Lunar and Planetary Science XXXI 1166.pdf

GEOLOGIC MAP OF THE MTM 85080 QUADRANGLE, PLANUM BOREUM REGION OF MARS. K. E. Herkenhoff, U. S. Geological Survey, 2255 N. Gemini Dr., Flagstaff, AZ 86001 ([email protected]).

Introduction: The polar deposits on Mars probably troughs that are asymmetrical in cross section [9]. record martian climate history over the last 107 to 109 Summertime albedo features such as dark bands are years [1]. The area shown on this 1:500,000-scale map often associated with topographic features. In some includes polar layered deposits and polar ice, as well as places terracing is resolved in MOLA profiles [9] and in some exposures of older terrain. This quadrangle was Viking Orbiter images on a scale similar to that ob- mapped in order to study the relations among erosional served in the south polar layered deposits [14]. How- and depositional processes on the north polar layered ever, Mars Orbiter Camera (MOC) images of the polar deposits and to compare them with the results of previ- layered deposits show layering on even smaller scales. ous 1:500,000-scale mapping of the south polar layered High-resolution MOC images of the north polar layered deposits [2,3]. terrain show evidence that individual layers are ex- The polar ice cap, areas of partial frost cover, the pressed as ridges rather than terraces in places [16]. layered deposits, and two nonvolatile surface units--the Stratigraphy and structure: The oldest mapped dust mantle and the dark material--were mapped in the unit, mantle material (unit Am), is distinguished by its south polar region [4] at 1:2,000,000 scale using a color rough, sometimes knobby surface texture. The mantle mosaic of Viking Orbiter images. We constructed Vi- material is exposed only at the southern edge of this king Orbiter rev 726, 768 and 771 color mosaics (taken quadrangle. The knobs and mesas of mantle material during the northern summer of 1978) and used them to that crop out within areas of smooth layered deposits identify similar color/albedo units in the north polar suggest that the mantle material was partly eroded be- region, including the dark, saltating material that ap- fore the layered deposits were laid down over them. pears to have sources within the layered deposits [5]. The layered deposits appear to cover the mantle mate- However, no dark material has been recognized in this rial except on steep scarps that expose the mantle mate- map area. There is no significant difference in color rial. The layered deposits may be more resistant to ero- between the layered deposits and the mantle material sion than the mantle material, so that the steep scarps mapped by Dial and Dohm [6], indicating that they are formed by more rapid erosion of mantle material be- either composed of the same materials or are both cov- neath layered deposits. Therefore, the mantle material ered by aeolian debris [3,4]. Therefore, in this map area in this area does not appear to have been derived from the color mosaics are most useful for identifying areas erosion of the polar layered deposits. of partial frost cover. Because the resolution of the The layered deposits (unit Al) are recognized by color mosaics is not sufficient to map the color/albedo their distinct bedded appearance, red color and lower units in detail at 1:500,000 scale, contacts between them albedo relative to the polar ice cap and frost deposits; were recognized and mapped using higher resolution they appear to be the youngest bedrock unit in this area. black-and-white Viking Orbiter images. The horizontal to subhorizontal beds that make up the No craters have been found in the north polar lay- layered deposits are especially well exposed in the map ered deposits or polar ice cap [7,8]. The observed lack area in dark bands that are free of residual polar ice. of craters larger than 300 m implies that the surfaces of Similar layered exposures have been recognized in the these units are no more than 100,000 years old or that south polar layered deposits [11-13]. In both polar re- they have been resurfaced at a rate of at least 2.3 mm/yr gions, layers are apparent at least partly because of their [8]. The recent cratering flux on Mars is poorly con- terraced topography, especially where accented by dif- strained, so inferred resurfacing rates and ages of sur- ferential frost retention [13,14]. Layer thicknesses of 14 face units are uncertain by at least a factor of 2. to 46 m were measured by Blasius and others [15] in Physiographic setting: The area of this map is regions of the north polar layered deposits outside this mostly within Planum Boreum, a plateau about 1,000 map area. Early MOC images show that layered deposit km across and up to 3 km high [9,10]. The plateau is exposures are rough, with evidence for deformed beds characterized by the smoothly sculptured landforms of and unconformities [16]. No definite angular uncon- the layered deposits, with about 1 km of relief at its pe- formities have been found within the south polar layered rimeter. The boundary of the plateau is near lat 80° S. deposits [2,3], unlike the north polar layered deposits, near this map area, except at the large reentrant where truncated layers have been recognized in higher Chasma Boreale. Areas of relatively complete frost resolution images [7,13]. Angular unconformities have cover are typically smooth and level (regional slopes been found in various locations within this map area, ~0.2°), whereas defrosted scarps slope 1° to 8° overall including lat 85.7° N., long 61° W., lat 82.6° N., long [9,15]. In many cases, the scarps form low-relief 82° W., and lat 83.6° N., long 90° W. Lunar and Planetary Science XXXI 1166.pdf

GEOLOGIC MAP OF THE MTM 85080 QUADRANGLE ON MARS: K. E. Herkenhoff

Structural deformation in this area appears to be preserved impact craters, resurfacing rates appear to be minimal or absent, as no faulting or folding has been at least 2.3 mm/yr [8]. observed. The scarps and troughs in the layered depos- Solar heating of the exposed layered deposits causes its are interpreted as erosional rather than structural sublimation of the water ice within them [21,22], features because of the lack of folded or offset layers. probably forming a residual deposit of nonvolatile mate- While faulting could have caused the angular uncon- rial. Such a layer would protect underlying water ice formities, the relations observed suggest that they were from further sublimation. The meter-scale roughness formed by erosion and deposition without folding or observed in MOC images of the north polar layered de- faulting. Future analysis of MOC images of these un- posits [16] suggests that the residual layer is either less conformities may aid in their interpretation. than about a meter in thickness or is competent enough The partial frost cover (unit Af) is interpreted as a to support moderate surface slopes. mixture of seasonal frost and defrosted ground on the References: basis of its albedo, color, and temporal variability. [1] Thomas P. et al. (1992) Polar deposits on Mars, in Bass and others [17] found that frost albedo reaches a Mars, University of Arizona Press, 767-795. minimum early in the northern summer, then increases [2] Herkenhoff K. E. and Murray B. C. (1992) USGS during the rest of the summer season. This behavior is Misc. Inv. Series Map I-2304; Herkenhoff K. E. and not observed in the south polar region [2]. The increase Murray B. C. (1994) USGS Misc. Inv. Series Map in albedo is interpreted as resulting from condensation I-2391; Herkenhoff K. E. (2000) USGS Misc. Inv. of H2O from the atmosphere onto cold traps in the north Series Map I-2686 (in press). polar region [17]. Because the images used for the base [3] Herkenhoff K. E. (1998) USGS Misc. Inv. Series and for mapping were taken in mid-summer, the extent Map I-2595. of the high-albedo units shown on this map is greater [4] Herkenhoff K. E., and Murray B. C. (1990) JGR, 95, than during early summer. 1343-1358. The albedo of the residual polar ice cap (unit Ac) is [5] Thomas P. C. and Weitz C. (1989) Icarus, 81, 185- higher than all other units on this map. The contact 215. with the partial frost cover (unit Af) is gradational in [6] Dial A. L. Jr. and Dohm J. M. (1994) USGS Misc. many areas, most likely because unit Af represents in- Inv. Series Map I-2357. complete cover of the same material (H2O frost) that [7] Cutts J. A. et al. (1976) Science, 194, 1329-1337. comprises unit Ac. The summer extent of the north [8] Herkenhoff K. E. and Plaut J. J. (2000) Icarus (in polar cap was the same during the Mariner 9 and Vi- press). king Missions [17], which suggests that it is controlled [9] Zuber M. T. et al. (1998) Science, 282, 2053-2060. by underlying topography. Albedo patterns in these [10] Smith D. E. et al. (1999) Science, 284, 1495-1503. summertime images are correlated with topographic [11] Murray B. C. et al. (1972) Icarus, 17, 328-345. features seen in springtime images. Areas of the high- [12] Cutts J. A. (1973) JGR, 78, 4231-4249. est albedos must be covered by nearly pure coarse- [13] Howard A. D. et al. (1982) Icarus, 50, 161-215. grained ice or dusty fine-grained frost [18,19]. The [14] Herkenhoff K. E. and Murray B. C. (1990) JGR, presence of perennial frost is thought to aid in the long- 95, 14,511-14,529. term retention of dust deposits [20], so areas covered by [15] Blasius K. R. et al. (1982) Icarus, 50, 140-160. frost all year are the most likely sites of layered-deposit [16] Edgett K. S. and Malin M. C. (1999) LPS XXX, formation. Abstract #1029. Geologic processes and history: The polar layered [17] Bass D. S. et al. (2000) Icarus (in press). deposits appear to have formed through deposition of [18] Clark R. N. and Lucey P. G. (1984) JGR, 89, 6341- water ice and dust, modulated by global climate changes 6348. during the last few million to hundreds of million years [19] Kieffer H. H. (1990) JGR, 95, 1481-1493. [7,11-13,21]. However, the details of the relation be- [20] James P. B. et al. (1979) Icarus, 68, 422-461. tween theoretical variations of Mars' orbit and axis and [21] Toon O. B. et al. (1980) Icarus, 44, 552-607. geologic observations are not clear [1]. [22] Hofstadter M. D. and Murray B. C. (1990) Icarus, With the exception of areas covered by the polar ice 84, 352-361. cap (unit Ac), the north polar layered deposits appear to have undergone net erosion in the recent geologic past, as strata are exposed in many places. Accumulation of layered deposits has likely occurred recently in areas covered by the polar ice cap, but deposition rates are poorly constrained. Similarly, rates of erosion cannot be accurately determined, but on the basis of the lack of