 The Utopia/Isidis overlap; possible conduit for mud volcanism.  E. M. McGowan1 and G. E.  McGill1, 1Department of Geosciences, Universiity of Massachusetts, 611 North Pleasant Street, 233 Morrill Science Center, Amherst, MA 01003 emcgowan@geo.umass.edu, gmcgill@geo.umass.edu   Introduction: Pitted cones, among the most pervasive putative water-related features on Mars, are widely distributed throughout the northern lowland. Most researchers believe pitted cones to be related to water e. g. mud volcanoes (1-5), pingos (6-9), hydrothermal spring deposits (2,10,11), and Icelandic pseudocraters (12). An aerially large, dense population of pitted cones, centered on the south western slope of Utopia basin where it is intersected by Isidis basin, is believed by some (1,4,5) to be mud volcanoes (FIG1). This paper will discuss local structure and processes that could facilitate mud volcanism at this site.  Multi- ringed impact basins: Both Utopia basin and Isidis basin are ancient overlapping multiringed impact basins. (13-16) The overlap of these two impact structures coincides with the location of the large population of pitted cones discussed in this paper. On Earth seismic investigation of Chicxulub Crater in Mexico, also a multi ring crater, shows that the rings located outside of the crater rim (outer rings) are normal faults extending to the MOHO (17).  The first outer ring of Isidis basin overlaps the crater rim of Utopia basin at the location of the pitted cones. A direct correlation between Isidis basin and Chicxulub crater cannot be made but, by analogy it is reasonable to assume that the outer ring of Isidis basin also is a deep normal fault.  Highland/Lowland boundary: The south western rim of the Utopia basin is the boundary between the southern highlands and northern lowlands between 10N-40N and 80E-125E. The elevation change between the highlands and lowlands at this location is approximately 3 km. This change in elevation would permit substantial deposition into the area occupied by the large population of pitted cones. Buczkowski and Cooke, (18) indicate the sediment cover in this area to be 1-2 km thick. A topographic profile line extending from the highlands through the pitted cone population shows a change of slope where the line crosses into the area of pitted cones (FIG2).  Water: Water  is  an essential  ingredient for mud volcanism.  Water ice, discovered by the Phoenix Lander, exists in the near surface in the high latitudes of the northern lowlands. An extensive cryosphere is believed to exist at depth (19-21) and there is evidence that at some time in the past the northern lowlands held a large body of water or ice (22-26). Methane: On Earth methane release from deep subsurface gas reserves or disassociation of clathrates has long been known to be associated with mud volcanism. Biogenic activity is not necessary for methane production; Scott et al. (27) showed through experimentation that on Earth large amounts of methane could be produced within the mantle. Methane release from the surface of Mars has been observed in three locations during the northern summer (28). Two of these areas, Syrtis Major and Nili Fossae, are located on the same outer ring of Isidis basin that intersects the large population of pitted cones.  Discussion: Examination of the evidence at the site of the large population of pitted cones indicates that mud volcanism is the likely scenario. A substantial cryosphere could supply the water for mud volcanism and clathrate formation. Water and/or clathrates could be released from the cryosphere through fractures created from the overlap of the outer ring of Isidis basin with the rim of Utopia basin. Looking at the history of sedimentation and water, the proximity of the highlands, the relationship to current methane release, and the potential for deep fracturing possibly to a cryosphere we conclude that the most likely origin of the pitted cones located on the southweestern slope of Utopia basin is mud volcanism.  [1] Tanaka K. L. et al. (2003) JGR, doi:10.1029/2002JE001908. [2] Ferrand W. H. et al. (2005) JGR doi:10.1029/2004JE002297. [3] McGowan E. M. and McGill G. E. (2009) LPS XXXX  Abstract #1295. [4] McGowan E. M. (2009) Icarus doi:10.1016/j.icarus2009.02,024. [5] Skinner J. A. and Mazzini A. (2009) Marine and Petroleum Geology, 26, 1866-1878. [6] Soare R. J. et al. (2005) Icarus, 174, 373-382. [7] Burr D. M. et al. (2005) Icarus 178, 5673. [8] Pablo M. A. and Komatsu G. (2009) Icarus 199, Issue, 49-74. 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[26] Perrone J. T. et al. (2007) Nature doi:10.1038/nature05873. [27] Scott H. P. et al. (2004) PNAS, 101, 39, 14023-14026. [28] (Mumma, M. J. et al, (2009) Science 10.1126/science.1165243. [29] ASU web based GIS software. Rim of Utopia Pitted cones Isidsouter rings Areasof methanerelease SyrtisMajor 20S 130E 70N 30E Outer ring of Utopia 120E 13N 80E 45N Figure 1.  Dicotomy boundary in the eastern northern hemisphere.  White boxes outline areas of summer methane release (28), orange circles denote rim of Utopia basin and outer rings of Utopia basin and Isidis Basin, and small white filled boxes are pitted cones.  Shaded topography from JMARS [29] is in a simple cylindrical projection.  The orange circles that define the crater rim and the outer rings of both Utopia and Isidis are correct in the vicinity of the pitted cones.  However, the remaining sections of the circles are distorted due to the projection. Figure  2. Profile from the highlands toward the center of Utopia Planitia (A-A') crossing pitted cones (white dots) as well as two types of giant polygons found in the area (yellow indicates subdued giant polygons and red indicates well defined giant polygons). On both the profile and the image the yellow lines indicate where one feature ends and another begins (highlands, lowlands, pitted cones, etc.). Notice the slight change of slope at 2 and 4 producing a slight convex profile where it crosses pitted cones and subdued giant polygons. 
