Icarus 242 (2014) 329–351 Contents lists available at ScienceDirect Icarus journal homepage: www.elsevier.com/locate/icarus Landscape formation at the Deuteronilus contact in southern Isidis Planitia, Mars: Implications for an Isidis Sea? ⇑ G. Erkeling a, , D. Reiss a, H. Hiesinger a, M.A. Ivanov b, E. Hauber c, H. Bernhardt a a Institut für Planetologie (IfP), Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Straße 10, 48149 Münster, Germany b Vernadsky Inst. RAS, Moscow, Russia c German Aerospace Center (DLR), Berlin, Germany article info abstract Article history: Two of the most widely studied landforms that are associated with a putative ocean that filled the north- Received 6 December 2013 ern hemisphere of Mars are (1) the Vastitas Borealis Formation (VBF), plain units that cover a larger por- Revised 19 June 2014 tion of the northern lowlands of Mars, and (2) a candidate paleoshoreline, e.g., the Deuteronilus contact, Accepted 11 August 2014 which represents the outer margin of the VBF. The VBF and the Deuteronilus contact are interpreted to Available online 20 August 2014 result from a short-lived Late Hesperian ocean that readily froze and sublimated. Similar landforms are also present in the impact basin of Isidis Planitia and suggest formation processes comparable to Keywords: those that formed the VBF and the Deuteronilus contact in the northern lowlands. Mars Our study of the Deuteronilus contact in Isidis revealed geologic evidence that possibly supports the Geological processes Mars, surface existence of a Late Hesperian/Early Amazonian Isidis Sea. For example, numerous valleys that are incised Mars, climate into the plains of the southern Isidis basin rim between 82°/90°E and 3°/6°N and trend a few tens of kilo- meters to the north following the general topographic gradient toward the center of Isidis Planitia. A few of them reach the Deuteronilus contact and continue as sinuous ridges in the Isidis Interior Plains (IIP). Based on our findings we conclude that the geologic setting along the Deuteronilus contact, including the valleys and ridges is a result of (1) Late Hesperian short-term fluvial activity, (2) a Late Hesperian/ Early Amazonian short-lived Isidis Sea that readily froze, (3) subglacial drainage and esker formation, and (4) subsequent sublimation of the proposed Isidis ice sheet. Although the fluvio-glacial model we introduce in our manuscript cannot fully explain the geologic setting, possible alternative formation models, including relief inversion and fluvio-volcanic scenarios are even less capable in explaining the observed geologic setting along the Deuteronilus contact. Ó 2014 Elsevier Inc. All rights reserved. 1. Introduction Mars (e.g., Malin and Edgett, 1999; Head et al., 1999, 2002; Tanaka et al., 2000; Kreslavsky and Head, 2002; Carr and Head, The debate about oceans on Mars has now lasted for more than 2003; Ghatan and Zimbelman, 2006; Poulet et al., 2007; Carter two decades and has revealed that there are strong arguments that et al., 2010; Salvatore et al., 2010; Ghent et al., 2012). both support and negate the existence of at least one large standing Two of the most widely studied landforms that are associated body of water that once possibly covered the northern hemisphere, with a hypothesized ocean that filled the northern hemisphere of i.e., up to one third of the planet’s surface, respectively. The debate Mars are (1) the Vastitas Borealis Formation (VBF), a plains unit that reflects one of the major unanswered scientific questions in Mars covers parts of the northern lowlands of Mars and possibly repre- research. While the existence of an ocean was repeatedly proposed sents a sublimation residue of frozen and sublimated outflow chan- (e.g., Parker et al., 1989, 1993, 2010; Baker et al., 1991; Head et al., nel effluents (e.g., Parker et al., 1993; Kreslavsky and Head, 2002; 1998, 1999; Clifford and Parker, 2001; Kreslavsky and Head, 2002; Mouginot et al., 2012), and (2) candidate paleoshorelines, e.g., the Carr and Head, 2003; Ghatan and Zimbelman, 2006; Erkeling et al., Arabia contact – the older, Late Noachian/Early Hesperian shoreline 2012; Ivanov et al., 2012; Mouginot et al., 2012) there is also – and the younger, Late Hesperian Deuteronilus contact, which rep- evidence that casts doubts on the presence of an ancient ocean of resents the outer margin of the VBF (e.g., Parker et al., 1989, 1993; Clifford and Parker, 2001; Carr and Head, 2003). The VBF and the Deuteronilus contact are interpreted as products of a short-lived ⇑ Corresponding author. E-mail address: [email protected] (G. Erkeling). Late Hesperian ocean that rapidly froze and sublimated. The http://dx.doi.org/10.1016/j.icarus.2014.08.015 0019-1035/Ó 2014 Elsevier Inc. All rights reserved. 330 G. Erkeling et al. / Icarus 242 (2014) 329–351 remnants of this ocean, together with accumulated wind-blown geologic units, which we dated with crater size-frequency distribu- materials, form the VBF (e.g., Kreslavsky and Head, 2002; tion (CSFD) measurements. We analyzed the morphometric Mouginot et al., 2012; Ivanov et al., 2012). parameters of the fluvial landforms in this region to characterize In terms of morphologic appearance, including transgressive the mode and intensity of fluvial erosion, transport and deposition. lobate contacts possibly resembling coastal contacts, widespread We also constrained the aqueous processes that resulted in the for- coned and knobby terrains and esker-like ridges (e.g., Crumpler mation of the Late Hesperian/Early Amazonian valleys and com- and Tanaka, 2003; Erkeling et al., 2012; Ivanov et al., 2012), similar pared them with those responsible for the formation of the landforms are also present in the Isidis impact basin (Kreslavsky significantly older dense and dendritic valley networks in the and Head, 2002; Crumpler and Tanaka, 2003; Erkeling et al., ancient highlands of the Libya Montes. Based on hyperspectral 2012; Ivanov et al., 2012). The geophysical properties, which point image data, we analyzed the mineralogy of the landscape between to widespread aqueous sediment deposition in the past and the the Libya Montes and the Deuteronilus contact. We critically dis- recent presence of sediments mixed with massive ice (Mouginot cuss the valley and ridge geometry along the Deuteronilus contact et al., 2012), suggest formation processes comparable to those that in the context of the fluvio-glacial formation scenario we introduce formed the VBF and the Deuteronilus contact in the northern low- in this study and alternative formation models including relief lands (Kreslavsky and Head, 2002; Crumpler and Tanaka, 2003; inversion. We compare and review our fluvio-glacial formation Erkeling et al., 2012; Ivanov et al., 2012). Also the stratigraphic model with the results of investigations of valleys and ridges else- sequence, for example comparable Late Hesperian model ages where on Mars and Earth (e.g., Pain and Ollier, 1995; Pain et al., and onlap geometries (e.g., Tanaka et al., 2005; Ivanov et al., 2007; Williams et al., 2009, 2013). Finally, we integrated our 2012) is indicative for similarities in the formation time of the Isi- results into a stratigraphic correlation chart that represents a dis plains and the VBF. In the Isidis basin, the origin of the plains self-consistent model of the geologic history and evolution of the units that fill the center of the basin and form the Deuteronilus valleys and ridges along the Deuteronilus contact in southern Isidis contact is also subject to debate. While there is substantial support Planitia. In summary, our investigations of the Deuteronilus con- for the ocean hypothesis (e.g., Lockwood and Kargel, 1994; tact and the fluvial and glacial landforms offer an excellent oppor- Hiesinger and Head, 2000; Kreslavsky and Head, 2002; Webb, tunity to provide significant insights into the late stage water- 2004; DiAchille and Hynek, 2010; Erkeling et al., 2012; Ivanov related geologic record of the southern Isidis basin. et al., 2012; Mouginot et al., 2012), alternative volcanic, eolian or Our work addresses the following questions: (1) Does the geo- tectonic formation scenarios have also been proposed (e.g., logic setting of valleys and ridges support the existence of an Isidis Grizzaffi and Schultz, 1989; Malin and Edgett, 1999; Tanaka Sea? (2) Which processes were responsible for the formation of the et al., 2000; Head et al., 2002; Carr and Head, 2003; Ghatan and valleys and ridges along the Deuteronilus contact in southern Isidis Zimbelman, 2006; McGowan, 2011; Ghent et al., 2012). Although Planitia? Which formation model is most consistent with the new a number of landforms in the Isidis basin, such as the cliffs and ter- observations? (3) What are the time limits for the formation of races of the Arabia contact and the lobate onlap geometry of the the valleys and ridges? In particular, were the valleys and ridges Deuteronilus contact are consistent with the assumption that both formed at the same time? How do they relate to the geologic history contacts represent shorelines of a putative Isidis Sea (Erkeling of the southern Isidis basin rim? For example, how do the younger, et al., 2012), they do not necessarily prove its existence. The main rare and isolated valleys relate to the ancient, widespread and source of ambiguity lies in the fact that different processes, i.e., flu- dense valley networks in the Libya Montes toward the south? In vial, volcanic or glacial processes can produce landforms that are addition, is there a temporal relation