Characteristics and ages of emplacement

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1 GEOPHYSICAL RESEARCH LETTERS, VOL. 33, L11201, doi: /2006gl025946, 2006 Recent high-latitude icy mantle in the northern plains of Mars: Characteristics and ages of emplacement V.-P. Kostama, 1 M. A. Kreslavsky, 2,3 and J. W. Head 2 Received 3 February 2006; revised 23 April 2006; accepted 2 May 2006; published 3 June [1] Martian high-latitude zones are covered with a smooth, layered ice-rich mantle containing characteristic polygonal patterns. The mantle is especially uniform and homogeneous in the northern plains. We performed a survey of decameter-scale mantle textures and circular features, including impact craters, in the northern lowlands down to 50 N latitude. Strongly altered and mantled craters of impact and non-impact origin form a population of subtle circular features below the mantle. Pits of non-impact origin are numerous in some regions at lower latitudes. Impact craters superposed on the mantle are small and very sparse. The inferred mean crater retention age of the mantle is 0.1 Ma. The spatial distribution of young craters suggests an age difference between the highest latitudes (younger) and some lower-latitude regions (older). Latitudinal trends in polygon textures agree with impact crater evidence for a latitudinal age progression of mantle properties. Citation: Kostama, V.-P., M. A. Kreslavsky, and J. W. Head (2006), Recent high-latitude icy mantle in the northern plains of Mars: Characteristics and ages of emplacement, Geophys. Res. Lett., 33, L11201, doi: /2006gl Introduction [2] A global zonality of surface textures on Mars was first seen in Mariner 9 images by Soderblom et al. [1973] who hypothesized the presence of eolian mantles at high latitudes. Different types of latitude-dependent terrain softening were observed in Viking images [e.g., Squyres et al., 1992]. Data collected by Mars Orbiter Laser Altimeter onboard Mars Global Surveyor (MGS) showed remarkable smoothing of subkilometer-scale topography at high latitudes, which was attributed to mantling deposits [Kreslavsky and Head, 2000]; this mantle has a specific decameter-scale surface texture clearly seen in high resolution images taken by Mars Orbiter Camera (MOC) onboard MGS [Malin and Edgett, 2001] (Figure 1). The mantle is at least tens of meters thick and is comprised of several layers [Kreslavsky and Head, 2002]. In mid-latitude zones (30 50 latitude) the mantle is dissected and eroded [Mustard et al., 2001], which has been attributed to desiccation of icerich material forming more continuous mantles at higher latitudes. The results from the gamma ray and neutron spectrometers (GRS suit) onboard Mars Odyssey [e.g., 1 Department of Physical Sciences, University of Oulu, Oulu, Finland. 2 Department of Geological Sciences, Brown University, Providence, Rhode Island, USA. 3 Also at Astronomical Institute, Kharkov National University, Kharkov, Ukraine. Copyright 2006 by the American Geophysical Union /06/2006GL025946$05.00 Boynton et al., 2002] showed that the mantle indeed contains much ice. The boundaries of the high hydrogen concentration according to the GRS data are located at about 60 latitude [e.g., Tokar et al., 2002]; at least the upper meter of the mantle at lower latitudes is desiccated. The mantle is geologically young, because it has few superposed impact craters [Mustard et al., 2001]. Formation of the mantle is probably related to redistribution of ice on Mars due to change of latitudinal insolation pattern controlled by obliquity variations. The uppermost mantle layers were proposed to form during the most recent obliquity peaks (0.4 2 Ma ago) [Head et al., 2003] by migration of H 2 O from polar caps, while Levrard et al. [2004] concluded that the mantle was formed due to the general decrease of oscillating obliquity 3 5 Ma ago by migration of H 2 O from equatorial ice deposits. [3] To learn more about the nature, age, formation and modification of the mantle, we performed a systematic survey of small impact craters and mantle morphology in the mantled high-latitude regions. Here we report on our results for the northern plains of Mars, where the geological units, and hence the mantle substrate, are exceptionally homogeneous. 2. Survey [4] For our survey we used narrow-angle MOC images of the northern lowlands taken during cycles E01 to E05 (February June 2001). This corresponds to the summer season in the northern hemisphere (solar longitude ), and the observational conditions were mostly good. To make the survey as homogeneous as possible, we used only images of 4.8 m/pixel resolution. We limited the survey to terrains northward of 50 N. In total, we examined about 700 images; 270 of them were of good quality and contained large areas of typical mantle. This small percentage of images with typical mantle does not reflect the percentage of area occupied by the mantle; it reflects the preferential targeting of MOC to certain features. [5] We systematically reviewed and classified the mantle texture, studied the morphology and measured the diameters of all circular features of 50 m to 2 km, as craters larger than this size are usually not completely covered by individual MOC images. Features that are smaller than 50 m in diameter are often identifiable, but size estimates for them are too inaccurate, and their morphological details are often not apparent. In total, we measured the diameters of more then 1100 circular features. Although we made every attempt to undertake the survey as evenly and as homogeneously as possible, the ability to identify features and textures sometimes differed from image to image due to differences in atmospheric conditions, illumination geome- L of5

2 Figure 1. Typical textures of the mantle in the northern plains: (a) Basketball texture, (b) linear texture, (c) wrinkle texture, and (d) polygonal texture. Portions of E01/01975, E04/00026, E01/01868, and E04/00028, respectively. Samples are km, illumination is from lower left. try, presence of contrasting albedo features, electronic settings of the camera, etc. Consequently, in analyzing the survey results, we remain aware of possible latitudinal (illumination) and regional (albedo pattern) biases. Nevertheless, we are very confident that in the surveyed images we identified all fresh features larger than 50 m that are superposed on (disrupt) the mantle. 3. Results 3.1. Mantle Textures [6] The mantle has a very distinctive decameter-scale surface pattern that is well seen in the images (Figure 1). The basketball texture, formed by very evenly spaced knobs, (Figure 1a; see also Malin and Edgett [2001], Figure 123d) is the most common and typical texture. The highest quality, highest-resolution (1.6 m/pix) low-sun images show that the knobs are dome-shaped, with the steepest slopes <6. In some places, the knobs are organized into highly coherent linear structures (Figure 1b, see also Malin and Edgett [2001], Figure 123e) forming a linear texture. The wrinkle texture (Figure 1c) is also very common. The difference between knobs and wrinkles is gradational. Wrinkle orientations are locally consistent but vary regionally; they do not have an obvious orientation preference. Basketball and wrinkle textures correspond to patterned ground types S2 and S3 in Mangold s [2005] global survey of patterned ground on Mars. The polygonal texture (Figure 1d; Mangold s [2005] type S1) is formed by small, rather irregular crack networks and occurs rarely and in relatively small patches (several km). The spatial scale of the textures differs within m range. The polygonal texture of the mantle differs from polygons that are observed on the floors of some large impact craters (type LT of Mangold [2005]); these polygons have an order of magnitude larger spacing and a different morphology. [7] Local variations of the patterns are often correlated with and modulated by kilometer-scale topography. The wrinkle texture on the slopes of km-scale knobs has a radial orientation. We observed a number of examples of barchan dunes that have apparently traveled over the basketball texture without modifying it, suggesting that the texture is made of rather competent material. [8] There are latitudinal variations in the occurrence of the mantle surface texture patterns (Figure 2). The common basketball texture occurs at all latitudes between N. The linear texture tends to occur more commonly at higher latitudes (70 80 N). The polygonal texture occurs only at high latitudes above 70 N. A tendency for the S1 pattern to occur the highest latitudes was also noted by Mangold [2005], although he also reported a few sites with such a pattern below 70 N. The wrinkle texture tends to occur at lower latitudes (50 70 N) Mantle Morphology [9] The mantle is almost continuous in the high latitudes (>65 N); only layered material of the polar deposits, icy polar cap outliers, dunes and a few very steep slopes are not covered by the mantle. There are several examples where the mantle is eroded on steep slopes, exposing its layered structure. In a number of images the uppermost meter-scalethick layer of mantle is not continuous, and forms patches outlined by a gentle scarp. [10] Below N erosion is ubiquitous and occurs not only on slopes, but also on flat surfaces. Often, erosion of the mantle produces specific dissected patterns described and surveyed by Mustard et al. [2001]. We analyzed many clear examples showing that the typical dissection described by Mustard et al. [2001] is dissection of the mantle originally characterized by basketball texture. In many sites, however, dissection does not take place, and the eroded edges of the mantle form lobate scarps Pits [11] In the N latitude zone, there are a number of circular or quasi-circular pits (Figure 3). They are different Figure 2. Latitudinal distribution of the (left) mantle texture occurrence and (right) pits and fresh craters. Plotted is the percentage of images of good quality having the specific features within the latitudinal zones. 2of5

3 Figure 3. (a) Pits covered with the mantle. Small sharp feature shown by arrow is interpreted to be a fresh impact crater (40 m). Portion of MOC NA image E02/01984, 1.4 km wide, illumination is from lower left. (b) This area is located approximately 50 km from that shown in Figure 3a and apparently belongs to the same huge cluster of pits. Impact crater (arrow) differs from the pits in its morphology (raised rim, circular shape). Portion of THEMIS VIS image V , 5.5 km wide, illumination from left. from typical raised-rim impact craters in their morphology. Some of them clearly have a collapse origin. Some of the pits are fresh; they cut the uppermost mantle layer, and disrupt the mantle texture, but typically they are softer and are covered with the textured mantle (Figure 3a). Some pits appear to be distributed randomly, but typically they form dense clusters. THEMIS images (Figure 3b) show that the clusters are huge and contain a large number of pits. Unlike the randomly distributed pits, these clusters of pits are observed in a single region of the northern plains to the N and NE of Alba Patera ( E). These pits are apparently related to the local geology of this region, characterized by lava flows from Alba Patera Mantled Crater-Like Features [12] There are numerous circular features at high latitudes interpreted by Malin and Edgett [2001] to be mantled craters (see their Figures 118 and 119). These features span a wide range of morphologies from well-expressed impact craters covered with a patterned surface (Figure 4a) to more subtle circular chains of lineaments and/or arcuate albedo markings (Figure 4b). We identified a continuous series of transitional morphologies, which could be considered as evidence of an impact origin of the subtle features. On the other hand, the rims of the circular features, when seen, sometimes are irregular, deviating from the more common circles of impact craters (Figure 5a). These features are clustered to a much greater degree than would be expected for a production population of impact craters. Cratered cones similar to those observed in Isidis Planitia [Bridges et al., 2003] (Figure 5b) outside the mantle extent, could be candidates for some of the mantled circular features. [13] The size-frequency distribution of the largest (1 2 km) mantled crater-like features in the N latitude zone is close to the production function for an Early Amazonian age (Figure 6). The deflection from the Amazonian/Hesperian boundary (the age of the Vastitas Borealis Formation (VBF), the substrate below the mantle in the Northern Lowlands) is less than a factor of two in crater density. This subpopulation of features may actually be the same as the production population of the VBF. For smaller Figure 4. (a) Impact crater, covered with mantle. (b) Subtle circular feature in the upper center, fresh small impact crater (60 m) in the lower right (arrow). Two portions of E02/ 01380, 1.4 km wide. Illumination is from lower left. Figure 4b has 30% higher contrast than Figure 4a. diameters (below 1 km), the crater retention age is much younger. Furthermore, a break in slope in the size-frequency distribution (arrow in Figure 6) may be related to the characteristic onset size of a non-impact subpopulation of circular features, such as those shown in Figure 5a Fresh Craters [14] We identified 26 small circular features >50 m in diameter, almost certainly impact craters superposed on the mantle (see examples in Figures 3a and 4b); the largest of these is 200 m. All these craters are somewhat degraded; they do not have apparent ejecta. In addition to that, small features that we conservatively classified as pits, could conceivably be somewhat softened impact craters superposed on the mantle. The mean density of superposed impact craters in the survey area, km 2, corresponds to a mean crater retention age of 0.1 Ma, according to the Neukum-Hartmann production function [Ivanov, 2001]. This estimate is uncertain (within an order of magnitude and perhaps more), because (1) the scaling from the Moon to Mars is accurate within a factor of 2 [Ivanov, 2001]; (2) the extrapolation of the projectile flux established for the hundreds of Ma time scale down to the 1 Ma time scale is Figure 5. (a) Lager crater-like features covered by the mantle. Portions of E05/00602, 1.4 km wide. Illumination is from lower left. (b) Impact (upper part) and non-impact (lower part) features in Isidis Planitia [Bridges et al., 2003]. Portion of M03/02859; 1.4 km wide. 3of5

4 Figure 6. Cumulative size-frequency distributions of mantled craterlike features (upper distribution with arrow) and of fresh superposed craters (in the lower left corner). Vertical lines correspond to 85% formal confidence interval. The arrow points to a faint but statistically significant break in slope. Thin lines show impact crater production isochrons dividing the periods of geological history of Mars (the shaded band is the Hesperian), as well as 0.1 Ma and 4.6 Ma isochrons. The isochrons are computed using the Neukum- Hartmann production function recalculated to Mars [Ivanov, 2001]. The dropdown of the isochrons at the right edge is due to removal of craters larger than 2 km from the production function. not firmly grounded; and (3) the atmospheric attenuation of the small projectile flux is uncertain. [15] Several images at latitudes below 70 N (e.g., E04/ 01263, E05/01597) contain a set of small superposed craters, apparently randomly scattered over the image; one or two of them are larger than 50 m, while a few others are smaller. This situation is very similar to that which would be expected for an accumulating population of impact craters. The crater density calculated over several such images gives an age estimate on the order of 2 Ma. On the other hand, a large number of images certainly do not have impact craters superposed on the mantle. This observation suggests that the crater retention age of the mantle in different locations may differ by at least an order of magnitude. Despite statistics of only a few craters, the retention age difference between individual images is statistically significant at a high confidence level. [16] Figure 2 clearly shows a concentration of superposed craters at lower latitudes. Despite poor statistics, the fivefold difference in crater density is statistically significant. Thus, in addition to the wide local age variations, there is a global latitudinal age trend. 4. Discussion [17] The inferred mean crater retention age of the mantle (0.1 Ma) formally corresponds better to the mantle formation during the last obliquity peaks (0.3+ Ma ago) [Head et al., 2003] than to the general decrease of obliquity 3 5 Ma ago [Levrard et al., 2004], although the latter cannot be confidently excluded. On the other hand, the observed wide age differences suggest that the formation of the mantle was occurring at least in part during the period from 5 to 3 Ma ago. [18] We propose two end-member scenarios to explain the relatively wide differences in the crater retention age of the mantle: (1) In the first scenario, older layer(s) of the mantle were emplaced in a wide area prior to 3 Ma ago. Then, during the last ice age from 2 to 0.3 Ma ago additional layer(s) of the mantle were emplaced in a smaller area, poleward from 60 N. The contacts between mantles of different age may not be recognized as such because the MOC high-resolution coverage is too patchy. (2) In the second scenario, the icy mantle material was deposited more or less simultaneously, but the accumulation onset of the observable crater population is delayed until 10s of meters of ice sublimate, and the surface texture is formed. At high latitudes this delay was essentially longer. Some of the patches of bare ice material outside the polar layered deposits (polar cap outliers) could be areas still undergoing delayed ice removal. An intermediate scenario may include formation of mantle layers of different age, and repeated deposition and removal of protective decameter-thick layers of pure ice. [19] The mantle texture has been interpreted as sublimation polygons [Marchant and Head, 2005; Mangold, 2005] that are formed due to thermal cracking of permafrost and further localized sublimation enhanced by cracks. Within this scenario the polygonal texture (Figure 1d) represents young polygons, while the basketball texture is a mature pattern. The concentration of the immature patterns at high latitudes is consistent both with the age trend inferred from craters and with the slower desiccation rates at higher latitudes. In the context of the second scenario, the local patches of polygonal texture are in the places that were last to lose the protective ice cover. References Boynton, W. V., et al. (2002), Distribution of hydrogen in the near surface of Mars: Evidence for subsurface ice deposits, Science, 297, Bridges, J. C., et al. (2003), Selection of the landing site in Isidis Planitia of Mars probe Beagle 2, J. Geophys. Res., 108(E1), 5001, doi: / 2001JE Head, J. W., J. F. Mustard, M. A. Kreslavsky, R. E. Milliken, and D. R. Marchant (2003), Recent ice ages on Mars, Nature, 426, Ivanov, B. A. (2001), Mars/Moon cratering rate ratio estimates, Space Sci. Rev., 96, Kreslavsky, M. A., and J. W. Head III (2000), Kilometer-scale roughness of Mars surface: Results from MOLA data analysis, J. Geophys. Res., 105, 26,695 26,712. Kreslavsky, M. A., and J. W. Head III (2002), Mars: Nature and evolution of young latitude-dependent water-ice-rich mantle, Geophys. Res. Lett., 29(15), 1719, doi: /2002gl Levrard, B., F. Forget, F. Montmessin, and J. Laskar (2004), Recent ice-rich deposits formed at high latitudes on Mars by sublimation of unstable equatorial ice during low obliquity, Nature, 431, Malin, M. C., and K. S. Edgett (2001), Mars Global Surveyor Mars Orbiter Camera: Interplanetary cruise through primary mission, J. Geophys. Res., 106, 23,429 23,570. Mangold, N. (2005), High latitude patterned grounds on Mars: Classification, distribution and climatic control, Icarus, 174, Marchant, D. R., and J. W. Head (2005), Equilibrium landforms in the Dry Valleys of Antarctica: Implications for landscape evolution and climate change on Mars, Lunar Planet. Sci., XXXVI, abstract Mustard, J. F., C. D. Cooper, and M. K. Rifkin (2001), Evidence for recent climate change on Mars from the identification of youthful near-surface ground ice, Nature, 412, of5

5 Soderblom, L. A., M. C. Malin, J. A. Cutts, and B. C. Murry (1973), Mariner 9 observations of the surface of Mars in the north polar region, J. Geophys. Res., 78, Squyres, S. W., S. M. Clifford, R. O. Kuz min, J. R. Zimbelman, and F. M. Costard (1992), Ice in the Martian regolith, in Mars, pp , Univ. of Ariz. Press, Tucson. Tokar,R.L.,W.C.Feldman,T.H.Prettyman,K.R.Moore,D.J. Lawrence, R. C. Elphic, M. A. Kreslavsky, J. W. Head III, J. F. Mustard, and W. V. Boynton (2002), Ice concentration and distribution near the south pole of Mars: Synthesis of odyssey and global surveyor analyses, Geophys. Res. Lett., 29(19), 1904, doi: /2002gl J. W. Head and M. A. Kreslavsky, Department of Geological Sciences, Brown University, Providence, RI 02912, USA. (misha@mare.geo.brown. edu) V.-P. Kostama, Department of Physical Sciences, University of Oulu, P.O. Box 3000, FIN-Oulu, Finland. 5of5