Mud volcanism and morphology of impact craters in Utopia Planitia on Mars: Evidence for the ancient ocean

Icarus 228 (2014) 121–140

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Mud volcanism and morphology of impact craters in Utopia Planitia on Mars: Evidence for the ancient ocean Mikhail A. Ivanov a,b,⇑,1, H. Hiesinger b,1, G. Erkeling b,1, D. Reiss b,1 a b

Vernadsky Institute, Russian Academy of Sciences, Kosygin St., 19, Moscow 119991, Russia Institut für Planetologie, Westfälische Wilhelms-Universität, Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany

a r t i c l e

i n f o

Article history: Received 15 January 2013 Revised 27 August 2013 Accepted 23 September 2013 Available online 8 October 2013 Keyword: Mars Utopia Planitia Mud volcanism Ocean

a b s t r a c t Results of our detailed geological mapping and interpretation of the nature and relative and absolute model ages of units and structures in the SW portion of Utopia Planitia (20–45°N, 100–120°E) suggest the following. (1) The size–frequency distribution (SFD) of craters that both are buried by materials of the Vastitas Borealis units (VB) and superpose its surface indicate that the absolute model ages of terrain predating the emplacement of the VB is 3.7 Ga. (2) Lack of craters that are partly embayed by materials of the VB in the SW portion of Utopia Planitia implies that the emplacement of the VB was faster than the rate of accumulation of impact craters and is consistent with the geologically short time of emplacement of the VB due to catastrophic release of water from outflow channels (e.g., Carr, M.H. [1996]. Water on Mars. Oxford University Press, New York, p. 229). (3) The SFD of craters that superpose the surface of the VB indicates an absolute model age of 3.6–3.5 Ga. The absolute model ages of etched flows, which represent the upper stratigraphic limit of the VB, are estimated to be 3.5 Ga. (4) The majority of the larger (i.e., >1 km) impact craters show ejecta morphologies (rampart and pancake-like ejecta) that are indicative of the presence of ice/water in the target materials. The distal portions of the pancake-like ejecta are heavily degraded (not due to embayment). This suggests that these craters formed in targets that contained higher abundances of volatiles. (5) The diameter ranges of the craters with either rampartor pancake-like ejecta are overlapping (from 2 to 60 km). Craters with pancake-like ejecta are concentrated within the central portion of the Utopia basin (less than 1000 km from the basin center) and rampart craters occur at the periphery of the basin. This pattern of the crater spatial distribution suggests that materials within the center of Utopia Planitia contained more ice/water. (6) Etched flows around the central portion of Utopia Planitia were erupted from beneath of the surface of the VB. Their morphology and pattern of degradation, however, are inconsistent with lava and, instead, indicate formation of the flows due to mud volcanism. (7) Etched flows are spatially associated with giant polygons and there is evidence that these features populated the center portion of Utopia Planitia before it was covered by the Elysium-derived units. The outer (southern) edge of the zone of polygonal troughs and etched flows approximately corresponds to the transition from pancake-like ejecta to rampart ejecta. This suggest that the outer edge of the zone of the polygons and flows may outline the deeper portions of the large body (2000 km across) of water/ice that likely existed in the center of Utopia Planitia in late Hesperian. Ó 2013 Elsevier Inc. All rights reserved.

1. Introduction The ancient, Early Noachian, age of the northern lowlands on Mars (Wilhelms, 1973; McGill, 1989; Frey and Schultz, 1988; Frey et al., 2002; Solomon et al., 2005) implies that the lowlands must have served as a major sink for materials formed and redistributed

⇑ Corresponding author at: Vernadsky Institute, Russian Academy of Sciences, Kosygin St., 19, Moscow 119991, Russia. Fax: +7 495 938 2054. E-mail addresses: [email protected] (M.A. Ivanov), hiesinger@ uni-muenster.de (H. Hiesinger), [email protected] (G. Erkeling), [email protected] (D. Reiss). 1 Fax: +49 251 83 36301. 0019-1035/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.icarus.2013.09.018

by volcanic, fluvial, glacial, and wind processes (Scott and Tanaka, 1986; Greeley and Guest, 1987; Kargel et al., 1995; Carr, 1996; Head et al., 2002; Tanaka et al., 2003a,b, 2005). The interpretation of the MOLA topographic data have shown that the northern lowlands have been covered by ridged plains of Early Hesperian age (Kreslavsky and Head, 2000) that resemble those in the upland volcanic provinces such as Hesperia and Lunae Planum (Head et al., 2002). In contrast to these expanses of ridged plains, the plains in the northern lowlands are characterized by systematically lower and more widely spaced ridges and by the presence of strongly degraded craters that have almost no discernible morphologic signatures and represent shallow topographic depressions (McGill, 1986; Buczkowski and McGill, 2002; Head et al., 2002).

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The subdued morphology of the ridges and craters has been interpreted to mean that materials of the Vastitas Borealis units, which are exposed over almost the entire floor of the northern lowlands (Scott and Tanaka, 1986; Greeley and Guest, 1987; Tanaka and Scott, 1987) overlay the ridged plains (Head et al., 2002). The surface of the Vastitas Borealis units shows a number of features that are consistent with the presence of a large reservoir of water/mud/ice within the northern lowlands. These features include thumbprint terrains that consist of curvilinear chains of pitted cones (e.g., Carr and Schaber, 1977; Lucchitta, 1981; Hiesinger et al., 2009), giant polygons (e.g., Lucchitta, 1981; McGill and Hills, 1992; Hiesinger and Head, 2000), assemblages of low sinuous/curvilinear ridges that resemble moraines and eskers (e.g., Kargel et al., 1995), impact craters with rampart- and pancake-like ejecta (e.g., Mouginis-Mark, 1979, 1987). In many studies, the VB has been interpreted either as sedimentary deposits formed by massive erosion from the southern highlands (Jons, 1985; Tanaka et al., 2001), or as materials deposited from a large standing body of water (an ocean) (e.g., Parker et al., 1989, 1993), or as a sublimation residue of such an ocean (e.g., Kreslavsky and Head, 2002), or as sedimentary deposits from the large outflow channels (e.g., Tanaka et al., 2005). The existence of the ocean is an important and still open issue in the martian geology. This hypothesis is supported by several boundaries that can be traced over large distance (thousands of kilometers) and separate morphologically distinguishable units within the VB at the periphery of the northern lowlands. These boundaries have interpreted as possible shorelines of the ancient ocean (Parker et al., 1989, 1993; Clifford and Parker, 2001). Some of these boundaries (e.g., Arabia contact (Clifford and Parker, 2001)) show topographic variations that exceed several kilometers and, thus, are inconsistent with the shoreline (Head et al., 1999; Carr and Head, 2003). The other boundary (Deuteronilus contact (Clifford and Parker, 2001)) is characterized by much smaller topographic fluctuations, 3792 ± 236 (Carr and Head, 2003), which is consistent with the shoreline interpretation. In some models, however, the long-wavelength trend of topographic variations along Arabia shoreline is explained as the result of true pole wander (Perron et al., 2007). The mean elevation of the Deuteronilus contact is close to both the mean elevation of the southern edge of the VB ( 3658 ± 282) (Carr and Head, 2003) and the morphologic termini of large outflow channels in Chryse Planitia ( 3724 ± 153) (Ivanov and Head, 2001). The channels, which that are thought to be carved by large-scale floods (Carr, 1979), are opening into the lowlands and are commonly considered as the primary sources of water that may have filled the northern lowlands in the late Hesperian (e.g., Parker et al., 1989, 1993; Baker et al., 1991; Clifford and Parker, 2001; Tanaka et al., 2005). The very large topographic depression of Utopia Planitia (2000 km in diameter and 1.5–2 km deep) could potentially contain a significant portion of the effluents of the outflow channels because the majority of Utopia Planitia is below the contour line of 3658 m (Smith et al., 2001) and materials of the VB cover almost the entire surface of Utopia (e.g., Greeley and Guest, 1987; Tanaka et al., 2005). Three types of features that occur in Utopia Planitia suggest the presence of a large reservoir of water in this region. (1) Heavily degraded/buried (ghost) craters (Carr, 1981; McGill, 1986) occur throughout Utopia Planitia. The morphology and topographic characteristics of these features have been interpreted as evidence of compaction of possibly sedimentary materials of the VB against the rims of buried craters (e.g., Buczkowski and McGill, 2002). (2) The southern edge of Utopia is one of the sites where thumbprint terrain (TPT) occurs (Lockwood et al., 1992). A variety of hypotheses, ranging from glacial (e.g., Grizzaffi and Schultz, 1989) to volca-

nic origin (e.g., Ghent et al., 2012), were proposed to explain the nature of TPT. In most of these hypotheses the presence of large amounts of ice/water is a requirement. (3) Giant polygons occur peripheral to the center of the Utopia basin (e.g., McGill, 1986). The key elements in many models of formation of these terrains are either wet sediments or the presence of a large standing body of water or ice (see (Hiesinger and Head, 2000) for the summary of the models). The large impact basin of Utopia Planitia (McGill, 1989) plays an important role in the geologic history of the northern lowlands, which has been portrayed in geological maps and correlation charts since the Mariner-9 and Viking missions (Scott and Carr, 1978; Scott and Tanaka, 1986; Greeley and Guest, 1987; Tanaka and Scott, 1987). The most recent map of this region (Tanaka et al., 2005) incorporates the data sets of the Mars Global Surveyor (MOLA gridded topography and dispersed MOC images) and MarsOdyssey (THEMIS infrared (IR), 100 m/px, and visible (vis), 17– 40 m/px, images) missions. The map shows that an extensive Vastitas Borealis interior unit (ABvi) covers the majority of Utopia Planitia and a suite of the younger volcanic and fluvial units and debris flows derived from the Elysium Mons region overlays the central portion of the Utopia basin. The interior unit, which appears to be homogeneous at the scale of the mapping, hosts both the giant polygons (closer to the center of the basin) and the occurrences of TPT near the southern edge of Utopia Planitia. These landforms probably reflect different environments that have occurred during their formation. These differences, however, were probably not obvious in the THEMIS IR images that have provided the most contiguous photobase for the mapping (the higher-resolution THEMIS–vis images were too sparse during the compilation of the map). In our study, we mapped the SW portion of Utopia Planitia (Fig. 1). The giant polygons characterize the northern portion of this area and TPT is concentrated in its southern segment. Thus, the selected area provides the possibility to trace changes of the sets of specific landforms and their morphology as a function of the distance from the edge of the Utopia basin. We have conducted our mapping using mostly images with higher resolution (THEMIS–vis, 17–40 m/px, HRSC, 12–50 m/px, and CTX, 6 m/px) that cover about 80–85% of the selected area. Gaps in the high-resolution coverage were filled by the daytime and nighttime THEMIS-IR images. The units and landforms have been first identified and mapped on the base of the high-resolution data and the characteristic morphologies of the mapped features were extrapolated to the medium-resolution images and traced within the data gaps of the high-resolution images. All available HiRISE images (0.25–0.5 m/px) were used to clarify the possible nature of some units and landforms. The topographic data were collected from the MOLA DEM (resolution 1/128 of a degree). In our study we addressed three major questions: (1) how specific landforms that characterized the VB are changed from the edges toward the center of the Utopia basin? (2) What these changes may tell about the presence or absence of a large putative water reservoir in Utopia Planitia? (3) Which features that were not seen previously in the lower-resolution data sets may provide an additional support (or contradiction) to the hypothesis of the large standing body of water/ice within the Utopia topographic depression?

2. Topographic configuration of our study area The study area covers a portion of the inner wall of the Utopia basin and is characterized by a very small and steady slope toward the northeast (Fig. 2a). The topographic profile from the edge of Utopia Planitia toward its center (Fig. 2a) shows that specific types

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Fig. 1c. Location of figures discussed in the text. The base is a MOLA shaded relief map, resolution is 1/64 of a degree; simple cylindrical projection.

Fig. 1. Regional context of our study area with major physiographic features. The study area extends from 20°N to 45°N and from 100°E to 120°E and portrays the SW portion of Utopia Planitia. The base is a MOLA shaded relief map (a); upper portion, resolution is 1/64 of a degree and color-coded gridded topographic map (b), lower portion, resolution is 1/128 of a degree; simple cylindrical projection. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

of features in the SW portion of the floor of Utopia Planitia occur at different elevations, which was noted previously by McGowan (2011). The pitted cones, which define the TPT in Utopia, occur near the periphery of Utopia at relatively higher elevations and the giant polygons and etched flows (the new type of landforms that is described in the following sections) are topographically lower. Such a difference in the topographic position may reflect the different environments at which these features formed. The southern edge of the Vastitas Borealis units in this region occurs almost at a constant elevation of about 3599 m (±54 m, one sigma) over a distance of about 1500 km and shows no detectable longwavelength (100s of kilometers) topographic trends, but local fluctuation around the mean elevation (Fig. 2b). In the area between the southern edges of both the VB and the younger Elysium-derived materials in the central portion of Utopia Planitia (Tanaka et al., 2005), the elevation changes from about 3600 to about 4700 m over a distance of about 600 km (the average slope is about 0.1°). At the northern boundary of the VB, there is a regional break of slope and the surface becomes nearly horizontal (the average slope is about 0.02°) in the central portions of the basin that are occupied by the younger (Amazonian) materials derived from Elysium Mons (e.g., Tanaka et al., 2005). The mean elevation in this

Fig. 2. Topographic profiles taken in the SW portion of Utopia Planitia. (a) Topographic profile across the study area from its lower left corner (20°N to 100°E) to the upper right corner, which is near the geographic center of Utopia Planitia (45°N to 120°E). (b) Topographic profile along the contact of the Vastitas Borealis units with smooth plains in our study area. Topographic data are MOLA gridded topography, resolution is 1/128 of a degree; ‘v.e.’ is for vertical exaggeration.

portion of the basin is changed for about 0.3 km over a distance of about 900 km (Fig. 2a).

3. Units and structures in the SW portion of Utopia Planitia Many of the units and structures, which we observe in the SW portion of Utopia Planitia, have been described in previous studies (e.g., McGill, 1986; McGill and Hills, 1992; Buczkowski and McGill,

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2002; Tanaka et al., 2005). In this section, we briefly list these features and describe their specific characteristics. The specific sets of morphologic features on the surfaces in our area allow definition of three major material units. They are described below. The unit of smooth plains (Hps, Fig. 3) occurs at the southern edges of Utopia Planitia along its transition to the highlands. The most distinguishable feature of this unit is that its morphologically smooth materials surround, embay, and partly cover numerous flat-topped and angular mesas and knob that are several kilometers across. In some places, structures resembling wrinkle ridges and rims of heavily degraded craters are seen on the surface of the smooth plains. By their geographical position, smooth plains are equivalent to the Utopia Planitia 2 unit (HBu2) and the Vastitas Borealis marginal unit (ABvm) defined and interpreted by Tanaka et al. (2005) as units formed due to erosion and re-deposition of various materials derived from the highlands and outflow channels. The abundant knobs, mesas, and degraded craters of unit Hps, which appear to represent erosional remnants of previous terrains, are consistent with this interpretation. The occurrences of wrinkle ridges within the smooth plains may suggest that the plains were partly formed by re-working of occurrences of volcanic plains because the ridges are typical structures of volcanic plateaus on Mars and the other terrestrial planets (Solomon and Head, 1980; Chicarro et al., 1985; Bilotti and Suppe, 1999), although the ridges may deform the sedimentary suits as well (Plescia and Golombek, 1986). By their morphology, the smooth plains at the southern edge of Utopia Planitia are very similar to the complex of units of knobby materials (Hmk) and smooth plains (Hps) at the periphery of Isidis Planitia (Ivanov et al., 2012). In the Isidis region, large clusters of knobs and mesas of unit Hmk are separated by materials of smooth plains and occur preferentially near the transition to Utopia Planitia (Ivanov et al., 2012). Narrow (a few hundreds of meters) sinuous ridges sometimes occur on the surface of the smooth plains (Fig. 3). One of the most widespread unit in the SW portion of Utopia Planitia is equivalent to the Vastitas Borealis interior unit, ABvi, (Tanaka et al., 2005). The surface of the VB is morphologically smooth and similar to the surface of unit Hps. Materials of the VB, however, seem to have different thermal properties.

Fig. 3. Example of smooth plains (unit Hps). Specific features of the unit are angular, flat-topped mesas (upper right). In places, there are narrow and sinuous ridges on the morphologically smooth surface of the unit (arrows). CTX image B20_017563_2027, resolution is 5.63 m/px, center of the image is at 21.49°N, 101.06°E. See Fig. 1c for location of this and the following figures.

Fig. 4. Contact between smooth plains (Hps) and the Vastitas Borealis units (VB). The VB shows broad lobes that are convex southward toward the smooth plains (dashed line). (a) Mosaic of THEMIS IR-daytime images showing that the surface of the VB is brighter. (b) THEMIS IR-nighttime images of the same area show that the surface of the VB is slightly darker. Both images suggest larger amounts of finegrained materials in the VB. Center of the images is at 20.5°N, 108.2°E, simple cylindrical projection.

Specifically, materials of VB are brighter in the THEMIS-IR daytime images (Fig. 4a) but slightly darker in the THEMIS-IR nighttime images than the adjacent smooth plains (Fig. 4b). These characteristics suggest that the VB contains more fine-grained materials than the smooth plains. Broad lobes (Tanaka et al., 2003a, 2005; Skinner et al., 2008, 2009) characterize the edges of the VB at the contact with the smooth plains (Fig. 4) and an important characteristic of the lobes is that they extend up slope, against the actual topographic gradient. The contact of the VB with the smooth plains in the SW portion of Utopia Planitia closely follows to a single contour line at about 3599 m (±54 m) over the distance of 1500 km (Fig. 2b). Three classes of specific landforms are associated with the vast plains of the Vastitas Borealis units. (1) Numerous small cone-like structures with summit pits (pitted cones (Tanaka et al., 2003a, 2005; Skinner et al., 2005; McGowan and McGill, 2008, 2009)) occur along the southern edge of the VB (Fig. 5a). The morphology (rounded cone-like mounds) and dimensions (a few hundred meters across) of the cones in Utopia Planitia are similar to those that populate the floor of Isidis Planitia where they form the thumbprint terrain (TPT) (e.g., Grizzaffi and Schultz, 1989; Hielscher et al., 2010; Ghent et al., 2012) and to the features that occur in Acidalia and Chryse Planitiae (Farrand et al., 2005; Oehler and Allen, 2010; Komatsu et al., 2012). In contrast to Isidis Planitia where the cones populate almost the entire floor of the basin (Hielscher et al., 2010), the cones in Utopia Planitia are concentrated within a broad (400 km wide) zone that extends from the outer edge of the VB toward the interior of Utopia Planitia. At the inner edge of this zone (approximately corresponding to the 4400 m contour line) the number of cones suddenly drops almost to zero and only a few cones can be seen in the interior of Utopia Planitia. Some lobes of the VB near its contact with the smooth plains are characterized by low and narrow ridges (Tanaka et al., 2005) that occur inside the lobes at about 1–2 km from their edges (Fig. 6). Following the margins of the lobes, the ridge loops around the obstacles and in places where two lobes come close to each other, their ridges merged into one structure (Fig. 6). In some places, the ridges are separated into pieces of elongated or circular hills with summit pits (Fig. 6, inset).

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Fig. 5. Morphologic features seen on the surface of the Vastitas Borealis units. (a) Small cone-like features with summit pits are concentrated near the outer edge of the VB. The cones are usually arranged in chains. CTX image P22_009585_2005_2027, resolution is 5.58 m/px, center of the image is at 20.56°N, 109.64°E. (b) Near the central portions of Utopia Planitia broad and shallow polygonal troughs cut the surface of the VB and form polygonal terrains. CTX image G01_018736_2102, resolution is 5.74 m/px, center of the image is at 30.58°N, 115.22°E.

These segments of the ridges resemble the cones of the TPT near the southern edge of the VB in Utopia Planitia (Fig. 5a). (2) The giant polygons (McGill, 1986, 1989; McGill and Hills, 1992) outlined by broad (2 km wide) and shallow (30 m deep) troughs (Hiesinger and Head, 2000; Cooke et al., 2011) characterize the internal portions of the VB in our study area (Fig. 5b). The exposed polygonal terrain forms a broad (a few hundred of kilometers wide) zone that lies topographically between about 4400 and 4700 m (Fig. 2a) and is located around the central portions of Utopia Planitia. Because the polygonal terrain in Utopia Planitia has been described in detail in many papers (see (Hiesinger and Head, 2000) for summary) we refer the readers to those publications for more information. (3) In close spatial association with the zone of polygons there are numerous features that represent broad (many tens of kilometers wide) and low (a few tens of meters high), flat-topped nappes with lobate edges (Fig. 7). These landforms are visible only in highresolution images (e.g., HRSC, THEMIS–vis, CTX) and have not been

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detected in previous studies of Utopia Planitia. Many of the nappes are associated with systems of narrow graben, emanate from them (Fig. 8a), and extend down the regional slopes, following the very gentle topographic gradients. In some other instances, the nappes radiate away from the domes (Fig. 8b). The characteristic structures of the source regions of the nappes, their lobate edges, dimensions, and topographic configuration collectively suggest that the nappes are, in fact, flows of low-viscous materials erupted from reservoirs beneath the surface of the Vastitas Borealis units. The flows generally consist of two parts with contrasting morphology. The topographically higher portion of the flows usually represents broad contiguous plateaus several (up to several tens) kilometers across bounded by low (a few tens of meters) and very sinuous, scalloped cliffs (Fig. 7). The topographically lower portions of the flows consist of numerous knobs and short and chaotically oriented ridges and are characterized by a very rough surface, which is in sharp contrast to the adjacent morphologically smooth surfaces of the VB (Fig. 7a). The knobs and ridges increase the highfrequency (at the scale of a few hundred meters) sinuosity (up to 1.2–1.5) of the broad flow lobes. Very faint flow-like assemblages of small knobs are seen sometimes at the continuation of the prominent lobes (Fig. 7b) and in places larger knobs are seen near the edges of the flows (black triangles in Fig. 7a); these likely represent detached pieces of the contiguous flows because isolated knobs and/or mesas are non-common features in the area of our study. The typical features of the upper plateau are rounded and slightly elongated rimless pits that can be arranged in chains and clusters of coalescing structures (white arrows in Fig. 7a). The pits show a very narrow distribution of sizes (a few hundred meters), which causes a very steep slope of the size–frequency distribution (SFD) of the pits of about 5 (Fig. 9). Both the lack of rims and the steep slope of the SFD of the pits strongly distinguish them from impact craters and suggest that the pits represent structures of different nature. The coalescence of the circular depressions that occur near the edges of the etched flows appears to characteristically define the scalloped margins of the flows. (Figs. 7a and 7c). In areas where the pits coalesce they destroy the surface and the plateaus gradually merge with the rough surface of the lower portions of the flows (black arrow in Fig. 7a). When coalescing pits penetrate deep into the upper plateau (Fig. 7c), they result in either isolated mesas (white arrows in Fig. 7c) or in groups of sinuous and sharpcrested ridges (black arrows in Fig. 7c). Due to the specific pattern of degradation of the flows, we will refer to the flows as etched flows. Materials derived from the Elysium magmatic center occupy the central (the deepest, starting at about 4700 m contour line) portion of Utopia Planitia. In the map of the northern plains (Tanaka et al., 2005) the Elysium-derived units are interpreted as (1) materials of volcanic origin, for example, lava flows (Elysium Rise unit, AHEe) and (2) materials related to possible magma/volatile interactions (units AEta and AEtb) presenting massive clastic, lahar-like flows (Christiansen, 1989; Russell and Head, 2003; Tanaka et al., 2005). In our map, we have combined these units into one unit of Elysium-derived materials (AEm, Fig. 10). This unit includes darker plains with a finely textured surface (‘‘a’’ in Fig. 10, corresponding to unit AHEe) and masses of brighter materials that either form gentle hills dissected by short ridges and valleys (‘‘b’’ in Fig. 10, corresponding to unit AEta) or fill broad channel-like troughs (‘‘c’’ in Fig. 10, corresponding to unit AEtb).

4. Morphology of impact craters The larger (>1 km) impact craters in our study area display distinctly different morphologies. The most degraded impact

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Fig. 6. Lobe-like features characterize the outer edge of the Vastitas Borealis units in our study area. Specific features of the lobes are depicted in the sketch map (right image). The lobes consist of higher and lower parts that are separated by a scarp. The edges of the lower parts are shown by dashed lines. A narrow ridges often occur at the base of the scarp. In places, the ridge becomes segmented and represents a chain of cone-like features with summit pits (left image, inset). CTX image B20_017497_2003, resolution is 5.6 m/px, center of the image is at 20.87°N, 103.11°E.

Fig. 7a. Example of an etched flow in the SW portion of Utopia Planitia. The upper portions of the flows are usually characterized by circular and elongated rimless pits (white arrows) that often coalesce and destroy the surface of the flows. At the ends of the flows, their upper portion becomes more degraded and gradually merges with heavily deformed lower portions of the flows (black arrow). In few places, large knobs appear to be completely separated from the main flow (black triangles). CTX image P02_001977_2123, resolution is 5.76 m/px, center of the image is at 31.6°N, 109.42°E.

structures that occur exclusively in the region covered by the Vastitas Borealis units are ghost craters (Fig. 11). These features have lost almost all original morphologic and topographic signatures of impact craters except the pronounced circularity (Carr, 1981;

Fig. 7c. Low, scalloped cliffs outline the upper portions of the etched flows. Edges of the cliff often show coalescing rimless pits. In places, the pits penetrate into the upper portion of the flow and separate small mesas from the main flow (white arrows). The heavily degraded surface of the lower portions of the flow consists of knobs, pits, and small sinuous ridges (black arrows). CTX image P15_006816_2100, resolution is 5.86 m/px, center of the image is at 28.69°N, 116.49°E.

McGill, 1986). The ghost craters show double concentric graben that likely formed due to compaction of the overlying materials of the VB over the rims of buried craters (McGill, 1989; Buczkowski and McGill, 2002; Buczkowski and Cooke, 2004). In our analysis we have used the diameter of the outer circular graben as a possible

Fig. 7b. Blow-up of the NW portion of the flow shown in Fig. 7a. Faint knobs (outlined by dashed lines) continue the trend of the more prominent flow. CTX image P02_001977_2123, resolution is 5.76 m/px, center of the image is at 31.6°N, 109.42°E.

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Fig. 9. The cumulative size–frequency distributions of rimless pits (solid curve) and impact craters (dashed curve) on the etched flows. The distribution of the pits is much steeper, reflecting the fact that the diameters of the pits are clustered near a typical value of 200–300 m.

Fig. 8. Source regions of the etched flows. (a) In many places the etched flows are associated with systems of narrow graben (white arrows), appear to emanate from them, and form flow-like features (extend in NW direction from the graben). THEMIS–vis image V27279025, resolution is 18.7 m/px, center of the image is at 31.885°N, 110.12°E. (b) In some other instances, the etched flows are associated with low domes. CTX image P17_007502_2195, resolution is 5.73 m/px, center of the image is at 37.44°N, 104.60°E.

measure of the size of the original craters in order to reconstruct the size–frequency distribution of the crater population that predates emplacement of materials of the VB. In the SW portion of Utopia Planitia we have identified and mapped 105 ghost craters in the diameter range from about 3.5 to 33 km. Two morphologic types of ejecta characterize the prominent craters: (1) multiple-layered ejecta with rampart termini (about 34% of the population of the larger craters, Fig. 12a) and (2) pancakelike ejecta (about 62% of the population, Fig. 12b and c) (Barlow et al., 2000). Craters with rampart and pancake ejecta occur in the same diameter range (from about 2 to about 60 km) and both types of ejecta have been interpreted to indicate the presence of volatiles in the target rocks (Head and Roth, 1976; Carr et al., 1977; Mouginis-Mark, 1979, 1987; Schultz and Lutz, 1988; Wrobel et al., 2006). An important characteristic of the larger (>1 km) impact craters in the SW portion of Utopia Planitia is that in this region there no evidence for partly embayed craters, which are common features of volcanic plateaus on Mars. All impact craters on the VB in our study area are either buried by its material (the

ghost craters) or are superposed on its surface (the prominent craters). The proximal portions of ejecta often consist of series of short (a few kilometers) lobes with sinuous fronts. The surface of the lobes appears morphologically smooth at the resolution of CTX images (Fig. 12b). The distal portions of all of the pancake ejecta are characterized by alcoves, notches, and bays penetrating deeply into ejecta blanket and narrow (several hundred meters) and short (several kilometers) arms extending into the surroundings (Fig. 12b). At some craters, there are both pieces of ejected materials completely separated from the main ejecta body and voids within the ejecta blankets where the surface of underlying terrains is exposed. The troughs of the polygonal terrain control the distribution of ejecta (Fig. 12b). Craters with the pancake ejecta can be either bowl- or cone-shaped or flat-floored (Fig. 12c). The interiors of the flat-floored craters are filled by deposits with a prominent pattern of concentric grooves and ridges on the surface. Evidence for the possible discrete sources of the deposits such as alcoves and gullies is absent both on the walls and near the rim of the flat-floored craters. There are also no detectable topographic and/ or morphologic features on the surface of the deposits that may suggest their preferential accumulation either at pole- or equatorward walls of the craters. The spatial distribution of the described units, landforms, and the larger impact craters is illustrated in the geomorphologic map of the SW portion of Utopia Planitia (Fig. 13). 5. Stratigraphic relationships and absolute model ages of units and structures Contacts between the major mapped units in the SW portion of Utopia Planitia (smooth plains, Vastitas Borealis units, and Elysium-derived materials) are very pronounced and clearly indicate

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Fig. 11. An example of a ghost crater. Structures that are similar to the one illustrated are broadly distributed in the area where the VB is exposed in our study area, but absent either within the smooth plains or in the region covered by the Elysium-derived units. CTX image P04_002623_2065, resolution is 5.69 m/px, center of the image is at 26.17°N, 113.43°E.

Fig. 10. Elysium-derived materials overlay the surface of the VB (to the north of dashed line) and embay polygonal troughs. The surfaces a, b, and c correspond to units AHEe, AEta, and AEtb, respectively (Tanaka et al., 2005). CTX image B17_016165_2184, resolution is 5.84 m/px, center of the image is at 38.73°N, 105.76°E. Inset (lower left corner) shows a portion of the THEMIS IR-nighttime mosaic for the same area (letters in the inset indicate same units as in the CTX image). Unit a, which is interpreted as lava plains (Tanaka et al., 2005), is much brighter than the surface of the VB.

the stratigraphic position of the units. Broad lobes characterize the edges of the VB at the contact with the smooth plains (Fig. 6). The lobes are convex southward toward the smooth plains and are bounded by a low scarp that runs around obstacles such as mesas and knobs of the smooth plains. The lobes overlap structures that occur within smooth plains (e.g., wrinkle ridges) and this indicates that material of the VB is stratigraphically higher. Because the area of the smooth plains in our study region is too small and polluted by secondary craters it is not possible to perform crater size–frequency distribution measurements. The onlap of the VB materials strongly indicates, however, that smooth plains formed prior to the emplacement of the Vastitas Borealis units (e.g., Tanaka et al., 2005). Crater statistics yield important information on the absolute model ages of mapped units (Neukum and Wise, 1976). In order to estimate the absolute model ages of the key units and structures in the SW portion of Utopia Planitia, we have counted craters using the THEMIS IR-daytime mosaic and CTX images within several

(from 3 to 8) count areas for each units/structures. Craters were measured and counted using the crater tool of the GIS ArcMap 9 software (Kneissl et al., 2011). The locations of each individual count areas are shown in Fig. 14 and detailed examples of our crater counts are shown in Fig. 15. The sizes of our count areas vary from 240 km2 to 1176 km2 (for counts on CTX images) and from 65,209 km2 to  90,354 km2 (for counts on the THEMIS mosaic). When craters were mapped and counted in individual areas, we have combined those related to each specific unit into one sample in order to increase the number of craters and make crater statistics to be more reliable. These enlarged samples were used for estimation of the model absolute age. For the VB, the combined Elysium-derived units, and the etched flows we selected different count areas (Fig. 14), which we dated separately. We derived absolute model ages by applying the production function of Ivanov (2001) and the chronology function of Hartmann and Neukum (2001). For this purpose, we used the CraterStats-II software (Michael and Neukum, 2010). The size–frequency distribution of craters that superpose the surface of the VB is approximated by one isochron in a wide interval of diameters from 0.8 to 8 km (Fig. 16a). The isochron corresponds to an absolute model age of 3.5 Ga. The size–frequency distribution shows an excess of craters in the diameter range of 2.5–5 km that corresponds to the age of about 3.6 Ga (Fig. 16a). Although we tried to avoid obvious secondary craters, a number of them may still contribute to the population of the craters that we have counted and the ‘‘bulge’’ on the crater count curve (Fig. 16a) may reflect the contribution of secondary craters. Nevertheless, we adopt the older age (3.6 Ga) as the possible time of emplacement of materials of the Vastitas Borealis units. We cannot rule out the possibility that the age 3.5 Ga may represent more correct estimate, but the difference between these two model ages (0.12 Ga) is not sufficient enough to introduce large changes in the stratigraphic scenario. In order to estimate the absolute model ages of the surfaces that predate the VB, we have counted both the ghost and prominent craters that occur within the same count areas. The size–frequency distribution of these crater populations suggests that the age of terrains that existed prior to the emplacement of the VB is 3.7 Ga (Fig. 16b). This age corresponds to the Early Hesperian

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Fig. 12. Morphological types of ejecta of the larger (>1 km) prominent craters in the SW portion of Utopia Planitia. (a) Ejecta of the rampart craters are characterized by superposing lobes with a low ridge at the end (arrows). There is no evidence for post-formational modification of the rampart ejecta. CTX image B21_017919_2015, resolution is 5.58 m/px, center of the image is at 23.38°N, 101.32°E. (b) Distal portions of the pancake-like ejecta are strongly degraded. There are large voids near the edges of contiguous ejecta blankets and some portions of ejecta are detached from the blanket (black arrows). Polygonal troughs control the distribution of the ejecta (white arrows). CTX image B18_016521_2109, resolution is 5.76 m/px, center of the image is at 31.43°N, 107.97°E. (c) In some places, craters with the pancakelike ejecta are filled by materials with a circular pattern of narrow grooves and ridges. CTX image B01_010139_2183, resolution is 5.94 m/px, center of the image is at 38.01°N, 103.12°E.

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epoch (Hartmann, 2005; Werner and Tanaka, 2011), which was characterized by massive eruptions of volcanic materials and formation of the extensive volcanic provinces (e.g., Hesperia Planum) of the Hesperian ridged plains within the southern uplands (Tanaka, 1986; Greeley and Guest, 1987; Tanaka and Scott, 1987; Hiesinger and Head, 2004; Ivanov et al., 2012) and on the floor of the northern plains prior to emplacement of materials of the Vastitas Borealis units (Head et al., 2002). The contact between the VB and the Elysium-derived units is very clear and unambiguously indicates the younger stratigraphic age of the Elysium materials (e.g., Tanaka et al., 2005). Several isochrons approximate the size–frequency distribution of craters superposed on the surface of the Elysium-derived units (Fig. 16c). The oldest age (2.2 Ga) is poorly supported by the crater statistics (four craters only) and appears to be the least reliable age. Thus, we have accepted the absolute model age of about 1 Ga (Fig. 16c) as a more statistically supported estimate of the time of emplacement of the Elysium-derived materials. We acknowledge, however, that formation of the Elysium-derived units may have occurred over an extensive time span (Neukum and Hiller, 1981; Mouginis-Mark, 1985; Christiansen, 1989; Skinner and Tanaka, 2001; Russell and Head, 2003; Werner, 2009; Pasckert et al., 2012). The age 578 Ma may correspond to the late resurfacing event. Etched flows clearly superpose the surface of the Vastitas Borealis units in our study area and, thus, represent its upper stratigraphic limit. The crater size–frequency distribution on the surface of the flows suggests that they may have been formed soon after the emplacement of the VB, at about 3.5 Ga (Fig. 16d). Despite the large error bars of this estimate, it is still within the late Hesperian and, thus, the flows are much older features than the Elysium derived units that also postdate the VB. The younger age estimates (2.97 Ga, 1.48 Ga, and 552 Ma) perhaps reflect the later resurfacing events, the nature of which is not clear. The polygonal troughs in the SW portion of Utopia Planitia cut the surface of the VB (Fig. 5b) and, thus, are younger structures. The Elysium-derived units outline the northeastern edge of the polygonal terrain in our study area and embay the troughs (Fig. 10). The troughs are very prominent within the VB and seem to gradually disappear toward the central portion of Utopia Planitia (Fig. 17a). However, in higher resolution images (e.g., CTX or THEMIS–vis) the heavily flooded troughs are still clearly visible within the areas covered by the Elysium units (Fig. 17b). The presence of the flooded troughs suggests that prior to the emplacement of the Elysium units the polygonal terrain was more extensive and also occurred in the central portions of the Utopia basin. Similarly, the small outliers of etched flows near the central portions of Utopia Planitia are heavily embayed by the Elysium-derived materials (Fig. 17c). In a few places in our study area, the polygonal troughs control the emplacement of materials of the etched flows (Fig. 18) and, thus, in these places the troughs are older structures. More frequently, however, the troughs cut the flows (Figs. 7a, and 8a) and, thus, are younger. These relationships suggest that although formation of the polygonal terrain and the etched flows in the SW portion of Utopia Planitia were partly overlapping in time, the troughs generally postdate the emplacement of the flows. The stratigraphic relationships among the units and structures in the SW portion of Utopia Planitia are summarized in the correlation chart that was calibrated by the estimates of the absolute model ages (Fig. 19). 6. Discussion The unusual morphology of the Vastitas Borealis units and features associated with them (e.g., ridged, knobby, polygonal, and

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Fig. 13. Geomorphologic map of the SW portion of Utopia Planitia. The map is in simple cylindrical projection.

mottled terrains (Lucchitta, 1981; Rossbacher and Judson, 1981; Scott and Tanaka, 1986; Tanaka and Scott, 1987; Greeley and Guest, 1987) have been recognized for a long time and served as the basis for the interpretation of materials of the VB as wateror ice-related sediments (e.g., Carr and Schaber, 1977; McGill, 1985, 1986; Kreslavsky and Head, 2002; Tanaka et al., 2005). The discovery of the features that have been interpreted as possible shorelines has lead to the hypothesis of the existence of a large standing body of water within the northern plains (Parker et al., 1989, 1993) and consequences of its evolution for the geological history of the planet (Baker et al., 1991; Clifford and Parker, 2001; Kreslavsky and Head, 2002; Carr and Head, 2003). Inspection of MOC high-resolution images showed little evidence in support for the coastal landforms (Malin and Edgett, 1999, 2001; Ghatan and Zimbelman, 2006) along the boundaries interpreted by Parker et al. (1989, 1993) as possible shorelines of the putative ocean. The existence of specific coastal landforms, however, would require wave action, which is less likely under the present climatic conditions (Ghatan and Zimbelman, 2006). More important, is that possible coastal features must be resistant to degradation for more than 3 byr to be still recognizable in modern imaging data sets.

Thus, the absence of specific coastal features does not necessarily argue against the ocean hypothesis. The more robust parameter that may or may not support the shoreline interpretation and the ocean hypothesis is the topographic positions of the proposed shorelines (Head et al., 1999). One of the contacts that have been mapped and interpreted as a shoreline, the Deuteronilus contact (Parker et al., 1989, 1993; Clifford and Parker, 2001), closely follows the outer boundary of the Vastitas Borealis units and shows relatively small topographic variations, 3658 ± 282 m, if a portion of the VB boundary that is influenced by the superposition of the younger Elysium-derived units is ignored (Carr and Head, 2003). The fact that the outer boundary of the VB is confined within a narrow interval of elevations was interpreted as important additional evidence supporting the ocean hypothesis (Carr and Head, 2003; Webb, 2004). In the SW portion of Utopia Planitia, the contact of the VB with the smooth plains has two important characteristics: (1) it closely follows a single contour line of 3599 ± 54 m over a distance of about 1500 km where the contact is visible in our study area and shows no systematic variations (Fig. 2b); the elevations of the contact are, thus, completely within the interval determined for the

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Fig. 14. Location of count areas for crater size–frequency distribution measurements. The map is in simple cylindrical projection.

entire outer boundary of the VB (Carr and Head, 2003) and (2) the contact consists of broad lobes (Fig. 6) that are convex toward the adjacent plains and overlay their surfaces within gentle local lows. These characteristics are consistent with an interpretation that materials of the VB were liquid (Parker et al., 1989, 1993; Clifford and Parker, 2001) or semi-liquid (Jons, 1987; Tanaka et al., 2003a,b) at the moment of emplacement. They were spreading along an equipotential surface in a broad region, and suggest, thus, an extensive flooding of the region. The broad (10–20 km across) lobes of the VB that extend up the regional slope and consist of the higher and lower parts that are separated by a scarp are dissimilar morphologically to margins of lava flows. In the THEMIS IRnighttime images, the surface of the VB appears darker than both the smooth plains (Fig. 4b) and the lava flows from Elysium Mons (Fig. 10). These characteristics are inconsistent with the interpretation of the Vastitas Borealis units in our study area as lava flows and require an alternative interpretation of the VB. Consequently, the VB has been interpreted either as deposits of a water reservoir or a body of ice (Parker et al., 1989, 1993; Head et al., 1999; Clifford and Parker, 2001; Kreslavsky and Head, 2002; Carr and Head, 2003; Tanaka et al., 2005). Two classes of features that occur within the SW portion of Utopia Planitia appear to be particularly important for our understanding of the nature of the materials of the VB and to define the extent of the possible water/ice reservoir in Utopia Planitia: these features

are impact craters (Figs. 11 and 12) and etched flows (Figs. 7 and 8). 6.1. Impact craters Three morphological types of impact crates occur in the SW portion of Utopia Planitia: (1) ghost craters (Carr, 1981; McGill and Hills, 1992) (Fig. 11), (2) craters with multiple-layer, rampart ejecta (Fig. 12a), and (3) craters with pancake-like ejecta (Fig. 12b and c) (Barlow et al., 2000). The subdued morphologic and topographic signatures of the ghost craters (McGill and Hills, 1992; Buczkowski and McGill, 2002; Buczkowski and Cooke, 2004) suggests that they represent relicts of impact craters that have been formed prior to the emplacement of materials of the VB (Buczkowski and McGill, 2002). The size–frequency distributions of both the ghost and the prominent craters in the same areas covered by the Vastitas Borealis units (Fig. 16b) suggest an Early Hesperian absolute model age (3.7 Ga) of the terrain predating the VB. Head et al. (2002) reported the same age of the surfaces underlying the VB within the much broader region of the northern plains. Morphologically, ghost craters are significantly different from craters that are completely buried by lava flows within some volcanic plateaus elsewhere on Mars. This suggests that the properties of the VB materials are different from those of common basaltic

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Fig. 15. Examples of count areas for crater size–frequency distribution measurements. (a) Craters counted on the THEMIS IR-daytime mosaic. Areas outlined white without marked craters correspond to large clusters of secondary craters. Thick white lines mark ghost craters. (b) Craters on the surface of the Elysium derived units. (c) Craters on etched flows.

lavas and has been interpreted as the result of compaction and slumping of materials of the VB overlying the buried craters (Buczkowski and McGill, 2002; Buczkowski and Cooke, 2004; Buczkowski et al., 2012). Thus the morphology of ghost craters provides additional evidence for the sedimentary nature of materials of the VB (Parker et al., 1989, 1993; Kreslavsky and Head, 2002; Carr and Head, 2003; Tanaka et al., 2005). In those parts of the SW portion of Utopia Planitia where materials of the VB are exposed (Fig. 13), there are no craters in a wide diameter range (from hundreds of meters to tens of kilometers) that are partly embayed. The morphology of craters in this region is bimodal: there are either completely buried craters (ghost craters) or craters that are superposed on the VB. This observation suggests that the emplacement of the Vastitas Borealis units was faster than the rate of accumulation of impact craters and is consistent with the emplacement of VB due to, for example, catastrophic floods from the outflow channels (Parker et al., 1989; Carr, 1996; Head et al., 1999; Carr and Head, 2003). In contrast, in the vast volcanic provinces (e.g., Hesperia Planum) partly embayed craters are common features that indicate multiple phases of eruption of volcanic materials in these regions. The larger (>1 km) prominent craters in our study region display either multiple-layered-rampart, or pancake-like ejecta. Both types of ejecta suggest the presence of water/ice in the targets at the moment of impact (Gault and Greeley, 1978; Greeley et al.,

1980; Wohletz and Sheridan, 1983), but probably reflect different amounts of volatiles in the target rocks (Woronow, 1981; Stewart et al., 2001). The distal portions of the lobes that form the multiplelayered ejecta are terminated by ramparts and at the resolution of CTX images (6 m/px) there is little evidence for the noticeable post-formational modification of the lobes (Fig. 12a). This may suggest that the amount of volatiles in the target rocks was high enough for liquefaction of the ejecta during an impact (e.g., Carr et al., 1977), but was insufficient for a noticeable degradation of the ejecta after its emplacement. In contrast, the distal portions of the pancake-like ejecta always show strong degradation patterns that are characterized by large voids where the underlying surface is exposed and remnants of ejecta that are completely separated from the main ejecta body (Fig. 12b). Both features require either removal of a portion of the ejecta or its embayment. In the vicinity of many craters with pancake-like ejecta, there are numerous polygonal troughs that cut the surface of the VB and, thus, postdate it. The troughs usually control the distribution of ejecta (Fig. 12b) and provide conclusive evidence that the shape of the ejecta blankets is not the result of embayment by materials of the VB. This implies that the specific pattern of degradation of the pancake-like craters was caused by enhanced erosion of their ejecta blanket. An escape of the volatiles could cause undermining of the ejecta, collapse of its surface, formation of voids in the ejecta blankets, and separation of the ejecta

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Fig. 16. The size–frequency distribution of craters of the VB, the Elysium-derived units, and etched flows in the SW portion of Utopia Planitia. See text for details.

outliers in places where the volatiles were more abundant. Thus, the unusual morphology of the pancake-like ejecta is indicative of the presence of abundant volatiles (water, ice, or both) in the target materials. Channels or other features that may indicate a fluvial activity are mostly absent at the edges of the pancake-like ejecta suggesting that instead of melting, sublimation of the volatiles was the principal process of ejecta degradation. Many of the craters with pancake-like ejecta show a low d/D ratio (0.04 ± 0.02, one sigma), compared with the bowl and coneshaped craters with either rampart- or pancake-like ejecta, 0.13 ± 0.03) and flat floors due to partial filling (Fig. 12c). Materials on the floor of the filled craters show a characteristic pattern of parallel grooves and ridges (Lucchitta, 1981; Carr, 1996) that is similar (except for the circularity caused by the crater geometry) to the pattern on the surface of the lineated fill that is formed by

flow of viscous, ice-saturated material into the troughs of fretted terrain (e.g., Morgan et al., 2009). Neither walls nor rims of the flat-floored craters in the SW portion of Utopia Planitia show, however, the alcove-like features that appear to be the sources of the fill of the troughs of the fretted terrain (Morgan et al., 2009). The lack of discrete source regions that are preferentially located on either pole- or equatorward walls and the circular symmetry of features on the surface of the fill suggest that its material was not accumulated by atmospheric transport and re-deposition of volatiles (Baker et al., 2010). The filling material likely represents viscous flows (Lucchitta, 1981) from the sources in the crater walls. This interpretation is consistent with the possible volatile enrichment of the pancake-like ejecta and is supported by the presence of the smaller flat-floored (filled) craters that are on top of the pancake-like ejecta near the rim of the main crater (Fig. 12c). In our

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Fig. 18. In some areas in the SW portion of Utopia Planitia the etched flows fill portions of the polygonal troughs and, thus, postdate the VB and are younger than the troughs. THEMIS–vis image V18993006, resolution is 18.9 m/px, center of the image is at 35.78°N, 100.08°E.

Fig. 17. Stratigraphic relationships of polygonal troughs and etched flows with the Elysium-derived units. (a) A mosaic of THEMIS IR-daytime images shows that the troughs are very prominent features of the VB. At the contact with the Elysium units, the troughs almost completely disappear. Center of the image is at 30.71°N, 113.37°E. (b) Within the areas covered by the Elysium-derived materials, the troughs are visible as very subdued elongated topographic depressions (arrows). CTX image B06_012077_2112, resolution is 5.75 m/px, center of the image is at 30.86°N, 113.35°E. (c) A small piece of an etched flow is embayed by the Elysiumderived materials. Heavily embayed and almost completely buried polygonal troughs are seen at the bottom of the image (arrows). THEMIS–vis image V28689012, resolution is 18.7 m/px, center of the image is at 30.93°N, 111.87°E.

Fig. 19. Correlation chart of the major units and structures in the SW portion of Utopia Planitia. Ages of the chronostratigraphic units are from Hartmann (2005).

study area, the flat-floored craters are abundant (114 craters, about 78 craters per 106 km2), occur exclusively near the central portions of the Utopia basin where they are intermixed with craters with pancake-like ejecta, and in their spatial distribution do not show

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evidence for latitude dependence. The reasons why some of the craters have cone- or bowl-shaped interiors while others are flatfloored may be due to either more advanced stages of crater modification of older craters, or larger amount of the volatiles in the target materials, or both. The morphology of impact craters in the SW part of Utopia Planitia (Fig. 12) suggests the presence of volatiles in the target materials. The spatial distribution of pancake-like and rampart craters indicates that volatiles were not evenly distributed within Utopia Planitia because the morphology of the ejecta of craters in our study area correlates with their geographic position. The multiple-layer, rampart-type ejecta, which presumably require smaller amounts of volatiles (Stewart et al., 2001; Pearce et al., 2011), characterizes craters that preferentially occur near the periphery of Utopia Planitia (Fig. 20a). In contrast, craters with pancake-like ejecta that likely indicate a larger abundance of volatiles in the target rocks are concentrated near the center of the basin (Fig. 20b). Such a spatial distribution of craters with different types of ejecta suggests that targets located less than 1000 km from the center of the Utopia basin were more volatile-rich than materials at the periphery of the basin. It must be emphasized that craters with either rampart- or pancake-type ejecta occur practically in the same interval of diameters (from 2 to 30–40 km). This implies that the depth of crater penetration is not the main factor in the excavation of more or less volatile-rich materials. In our interpretation, the spatial distribution of the different types of ejecta reflects an increase of amount of the volatiles toward the central portion of Utopia Planitia and suggests that this region likely was a large reservoir of water/ice. 6.2. Etched flows The stratigraphic relationships between the flows, polygonal troughs, and materials of the VB indicate that the flows postdate the emplacement of the Vastitas Borealis units (Figs. 7 and 8) and, thus, represent its upper stratigraphic limit. The absolute model age of the formation of the flows is 3.46 Ga ago (Fig. 16d) and is indistinguishable from the absolute age of the VB in the SW portion of Utopia (3.49 Ga, Fig. 16a). Together with additional stratigraphic evidence this suggests that the emplacement of the VB in this region was immediately followed by the formation of the etched flows. The pattern of the flow emplacement and the specific features of their source regions (fractures, low domes, Fig. 8a and b) suggest that the flows are results of effusive eruptions of apparently lowviscous materials from reservoirs beneath the surface of the VB. Although some features of the flows (e.g., the upper plateaus) are seen in inflated lava flows (Keszthelyi et al., 2000), the large distance of the flows from known volcanic centers and several important characteristics of their morphology are inconsistent with the interpretation of the flows as common volcanic features.

Fig. 20. Morphology of ejecta of the larger craters in the SW portion of Utopia Planitia changes as a function of the distance from the center of the Utopia basin (upper right corner of the map). (a) Craters with rampart ejecta tend to occur at the periphery of the basin. (b) Craters with the pancake-like ejecta are concentrated in the central portion of the basin. Thick dashed lines show the approximate position of the outer edge of the zone of etched flows. Gray line shows the outer edge of the Elysium-derived units. The maps are in simple cylindrical projection.

(1) The upper plateaus (layers) of the flows (Fig. 7) show specific features that are suggestive of the presence of volatile components during the emplacement of the flows. The margins of the upper layers typically appear as low cliffs with very sinuous, scalloped, or digitate edges with gulfs and bays formed by coalescing pits that penetrate deeply into the layer (Fig. 7). Lava flows more often are characterized by festoon-like margins related to advancing local flow fronts. The scallops and notches, in contrast, are usually collapse features that may indicate either undermining of the layer edges, low strength of the layer materials, the presence of volatiles, or all of the above.

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(2) The upper plateaus of the etched flows quite often show sets of isolated mesas with scalloped edges separated from a contiguous layer (white arrows in Fig. 7b). The mesas suggest that the material of the upper plateaus is friable, which may be, for example, due to abundant vesicles (e.g., pumice flows). Thus, the presence of the mesas may indicate the low strength of materials of the flows (consistent with the scalloped bounding cliffs) and possible presence of volatiles in etched flows. (3) Shallow, rounded and elongated rimless pits are characteristic features on the uppermost surface of etched flows (Fig. 7). The pits vary in size but most of them are a few hundred meters in diameter and this causes a very steep slope of the size–frequency distribution curve (Fig. 9). In contrast, a much shallower slope characterizes the distribution of impact craters superposed on etched flows (Fig. 9). Usually, the pits are broadly distributed over the surface of the flows, but sometimes they are arranged in chains and clusters (white arrows in Fig. 7a). When the pits of the clusters are coalescing they form either shallow sinuous depressions within the layer or broad flat-floored niches at its edges. In both cases, the walls of these features are scalloped and very sinuous. All these characteristics of the pits strongly distinguish them from regular impact craters and suggest that they are, in fact, collapse features. In no case, however, have we observed evidence that the pits are tectonically induced features, which was proposed, for example, for the chains of pits within some graben on the flanks of Alba Patera (Scott et al., 2002). Thus, we interpret the flow pits to indicate internal voids in the flows unrelated to tectonic deformation. The sizes of the pits (a few hundred meters) are inconsistent with the size of volatile vesicles in magmas and require much larger inclusions of volatile-rich materials, for example, fragments of ice. (4) At the edges of the upper plateau coalescing pits completely destroy the cliff and form the hummocky-pitted-ridged surface that characterizes the topographically lower portions of the etched flows (black arrows in Figs. 7a and 7b). The surfaces of the lower parts of the flows are heavily degraded and in contrast to this, the surface of adjacent materials of the VB is morphologically smooth (Figs. 7 and 8). In many places, faint, flow-like zones of chaotically organized low ridges, knobs, and hummocks are attached to more prominent flow lobes and continue the trend of the lobes (white triangles in Fig. 7a). Sometimes, small fragments that are completely separated from the contiguous flows are seen (black triangles in Fig. 7a). Because the flows postdate the VB, this style of modification of the flow edges cannot be explained by the embayment of the flows and implies more advanced stages of degradation of the flow materials than for materials of the VB. The pattern of degradation of the flows resembles that of the distal portions of the pancakelike ejecta (Fig. 12b) and suggests that material of the flows likely contained volatile components, whose escape caused the collapse of the surface and the formation of the observed morphologies of the flows. All the above (1–4) morphological features of the etched flows distinguish them from common low-viscous lava flows (Fig. 21). Features that may suggest explosive activity (e.g., patera-like structures, low and broad cones with large summit pits and dissected flanks, etc.) were not observed in association with the etched flows in the SW portion of Utopia Planitia. Possible reconstructions of the original morphology of the flows suggest that a significant portion of the initial volume of the flows (may be as much as 30–50%) is missing due to their degradation. Under the

Fig. 21. The comparison of the etched flows in Utopia Planitia with common lava flows (e.g., from Syria Planum) shows that that specific features of the etched flows such as (1) a heavily degraded surface, (2) scalloped or (3) digitate edges, and (4) collapse pits are not observed in lava flows. (a) THEMIS–vis image V21938013, resolution is 18.6 m/px, center of the image is at 35.01°N, 106.94°E. (b) THEMIS–vis image V32078001, resolution is 17.4 m/px, center of the image is at 17.5°S, 257.1°E.

low atmospheric pressure on Mars, however, a large proportion of dissolved gases in magma would result in an explosive activity (Wilson and Head, 1994), the evidence of which is absent in association with the flows. The typical height of etched flows is several tens of meters, but they can extend for tens of kilometers. These dimensions of the flows suggest eruption of very liquid materials and are inconsistent with emplacement of gas-saturated, pumicelike lavas, whose viscosity is significantly larger due to their abundant gas vesicles (Pavri et al., 1992; Head and Wilson, 1992). Because the characteristics of the etched flows are different from those that are observed or expected at common volcanic features, we exclude a volcanic origin (‘‘hot’’ volcanism) of the flows. Alternative explanations of the origin of the flows are eruptions of mixtures of rocks and water/ice due to mud volcanism. This interpretation is consistent with the structures at the flow source areas, but mostly based on the very specific pattern of degradation of the flows, which is very difficult or impossible to explain in the framework of the usual, hot-volcanism model. Thus, we interpret the etched flows as evidence for widespread (Fig. 13) effusive mud volcanism in the SW portion of Utopia Planitia.

6.3. Model of geological evolution of Utopia Planitia The change of morphology of crater ejecta toward the central portion of Utopia Planitia (Fig. 20) and the interpretation of the etched flows as mud flows are of key importance for the understanding of the history of volatiles in Utopia Planitia and in the northern plains of Mars.

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The diameter ranges of craters with rampart and pancake-like ejecta are significantly overlapping. This means that craters of both types excavated materials from approximately the same depths. The craters with pancake-like ejecta are concentrated within the central portions of Utopia Planitia (Fig. 20b). This suggests that the the thickness of volatile-rich materials was greater in the central portion of the Utopia basin. The transition from rampart to pancake-like ejecta occurs in a rather narrow zone, which may suggests rapid increase of the thickness of the volatile-rich layer toward the center of Utopia Planitia and the existence of a water/ ice reservoir in this region. We interpret the etched flows as results of eruptions of mud. This interpretation requires the presence of liquefied (mud-like) materials in the reservoir(s) beneath the surface of the Vastitas Borealis units. The observations that materials of the Elysium-derived units embay both the polygonal troughs and the etched flows (Fig. 17) suggest that these structures may have been more extensively distributed and occurred in the central portions of Utopia Planitia. It is important to note that the transition from rampart to pancake-like ejecta approximately coincides with the outer edges of the zone of etched flows in the SW portion of Utopia Planitia (Fig. 20) Thus, the independent classes of features, impact craters and etched flows, consistently indicate transition from smaller to larger amounts of volatiles in the sub-surface. If this is the case and if the reservoir was symmetric relative to the center of the Utopia basin, then its diameter may be as large as about 2000 km. Such a great extent suggests that the reservoir may represent the remnants of a former body (or bodies) of water ponding in the topographic low of Utopia Planitia and may be a consequence of the existence of an ocean in the northern lowlands of Mars. The absolute model age of the etched flows (3.5 Ga) is inconsistent with the hypothesis of their formation by squeezing out of ice-saturated material under the load of the, Elysium-derived units; Amazonian in age (Fig. 16) and suggests that the flows were erupted soon after emplacement of materials of the VB (Fig. 16) from either partly or completely liquefied sources. While the liquefaction might be related to the thermal influence of the Elysium magmatic center, in our study area we did not observe features that would indicate the presence of dikes at depth (e.g., long, narrow, linear graben (e.g., Lister, 1990)) or exhumed dikes of dike swarms (Head et al., 2006; Korteniemi et al., 2010; Pedersen et al., 2010) extending from the Elysium magmatic center toward the zone of the flows. Also, no features were observed in association with the etched flows that are indicative of a magma/ ice interaction at shallow depths such as explosive calderas, outflow channels, moberg ridges, and lahars. (Lucchitta, 1981; Christiansen, 1989; Russell and Head, 2003; Pedersen and Head, 2010). The absence of these features does not favor the hypothesis of the mud volcanism in Utopia being triggered by thermal pulses from Elysium. Freezing of a standing body of water (or water and mud) provides an alternative mechanism for the formation of the etched flows. In this scenario, floods from the outflow channels fill a portion of the northern lowlands (e.g., Baker et al., 1991; Carr and Head, 2003) and form a large body of water in Utopia Planitia (Fig. 22a). This body may or may not be connected with the other lowlands that may represent additional water traps (i.e., the Isidis and North Polar basins (e.g., Carr and Head, 2003)). The possible fate of a large standing body of water on Mars under the present climatic conditions was considered in a number of papers (e.g., Carr, 1983; Moore et al., 1995; Kargel et al., 1995; Kreslavsky and Head, 2002). All models derived by these authors agree on the geologically short time of freezing of such a body, from 104 (Moore et al., 1995; Kargel et al., 1995;) to 105 years (Kreslavsky and Head, 2002). The rate of the following sublimation

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Fig. 22. A generalized model of the sequence of major events in Utopia Planitia. Not to scale. See text for discussion.

of ice in the freezing body strongly depends on the rate of accumulation of a surficial debris layer. Such a layer only a few meters thick would effectively decrease sublimation (Farmer and Doms, 1979; Carr, 1990; Carr and Head, 2003) and favor gradual freezing of the rest of the water body. In this scenario, the layer of debris accumulating on top of the ice due to its sublimation represents the Vastitas Borealis units (Fig. 22b). As the body of water continues to evolve, both the progressive freezing and load of the growing layer of ice and debris build up the internal pressure. This causes eruptions of still liquid mixtures of rocks/soils, water, and ice to the surface and the formation of the etched flows (Fig. 22c). As soon as the flows were emplaced, their volatile components began to sublimate and/or melt and this resulted in the specific degradation pattern of the flows. This mechanism of the formation of the flows suggests that the flow materials should be enriched by salts. In order to test this consequence, we have inspected all available images provided by the CRISM instrument. Unfortunately, the SW portion of Utopia Planitia is mostly covered by dust and CRISM data do not show discernible features. Increase of the volume of the freezing body of water should lead to its net expansion and may have caused deformation of the overlying layers (Fig. 22c). The shape and orientation of the polygonal troughs in Utopia Planitia indicate that they formed under conditions of isotropic extension (e.g., Hiesinger and Head, 2000). This is consistent with the deformation of the surface due to expansion of the growing body of ice. In relatively rare cases, the polygonal troughs predate the emplacement of the etched flows (Figs. 8b and 18), but the majority of the troughs postdate the flows (Figs. 7 and 8a). These relationships relax the stratigraphic constraints on the formation of the troughs and allow for alternative formation mechanisms, for example, an uplift of the surface after sublimation of a significant portion of ice from the water body in Utopia Planitia (Hiesinger and Head, 2000). Near the final stages of the evolution of the body of water, either a thicker ice layer seals the sources of etched flows and prevents their continued eruptions or the body becomes completely

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frozen (Fig. 22d). The net volume of the system, however, should increase with freezing, and this creates favorable conditions for expansion of the overlying surface and progressive formation of the majority of polygonal troughs (Fig. 22d) after the emplacement of the etched flows. In the central portion of Utopia Planitia, the Elysium-derived units of the Amazonian age (Fig. 22e) have buried both the mud flows and the giant polygons (Fig. 17) and in of our study area left these features exposed along the edges of the deepest portions of the former water reservoir (Fig. 22e). This part of the reservoir may have retained greater amounts of volatiles, which is reflected by the formation of craters with pancake-like ejecta (Fig. 20). Closer to the periphery of the basin where the original body of water was shallower, a significant portion of the volatiles could have been lost due to sublimation and this is manifested by rampart ejecta (Fig. 20). 7. Summary and conclusions Results of our study of the SW portion of Utopia Planitia can be summarized as follows. (1) The size–frequency distributions (SFD) of both ghost and prominent craters within the SW portion of Utopia Planitia suggest that the absolute model ages of terrain predating the emplacement of the VB are about 3.7 Ga, which corresponds to the time of formation of vast volcanic plains in the Early Hesperian (e.g., Tanaka, 1986). (2) In the SW portion of Utopia Planitia there are no craters that are partly embayed by materials of the VB. This implies that the emplacement of the Vastitas Borealis units was faster than the accumulation rate of impact craters and is consistent with the geologically short time of emplacement of the VB related to, for example, large floods from the outflow channels (Parker et al., 1989, 1993; Head et al., 1999; Carr and Head, 2003; Tanaka et al., 2005). (3) The SFD of prominent craters yields an absolute model age of the VB of 3.6–3.5 Ga. The ages of the etched flows that overlay the VB are 3.5 Ga. These absolute model ages suggest the geologically rapid emplacement of the VB, which is consistent with the absence of partly embayed craters. (4) The majority of the larger (>1 km) impact craters show ejecta morphologies (rampart and pancake-like ejecta) that are indicative of the presence of ice/water in the target materials. The distal portions of the pancake-like ejecta are heavily degraded, however, their embayment by materials of the VB as the cause of the degradation can be excluded because the craters postdate the VB. This means that the target properties were the main reason of the enhanced degradation of the ejecta. We interpret the unusual morphology of the distal portions of the pancake-like ejecta as evidence for a higher abundance of volatiles (ice and/or water) in the target materials. Craters with rampart ejecta do not show recognizable signs of the post-formational degradation. This suggests that the amount of volatiles in the target materials was insufficient to cause noticeable ejecta modification. (5) Although the diameter ranges of the craters with either rampart- or pancake-like ejecta are overlapping, the areal distributions of the craters appear mutually exclusive. Craters with pancake-like ejecta are concentrated within the central, deepest, portion of the Utopia basin (less than about 1000 km from the basin center) and rampart craters occur at the periphery of the basin. This spatial distribution of crater types suggests that the materials within the center of Utopia Planitia contained more ice/water.

(6) Etched flows around the central portion of Utopia Planitia were erupted from beneath the surface of the VB. Their morphology and pattern of degradation, however, are inconsistent with lava and, instead, indicate a formation of the flows due to mud volcanism. Thus, etched flows provide additional and important evidence for the existence of a large body of water in the central portion of Utopia Planitia. (7) Etched flows are spatially associated with giant polygons and there is evidence that these features populated the center portion of Utopia Planitia before they were covered by the Elysium-derived units. The outer (southern) edge of the zone of polygonal troughs and etched flows approximately corresponds to the transition from pancake-like ejecta to rampart ejecta. This suggest that the outer edge of the zone of the polygons and etched flows may indicate an edge of the deeper portions of a large body (2000 km across) of water/ice that likely existed in the center of Utopia Planitia in the late Hesperian. (8) The elevation of the outer edge of the zone of polygons/ etched flows is very close to the height of the barrier between the Utopia basin and the North Polar basin, which occurs at 4350 m (Carr and Head, 2003). In Acidalia Planitia, which is the southern portion of the North Polar basin, the same set of morphologic features such as giant polygons (Lucchitta, 1981), craters with pancake-like ejecta, and small cones interpreted as evidence of mud volcanism (Oehler and Allen, 2010; Komatsu et al., 2012) are abundant and occur at about the same elevations as in Utopia Planitia. However, the presence of the etched flows in this region has not been reported yet. This can be readily explained by the fact that the high-resolution/high-quality images that are necessary for detection of the etched flows were not available until relatively recently. In our future study, we plan to conduct a systematic search of the potential mudflows in the region of Acidalia Planitia. The presence of the flows will imply that the complete set of features that may be indicative of the presence of a large standing body of water/mud in Utopia Planitia is repeated in Acidalia Planitia and, thus, both regions may have evolved in similar geological environment. In contrast, if the flows are absent in Acidalia Planitia, it would suggest that the geologic histories of these regions were probably different, which is poorly consistent with the hypothesis of a single reservoir of water/mud that has existed within the northern lowlands in the geological past of Mars.

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