Extensive Noachian fluvial systems in Arabia Terra: Implications for early Martian climate

Valley networks are some of the strongest lines of evidence for extensive fluvial activity on early (Noachian; >3.7 Ga) Mars. However, their purported absence on certain ancient terrains, such as Arabia Terra, is at variance with patterns of precipitation as predicted by “warm and wet” climate models. This disagreement has contributed to the development of an alternative “icy highlands” scenario, whereby valley networks were formed by the melting of highland ice sheets. Here, we show through regional mapping that Arabia Terra shows evidence for extensive networks of sinuous ridges. We interpret these ridge features as inverted fluvial channels that formed in the Noachian, before being subject to burial and exhumation. The inverted channels developed on extensive aggrading flood plains. As the inverted channels are both sourced in, and traverse across, Arabia Terra, their formation is inconsistent with discrete, localized sources of water, such as meltwater from highland ice sheets. Our results are instead more consistent with an early Mars that supported widespread precipitation and runoff. INTRODUCTION Multiple lines of geologic evidence indicate that liquid water existed at the surface on early Mars, but a key question is whether Mars’ early climate was warm and wet with widespread precipitation, or cold, with liquid water sourced mainly from sporadic melt of glaciers and ice sheets. Most Noachian-aged (>3.7 Ga; Michael, 2013) surfaces below 60° latitude are dissected by valley networks, interpreted to have formed by fluvial surface erosion (e.g., Carr, 1995; Hynek et al., 2010). A natural interpretation is that precipitation-driven surface runoff carved the valley networks (e.g., Hynek and Phillips, 2001). However, some Noachian terrains for which models of Mars’ Noachian climate (Wordsworth et al., 2015) predict precipitation appear poorly dissected by valley networks. The most extensive of these Noachian “poorly dissected regions” is Arabia Terra. As an alternative, Wordsworth et al. (2013) proposed the “icy highlands” model, in which water sources are localized to high-elevation regions (i.e., the equatorial highlands) as ice sheets, and that formation of valley networks was driven by the sporadic melting of such ice. Arabia Terra comprises the most northerly portion of Mars’ ancient cratered highlands. It is one of the oldest terrains on the planet, mainly dating from the midto late Noachian (Tanaka et al., 2014), ca. 3.7–3.9 Ga (Michael, 2013). Remnants of thick, layered sedimentary units that mantle topography (herein referred to as “etched units”) and surround inliers of older Noachian material demonstrate that Arabia Terra was subject to widespread episodes of resurfacing and denudation (e.g., Hynek and Phillips, 2001). The etched units are generally formed of horizontally bedded strata, as much as hundreds of meters thick, that mantle the topography (e.g., Moore, 1990; Fassett and Head, 2007). They are more eroded in the north, where they become discontinuous and then absent (e.g., Zabrusky et al., 2012). In the south, the Meridiani Planum region is covered by the youngest etched unit, interpreted to be early Hesperian (ca. 3.6–3.7 Ga; Hynek and Di Achille, 2016). These observations suggest that valley networks in Arabia Terra might have been buried by the etched units and/or removed by later erosion. While some valley systems do traverse the central region of Arabia Terra (e.g., Irwin et al., 2005b), most are truncated by the etched units, and valleys are generally absent in the wider region (Hynek et al., 2010). Similarly, although paleolakes occur in parts of Arabia Terra (Fassett and Head, 2008b; Goudge et al., 2016), they are absent in much of the region, notably western Arabia Terra. Two scenarios therefore exist to explain the apparent lack of valley networks in Arabia Terra: they either formed but are no longer visible (the “burial and erosion” explanation) or did not form here at all (consistent with an “icy highlands” interpretation; ice sheets did not extend into the low elevations of Arabia Terra). To test which scenario best fits the geologic evidence, we conduct a new mapping study encompassing most of Arabia Terra and using recent, high-resolution data (Mars Reconnaissance Orbiter [MRO] Context Camera [CTX] images; ~6 m/pixel with near global coverage; Malin et al., 2007; Fig. DR1 in the GSA Data Repository1). Using these data, rather than the >100 m/pixel images previously used to identify valley networks (e.g., Hynek et al., 2010), we identify and map the distribution of previously unrecognized fluvial systems in Arabia Terra and evaluate their depositional context. Finally, we consider the implications of their presence for paleoclimate scenarios for early Mars. INVERTED CHANNEL SYSTEMS: IDENTIFICATION AND INTERPRETATION Our CTX image mapping of Arabia Terra shows the existence of extensive systems of relatively continuous sinuous ridges superposed on a generally northwest-sloping surface (Figs. 1 and 2). Throughout Arabia Terra, the ridges conform to regional topography and do not cross topographic divides. Ridge orientations show significant variability, but the longest preserved segments are commonly oriented northwest-southeast or north-south. The ridges typically form single-thread topographic features (~10–60 m high with generally flat tops; Fig. DR2), although 1 1GSA Data Repository item 2016280, Figure DR1 (image coverage and context map), Figure DR2 (DEM of inverted channel), Figure DR3 (inverted channel layering), Figure DR4 (images of inverted channels), Figure DR5 (map showing former extent of etched units), Figure DR6 (schematic of inverted channel formation), Table DR1 (summary of inverted channel characteristics), and Table DR2 (instrument and image ID numbers), is available online at www .geosociety .org /pubs /ft2016.htm, or on request from [email protected]. *E-mail: [email protected] GEOLOGY, October 2016; v. 44; no. 10; p. 1–4 | Data Repository item 2016280 | doi:10.1130/G38247.1 | Published online XX Month 2016 © 2016 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. 2 www.gsapubs.org | Volume 44 | Number 10 | GEOLOGY anabranching morphologies are locally observed (Fig. 2A). The sinuosity of the longest segments ranges from 1.08 to 1.64. Tributary ridge systems merging into a single thread are commonly observed (Figs. 2B and 3). Individual ridges range from tens of meters to ~1–2 km in width. The longest observed segment is ~200 km long; the total cumulative ridge length is ~17,000 km (Table DR1 in the Data Repository). MRO High Resolution Imaging Science Experiment (HiRISE; McEwen et al., 2007) images indicate that many of the ridges show subhorizontal meterto decameter-scale internal layering (Fig. DR3). The sinuous single-thread form of the ridges, together with the presence of anabranching geometries and tributary junctions, is strong evidence that these systems represent paleo-fluvial channel systems now preserved as inverted relief. The internal layering, visible in some channel margins, is evidence that the infill of these channels comprises indurated sedimentary deposits. These systems strongly resemble similar ridges interpreted to be fossilized paleo-fluvial channels on Earth (e.g., Maizels, 1987) and “raised channel” systems on Mars (e.g., Pain et al., 2007; Williams et al., 2009; Burr et al., 2010). Thus, we interpret the Arabia Terra ridge systems mostly as ribbon paleo-channel bodies that were later topographically inverted due to differential erosion. Alternative interpretations exist: the ridge systems could be interpreted as (1) eskers or (2) exhumed infills of bedrock-incised valley networks. We discount the esker interpretation because of a lack of associated glaciated features and because the ridges conform to regional topography. The absence of preserved valley margins in the majority of cases indicates that the ridges are generally not exhumed fills of valley networks incised into bedrock. Moreover, the idea that bedrock was regionally eroded while channel fill material remains seems implausible. 500 km N Inverted c a h nnels not associated with valley networks Valley networks transitioning into inverted channels Cassini Crater Schiaparelli Crater Fig. 2b Fig. 2a Fig. 3 General drainage direction? General drainage direction? Few inverted channels Few inverted channels Inverted channel Valley network Study area