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Planetary and Space Science 59 (2011) 1143–1165

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Planetary and Space Science

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Mars: The evolutionary history of the northern lowlands based on crater counting and geologic mapping Ã S.C. Werner a, , K.L. Tanaka b, J.A. Skinner Jr.b a Physics of Geological Processes, University of Oslo, Norway b Astrogeology Science Center, U.S. Geological Survey, Flagstaff, Arizona, USA article info abstract

Article history: The geologic history of planetary surfaces is most effectively determined by joining geologic mapping Received 15 April 2010 and crater counting which provides an iterative, qualitative and quantitative method for defining Received in revised form relative ages and absolute model ages. Based on this approach, we present spatial and temporal details 18 February 2011 regarding the evolution of the Martian northern plains and surrounding regions. Accepted 29 March 2011 The highland–lowland boundary (HLB) formed during the pre-Noachian and was subsequently Available online 9 April 2011 modified through various processes. The Nepenthes Mensae unit along the northern margins of the Keywords: cratered highlands, was formed by HLB scarp-erosion, deposition of sedimentary and volcanic Mars materials, and dissection by surface runoff between 3.81 and 3.65 Ga. Ages for giant polygons in Geologic evolution Utopia and Acidalia Planitiae are 3.75 Ga and likely reflect the age of buried basement rocks. These Lowlands buried lowland surfaces are comparable in age to those located closer to the HLB, where a much Crater statistics Paleao-ocean thinner, post-HLB deposit is mapped. The emplacement of the most extensive lowland surfaces ended between 3.75 and 3.4 Ga, based on densities of craters generally 43 km in diameter. Results from the polygonal terrain support the existence of a major lowland depocenter shortly after the pre-Noachian formation of the northern lowlands. In general, northern plains surfaces show gradually younger ages at lower elevations, consistent local to regional unit emplacement and resurfacing between 3.6 and 2.6 Ga. Elevation levels and morphology are not necessarily related, and variations in ages within the mapped units are found, especially in units formed and modified by multiple geological processes. Regardless, most of the youngest units in the northern lowlands are considered to be lavas, polar ice, or thick mantle deposits, arguing against the ocean theory during the Amazonian Period (younger than about 3.15 Ga). All ages measured in the closest vicinity of the steep dichotomy escarpment are also 3.7 Ga or older. The formation ages of volcanic flanks at the HLB (e.g., Alba Mons (3.6–3.4 Ga) and the last fan at Apollinaris Mons, 3.71 Ga) may give additional temporal constraint for the possible existence of any kind of Martian ocean before about 3.7 Ga. It seems to reflect the termination of a large-scale, precipitation-based hydrological cycle and major geologic processes related to such cycling. & 2011 Elsevier Ltd. All rights reserved.

1. Introduction northern and the southern hemispheres of about 5 km (e.g., Smith et al., 1998). In the eastern hemisphere along Terrae Sabaea and One of the dominating features of the Martian physiography is Cimmeria, the dichotomy boundary is characterized by a promi- its division into heavily cratered highlands, which occupy the nent scarp and possible extensional and contractional features southern hemisphere, and mostly featureless low-lying plains, (Watters, 2003), while other parts in the western hemisphere which occupy the northern hemisphere. The lowland region, along Arabia and Tempe Terrae have mainly gentle slopes. roughly centered on the north pole, covers one-third of the planet Tharsis, one of the large volcanic provinces, and the possible and is broadly characterized by a smooth, gently sloping surface volcanically formed Medusae Fossae formation partially bury the (at kilometer-scale) (Kreslavsky and Head, 2002), with elevations dichotomy escarpment, and several large impact basins appear to below the mean planetary radius (e.g., Smith et al., 1999). have contributed to its current exposure. Various formation Topography data reveal a difference in elevation between the mechanisms have been suggested for the northern plains basin; for example, its origin has been attributed to tectonism driven by mantle dynamics (e.g., Wise et al., 1979; McGill and Dimitriou, Ã Corresponding author. Tel.: þ47 22856472. 1990; Sleep, 1994; Zhong and Zuber, 2001; Zuber, 2001)ortoimpact E-mail address: [email protected] (S.C. Werner). cratering (e.g., Wilhelms and Squyres, 1984; Frey and Schultz, 1988;

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McGill, 1989; Marinova et al., 2008; Nimmo et al., 2008; Andrews- affecting smaller diameter craters); and (3) the exclusion of Hanna et al., 2008). However, any clear morphologic evidence has highly degraded craters in the crater catalog, so that only been obscured by resurfacing events. Gravity signatures indicate the relatively pristine craters with preserved ejecta blankets were Martian crust thins northward in the vicinity of the highland– included (these were interpreted to superpose and thus postdate lowland boundary (Janle, 1983; Zuber et al., 2000). Crustal magnetic the unit). anomalies are generally stronger in the highlands compared to the By investigating the full cratering record using the crater size- lowlands, indicating that the Martian dynamo had ceased prior to (or frequency distribution, it is not necessary to exclude or pre-sort perhaps as a consequence of) the formation of the northern plains crater populations. Significant resurfacing will be visible as devia- (Connerney et al., 2001). tions from the fitted crater curve in cumulative crater plots and Prior to Mars Global Surveyor (MGS; Orbital insertion date: 11 different events of sufficient magnitude can be separated (Werner, September 1997. Launch date: 7 November 1996), mapping and 2009). Northern plains surfaces reflect complex resurfacing his- stratigraphic analysis of Mars’ global geology was largely based tories, which may relate to a variety of processes occurring at a on Viking Orbiter image data (Scott and Tanaka, 1986; Greeley range of rates and temporal overlap. These complexities make the and Guest, 1987; Tanaka and Scott, 1987). However, a clearer northern plains favorable for detailed analysis through the com- understanding of the physiographic characteristics and geologic bination of map- and crater-based stratigraphic interpretations. evolution of the northern lowlands was afforded by topographic Here, we present a sample of crater size-frequency distribu- data of the Mars Orbiter Laser Altimeter (MOLA) and additional tions for representative localities of major geologic units of the image data of the Mars Orbiter Camera (MOC), both onboard northern plains. In combination with geologic map relations, we MGS. In particular, Tanaka et al. (2003, 2005) used these datasets discuss the evolution of the northern lowlands and surrounding to significantly revise the boundaries and ages of geological units terrains that is implied by these counts. The main focus in the first in the northern plains. The main definition of the new lowland part of the study is to detail the modeled ages for major episodes units was guided by the demarcation between predominantly of geologic activity within the lowlands, based on key areas guided Noachian (4 3.7 Ga) densely cratered materials that characterize by the most recent geological mapping (Tanaka et al., 2005). In the the equatorial and southern highlands of Mars and the younger, second part of the paper, we present detailed properties of the mostly plains-forming materials that typically lie below an lowlands’ crater size-frequency distributions observed in Utopia elevation of 2000 m. Major geographical features include the Planitia and giant polygonal terrain of Utopia and Acidalia Plani- Borealis, Utopia, and Isidis basins, which are largely defined, tiae. We discuss these regional observations in the context of the respectively, by the Vastitas Borealis, Utopia Planitia, and Isidis northern plains geologic record, with specific focus on the exis- Planitia plains regions. All geologic units and structures that occur tence of a postulated Oceanus Borealis (Baker et al., 1991). within the boundaries of these lowland topographic basins are distinctive because of their separated geologic evolution since early in Mars’ evolution. As such, these became repositories of 2. Absolute ages in resurfaced units—method regional sequences of materials marked by a unique assemblage and uncertainties of landforms. The post-Viking geological map of the northern plains (Tanaka Remote-sensing based delineation of geological units by rela- et al., 2005) resulted in two new definitions applied to the global tive age, following morphologic or spectral characteristics, relies stratigraphic scheme of Tanaka (1986). First, the pre-Noachian on superposition principles and other stratigraphic relations period was defined as the age of primordial crustal rocks under- among surface units and features (e.g., Hansen, 2000; Tanaka lying and predating but incorporated as fragments within Early et al., 2005). However, these are largely qualitative assessments Noachian outcrops. The northern plains depression and Utopia and do not provide information on absolute age. Because of the basin within it are topographic features that apparently formed local variability in crater densities, separated outcrops of equiva- during the pre-Noachian as no outcrops of materials predating the lent units generally cannot provide consistent ages relative to one features are evident. The second major change was a refinement another. For many planetary surfaces, the relative abundances of the Hesperian/Amazonian boundary (3.15 Ga) as the surface (and size-frequency distribution) of impact craters distinguish age of Vastitas Borealis marginal and interior units. This mod- outcrops of geological units quantitatively by relative age. Deter- ification from previous geological map depictions of the Vastitas mination of a surface’s geologic history most effectively depends Borealis units continues to provide a basis for subdividing on not only geologic mapping, which delineates materials by primary (depositional) and secondary (modificational) geologic surface and natural breaks in the rock sequence, but also crater processes within the Martian northern plains. counting, which provides a quantitative tool for defining relative The time-stratigraphic classification of the northern plains ages and modeling absolute ages (e.g., Tanaka, 1986; Hartmann geologic units of Tanaka et al. (2005) is based mainly on super- and Neukum, 2001). Although a combination of these two position relations among the geologic units as well as the overall approaches is optimal, the application of crater-based ages is density of craters larger than 5 km in diameter for each unit inherently more complex and, thus, requires more knowledge of (NcumðDZ5kmÞ) from the crater catalog published by Barlow and adherence to theory, method, and procedure. (1988, 2001) and the NcumðDZ5kmÞ epoch boundary values given The theoretical concept and mathematical background of age- by Tanaka (1986). Tanaka et al. (2003) discussed the relevance of dating techniques based on crater counts was developed in the these crater frequencies with respect to their correctness and 1960s and 1970s and summarized by the Crater Analysis Techni- stated three problems with their approach: (1) the misregistra- ques Working Group (Arvidson et al., 1979). Absolute ages on tion between the topographically based geological maps and the Mars and other solid surface objects are determined by a crater Viking image-based crater catalog, which might attribute craters production function and an applied cratering chronology model. to a unit to which they do not belong or vice versa, affecting The crater production function describes the expected crater size- smaller units the most; (2) taking the cumulative number for frequency distribution recorded in a geological unit at a specific a single reference diameter (here 5 km and larger) instead of time, and is, when scaled, comparable between planetary bodies the entire crater size-frequency distribution, which inherently such as the Moon and Mars (e.g., Neukum and Wise, 1976; obscures the ability to discern resurfacing event(s) within the Neukum and Ivanov, 1994; Ivanov, 2001). The transfer of crater- crater population, particularly of small vertical scale (i.e., those based functions from the Moon to Mars requires the knowledge of Author's personal copy

S.C. Werner et al. / Planetary and Space Science 59 (2011) 1143–1165 1145 the impacting body flux ratio between the lunar case to Mars, removes or obscures the cratering record starting at the smaller with consideration of celestial mechanics and how the conditions size range. Alternatively, resurfacing can altogether reset the of crater formation differ from one planet to another. The latter cratering record in instances where the surface is completely conditions are mainly defined by the respective gravity and target renewed. In either case, the determination of a resurfacing event properties of the impacted bodies (Ivanov, 2001). based on crater density is dependent on the event’s magnitude. The transfer of relative into absolute ages by the use of a A distinct drop in the crater size-frequency distribution of a chronology model is based on the assumptions that (1) the crater resurfaced geologic unit signifies a resurfacing event (Fig. 1AorB); production function—the original size-frequency distribution—is multiple episodes of resurfacing can be implied by multiple drops in accurately known (Ivanov, 2001, originally in Neukum et al., the distribution. In cases of ongoing resurfacing (e.g., on steep slopes 1975), (2) the flux of projectiles onto the surfaces is isotropic, of Martian surface structures) or saturation, the crater size-fre- and (3) any target-property variations are negligible. With these quency distribution continually crosses isochrons and cannot be assumptions, the crater densities measured on planetary surfaces linked to a discrete age (Fig. 1C). that have been exposed for the same amount of time with respect For units where resurfacing has been observed the determina- to diameter are equivalent in age. Relative ages based on Ncum ðD,tÞ tion of ages has to be limited to a certain diameter range (below are given for a fixed diameter D (e. g., 1, 2, 5, 16 km); these are or above a diameter d% , where the resurfacing has influenced the commonly referred to as crater-retention ages (Hartmann, 1966). crater size-frequency distribution). The oldest age is derived The crater chronology model applied herein is based on straightforwardly, by fitting the production function to the measurements of the crater size-frequency distribution on areas large-diameter range. For the resurfacing event, if no correction of the Moon, which are linked to radiometric ages retrieved from has been applied, the age is overestimated (dependent on the lunar rocks sampled and returned during the Apollo missions. magnitude of the resurfacing event) due to the cumulative This distribution is transferable to other (inner solar system) character of the crater size-frequency distribution. This problem planets and solid surface bodies, assuming that all bodies under- is avoided in incremental distribution plots (Arvidson et al., 1979, went the same bombardment (e.g., Neukum and Wise, 1976; and customarily used by W. K. Hartmann). Neukum and Hiller, 1981; Hartmann, 1973, 1977, 1978; and The general description of the cumulative crater size-fre- unified in Hartmann and Neukum, 2001). quency distribution Ncum ðD,tÞ is given by the cumulative number Three phenomena—saturation, contamination by secondary of craters per area for a given diameter D at a certain time t: — Z Z cratering, and geological resurfacing have the potential to cause 1 t 0 0 0 0 deviation from the crater production function. In densely cratered NcumðD,tÞ¼ gðD Þ dD f ðt Þ dt ð1Þ (i.e., old) units, the rate of production and destruction eventually D 0 becomes steady, starting at small diameters. Such an equilibrium for an undisturbed geological unit (Fig. 2A) a simple solution can distribution exhibits a characteristic cumulative slope on a power- be found law fit of b¼2 at the small crater range (e.g., Woronow, 1977; NcumðD,tÞ¼GðDÞFðtÞ, ð2Þ Richardson, 2009). If measurements are contaminated by secondary craters, the observed crater size-frequency distribution may where G represents the cumulative crater size-frequency distri- appear steeper than the crater production function suggests (e.g., bution and F the flux of projectiles. It is required that the crater Shoemaker, 1965). Other deviation from the crater production size-frequency distribution is stable over time, and only the flux function typically implies a resurfacing event (erosion or deposi- changes (decays, e.g., Hartmann and Neukum, 2001). tion). Although separation of different surface units might some- In a unit where subsequent processes (erosion and deposition) times not be possible due to limited image resolution or lack of occurred, the smaller crater population has been modified pre- stratigraphic contrasts, effects of geologic resurfacing are detect- ferentially (compare Fig. 2A and B). Subsequently, the surface able in the crater size-frequency distribution (as long as there are accumulates craters as if it is a fresh surface (Fig. 2C). On a statistically sufficient crater populations). In such cases, the age of planetary surface, a proper remote-sensing method to distinguish the resurfacing event can be estimated because it effectively both populations is not known (compare Fig. 2C and D). An

] Single Resurfacing Multiple Resurfacing Continual Resurfacing -2 Cum. Crater Frequency N(D> 1km) [km Frequency Crater Cum.

Crater Diameter D [km]

Fig. 1. Three different schematic crater size-frequency distributions show resurfacing events which have (A) a distinct timing visible in the re-steepening of the crater size- frequency distribution at smaller sizes, (B) is followed by a second distinct event, and (C) is occurring continuously. At the smallest crater range a ﬂattening of the curve is observed which is related only to the resolution limit of the image data. Author's personal copy

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] 3.3 Ga 2.2 Ga -2 3.7 Ga 3.3 Ga 3.7 Ga

2.17 Ga

3.70 Ga Cum. Cra Cum. equency N (D> 1km) [km

Crater Diameter D [km] Crater Diameter D [km]

Fig. 2. Schematic description of a (A) cratered surface, (B) the same surface affected by erosion erasing craters of smaller diameters, (C) the same surface after continued cratering. In reality the surface would look as shown in (D), where the old and younger population cannot be distinguished (stippled and solid lines in C). (E) depicts the resulting crater size-frequency distribution plotted with isochrons, and (F) shows the same crater size-frequency distribution but also the smaller range crater size- frequency distribution corrected for the resurfacing event. This results in a reduced age for the resurfacing event compared to the uncorrected one.

example of a measured resurfacing crater size-frequency distri- (Ncum—value of an individual diameter bin) is not taken, but rather bution is given in Fig. 2E. Any attempts to distinguish between the best representing crater-production function (Ivanov, 2001)of erosional state or other parameters of the craters can be strongly the observed crater size-frequency measurements. The fitting misleading (and are subjective). To understand the crater popula- procedure follows the Marquardt–Levenberg algorithm, a weighted tion accumulated on the surface after the resurfacing event, a non-linear least-squarespffiffiffiffi fit (Levenberg, 1944; Marquardt, 1963). method is needed to separate both populations. A first approach is The statistical error ( N) of the measurement indicates the described by Werner (2005, 2009), which was later adopted by uncertainties of the relative ages and the fit quality depends on Michael and Neukum (2010) using a different numerical imple- the number of bins which can be used as fit range. The uncertainty mentation. An iterative treatment is necessary to determine the of such a fit for making use of the whole size range for the age(s) of resurfacing event(s) by fitting the well-known crater derivation of a mean N-value at a certain diameter is of the order production function to the crater-size range below a certain of the statistical error of 2s and less than the individual bin error of diameter to restore the full crater size-frequency distribution of 1s. The level of uncertainty of the scaled cumulative number N per the resurfacing event and the age of the younger surface (Fig. 2F). bin is given by In cases where additional resurfacing processes took place, the N1=2 correction has to be repeated at the next cut-off diameter. 7s ¼ 7 ð3Þ N A Prerequisites for the crater count statistics are (1) careful geologic mapping in order to delineate homogeneous, temporally for each bin. Here, the reliability of the ages is improved by unique units, and (2) exclusion of sublimation pits, volcanic and statistical measures if a larger area and/or a larger number of secondary craters, and other non-primary impact craters to the craters is counted. The fitted crater frequencies given for a extent possible. Contamination due to unrecognized global sec- reference diameter are translated to absolute ages by applying a ondary craters unwittingly included in the measurements is less cratering chronology model. In the course of this work the than 10% (old surfaces), and in most cases less than 5% (Werner, reference diameter is Ncum ¼ DðZ1kmÞ. 2005; Werner et al., 2009). For Mars, the applied chronology model is transferred from the For determination of initially relative ages and subsequently Moon (Hartmann and Neukum, 2001; Ivanov, 2001). A variety of absolute ages, the cumulative number at a certain diameter lunar cratering chronology models existed previously and agreed Author's personal copy

S.C. Werner et al. / Planetary and Space Science 59 (2011) 1143–1165 1147 within a factor of 2–3. These different models have converged to a terrain characterizing most of the highland surface. The uniﬁed model (Hartmann and Neukum, 2001; Ivanov, 2001). The Nepenthes Mensae unit consists of knobs and mesas of highland cratering chronology used in this paper is given by rock and interposed slope and plains material, which forms much

14 6:93T 4 of the highland–lowland margin. Locally, relatively thick lava NcumðZ1kmÞ¼3:22 10 ðe 1Þþ4:875 10 T ð4Þ plains marked by wrinkle ridges make up the Lunae Planum and according to Hartmann and Neukum (2001). However, they do Tempe Terra units. The highland units are represented by a not present this equation numerically but as a diagram. Any number of type areas for which crater counts were performed possible uncertainties inherent in the lunar cratering chronology (Appendix A, Fig. A1). Their distribution and locations are shown model might be found in the cratering chronology model for Mars in Fig. 3 and the ages and dimensions are summarized in Table 1. (Hartmann and Neukum, 2001; Ivanov, 2001) because of the These older units generally range in age from 4.04 to 3.6 Ga. transfer from Moon to Mars. Therefore, this uncertainty of the Geologically, the Noachis Terra unit (the oldest unit counted in Martian cratering chronology, causes a possible systematic error this study in areas 20, 37, and 47) is interpreted to reflect a long in model ages (up to a factor of 2 for younger, Amazonian surfaces history of resurfacing due to secondary erosional and depositional and about 100 Ma for older, Hesperian and Noachian surfaces). processes as well as superposed tectonism. These regions are Possible inherent uncertainties in the cratering chronology characterized by high crater densities, valley networks, isolated may be significantly larger than our quoted uncertainties (e.g., depressions (which may be subdued, eroded impact craters), as Hartmann and Neukum, 2001). However, only these statistical well as ridges, scarps, and troughs. Although resurfacing events in uncertainties from fitting the isochrons are listed. these units are recognized in the crater size-frequency distribu- Other uncertainties may be introduced if the counting units tion, gradual deviation from the expected crater-production and crater diameters are determined on inappropriately projected function suggests that such events are generally not limited to image maps. We have tested the range of uncertainties resulting single episodes, but rather continual processes of surface degra- for the area determination based on different map projections dation and alteration. Werner (2008) argued that there is no used for our image data and a sinusoidal (equal-area) projection, mapped unit anywhere on Mars older than about 4.1 Ga, which is and found differences o2% for the area sizes. However, these close to the crater-retention age found for Noachis Terra. variations in areal size are insignificantly small compared to the statistical uncertainties of the ages determined here (o0:002%). Measurements resulting in considerable age ranges can be 3.1. The Chryse region caused by complex resurfacing histories (or random statistical fluctuations in crater populations). Crater populations measured The physiographic setting is dominated by Chryse Planitia, a on image data of relatively low resolution (in our case 231 m/pxl) topographic depression about 1500 km in diameter proposed to do not permit a discussion of any resurfacing events acting on the be an impact basin (Schultz et al., 1982). The largest and most removal of craters in the small-size range (less than about 1 km in prominent outflow channels on Mars (Kasei, Maja, Shalbatana, diameter). Craters smaller than 1 km in diameter cannot be Simud, Tiu, Ares, and Mawrth Valles) emanate from chaotic detected due to resolution limits, therefore, possible resurfacing terrains and drain into the roughly circular Chryse Planitia, which effects on these smaller craters cannot be assessed. opens northward into the lowland plains. Except for the arcuate Xanthe Montes, which forms a possible basin rim on the western margin of Chryse Planitia, clear evidence for a Chryse impact 3. Northern lowland stratigraphy basin cannot be confidently identified, perhaps because of the removal of rim structures by earlier fluvial processes. The Chryse We selected representative type areas that have simple out- basin does not show a distinctive gravity anomaly that would be lines and contain a single geological unit for crater size-frequency indicative of a large impact (Yuan et al., 2001). distribution measurements. These areas were extracted from the The circum-Chryse outflow channels were mainly active dur- Viking-based Mars Digital Image Model (MDIM) 2.1 global dataset ing the Hesperian (Tanaka et al., 1992; Rotto and Tanaka, 1995), (a complete orthorectified mosaic at a resolution of 231 m/pxl; postdating the development of highland valley networks. Baker Archinal et al., 2003) as well as from the THEMIS daytime IR (1982) argued that the Chryse channels are large-scale complexes global mosaic (a mosaic at a resolution of 99 m/pxl available at of fluid-eroded troughs that emanated from collapse zones in the USGS). The location of the crater-count areas is shown in regional chasmata and chaos regions. Channel relics, floodplains Fig. 3. The image clippings and measured crater size-frequency along channel courses, depositional streamlined bars, and ero- distributions are only collectively presented in Appendix A. Based sional islands led to the interpretation that channel-system- on the crater size-frequency measurements, the resulting relative forming floods originated from aquifer outbreaks (Carr, 1979). and absolute surface ages and the time-stratigraphic relations of Although catastrophic flooding is the most commonly accepted the investigated units are described in the following section and interpretation for the origin of the circum-Chryse outflow chan- will be discussed with respect to previously suggested time- nels, morphologic features indicate that these have been addi- stratigraphic relations (e.g., Scott and Tanaka, 1986; Greeley and tionally sculpted by diverse erosional forces including winds, Guest, 1987; Tanaka and Scott, 1987; Tanaka et al., 2003, 2005). debris or mud flows, glaciers, and lavas (e.g., Chapman et al., Special focus has been given to four different zones, which are 2010a,b). However, these interpretations are consistent with the interpreted to represent contrasting geologic characteristics and hypothesis that the Vastitas Borealis units represent ocean sedi- evolutionary histories within the northern plains, based on results ments derived from material discharge from the Chryse channels from geologic mapping. These zones include the Chryse region (1), (e.g., Parker et al., 1993; Baker et al., 1991). Some channel systems the Utopia and Isidis basins (2), the Amazonis and Elysium show no obvious source regions, but possibly emanate at fracture Planitiae (3), and the Scandia region (4). and fissure zones, including those associated with impact craters. Densely cratered units are common at the margins of the Most source-region situations are believed to be causatively northern plains and generally grade morphologically into one related to volcanic and tectonic processes due to their occurrence another. The Libya Montes is the oldest unit, and forms massifs within or near volcanic materials and/or zones of deformation, and other high-standing terrains characteristic of ancient, large including Valles Marineris and the Tharsis and Elysium rises impact basin rims. The Noachis Terra unit makes up rugged (Basilevsky et al., 2009). Author's personal copy

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Elevation (km) <0-8 >21

Fig. 3. The northern hemisphere of Mars with geological units as mapped by Tanaka et al. (2005) shown with the MOLA derived elevation model (A) and color-coded geological units (B). The outline of the numbered crater-counted areas are given, for detailed information such as unit name, stratigraphic classiﬁcation, average (median) elevation, area size, crater statistics, crater density and model age information see Table 1. Author's personal copy

S.C. Werner et al. / Planetary and Space Science 59 (2011) 1143–1165 1149 03 04 04 03 13 05 03 04 08 03 05 05 22 05 09 06 06 13 06 10 13 07 05 06 27 09 22 03 05 05 09 05 07 24 17 31 04 : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 00 04 06 06 04 28 08 04 07 29 04 09 08 22 09 25 13 13 25 12 38 21 09 12 28 33 23 04 09 10 34 08 19 26 18 32 06 : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ 3.85 3.55 3.74 3.15 3.61 3.63 1.76 3.36 2.04 2.91 1.07 3.25 2.06 1.83 1.76 3.85 3.47 3.48 4.04 3.96 3.51 3.78 3.66 3.61 3.69 3.55 3.57 3.57 3.58 3.48 3.52 4.04 3.81 3.65 3.76 3.89 3.63 (1 km) values cum Fig. 1 Fig. 1 ) Age (Ga) 2 % % % % % % % % % % % % % % % % % % 2 2 3 2 3 3 3 3 3 3 3 3 3 3 3 3 3 4 3 2 2 3 3 2 3 3 3 4 4 3 4 4 2 3 3 3 3 (1 km); (km cum 1.62e 4.48e 4.84e 1.43e 3.29e 2.86e 2.89e 4.12e 9.51e 4.48e 5.12e 4.12e 8.50e 5.91e 3.29e 3.53e 1.78e 3.53e 3.66e 2.65e 8.92e 2.98e 4.84e 1.13e 2.65e 4.89e 7.63e 1.63e 8.58e 2.06e 9.95e 1.44e 5.21e 1.00e Resurfacing, compare 8.58e 1.43e 2.59e Resurfacing, compare ) ) h. 04 10 : : 06 38 : : 03 04 04 05 04 08 03 05 05 05 06 06 10 13 07 05 06 27 09 05 09 05 07 04 : : : : : : : : : : : : : : : : : : : : : : : : 00 0 0 04 06 06 08 07 29 04 09 08 09 13 12 38 21 09 12 28 33 10 34 08 19 06 : : : : : : : : : : : : : : : : : : : : : : : : 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ 3.85 3.55 3.61 3.63 3.25 1.83 3.48 4.04 3.76 4.04 (3.83 (3.45 3.81 3.65 3.48 3.96 3.51 3.78 3.66 3.61 3.69 3.55 3.57 3.57 3.58 3.52 ) Age (Ga) N 2 sidered inaccurate and are rejected for the discussion. N % % % % % % % 2) 3) 2 2 2 3 3 3 3 4 3 2 3 3 2 3 3 3 3 3 3 3 3 3 3 3 (1 km); (km cum N 1.43e 3.29e 4.12e 4.48e 1.78e 8.92e 2.65e ) Viking image-based Themis IR image-based 2 . Relative and absolute ages were derived first for crater size-frequency distributions measured Fig. 3 . They are translated to absolute ages by applying a cratering chronology model as given in Section 2, Eq. (4). Þ 1km Z ð D ¼ cum N 3083.04 (423.32) 78565.80 4.84e 4286.79 (112.33) 122717.67 2.65e 2030.61 (353.55) 29343.161181.81 (129.97)3064.58 (154.12) 2.86e 28183.273340.09 (163.03) 43710.23 2.89e 3316.9 (174.65) 51913.23 (1.27e 2608.92 (287.89) 9.51e 19709.693002.49 (66.18) 49003.483711.45 5.12e (78.50) 4.12e 15150.803886.19 (86.64) 36851.95 8.50e 3758.09 (91.50) 57768.01 5.91e 3870.77 (72.35) 57338.82 (2.46e 3791.84 (251.22) 45956.70 3.29e 4897.44 (84.97) 48466.58 3.53e 3.53e 75354.353934.39 (64.76) 3.66e 53615.86 2.98e 1266.73 (448.68)2047.79 (451.97) 25101.61 54652.38 1.13e 4.89e 29.32 36.89 44.23 9.39 62.4326.05 34.00 28.05 18.89 45.25 21.00 52.07 16.97 45.80 19.39 47.68 14.40 24.71 40.73 45.57 28.74 39.46 27.83 39.12 23.70 36.42 12.99 57.76 38.13 30.06 Stratigraphy Longitude Latitude Mean elevation (m) Area (km ) zone 1 zone 2 — — Tanaka et al., 2005 are resurfacing ages and are derived as described in Section 2. The elevation value is the mean elevation and its standard deviation given for each patc % (following 47 Noachis Terra Nn 6.73 39.74 Chryse Planitia region Results of the crater# size-frequency measurements Geological unit 46 Vastitas Borealis interior ABVi 47 47 3737 Noachis Terra37 3433 Lunae Planum33 Chryse Planitia32 132 Chryse Planitia39 1 Nn 41 Chryse Planitia41 1 Chryse Planitia38 1 HNTl 38 HNCc1 Chryse Planitia30 230 HNCc1 Chryse35 Planitia 235 HNCc1 Chryse Planitia40 HNCc1 240 Chryse31 Planitia HCc2 331 Chryse Planitia28 HCc2 428 Simud36 Vallis HCc2 36 Vastitas Borealis HCc3 interior46 29 HCc4 29 Vastitas ABVi Borealis marginalUtopia Planitia region 20 Hcs 20 Noachis20 Terra ABVi 2222 Nepenthes Mensae0808 Nepenthes Mensae Nn HNn HNn 113.95 113.75 5.12 67.83 9.20 30.1408 (245.26) 36.20 21992.89 4.84e represent the best fit to the measured crater size-frequency distributions at the reference diameter Ages annotated by Table 1 Listed absolute ages and other statistical parameters of the crater size-frequency statistics measured for the patches shown in on Viking image data and refined through crater size-frequency distributions measured on THEMIS daytime IR. Viking-based ages in parentheses are con Author's personal copy

1150 S.C. Werner et al. / Planetary and Space Science 59 (2011) 1143–1165 ) 21 16 : : 23 17 07 : : : 12 : 03 04 05 06 10 40 13 09 38 07 21 08 17 09 04 06 08 08 20 42 52 04 45 05 04 29 0 0 03 51 21 25 17 08 20 05 : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 80 90 04 07 09 12 40 55 38 39 14 22 20 83 28 07 13 20 18 56 81 07 47 09 06 35 0 0 0 05 52 50 27 19 29 21 08 : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 þ þ 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ 3.46 2.02 2.73 3.08 1.66 1.24 1.30 0.95 0.894 0.845 3.38 2.32 2.40 2.69 3.80 3.65 3.61 3.45 3.52 3.49 3.48 3.27 3.62 3.55 3.60 3.32 2.62 2.65 3.66 3.57 3.65 3.56 3.44 3.50 3.48 3.46 (3.43 Fig. 1 Fig. 1 Fig. 1 Fig. 1 Fig. 1 ) Age (Ga) 2 % % % % % % % % % % % % % % 3) 3 3 4 3 3 3 3 3 3 3 4 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 4 4 4 3 3 3 4 3 3 4 (1 km); (km cum 8.50e 2.65e 2.52e 3.40e 4.89e 4.12e 9.85e 2.46e 1.34e 2.98e 1.56e 2.73e 8.09e 2.65e 1.82e 4.63e 2.40e 4.29e 3.29e 3.96e 2.13e 1.93e 1.28e 1.29e 5.12e 1.13e 3.53e 4.89e 1.32e 2.80e Resurfacing, compare 2.52e Resurfacing, compare 6.05e 6.34e Resurfacing, compare Resurfacing, compare 4.36e 4.12e 1.17e Resurfacing, compare ) ) ) 04 06 05 : : : 06 14 10 : : : 03 05 06 10 40 13 09 38 08 17 09 04 06 08 20 42 52 04 45 05 04 29 51 21 27 08 05 : : : : : : : : : : : : : : : : : : : : : : : : : : : 80 90 0 0 0 04 09 12 40 55 38 39 20 83 28 07 13 18 56 81 07 47 09 06 35 52 50 28 29 08 : : : : : : : : : : : : : : : : : : : : : : : : : : : 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ 2.65 2.02 2.73 3.08 1.66 0.57 3.38 2.32 2.69 3.56 (3.56 3.44 3.50 (3.76 (3.61 3.61 3.62 3.55 2.62 3.66 3.65 3.45 3.52 3.49 3.48 3.27 3.60 3.32 3.57 3.65 ) Age (Ga) N 2 % % % % % % % % 3) 3) 3) 3 3 3 4 3 3 4 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 4 3 3 (1 km); (km cum 1.29e N 9.85e 1.34e 1.56e 8.09e 2.78e 2.13e 1.28e 1.13e 1.32e ) Viking image-based Themis IR image-based 2 1538.17 (1191.75) 82849.72 3.40e 2661.78 (77.57) 63195.11 5.12e 2610.9 (237.81) 29623.393505.56 (58.19) (8.50e 76588.43 4.12e 3787.93 (120.63)3596.83 (44.07) 88187.19 72416.07 4.29e 3.29e 2917.93 (202.27) 50116.063521.39 (144.19) 4.89e 4727.12 (55.12) 28897.434161.61 (44.28) 2.46e 31919.114598.41 (77.14) 37639.36 2.98e 4334.67 (79.34) 82297.54 2.73e 4937.72 (30.21) 56956.72 (3.40e 3827.67 (50.01) 134931.48 2.65e 1.82e 24735.53 2.40e 3815.02 (67.27)3817.26 (92.05) 60256.334304.55 (112.52) 26260.76 (4.11e 4207.89 (83.87) 40096.88 3.96e 1.93e 238625.693802.92 (97.84)1911.31 (292.88) 130296.541029.86 (757.63) 135372.49 3.53e 4.89e 121332.66 2.80e (2.35e Stratigraphy Longitude Latitude Mean elevation (m) Area (km ) zone 3 — Tanaka et al., 2005 ) (following continued ( 19 26 Syrtis Major Planum HIs 77.80 9.79 Results of the crater# size-frequency measurements Geological unit 21 Utopia Planitia 127 Utopia Planitia 2 HBu1 113.55 HBu2 14.83 94.41 20.51 12 Astapus10 Colles Vastitas Borealis marginal ABVm ABa 67.01 82.65 53.27 44.22 26 21 0909 Utopia Planitia 127 2323 Utopia Planitia14 214 Elysium rise15 HBu115 Elysium rise1616 Tinjar Valles17 HBu2 73.33 a17 Tinjar Valles18 a18 39.46 AHEe Tinjar Valles13 113.40 a13 AHEe Isidis Planitia 19.29 12 AEta 112.8310 AEta 127.3111 33.44 11 AEta 132.13 Vastitas25 Borealis interior 26.61 25 AIi 127.53 Vastitas Borealis24 41.07 interior24 110.66 Vastitas ABVi Borealis19 32.76 interior Vastitas ABVi 86.95 Borealis 46.37 interiorAmazonis Planitia region 04 ABVi 78.1004 6.42 Utopia01 Planitia 2 ABVi 95.5501 58.64 Arcadia02 Planitia 112.7002 28.07 Elysium03 rise 111.5003 26.18 Elysium rise HBu2 64.81 HAa 136.39 AHEe 170.32 4.00 AHEe 161.07 36.56 139.33 22.58 16.32 Table 1 Author's personal copy

S.C. Werner et al. / Planetary and Space Science 59 (2011) 1143–1165 1151

The Ares and Simud Valles units represent materials covering 08 18 29 02 01 : : : : : 09 20 32 03 01 : : : : : 69 18 65 09 09 12 06 0 0 0 12 0 0 13 : : : : : : : : 10 81 77 28 21 56 14 0 0 0 : 13 0 0 13 parts of the outflow channel floors, whereas the Chryse Planitia : : : : : : : : : 0 0 0 0 0 0 0 0 0 þ þ þ þ þ 0 1 0 0 0 0 0 0 0 þ þ þ þ þ þ þ þ þ 1–4 units represent materials emplaced by outflow channel dissection as well as by mass wasting of the highland margin 0.399 0.299 0.039 0.056 1.00 1.04 0.948 3.36 3.27 1.39 3.37 3.56 3.33 1.78 (Tanaka et al., 2005). Scoured channels extend across the Chryse Fig. 1 region but are buried by the Vastitas Borealis interior and marginal units (Tanaka et al., 2005, and references therein), which themselves appear to be resurfaced vestiges of a precursor unit. The landforms, broadly characterizing the Vastitas Borealis inter- % % % % % % % 4 3 4 3 4 4 5 4 3 3 3 5 4 4 ior and marginal units, are indicative of periglacial-like resurfacing, localized sedimentary diapirism and mud volcanism, and 2.06e 1.96e 1.95e 1.82e 6.78e 2.09e 3.40e 8.68e 1.46e 1.90e 2.73e 4.88e 5.07e Resurfacing, compare 4.80e cyclic deposition and erosion of decameter-scale mantle deposits. In total, 16 type areas were used to describe the timing of

) the evolution in this region (Fig. 3). They were selected to be ) ) 58 : 68 : 08 : 07 07 09 0 : : : 13 20 0 representative for each geological unit in this region. Most of the : : 18 65 09 06 0 : : : : 10 0 0 0 77 28 14 þ : : : : 0 0 0 0 0 0 þ 1 0 0 0 þ þ þ þ þ þ counted areas represent units belonging to the Chryse Planitia’s inner slope region. Ages determined for the Chryse type areas (the 0.399 (0.965 (3.53 3.37 3.56 (3.64 3.27 1.39 Chryse Planitia 1–4 units) range from 3.83 to 3.25 Ga. The variations in age are reflected by changes in morphology and support spatially linked areas that show erosional activity and depositional units. Many depositional units correlate with younger ages as they are found at lower elevation. For example, % 3) 4) 3) we sampled the Chryse Planitia 1 unit in four type areas (numbers 4 3 4 3 3 þ : 0 03 32, 33, 39, and 41), and the ages range from 3.89 0:04 to 3.61 þ 0:05 0:08 Ga. The crater size-frequency distributions measured in 1.95e these areas of the Chryse Planitia 1 unit dominantly result in the formation and resurfacing surface ages around 3.63 Ga (averaged), but crater counts for two of these areas (numbers 32 and 33) reveal an older age (about 3.85 Ga, averaged). In particular, the surface in the southern part of area 33 resembles degraded highland terrain, while other parts that resemble smooth plains with wrinkle ridges and fluvial overprint are visible along with many knobs (Appendix A, Fig. A1). Areas representing the Chryse Planitia 2 (30, 35, 38) and 3 (40) units generally fall into the same age range (3.67 Ga, averaged), but all of these surfaces show resurfacing events constrained by their crater size-frequency distributions. The outflow channel activity is illustrated by type 4488.57 (177.25) 87061.21 3.40e 2618.85 (281.98) 128222.94 2.09e 3879.03 (51.99)4047.82 (39.02) 112428.17 (4.70e 180451.263908.2 (25.74) 1.82e 231954.99 6.78e 3449.75 (198.79)3934.83 (115.81) 140044.89 (3.08e 44483.76 (4.68e areas from the Simud Vallis (28) and Chryse Planitia 4 (31) units þ 0:07 with an age of 3.57 0:21 Ga. In the Chryse Planitia 4 unit, a distinct þ 0:13 resurfacing event is indicated at about 3.25 0:38 Ga ago, while the resurfacing event observed in the crater size-frequency distribution measured in the Simud Vallis unit ended more recently þ 0:31 (2.06 0:32 Ga). Surface morphology and crater counts collectively suggest a resurfacing that is continuously erosional rather than due to depositional activity. The surface downslope of Simud Vallis appears to be resurfaced in a more homogeneous manner, perhaps 128.10 72.44 116.84 58.21 146.71172.10 36.90 47.48 161.67 29.85 119.24122.83 63.52 67.63 by fluvial deposition, as suggested by surface morphology. The Vastitas Borealis (29, 36, 46) units show slightly younger formation ages of around 3.52 Ga with resurfacing around 1.8 Ga. Hesperian ridged plains materials such as those mapped in Lunae Planum are interpreted to be the bulk of materials scoured by the Chryse outflow channels. Ages of Lunae Planum materials indicate a presumptive upper-time limit for the start of the

zone 4 erosive phase of outflow channel activity. However, Lunae Planum — unit type area (in this case from Tempe Terra (34), Fig. 3) has an þ 0:05 age of 3.51 0:08 Ga, contradicting the view that these materials entirely pre-date circum-Chryse erosion. Ages for the nearby outflow channel units are found to be older, which contrasts with geologic mapping results. The favored interpretation, then, might be that the process(es) that emplaced (or modified) Lunae Planum ended after the chief formational phase of the Kasei channel system. Alternatively, this might be a classic example of crater- based superposition details that are insufficiently observed in the Viking-MDIM 2.1 global dataset. Detailed studies of Kasei Valles (Tanaka and Chapman, 1992; Chapman et al., 2010a) suggest that 45 Vastitas Borealis interior ABVi 42 Alba Mons HTa 0606 Amazonis07 Planitia 1 N07 Amazonis07 Planitia 1 N0505 AAa1n Amazonis Planitia 2 NBetween Alba Mons and the North AAa1n Pole 43 4343 AAa2n Scandia region4444 Vastitas Borealis marginal45 ABVm HTa the Lunae Planum unit was resurfaced during the Kasei flooding. Author's personal copy

1152 S.C. Werner et al. / Planetary and Space Science 59 (2011) 1143–1165

Major parts of the channels were excavated before 3.5 Ga ago, rise (14, 15) unit in Utopia basin has a model age of about 3.5 Ga, while any subsequent resurfacing (fluvial and/or volcanic) only þ 0:21 but for both units resurfacing occurred that ended at 3.08 0:50 Ga marginally shaped the inner landforms and surfaces of the þ 0:38 (14) and 1.66 0:39 Ga (15). The crater size-frequency distribution channels (Chapman et al., 2010b). Similar findings were made at measured for the Tinjar Valles units (16–18) indicates the forma- the confluence of Tiu and Ares Valles (Marchenko et al., 1998). tion ages of 3.48 Ga, followed by more or less continuous resurfacing ending more than 1 Ga ago. The surface formation age of the Vastitas Borealis interior (19) unit seems to be the 3.2. The Utopia and Isidis basins and vicinity þ 0:5 youngest, about 2.65 0:8 Ga. However, the crater size-frequency The Utopia basin has also been considered to be of impact distribution measured on the THEMIS mosaic appears to have an origin and appears relatively circular, about 3200 km in diameter excess of craters for diameters between 2 and 5 km, and can be þ 0:07 and 1–3 km deep (Thomson and Head, 2001). No impact basin rim fitted by an isochron of 3.43 0:12 . This apparent modification of features are preserved, though their existence has been postu- large-diameter craters was first described by McGill (1986) and lated by the occurrence and distribution of surficial landforms will be further discussed in Section 4. including knob fields, subtle topographic depressions, and cone TheagesofUtopiaPlanitia1unitwerecomparedbetweenthe and vent features (McGill, 1989; Skinner and Tanaka, 2007). þ 0:03 þ 0:05 eastern flank (3.80 0:05 Ga; 21) and western flank (3.65 0:09 Ga; 9). Overlapping Utopia, the Isidis basin rim is preserved on its Similarly,theUtopiaPlanitia2unit,whichwasmappedbasedon southern flank as the uplifted and eroded massifs of Libya Montes. broad morphologic uniformity, shows different surface ages deter- The Nili Fossae are arcuate grabens that partly demarcate the þ 0:06 þ 0:1 mined for areas 27 (3.61 0:12 Ga) and 23 (3.45 0:4 Ga); these ages northwestern outline of the impact structure. Both the Utopia and appear to agree with local stratigraphic sequences of progressively Isidis basins display high-amplitude gravity anomalies indicating younger surfaces with decreasing elevation but also might indicate buried mascons (e.g., Mohit and Phillips, 2007; Muller and poor regional correlation in the ages. This results in some type areas Sjogren, 1968). (8, 9, 12) that represent stratigraphically and morphologically differ- Utopia basin as mapped by Tanaka et al. (2005) is surrounded ent units to have similar crater ages around 3.6 Ga. In other words, by highland materials made up of the Libya Montes, Noachis even though the morphologic stratigraphy is consistent from region Terra, Nepenthes Mensae, and Syrtis Major units. The Utopia to region, the unit surface ages are time transgressive at a regional basin margin forms undulating plains that slope down from scale. Alternatively, geologic contacts may be misplaced in some the highland margin and are made up of the Utopia Planitia regions, particularly where their lateral continuity is uncertain (as 1 and 2 units at progressively greater distance from the highland– identified by approximate contacts). lowland boundary margin. Sheet lavas and volcanic debris flows Crater counts for area 13 on the Isidis Planitia floor provide an emanate from the western flank of the Elysium rise and extend þ : age of about 3.44 0 09 Ga, while the basin itself originated at westward more than 2000 km into the Utopia basin, covering 0:28 about 3.9670.01 Ga (Werner, 2008). Nili and Meroe Paterae, the much of its floor and burying Early Amazonian Vastitas Borealis probable source for Syrtis Major volcanic flows, show an age of materials. These units include the Elysium rise and Tinjar Valles about 3.73 Ga with resurfacing ending at 2.5 Ga (Werner, 2009)in (a and b) units (Fig. 3). The Tinjar Valles units source from agreement with Hiesinger and Head (2004). Here, the crater northwest-trending fissures and troughs of Elysium Fossae and counts for this unit (area 26) indicate an age of 3.56 þ 0:03 Ga, generally bury lava flows of the Elysium rise unit. The Tinjar 0:04 Valles b unit delineates materials emplaced through confined which is close to the latest-stage activity of other paterae flow to form the Granicus, Tinjar, Apsus and Hrad Valles, while (Werner, 2009). the Tinjar Valles (a) unit represents the complex and rugged overbank and sheet flow units that generally define the boundary 3.3. The Elysium volcanic province and Amazonis Planitia of the Utopia channeled lobe. On the northwest margin of Utopia basin, the Astapus Colles unit thinly buries other lowland units The third zone of interest includes the volcanic province and is interpreted to represent a degraded ice-rich mantle deposit Elysium and surrounding plains units (Fig. 3). Three volcanoes, (e.g., Mustard et al., 2001). Isidis basin is also surrounded by Elysium Mons, Hecates Tholus and Albor Tholus, as well as other ancient highland materials and is bounded to the west by the local fissure vents on the southern flank of the Elysium rise and in volcanic flows of the Syrtis Major Planum unit and abuts Utopia Amazonis and Arcadia Planitiae, constitute major volcanic vents basin along a low topographic saddle covered by the Utopia in this region. From these, several volcanic lobate flow units were Planitia 2 unit. The Isidis Planitia unit makes up almost the entire sourced that cover a rather large area of the northern lowlands Isidis basin floor. Flow tongues are visible in the southeast, (Tanaka et al., 2005). Amazonis Planitia was the original type originating from Amenthes Planum. region for the youngest of the Martian stratigraphic periods (Scott We crater counted 19 areas representing nearly all the afore- and Carr, 1978). mentioned geological units (see Fig. 3 and Table 1; for the crater The Arcadia Planitia unit represented by area 1 has an age of size-frequency distributions and image data, Appendix A). In this þ 0:05 3.57 0:09 Ga. Areas 2 and 3 sample widely separated surfaces of region, the best-fit ages range from the formation ages of 4.04 þ 0:05 þ 0:04 þ 0:16 the Elysium rise unit, yet both yield crater ages near 3.60 0:1 Ga. : (Early Noachian) to resurfacing ages of 0.845 : Ga (Late 0 06 0 17 A similar age (3.66 þ 0:04 Ga) is found for area 4 (Utopia Planitia Amazonian). The oldest surface of this region occurs within the 0:07 Noachis Terra unit (20), likewise as for the circum-Chryse region. 2 unit) SSW of Elysium Mons. In other areas surrounding the Roughly along the 1201E-meridian, several type units provide Elysium rise, such as around Athabasca Valles, the same age of a surface-age sequence across the Utopia basin (areas 14, 18–24). 3.66 Ga has been found in earlier investigations (Werner et al., 2003; Murray et al., 2005). The resulting surface ages gradually decrease with decreasing elevation and toward the center of the Utopia basin. All units Two areas representing units in northern Amazonis Planitia west of Olympus Mons indicate ages of Middle to Late Amazonian involved show at least one resurfacing event, and all units were þ 0:65 þ 0:69 resurfaced (20–22) or formed (14, 18, 19, 23, 24) at about 3.48 Ga. (1.39 0:77 Ga, area 5; 1.78 0:81 Ga, area 6 with a resurfacing þ 0:08 The stratigraphic profile line is apparently interrupted by areas event ending 0.4 0:08 Ga, which is reflected in surface expression representing the Elysium rise and Tinjar Valles units. The Elysium by mantling). Although areas 6 and 7 belong to the same unit Author's personal copy

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(Amazonis Planitia 1 north unit), area 7 appears to represent a (Werner, 2005). The are plotted in Fig. 4. These ages decrease in þ 0:18 age from old to young across the dichotomy and correlate with much older surface with an age of about 3.27 1:1 Ga. Most of the large craters in area 7 are embayed or partly covered by younger the south-to-north decreasing elevation from the highlands to the deposits and the shape of the crater distribution indicates con- lowlands. Within given geological units, younger ages are found tinuous resurfacing at craters smaller than 3–4 km in diameter. in western versus eastern outcrops. The highland surface ages Similar measurements across the entire Amazonis Planitia range between 3.98 and 3.89 Ga. These Early to Middle Noachian demonstrated a strong spatial variation in crater densities ages are representative (see above) and are in good agreement (Werner, 2005). Although the eastern part of Amazonis Planitia with an almost homogeneously formed highland plateau, where (close to the Olympus Mons aureole) is characterized homoge- large-scale surface modifying processes are dominated by impact neously by Amazonian plains-forming lava flow units, their cratering. The crater size-frequency distribution measured close emplacement may have began in the Hesperian (as the number to the Isidis basin reveals the oldest surface age and resurfacing of buried craters suggest) and may be not as thick as in other reflecting an eventful history since impact basin formation (Frey areas of Amazonis Planitia. THEMIS daytime-infrared images et al., 1988). The eastern highland–lowland boundary is expressed reveal multiple flow features, and suggest a temporal subdivision, by an escarpment that originated through dissection of highland although the process forming the plains stayed the same. plateau material. Here, a broad age range for surfaces at the base of the escarpment between 3.83 and 3.67 Ga (Late Noachian) is observed. Areas measured closer to the escarpment appear older 3.4. Alba Mons to the north pole than those more distant from the escarpment. Larger craters are still visible on the remaining blocks and mesas of the dissected This region includes the broad volcanic edifice of Alba Mons, highland plateau, while farther away smooth lowland plains marking the northernmost extent of the Tharsis rise, and its dominate the terrain. adjacent plains. From north to south, this surface is composed of As a general trend, younger surface ages are found towards the Scandia region, Vastitas Borealis marginal and interior units, topographic lows in the lowlands. Plains units east of Lyot formed and the Alba Mons unit (Tanaka et al., this issue). We produced during the Late Hesperian. Compared to the eastern part of crater counts for each of these units along a profile roughly the investigation area, only a single transition unit is present. following the 2401E-meridian. The selected areas follow a gentle The ages for this unit vary between 3.74 and 3.68 Ga, or Late slope down into the Borealis basin. Ages based on crater size- Noachian. In the Deuteronilus region, we obtain Noachian ages if frequency distributions for areas 42, 43, 44, and 45 indicate a measuring all craters on mesas and the lower units together, but surface age ranging from 3.56 to 3.37 Ga. All four units show if just the lowland parts are considered, the age is as young as the resurfacing ending at about 1 Ga. lowland units west of Lyot (about 3.15 Ga, Early Amazonian). The Differences between crater counting ages performed on Viking impact structure Lyot formed about 3.470.05 Ga ago (Werner, and THEMIS image bases show the most dramatic differences 2008). The temporal separation between the eastern and western for surfaces in this region. Not only do slight age variations of part of the investigated dichotomy boundary area, indicated in 100–200 Ma result, but an erroneous age sequence would be morphology, topography and gravity anomalies, has been con- derived if only based on Viking image data (surface ages are firmed by crater counts (Werner, 2005). overestimated for the units represented by the patches 43 and 44, see Table 1). In these cases, clouds and haze in the Viking images 3.6. Chronostratigraphical inventory of the northern lowlands limited crater detectability, so that some albedo variations were erroneously interpreted as craters. In the Chryse and Utopia–Isidis basin regions, the Noachis Terra unit surfaces formed between 4.04 and 3.96 Ga. This unit generally 3.5. The highland–lowland dichotomy boundary forms the lowland-bounding cratered highlands, where the long- evity of cratering processes appears to have resulted in resurfacing The varying geologic, morphologic, and stratigraphic charac- of highland rocks (based on multiple intersections of isochrons). In teristics of the highland–lowland boundary are perhaps best Utopia Planitia, the Nepenthes Mensae unit ages reflect the mor- seen between Chryse and Isidis Planitiae. Topographically, the phologic transition between highlands and lowlands, an equivalent highland–lowland boundary changes from a gently sloping sur- interpretation to map-based stratigraphies. Similarly, in the circum- face in Arabia Terra to a steep escarpment bordering Terrae Chryse region, the Chryse Planitia 1–4 units that make up the Sabaea and Cimmeria. The transition longitude is at about region’s southern slope have ages from 3.89 to 3.6 Ga. These units 3301W, and is marked coincidentally but not causatively by the generally tend to become gradually younger with decreasing eleva- most prominent fresh impact structure of the northern lowlands, tion and increased latitude. Many mapped units (although discri- the 221.2-km-diameter, double-ringed Lyot crater (centered at minated primarily by superposition relationships) are interpreted to 501N, 3311W). The location of this crater marks also a change in overlap in both age and morphologic expression. Outflow channel Bouguer gravity anomalies and calculated crustal thickness (Janle, floors mostly formed prior to 3.55 Ga; those adjacent to floodplains 1983; Neumann et al., 2004). The heavily cratered highlands appear slightly younger. The ages of Vastitas Borealis units in crustal thickness is estimated to be nearly everywhere thicker theChryseregionarearound3.4570.05 Ga, similar to their ages than 60 km, while the lowland crust ranges from 20 to 40 km in in the Scandia region between the north pole and Alba Mons thickness. Although Arabia Terra displays heavily cratered high- (3.33–3.56 Ga). The ages of Vastitas Borealis units in the Utopia land morphology, its relatively low elevation as it slopes into the basin and its vicinity are similarly distributed, whereas the Utopia lowlands results in a thinning crustal thickness profile and basin-marginal plains ages are about 3.60 Ga, with higher elevations smaller lowland crustal thicknesses (Zuber et al., 2000). showing older ages (3.85 Ga). Especially in the Utopia basin, a Using preliminary northern plains map units and boundaries gradual decrease in ages towards topographic lows is observed, of Tanaka et al. (2003), Werner (2005) investigated the crater-size with a minimum age of about 3.32 Ga for the lowest lying parts of frequencies in order to unravel the differences in topography and the Vastitas Borealis interior unit (24). Lava flows, lahars, and crustal structure between regions east and west of Lyot crater, channels (Elysium rise and Tinjar Valles units) occurred less than which also correspond to correlating or different episodes of unit 3.5 Ga ago, followed by later phases of resurfacing between 1.5 and development. Ages in this area range from about 4 to 3.1 Ga 1Gaago. Author's personal copy

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LH 3.5 3.6 EH 3.7 ModelAge[Ga] Cratering −2 LN 10 Themis IR day formation age 3.8 resurfacing age Cum.CraterFrequency[km MN 3.9 Viking MDIM 2.1 formation age resurfacing age 4.0 EN formation age Viking MDIM 1 formation age 4.1 10−1 Lunae Planum lba Patera rcadia Planitia unit mazonis Planitia 1−2 unit res/Tiu Valles(#) stapus Colles unit astitas Borealis interior astitas Borealis marginal A A A A A Noachis Terra unit Syrtis Major Planum V Utopia Planitia 1−4 unit Dichotomy Nepenthes Mensae unit Elysium rise/Tinjar Valles Kasei Valles (*) Chryse Planitia 1−4 unit V Lyot Isidis Planitia

Fig. 4. Summary of crater frequencies Ncum (1 km) (left scale) and model ages derived applying the cratering chronology model by Hartmann and Neukum (2001) (right scale) for outﬂow channels, lowland units, highland areas, and volcanic deposits and plains units. These measurements have been performed on Viking MDIM2.1 mosaics (squares and dashed empty circles, the latter symbolize resurfacing events) and Themis IR daytime moasaiced images (circles, empty circles symbolize resurfacing events). These datasets are supplemented by studies around the highland–lowland dichotomy boundary (black squares, compare Section 3.5), in Ares and Tiu Valles (black stars, Marchenko et al., 1998), and Kasei Valles (black stars, Lanz, 2004), all measured on Viking image data. Horizontal lines show the epoch boundaries as published in Hartmann and Neukum (2001), they vary only for the Amazonian boundaries (dashed are Hartmann’s favored boundaries, solid the ones favored by Neukum.)

Most volcanic activity in and around the northern lowlands 4. Giant polygonal trough units in the northern lowlands may have occurred within a few hundred million years. The Lunae Planum unit appears to have been emplaced up to 3.51 Ga. The Landforms such as polygonal patterned terrain so-called giant Syrtis Major Planum unit shows a similar age (3.56 Ga), though polygons, are result of major resurfacing events. Utopia, Elysium, the surface of Isidis Planitia is younger (3.44 Ga). The Elysium and Acidalia Planitiae are occupied by extensive areas of these flank and adjacent plains flows of Arcadia Planitia were emplaced giant polygons. Originally, this polygonally fractured terrain was between 3.57 and 3.48 Ga. Our crater counts in the extensive observed on the Mariner 9 B-frames (Mutch et al., 1976) and Amazonis Planitia 1 north unit suggest multiple events. Although revealed, accompanied by other morphologies, that large portions these multiple events are not recognized everywhere, morpholo- of the Martian surface likely was modified by the action of water. gically we interpret these plains to have formed by the emplace- Later, high-resolution Viking photographs resolved in greater ment of sheet lavas, and the time frame is very extended. detail the locations and morphology of the giant polygons. Determined ages range from 3.27 to 0.04 Ga. The construction In general, they consist of 200–800-m-wide steep-walled and of northern Alba Mons concluded around 3.37 Ga, coinciding with flat-floored troughs, some tens of meters deep, the intersections the formation of the characteristic landforms observed in the of which form polygons that are 5–30 km in diameter (Pechmann, Scandia region unit (3.36 Ga) situated north of the Alba Mons. 1980). These features occur in southeastern Acidalia Planitia, However, the emplacement and/or major resurfacing of most northwestern Elysium Planitia, and the low elevation margins of sedimentary units located within the Martian lowlands appears to Utopia Planitia, mostly in the Martian northern mid-latitudes. be broadly post-date the formation of major volcanic constructs. These giant polygons are much larger than small-scale polygons Major plains-related volcanic flows (e.g., in Utopia and Amazonis that are interpreted to form by periglacial processes at higher Planitiae) overlap with the emplacement and/or resurfacing of latitudes (Rossbacher and Judson, 1981). lowland units. Although most constructs were in place by the In previous mapping approaches (Scott and Tanaka, 1986; Early Amazonian, it appears that volcanic and sedimentary units Greeley and Guest, 1987; Tanaka and Scott, 1987), the giant may complexly interfinger along the margins of the Martian polygonal terrain occupying regions were mapped as the grooved lowlands. member (Hvg), one of four members of the Vastitas Borealis When translating the absolute surface ages (Table 1)to Formation. Textural and albedo differences as well as morpholo- geological epochs (Fig. 4; see also Section 5), most lowland gic characteristics have been used to outline these different geologic units appear to have been emplaced (or underwent members. Pechmann (1980) concluded that the polygonal trough major resurfacing) during the Hesperian, with the exception of units intersect units of all other Vastitas Borealis members. McGill the highland units (Noachian age). Only a few regions, such as the (1986) investigated crater distributions of these members, which plains northwest of Olympus Mons, are Amazonian in age. revealed apparent variability in the distribution shapes. However, there is morphologic and stratigraphic evidence that A discussion about the origin of the giant polygons and crater major resurfacing was common in regions of the lowlands. distribution variability is inherently linked to the question: Did Author's personal copy

S.C. Werner et al. / Planetary and Space Science 59 (2011) 1143–1165 1155 large standing bodies of water fed by catastrophic flood events þ 0:02 a surface age of 3.52 0:03 Ga for craters larger than 1.4 km in exist in the past? We find that our crater count and map-based diameter for both locations. Additionally, the distribution of the geologic and stratigraphic assessments can provide some infor- ringed-graben structures was measured assuming that they are mation for addressing this outstanding question. the imprinting crater population of the underlying surface or basement. Locations of both ghost and visible craters are shown in Fig. 5B and E. The distribution of visible craters was stacked 4.1. Crater distributions in polygonal troughed terrain (Acidalia and with the population of ghost craters (Cruikshank et al., 1973) for Utopia Planitiae) both regions in Utopia and Acidalia Planitiae (Fig. 5C, F). The combined visible and ghost crater populations yield an age of Various hypotheses for the formation of giant polygons have þ 0:02 3.77 : Ga for Utopia Planitia for craters larger than 6 km in been proposed, including thermal cooling and contraction in 0 02 diameter and a slightly younger age of 3.68 þ 0:02 Ga for craters permafrost (Carr and Schaber, 1977; Carr et al., 1976), desiccation 0:03 larger than 3 km in diameter for Acidalia Planitia’s basement. This of water-saturated sediments (Morris and Underwood, 1978), implies that a substantial amount of deposition took place in cooling of lava (Morris and Underwood, 1978; Masursky and parts of Acidalia and Utopia Planitiae. Using estimates of Garvin Crabill, 1976; Carr et al., 1976), or tectonic deformation (Mutch et al. (2003), in order for buried craters to form circular grabens et al., 1976; Carr et al., 1976). Pechmann (1980) showed that no with diameters about 30 km, a layer of compactable (sedimen- terrestrial analogs satisfactorily describes the scales observed or tary) materials must be 4350 m thick. the mechanisms involved. McGill and Hills (1992) explained the observed large size of the polygons and accounted for the stresses responsible for their formation by plate-bending and finite- 4.2. Cratering variations in Utopia Planitia element models. These assessments suggested the polygons formed by thermal contraction and vertical compaction, perhaps Crater morphologies observed in Utopia Planitia seem to be during the cooling of water-saturated sediments or volcanic rocks characteristically of Mars. However, craters as large as about accompanied by differential compaction over buried topography. 14 km in diameter are bowl-shaped, like simple craters on the The giant polygonal pattern is accompanied by single or double Moon or Mercury. A similar size-range for the transition from ring-like graben structures, which are assumed to occur above simple-to-complex interior crater morphology in the Martian buried craters. These have been used to estimate the thickness of lowland plains has been noted by Wood et al. (1978). In principle, the deforming overburden material (McGill, 1986; McGill and central peak morphology is expected for craters larger than 5 km Hills, 1992). Lucchitta et al. (1986) suggested that the troughed (Pike, 1980), but has been observed for craters as small as 1.5 km material is of sedimentary origin and was deposited in a standing in diameter in heavily cratered highland units (Mouginis-Mark, body of water by emphasizing the spatial correlation of giant 1979). Pristine craters larger than 15 km are not present in this polygons, outflow channels mouths, and topographic lows within part of Utopia Planitia. Many craters show ejecta morphologies the lowlands. Another explanation for the origin of undulating which are single- or double-lobed ramparts, so-called fluidized surfaces and polygonal crack patterns of this size is based on the ejecta blankets. They are believed to indicate the presence of formation through Rayleigh convection in rapidly emplaced volatiles (e.g., water ice or water in the subsurface, Carr et al., water-rich sediments (Lane and Christensen, 2000). Elastic 1977). On Earth, craters which were formed in shallow marine rebound has also been proposed for the formation of giant (sedimentary) environment commonly show a diminished relief polygons, a process which implies uplift of the northern Martian compared to their on-shore counterparts (e.g., Shuvalov et al., lowlands crust following the evaporation or sublimation of 2002). A number of craters show ballistically emplaced ejecta, a water or ice bodies (Hiesinger and Head, 2000). spreading outward over distances of less than one crater dia- As shown in Fig. 6, even though craters that superpose fields of meter. For both crater ejecta types, no clear diameter dependence giant polygons may appear rather pristine, they are embayed and/or is found. For a few of the larger craters, no ejecta are visible at the subdued by later deposits. These deposits appear to drape the given image resolution, suggesting that they were formed before original relief and form circular graben structures within the the upper part of the surrounding surface unit was emplaced or polygonal troughed terrain. These circular grabens have diameters that the surface was degraded after the crater was emplaced. between 8 and 30 km, and are believed to be fracture patterns They appear embayed or draped by later layers of volcanic or tracing buried craters (Carr, 1981; McGill, 1986), yielding a regional sedimentary deposits. cover thickness of about 600 m (McGill and Hills, 1992). These Based on the Viking-MDIM-2.1 image data, new crater size- single- and double-ringed grabens are associated with slight topo- frequency measurements were performed and ages were deter- graphic depressions seen in the MOLA topography (Buczkowski and mined in selected areas of the Utopia region, which cover McGill, 2002). Based on the modeled graben spacing of double- polygonal terrain and surrounding units (Fig. 7a–c). Additionally, ringed troughs, Buczkowski and Cooke (2004) suggested a cover a mosaic of 28 high-resolution Viking frames (orbit 430B) with a thickness of 1–2 km. Furthermore, MOLA topographic data also pixel resolution of 15 m was prepared (Fig. 6D), and, to cover the revealed the presence of roughly circular basins with diameters of full range of crater sizes (in this case from 50 km down to 10 m), several tens of kilometers in both the Martian highlands and small-scale information was retrieved from a MOC-NA image lowlands, which are usually not associated with any structural M08/03489 at a resolution of 3.03 m (Fig. 7e). The location feature in image data. These were called quasi-circular depressions of the Viking 430B frames and the MOC-NA image is shown and are potential candidates for large, buried or deeply eroded in Fig. 7a. impact structures (e.g., Frey et al., 2002. Buried basins of diameter All crater size-frequency distributions of the selected units ranging between 20 and 70 km in diameter would refer to a cover (Fig. 7a–c) converge in the smaller crater diameter size range and thickness between 1 and 2 km, respectively, (Garvin et al., 2003). give an age of 3.4 Ga (Fig. 8). This plains age is observed in many We measured the crater size-frequency distribution specifi- measurements in other lowland regions (see above) and is also cally for the polygonal-troughed terrain region in the Utopia and confirmed by the measurements of the crater size-frequency Acidalia Planitiae (Fig. 5A and D) and recorded an unusual distribution in mosaicked image data at a pixel resolution of deficiency of large craters (Fig. 5C and F). Initially, crater distribu- 15 m observed during Viking orbit 430B. A later resurfacing event tions of clearly visible craters were measured and resulted in is observed, which is also supported by the crater counts on Author's personal copy

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120°0'0"E 30°0'0"W

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cum

Cum.Crater Frequency N Cum.Crater Frequency N ghost + pristine craters ghost + pristine craters ghost craters ghost craters pristine craters pristine craters

Crater diameter D [km] Crater diameter D [km]

Fig. 5. In Utopia Planitia (A) and in the Acidalia Planitia (D) giant polygonal terrains are shown (hatched) with respect to the map units of the northern lowlands (Tanaka et al., 2005). For these areas, crater counts of pristine and so-called ghost craters were performed. The position of the visible (pristine) craters are indicated in black, and the so-called ghost craters in red (for Utopia Planitia, B, for Acidalia Planitia, E). The resulting crater size-frequency distributions for the visible (pristine) crater distribution (squares) show a clear deﬁciency of large craters. The distribution of ghost craters (empty circles), all larger than 2 km, is recognized. The summed distribution of visible (pristine) craters and the buried crater population (ghost craters) is shown (ﬁlled circles). Both areas have been resurfaced about 3.52 Ga ago, but the detectability of ghost craters varies.

the MOC-NA image (Fig. 8). For the larger-crater diameter- other surface structures, but mostly with the presence of either size range (larger than 3 km), a variety of shapes for the crater subdued (Fig. 7b) or embayed (Fig. 7c) large craters, indicating size-frequency distribution is observed for the different areas of resurfacing processes have been acting in a different manner. Utopia Planitia, conﬁrming earlier observations by McGill (1986). The diversity or deviation from the expected crater-production These variations correlate with the presence of giant polygons and function (Ivanov, 2001) for the larger-crater diameter-size range Author's personal copy

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Fig. 6. In the central part of the Utopia Planitia a diversity of crater ejecta morphologies are found: (A) crater with ﬂuidized double lobate ejecta blanket, (B) similarly sized crater with single ejecta blanket (C) rather ballistically emplaced than ﬂuidized ejecta (D) pancake ejecta (E) double-ring graben suggesting buried crater (F) single-ring graben, and superposed by smaller craters. The areas of giant polygons are characterized by ring- or double-ringed graben features interpreted as buried impact structures.

(larger than 3 km) most likely depends on different target proper- which would imply a very slow water regression, if indeed this ties or variations in geological evolution. However, it is unclear why variation resulted from a Hesperian ocean. there is a lack of large craters in certain regions, yet in neighboring regions it appears that the large crater population of the lowland basement is exposed. In units (Fig. 7c) that show an apparent excess 5. Implications for the evolutionary history of the of larger craters in the measured crater size-frequency distribution highland–lowland boundary, and the northern lowlands (Fig. 8), the largest craters apparently formed on the surface of 3.75 Ga and survived embayment by younger volcanic or sedimen- Summarizing the results of our investigation, we constrained tary deposits at about 3.4 Ga (Fig. 7b). Crater counts on high- that the highland–lowland dichotomy boundary developed dur- resolution images constrain the resurfacing at about 3.4 Ga and ing the pre-Noachian and was later modified. The Noachis Terra suggest that subsequently only minor resurfacing such as mantling unit formed during a period between 4.04 and 3.96 Ga, whereas affected craters at diameters smaller than 300 m. the slightly younger Nepenthes Mensae unit formed between 3.81 Irrespective of the cause of the diversity or obscuration of the and 3.65 Ga. The Nepenthes Mensae unit, which marks the edge measured crater size-frequency distribution in the larger-size of the highland plateau, was resurfaced by inter-crater plains range compared to the expected crater-production function, formation (sedimentary and volcanic) and erosion by surface resurfacing occurred between 3.4 and 3.75 Ga in areas where water runoff (valley networks) during the Hesperian. polygonal terrain is observed. The earlier crater population may Examining the dichotomy between Chryse and Isidis Planitiae, a have been buried by a 1–2-km-thick layer as previously proposed morphologic difference of the regions eastward and westward of (Buczkowski and Cooke, 2004). In other areas of Utopia Planitia, the impact basin Lyot is found. This separation is seen in crater ring-graben structures and a lack of large craters are not counts, morphology, topography and gravity anomalies. However, it observed. These areas appear to be at the outer edge of the Utopia is unclear why these boundary parts developed differing topo- basin, and this conforms with sedimentation concentrated in the graphic expressions. The original morphology of the dichotomy Vastitas Borealis interior unit as might have occurred in a palaeo- boundary has been partly obscured by volcanic, fluvial, and crater- ocean in the northern lowlands. Therefore, the interpretation that ing processes, which is reflected in ages measured in this study. the polygons emerged through desiccation and differential com- Fluvial activity, most dramatically displayed in the Chryse region, paction of sediment over buried topography is strongly sup- was less effective in obscuring the dichotomy boundary than ported, although not necessarily a unique interpretation. It also volcanic accumulations (e.g., as at Alba Mons and Olympus Mons). implies the slow (or frequently episodic) emplacement (sedimen- Four volcanic provinces superpose the lowlands (Elysium rise tation over roughly 350 Myrs) of sediments within these regions and Apollinaris Mons), overlap the dichotomy (Tharsis rise), or (isochron-cutting crater distribution, compare Fig. 1C), so that the cover adjacent highlands (Syrtis Major Planum). Therefore, infor- formation of the giant polygons is unlikely to have formed mation for the time frame for the existence of a Martian ocean through Rayleigh convection during the deposit emplacement as may be indicated in the formation age of these volcanic flanks, suggested by Lane and Christensen (2000). The elastic rebound of and in the flank base morphology of the volcanoes along the the crust due to the removal of water, a driving force suggested by highland–lowland boundary. Alba Mons and the Elysium rise Hiesinger and Head (2000), is possible and could aid the pattern show smooth slopes, characterized by superposing lava flows or formation after the disappearance of a possible ocean. Consider- similar volcanic morphologies, formed during the Late Hesperian. ing the analysis of the northern-lowland plains units, we observed On the other hand, the Apollinaris Mons flank base forms a steep a correlation between younger ages and topographic lows. These escarpment towards the lowlands. While the undisturbed flank ages were found to cover an age range between 3.6 and 2.6 Ga, formation of Alba Mons and the Elysium rise occurred only in the Author's personal copy

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Fig. 7. The central part of the Utopia Planitia (265–2401W and 30–371N) is shown here (Viking MDIM2.1) to illustrate the selected areas, which were used for crater counting (a–e). Additionally, the location of the Viking high resolution image mosaic (15 m/pxl taken during orbit 430B) and the location of the MOC (M0803489) image are plotted. The image data are shown as well. The subunits were selected according to their diverse crater morphology. The counts resulting crater size-frequency distributions are shown in Fig. 6.

200 Ma 0 Utopia region 3.40 Ga formation 3.75 Ga −1 resurfacing Chryse region −2 formation resurfacing

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Fig. 9. The relation between age and elevation for the Utopia and the Chryse region.

Late Hesperian, the lowland-facing cliff-like ﬂank of Apollinaris Mons formed already before the Early Hesperian. However,

Cum. Crater Frequency N [km Frequency Crater Cum. Apollinaris Mons’ last volcanic episode is manifested in a south- ward erupted lava fan at about 3.71 Ga (Werner, 2009) at the transition between Noachian and Hesperian. This difference in morphology and timing between the main edifice of Apollinaris 15m VIKING MOC Mons and Alba Mons may imply that early stages of Apollinaris MDIM2 .1(all) Mons formed in the presence of a standing body of water, while MDIM2.1 (many visible large craters) Alba Mons and the youngest fan of Apollinaris Mons did not. MDIM2.1 (no visible large craters) A general water cycle, including precipitation and surface runoff (valley networks, Fassett and Head, 2008), may have persisted until then (3.71 Ga ago). Crater Diameter D [km] The formation of the most extensive surfaces in the lowlands, as defined by densities of craters generally 43 km in diameter, Fig. 8. The crater size-frequency distributions are measured in units shown in Fig. 7. We counted on Viking MDIM2.1 and 15 m/pxl resolution Viking image data, ended between 3.7 and 3.4 Ga ago. All ages measured in the as well as on a MOC image. The subunits b and c were selected according to the closest vicinity of the steep dichotomy escarpment are around presence or deficiency of large craters (diameters around 15 km). The crater size- 3.7 Ga or older. By trend, neighboring northern lowland units frequency distributions of these selected areas confirm a diversity at the large-size show gradual changes to younger formation and resurfacing ages range. They converge for craters smaller than about 2 km in diameter. Crater continuing down-slope toward topographic lows (Fig. 9). These counts at higher resolution reveal one resurfacing event which is pronounced at about 300 m (compare Fig. 1A) followed by a more continuous resurfacing pattern ages cover an age range between 3.6 and 2.6 Ga. This could imply as illustrated in Fig. 1C. a gradual water regression (or other resurfacing processes). Author's personal copy

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Fig. A1. Inventory of the clippings of the Viking MDIM2.1 mosaics on which the measurements have been performed. They represent exactly the area which was counted. The resulting crater size-frequency distributions for each patch are shown together with the best-fit model ages (thin line), derived by fitting crater production function by Ivanov (2001) and applying the cratering chronology model by Hartmann and Neukum (2001). The fit range is given by diameter values for which the curve is plotted (thick line). Statistics for these measurements are given in Table 1. Resurfacing events are plotted corrected already, see Section 2 for details. Author's personal copy

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Fig. A1. (continued)

The youngest units in the northern lowlands are considered to be If a (transient) Amazonian ocean existed, it did not result in lavas or polar ice (Tanaka et al., 2005), arguing against the ocean any signiﬁcant deposition or resurfacing, which we consider theory during the Amazonian Period (younger than about 3.1 Ga). unlikely. Author's personal copy

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Fig. A1. (continued)

Detailed studies of the giant polygonal terrain observed in the basement ages (up to 3.77 Ga) as deﬁned by entirely exposed craters Utopia and Acidalia Planitiae indicate that the distribution of visible closer to the highland–lowland dichotomy boundary (e.g., Utopia craters and the ring-graben structures (ghost craters) yield the same Planitia 1 and Chryse Planitia 1 units), where a much thinner deposit Author's personal copy

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Fig. A1. (continued)

post-dating the boundary retreat is expected. The lowland Vastitas the existence of a proposed depocenter (in a water or mud ocean) Borealis interior unit with an age of about 3.4 Ga mostly appears to shortly after the crustal formation of the northern lowlands. constitute deposits at least 1–2 km thick. Results found in relation to These ages are consistent with the last time period expected the polygonal terrain temporally and morphologically conform with for the existence of any kind of Martian ocean. A general water Author's personal copy

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Fig. A1. (continued) cycle, including precipitation and surface runoff (valley networks, S. C. Werner is supported by the Norwegian Research Council through Fassett and Head, 2008), may have persisted only until about aCentreofExcellenceGranttoPGP. 3.7 Ga ago.

Appendix A. Image data

Acknowledgments Inventory of the clippings of the Viking MDIM2.1 mosaics and the resulting crater size-frequency distributions for each patch This paper beneﬁted greatly from thorough reviews by Brian are shown in Fig. A1. Hynek and an anonymous reviewer. We appreciated comments by Rosalyn K. Hayward and Corey Fortezzo during internal USGS References reviews. We are thanking Ursula Wolf (Freie Universitat¨ Berlin) for her help counting on Viking image data. An earlier stage of this study Andrews-Hanna, J.C., Zuber, M.T., Banerdt, W.B., 2008. The Borealis basin and the was funded by the Deutsche Forschungsgemeinschaft (DFG, through origin of the martian crustal dichotomy. Nature 453, 1212–1215. doi:10.1038/ nature07011. G. Neukum); also funding was provided by the NASA Planetary Archinal, B.A., Kirk, R.L., Howington-Kraus, E., Rosiek, M.R., Soderblom, L.A., Lee, Geology and Geophysics Program for K. L. Tanaka and J. A. Skinner. E.M., 2003. Mapping Mars at global to human scales. In: IAU Special Session, 1. Author's personal copy

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