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Lunar and XXXIII (2002) 2027.pdf

MOLA TOPOGRAPHIC EVIDENCE FOR A MASSIVE OCEAN ON .. T. J. Parker1, J. A. Grant2, F. S. Anderson1, and B. J. Franklin1. 1Jet Propulsion Laboratory, 4800 Oak Grove Dr., Pasadena, CA 91109, timothy.j.parker@jpl..gov. 2Center for and Planetary Studies, National Air and Space Museum, Smithsonian Institu- tion, 4th and Independence SW, Washington, DC 20560-0315.

Introduction: The recent identification of putative chotomy boundary at this elevation. The second step lies shorelines in the Utopia Basin and in the north of plainward of Contact 2 [or Deuteronilus Level, 4]. Alba Patera [1] in MOLA topography shows that a number Our second example, from the crustal dichotomy of broad topographic terraces may not be apparent in the of , verifies an earlier assertion in [5,6] that Viking MDIMs, possibly due to subsequent draping by the mouths of valley networks that debouche across the di- younger deposits. At about the time of the study by Head chotomy boundary nearly always terminate at an elevation and others, one of our team (Franklin) realized that similar that may be hundreds of meters to kilometers up into the terraces are evident in the topography in the east Acida- highlands even though regional slopes continue beyond lia/west region, where we had ex- them toward the plains. This suggests a common base level tended our proposed shoreline maps as part of the Mars for the valley networks which were inferred to terminate in Geologic Mapping Program. In this area, the proposed an ancient ocean at that elevation. Viking-based topography shorelines parallel topography and often fall along what was too coarse and uncertain to verify this assertion at the appear to be erosional escarpments, though not preferentially time. Now, using MOLA topography (Fig. 2) it is possible either at the top or base of the escarpment. The escarpments, to verify that Al Qahira, Durius, and Ma'adim Valles all and associated broad topographic terraces between them, terminate at approximately the same elevation, about -1400 appear similar to those described by [1] in Utopia, in that to -1500m, though the regional slope continues northward they are difficult to see even in regional Viking Orbiter im- and down for at least another kilometer into southern Ely- ages unless associated with prominent contacts between sium Planitia. geologic units. If these features are old coastal terraces that Implications: There are at least two implications of the predate the contacts identified in Viking and MOC images results we have compiled to date. First, the extrapolation of [e.g., 2], then a detailed examination of the MOLA topogra- Contact 1 [5,7] eastward from Deuteronilus Mensae along phy may reveal additional, similarly degraded topographic the dichotomy boundary to the region between Isidis and terraces parallel to the dichotomy boundary, possibly even at is probably in error. Contact 2 can be traced higher elevations than those identified to date. nearly continuously around Isidis and north of the Elysium Topographic Expression of Ancient Erosional Ter- volcanic rise, placing it over 2 km lower in elevation than races: We have begun examination of selected areas along Contact 1 as mapped in these . The elevation of the dichotomy boundary for topographic evidence for re- this “Contact 1” is more comparable to that of the southern gionally extensive erosional terraces, using MOLA data contact of the TES “Hematite” unit, or Meridiani Level analysis software customized for this process by F. S. [4,8]. Anderson. To enable this analysis, we have gridded all the The second implication of our interpretation of these released MOLA data at a spatial scale of a few hundred me- erosional terraces as coastal is that an ocean upwards of 5 to ters per pixel and applied a contour interval of a few tens to 7 km deep would be required by the maximum elevation of one hundred meters (depending on the areal coverage of the terraces identified south of Elysium. The highest terrace map). identified to date is at 2200 m elevation. Unless there has Beginning with the east Mensae through been significant vertical crustal motion (a big uncertainty), northwest region, we have identified a number surface water at this elevation would extend far into the of regional terraces parallel to the dichotomy boundary. southern highlands, filling both the Argyre and Hellas basins Most notable are two "steps" at about -3900m and at about - and even inundating much of the south polar region. 4100m elevation. The first of these, at -3900m, corresponds References: [1] Head J. W., et al. (1999) Science, to the location of the gradational boundary [3], or Arabia 286,2134-2137. [2] Parker T. J. et al. (2000) LPS XXXI, 2p. Level [4]. The primary indication that this boundary or level [3] Rossbacher L. A. (1985) in Woldenberg J. J. (ed), Mod- is an erosional feature is shown in Fig. 1. Craters along this els in Geomorph., 343-372. [4] Clifford S. M. and T. J. boundary have been eroded, not just buried, because hun- Parker (2001) Icarus in press. [5] Parker T. J. et al. (1993) dreds of meters of rim material is "missing" from the craters’ JGR, 98, 11061-11078. [6] Parker T. J. and P. M. Schenk plainward rims. Topography above -3900m elevation is (1995), LPS XXVI, 1105-1106. [7] Parker T. J. et al. (1989) degraded (and crater rims destroyed), and below -3900m is Icarus, 82, 111-145. [8] Edgett K. S. and T. J. Parker (1997) filled in, resulting in a topographic "terrace" that can be GRL, 24, 2897-2900. traced laterally through neighboring craters along the di-

Lunar and Planetary Science XXXIII (2002) 2027.pdf

BIG NOACHIAN OCEAN: T. J. Parker, J. A. Grant, F. S. Anderson, and B. J. Franklin

Figure 1: Topographic map of four large craters at the Arabia Level in eastern Cydonia-northwestern Arabia Terra region. Note that all four craters have been both "planed off" and filled in at about -3900m elevation. This cannot be due to simple emplace- ment of plains materials from the north, as they drop quickly in elevation away from the Arabia Level northward (northwest cor- ner of map). Simple Cylindrical projection. MOLA topography derived from data released through orbit 15507. Contour inter- val: 50 meters. Topomap overlaid on MDIM-1.

Figure 2: Topographic map of the crustal dichotomy between Al Qahira Vallis (left) and Ma'adim Vallis (right). These two val- ley networks, and Durius Valles (near center) all terminate at about -1400 to -1500m elevation, though the regional gradient con- tinues northward and down for at least an additional kilometer of elevation. This relationship suggests that they all formed at the same base level, which was elevated with respect to Elysium Planitia at the time (possibly an ocean shoreline). Note also the progressive degradation of crater forms from south to north, particularly the complete destruction of the north rim of the large crater above center into which Durius Valles drained. Simple Cylindrical projection. Map corners: 10-20S , 160-180E (MOLA overlay in areocentric coordinates). MOLA topography derived from data released through orbit 15507. Con- tour interval: 100 meters. Topomap overlaid on MDIM-1.