Lunar and Planetary Science XXX 1894.pdf UTOPIA BASIN, MARS: A NEW ASSESSMENT USING MARS ORBITER LASER ALTIMETER (MOLA) DATA B. J. Thomson and J. W. Head III, Dept. of Geological Sciences, Brown University, Box 1846, Providence, RI, 02912. [email protected], [email protected]. Introduction broad ridge on the north side of the basin can be used to define Utopia Planitia is a large plain located in the northern a ring around the basin center approximately 2100 km in di- lowland region of Mars. It was selected as the landing site for ameter. A circle fit to the dichotomy boundary between Viking 2 because of its high scientific interest coupled with 240°W and 300°W has been suggested as a possible second spacecraft survivability concerns [1]. We are now able to ring ~4600 km in diameter [2], but there is no compelling utilize high-precision topographic data from the Mars Orbiter topographic evidence of a ring with this diameter continuing Laser Altimeter (MOLA) to reexamine this area. We address into higher latitudes in the northern portion of the basin. three topics: 1) evidence of the presence of ancient impact basins, 2) the relation of Utopia to channel formation and the Utopia Basin Volumetric Considerations fate of water from outflow channels, and 3) evidence for pos- One of the most long-standing unresolved issues about sible standing bodies of water in its past history. Mars is the presence of large standing bodies of water. This issue has important implications for the possible origin and Evidence of a Basin in Utopia Planitia evolution of Martian life. Most investigators agree that fluvial The idea that Utopia Planitia is a large impact basin was processes operated at some time during Martian history [6], proposed by McGill [2]. Before accurate topographic data but there is a great deal of debate about the total amount and were available, McGill mapped the distribution of geologic persistence of water in the Martian hydrological cycle. Some features such as knobs, partially buried craters, and mesas in authors have suggested that there was once an ocean in the Utopia Planitia. In general, these features tend to be abundant northern hemisphere of Mars [7]. Estimates of the large vol- near the margins of Utopia, particularly in the south near the ume of water necessary for an ocean lie near the upper end of crustal dichotomy boundary, but they gradually become less estimates of the total amount of water that might have been abundant as one moves radially inward toward the geographic available on Mars. At the other end of the spectrum, the vari- center of Utopia. Material surrounding and embaying areas of ous basins in the northern hemisphere which lie at the mouths abundant knobs and partially buried craters indicate a thin of outflow channels from the southern highlands could have veneer of younger plains material. The thickest plains depos- possessed significant bodies of standing water, even if there its occur where these features are not present. In the center of was not enough water available to fill an ocean [8]. The esti- Utopia there is a large, somewhat circular area where these mates of the volumes of these smaller seas or lakes lie well features are completely absent, which has been interpreted as a below the total estimated volume of water discharged from buried impact basin interior [2]. outflow channels [7, 9]. Recently acquired topographic data from MOLA has con- Many authors have suggested that Utopia Basin once con- firmed McGill’s hypothesis that Utopia Planitia is a large, tained ponded water/ice [4, 10-12]. Using MOLA topographic circular basin of likely impact origin. On the basis of the best data, we are now able to calculate an accurate volume estimate pre-MOLA topographic data published by the USGS [3], there for this basin and are able to assess the implications of the has been some disagreement over the placement of the basin locations of shorelines and submergence represented by vari- center. Based on this previous topographic data, some authors ous stages of filling. We used a progressive flooding model to have assumed that the basin center lies at about 35°N, 250°W determine that the maximum ponded volume of water that [e.g. 4]. Others have placed the basin center at 48°N , 240°W could fit into Utopia Basin prior to spilling into adjacent low- [2, 5]. With the new MOLA topography, we have observed lying areas is 1.2 ´ 106 km3. that the center of Utopia (defined as the topographically low- est point of the basin) is actually at about 45°N, 248°W. This represents a shift northward of over 500 km from its previ- ously inferred position on USGS maps. Utopia has been interpreted to be a catchment basin for outflow channels and there has been interest in where fluids filling the basin would overflow into adjacent topographic lows. There is a broad, arcuate ridge in the northern portion of the basin that marks the boundary between Utopia Basin proper and Vastitas Borealis. The topographic low along this ridge is the point at which the basin would first overflow and spill over into the adjacent North Polar Basin [8] if Utopia Basin was filled with water. The USGS topography shows this saddle point at about 45°N, 250°W on a pronounced ridge, while MOLA topography shows that this saddle point actually lies at 60°N, 237°W on a more subdued, broad ridge [8]. Figure 1: MOLA DEM with superposed Viking Orbiter mosaic. Despite the circular appearance of the basin, the evidence Perspective is facing southeast, and image boundaries are 0°N to 60°N, 210°W to 285°W. Water level represents flooding to just below for multi-ringed structures remains problematic. The subdued, the -4350m contour. Vertical exaggeration in the image is ~200´. Lunar and Planetary Science XXX 1894.pdf UTOPIA BASIN: B. J. Thomson and J. W. Head III This volume of water would fill the basin up to approxi- These matched terraces are possibly analogous to terres- mately the -4350 m level (Fig. 1). Excess water would spill trial wave-cut terraces and strandlines (Fig. 2). Terraces at the over to the northeast into the adjacent North Polar basin. Our -4350 m elevation are particularly significant because this data show that this maximum volume of water would form a level represents a potentially stable water surface. Once the circular lake approximately 2100 km in diameter (Fig. 1). basin was filled to this level, the water level would have re- Assuming no subsidence due to loading, the greatest depth of mained there if infilling continued because additional water this lake would be 650 m at the basin’s center. would overflow into the adjacent North Polar Basin [8]. Isidis Basin Conclusions We have also examined Isidis Basin, a small, circular sub- MOLA topography shows that Utopia is the location of a basin that lies on the southwest margin of Utopia basin. Un- circular topographic basin about 2000 km in diameter at the fortunately, the MOLA coverage of Isidis Basin is insufficient location originally proposed by McGill [2] to be a large de- to yield an extremely accurate volume estimate, but our cur- graded impact basin. The Utopia Basin is one of two large rent coverage indicates that this sub-basin has a volume <105 basins in the northern lowlands. If outflow channels emptied km3. The data show that Isidis lies on a small plateau that is water into the northern lowlands, both the Utopia and North elevated over 500 m from the elevation of the maximum vol- Polar basins would serve as catchment areas. MOLA topo- ume of water that would fill Utopia, -4350m. This result dif- graphic data show that water would fill the basin to a depth of fers from previous topography which showed the floor of Isi- about 650 m; at this point, additional inflow would spill over dis at the same elevation as the floor of Utopia [3]. to the north into the North Polar Basin. Thus, if water once filled the Utopia Basin, we would expect this level (-4350m) Evidence of a Possible Paleolake to relatively stable. Interestingly, we observe a distinctive There are a variety of small scale features in Utopia basin topographic terrace at this level (Figs 1, 2). The extreme reg- that might be morphological indicators of ponded water. ularity of this terrace seems unusual for impact basin ring to- These include smooth infilled crater floors, channel deposits in pography. Because of this coincidence (terrace formation at and around the basin, shoreline terraces, and lineations. Other the peak fill topographic level), a more likely interpretation more enigmatic terrain which may be related to water or may be that that this circumferential terrace formed at the ground ice interaction includes thermokarst, fingerprint ter- margins of a standing body of water in a degraded circular rain, polygonal terrain, and small cratered cones which might impact basin. be pingos or pseudocraters. Among the most suggestive previously mapped morpho- logical indicators of standing bodies of water/ice are possible subice features on the east side of the Utopia basin around the northwestern flank of Elysium Mons [10]. On the basis of features interpreted as hyaloclastic ridges and hills, table mountains, associated jökulhlaup deposits, and fluvial chan- nels, Chapman [10] hypothesized that a frozen paleolake ex- isted in Utopia basin as recently as 1.8 billion years ago. Three MOLA passes intersect the edge of this proposed pa- leolake, which in some places forms a pronounced scarp. The altitude of this scarp as revealed by MOLA is about -3600 m.