Valleys in pit craters on Mars: Characteristics, distribution, and formation mechanisms

Icarus 225 (2013) 272–282

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Valleys in pit craters on Mars: Characteristics, distribution, and formation mechanisms Samantha E. Peel ⇑, Caleb I. Fassett Department of Astronomy, Mount Holyoke College, South Hadley, MA 01075, United States

a r t i c l e

i n f o

Article history: Received 4 September 2012 Revised 20 March 2013 Accepted 28 March 2013 Available online 10 April 2013 Keyword: Mars, Surface Geological processes Mars, Climate Cratering

a b s t r a c t New observations of central pit craters on Mars reveal that some craters contain prominent interior valley networks that drain into their central pit. In this study, we catalog the distribution of pit craters with associated valleys, and characterize in detail five diverse, well-preserved examples. In some instances, sedimentary deposits we interpret as either alluvial fans or deltas are found in the central pits, and evidence suggests that at least some of these pits once contained lakes. Fluvial activity within the five pit craters we detail, appears to have occurred relatively late in Mars history, during the Hesperian or Amazonian periods. For this reason, further understanding the formation mechanisms of these valleys has important implications for the history of water on Mars. Ó 2013 Elsevier Inc. All rights reserved.

1. Introduction and background Central pit craters have been found at many locations on Mars (e.g., Smith, 1976; Hodges et al., 1980; Barlow, 2010), Ganymede (e.g., Croft, 1983; Moore and Malin, 1988; Schenk, 1993; Alzate and Barlow, 2011), and Callisto (Croft, 1983; Schenk, 1993). They are defined as complex craters that contain a central depression formed either on the crater’s floor (‘‘floor pits’’) or central peak (‘‘summit pits’’) (Wood et al., 1978; Hale and Head, 1981; Barlow, 2010). Some, but not all, floor pits are bounded by raised rims (Garner and Barlow, 2012), which can lead to some ambiguity between floor and summit pits. For this study, we use the definition of Barlow (2010) that floor pits are those where the bottom of the pit is at an elevation at or below the elevation of the surrounding crater floor, and summit pits as those where the pit floor is at a higher elevation than the crater floor. Mechanisms that have been proposed for the formation of central pits include vaporization (e.g., Wood et al., 1978; Bray et al., 2012), impact melt drainage (Croft, 1981; Senft and Stewart, 2009; Elder et al., 2012; Bray et al., 2012), and collapse of a central peak (Croft, 1981; Passey and Shoemaker, 1982). Many of these models invoke modification of the impact process due to the presence of subsurface water or water–ice (e.g., Wood et al., 1978; Croft, 1981; Elder et al., 2012; Alzate and Barlow, 2011). Alternatively, central pit craters may potentially result from crater ⇑ Corresponding author. Present address: Department of Earth and Planetary Sciences, University of Tennessee, 1412 Circle Dr., Knoxville, TN 37996, United States. E-mail address: [email protected] (S.E. Peel). 0019-1035/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.icarus.2013.03.031

formation in layered targets (e.g., Greeley et al., 1982), although pit craters are morphologically distinct from examples where this is known to have occurred (e.g., Oberbeck and Quaide, 1967). In some instances, pit craters are modified by fluvial erosion, which is the focus of this study. This paper is the first to characterize these features. Here, we describe the distribution of pit craters that contain interior valleys draining into their central pit (‘‘pit valleys’’). We then discuss conditions that may have led to the formation of pit valleys, focusing on several specific examples where the geomorphology of the pit crater, valleys, and sedimentary deposits are well-preserved.

2. Data A variety of datasets were used to both survey the distribution of valleys within pit craters and assess the characteristics and formation mechanisms of the pit valleys that were observed. The primary data utilized for surveying, characterization, and analysis were 5 m/px resolution images from the Mars Reconnaissance Orbiter Context Camera (CTX) (Malin et al., 2007), which were processed through imaging phase G17. In addition, we also relied upon both gridded topography and profiles from the Mars Orbiter Laser Altimeter (MOLA) (Smith et al., 2001) experiment. MOLA profiles have 300 m point spacing but sample only part of the surface; the gridded MOLA data has 463 m/px resolution and interpolates between these profiles. We also used stereo digital terrain models (DTMs) from the High Resolution Stereo Camera (HRSC) (Neukum et al., 2004) (50–150 m/px horizontal resolution) and CTX (15 m/px horizontal resolution).

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The HRSC stereo digital terrain models have been produced by the HRSC team as part of their systematic processing effort (Gwinner et al., 2010), whereas the CTX DTMs were constructed for this project using the Ames Stereo Pipeline (Moratto et al., 2010). The ArcMap Geographic Information System was used for all analysis and measurement. 3. Distribution and general observations The distribution of pit craters on Mars has been established by several earlier studies. Barlow (2010) cataloged pit craters on Mars with diameters ranging from 5 km to 156.9 km, although most were within the range of 10–20 km in diameter. Pit craters are broadly distributed on Mars (80° to 80°N), though they are more numerous at low-to-mid-latitudes (40° to 40°N) (Barlow, 2010; Robbins and Hynek, 2012). Robbins and Hynek (2012) reported that 92% of central pit craters are qualitatively fresh, consistent with the interpretation that they are mostly relatively young. However, a spectrum of degradation states for pit craters is observed (Hodges et al., 1980; Barlow, 2010, 2011; Robbins and Hynek, 2012), suggesting that the conditions that allowed for their formation existed over a significant portion of martian history. We used a global catalog of craters on Mars that included a classification of pit craters (Robbins and Hynek, 2012) to examine which pit craters had valleys on their interior. This database was cross-correlated with footprints of existing CTX images in order to examine their detailed morphology. Eighty percent of the 2513 pit craters mapped with THEMIS by Robbins and Hynek (2012) had CTX coverage (2008 craters), and of this eighty percent, five percent had valleys on their interior (96 craters). Given this relatively high density of CTX coverage, the frequency of pit craters with valleys within CTX data is interpreted to be representative of the population as a whole. Fig. 1 shows the distribution of pit craters on Mars from Robbins and Hynek (2012), as well as the locations of pit craters we observed to have associated interior valleys (see also Supplemental data). This distribution suggests that pit craters that have fluvial modification on their interior are more common in the mid-latitudes (near 30°N and S) than would be expected if they were a random geographic subset of the pit crater population as a whole. Amazonian ice-rich units are common in the mid-latitudes as well (Squyres and Carr, 1986; Head et al., 2005, 2010; Sousness et al., 2012), so the enhanced concentration of valley formation in the pit craters within these regions may be related and pre-existing ground ice is a potential source of water. However, pit craters with fluvial modification are not strictly confined to those latitudes and some are observed in equatorial latitudes as well.

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There are substantial differences in the magnitude of fluvial modification in pit craters where valleys are observed. The morphology of valleys also varies a great deal between craters. Central pits can have extensive and complex networks of associated tributaries, or more limited tributary networks (e.g., Fig. 2). Fresh pit craters that do not have valleys draining into their pits often have fractures or furrows that extend radially from the rims of their pits. These fractures may act as initial pathways for drainage in pit craters with valleys, particularly when the pits have full rims, although many pits are only partially rimmed (Garner and Barlow, 2012). Valleys draining into the central pits have morphologies from sinuous or meandering to rectilinear in nature. Rectilinear valleys leading into central pits may have been structurally controlled or follow pre-existing fractures. In some instances, the drainage networks leading to the central pit extend to the crater rim, whereas in other instances valleys are limited to the crater floor. Surficial mantling, which is common at mid-to-high latitudes on Mars (Soderblom et al., 1973), is found in many of the craters we investigated. This material has been interpreted as being icerich and very young (less than a few million years old) (Mustard et al., 2001). The importance of this mantling unit varies within and between the pit craters, ranging from a discontinuous residual deposit, to a thin uniform veneer, to an intact, thicker unit that obscures larger sections of a crater’s wall. In all instances, we interpret this mantling material as post-dating fluvial activity within the craters we describe. 4. Specific examples Five craters (Figs. 3–7) were investigated in detail to characterize the formation conditions and hydrology of pit craters with interior valley networks. These craters represent some of the best preserved examples of fluvially-modified pit craters that we cataloged, with evidence for fluvial activity found both as valleys and, in some cases, in inverted relief (e.g., Williams and Edgett, 2005; Pain et al., 2007). Table 1 presents a summary of the measurements characterizing the features found interior to the craters including pit volume, pit fan volumes, and total valley volume. The five craters are all located at low-to-mid southern latitudes, and range in diameter from 20 to 40 km. 4.1. Chronology We conducted crater counts on the ejecta of each of the craters of interest by using CTX data to map the extent of the continuous ejecta deposit and measure craters superposed on the ejecta with

Fig. 1. Distribution map of pit craters (white) based on catalog of Robbins and Hynek (2012) superposed on MOLA hillshade. Pit craters with evidence for post-formation fluvial modification are shown in black (96 out of the 2008 craters with CTX coverage have valleys). The five pit craters with valleys we describe in detail are labeled 1–5.

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Fig. 2. Examples of different drainage patterns for valleys that lead to central pits. (a) Dendritic drainage northeast of the central pit in crater 5 (CTX image B19_016981_1432_XN_36S201W). (b) Rectilinear valley south of the crater 4 central pit (CTX image B21_017751_1609_XN_19S347W).

Fig. 3. Pit crater 1 (crater is 37 km in diameter). Valleys that lead to the central pit are visible on the crater floor; ice-rich units are located on terraces near the crater wall. (CTX images B06_011917_1385_XI_41S188W, P05_002819_1385_XN_41S189W and P18_007935_1401_XN_39S188W). In this and all subsequent images, due north is up.

CraterTools (Kneissl et al., 2010). The resulting size–frequency distributions (CSFDs) were then assigned a tentative, best-fit absolute age using the Neukum Production Function (Ivanov, 2001) and compared against period boundaries (Werner and Tanaka, 2011). The results of this measurement suggest that the craters we focus on in detail are of Hesperian or Amazonian age (Fig. 8). This is consistent with morphological characteristics of the craters such as their well-preserved ejecta and sharp rims (e.g., Craddock et al., 1997; Mangold et al., 2012). Crater counting on the ejecta of individual craters does have uncertainties beyond those common when crater counting on geologic units. Two particular concerns are that some craters on the ejecta may be auto-secondaries (e.g., Plescia and Robinson, 2011), or that pre-impact craters were not erased during ejecta emplacement and may contaminate the count. Both of these factors would tend to cause the ages we measure to be overestimates. Qualitative indicators suggest that these effects are not major limitations to our results. First, in past work where auto-secondaries have been suggested to be important, the slope of the CSFD has generally

Fig. 4. Pit crater 2 (crater is 31 km in diameter). Valleys leading to the pit are prominently incised in the western half of the crater floor. (CTX images B19_017185_1542_XN_25S012W and P05_003063_1523_XI_27S012W superposed on THEMIS IR day mosaic).

been steeper than model isochrons, which is not the case for our results. Second, during mapping at CTX resolution, pre-existing impact craters were generally easy to recognize and exclude. Thus, we interpret these crater statistics as implying the pit craters we examine in detail are Hesperian or Amazonian, and constrain the fluvial activity within the craters to be younger than that. We thus interpret these features as part of a growing record of fluvial erosion associated with the interiors and ejecta of relatively young craters (Mouginis-Mark, 1987; Williams and Malin, 2008; Morgan and Head, 2009; Harrison et al., 2010; Grant and Wilson, 2011; Jones et al., 2011; Mangold et al., 2012). The existence of liquid water during the Hesperian and Amazonian periods still appears to have been geographically confined and reflective of localto-regional conditions rather than global conditions, unlike what may have occurred earlier (e.g., Fassett and Head, 2008a, 2011). 4.2. Crater 1 Crater 1 (Fig. 3) is 37 km in diameter with two main meandering valleys on its floor, east and west of the central pit. Each of

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Fig. 5. Pit crater 3 (crater is 23 km in diameter). Valleys lead to the central pit from every direction except the east, although the headwaters of some of the valleys are obscured by shadow. (CTX Image B20_017412_1407_XN_39S088W superposed on THEMIS IR day mosaic).

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Fig. 7. Pit crater 5 (crater is 39 km in diameter). Extensive valleys that lead into the central pit are seen to the southwest and northeast of the pit. Alluvial fans and bajadas are found adjacent to the wall of the crater; these alluvial fans appear stratigraphically superposed on the valleys leading into the central pit. (CTX images B18_016770_1429_XI_37S201W, B19_017192_1443_XI_35S202W and B19_016981_1432_XN_36S201W).

end of valleys extending from the crater wall. For this reason, it is inferred that valleys on the crater wall were likely tributaries to the pit valleys despite their current lack of connectivity. 4.4. Crater 3 Crater 3 (Fig. 5) is 23 km in diameter. Valleys drain into the pit from the north, west, and south, with little to no valley development on the pit’s eastern side. Most of the pit valleys appear to connect to tributaries on the crater walls, with the exception of a valley southeast of the pit that originates in an area with unusual morphology and irregular topography. The pit valleys and other parts of the crater are superposed by an ice-rich mantling unit and thicker deposits of ice-rich crater fill is found along the crater walls, particularly to the north. No unambiguous sedimentary deposits from the pit valleys are observed within the pit. 4.5. Crater 4 Fig. 6. Pit crater 4 (crater is 28 km in diameter). Prominent valleys lead off the craters northern and southern wall, and lead to the deposition of sediment within the central pit. (CTX Image B17_016261_1615_XN_18S347W and B21_017751_1609_XN_19 S347W).

these main valleys has a fan-shaped deposit at its terminus (Fig. 9a). There are two smaller pit valleys that drain into the southern portion of the pit. The headwaters of the valleys are not clear, potentially because they are obscured by (possibly ice-rich) mantling material superposed on the crater’s floor, terraces, and walls. Valleys or furrows are also common on the crater walls as well as on the interior and exterior rim. 4.3. Crater 2 Crater 2 (Fig. 4) is 31 km in diameter. Three large valleys drain into the central pit from the northwestern quadrant of the crater floor, although two of these valleys are superposed and partially obscured by a younger impact crater. Sedimentary deposits from the valleys are apparent in the pit (Fig. 9b). The headwaters of the pit valleys are on the crater floor, but are situated near the

Crater 4 (Fig. 6) is 28 km in diameter and has several valleys that drain into its central pit, the most prominent of which is a large valley that enters the pit from the northeast. This valley drains most of the northern half of the crater; the two valleys that enter the crater from the south have smaller watersheds. A few additional valleys exist that are not as well preserved (e.g., entering the pit from the west and east). Channels are also preserved as inverted relief within the crater: for example, along the margins of alluvial fans on the northern crater wall (north of the central pit) (Fig. 6). A substantial sedimentary deposit is associated with the valley entering from the south, and smaller, more diffuse deposited are associated with the valleys entering the crater from the west and north, respectively (Fig. 9c). The CSFD for crater 4 (Fig. 8d) differs substantially in shape from isochrons. Inspection of the surroundings of this crater revealed that it is only 190 km from the rim of the 150-km-diameter Bakhuysen crater. Visual inspection suggests that many of the craters superposed on crater 4’s ejecta are Bakhuysen secondaries. The age of Bakhuysen itself is somewhat uncertain: it has wellpreserved facies and it has secondary craters superposed on

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Table 1 Table showing some of the basic measurable characteristics of the craters investigated. The craters that show ‘‘–’’ for fan ID and fan volume measurements are those where MOLA PEDR did not sample fans if present (no fans are observed in crater 3). None of the pits had sufficient topographic coverage to make volume measurements on more than one of its fans. The pit valley volume refers only to the valley which fed the particular pit fan whose volume we infer. A discussion of the uncertainties for these measurements is included in the Appendix A. We estimate that the pit volumes we infer have uncertainties of 20–30% due to the sparse sampling of the topography from MOLA. The estimated pit fan and pit valleys volumes are hard to measure because of their smaller scale and have an uncertainty of at least a factor of two due to sparse sampling of fan and valley cross-sections by topographic data. Crater #

Location (lat., lon.)

1 2 3 4 5

41.2N, 27.5N, 39.2N, 19.3N, 36.3N,

171.2E 12.2E 88.5E 12.9E 158.2E

Location of measured fan

Pit vol. (km3)

Pit fan vol. (km3)

Pit valley vol. (km3)

W – – NE NE

3.6 4.6 0.3 1.5 10.3

0.2–0.3 – – 0.3–0.4 0.3–0.4

0.05–0.1 – – 0.5–1.0 0.1–0.3

Fig. 8. Cumulative crater size–frequency distribution of the five example pit craters. The shape of the period boundaries are from the Neukum Production Function (Ivanov, 2001). Solid lines are Noachian/Hesperian and Hesperian/Amazonian boundary, and intermediate subperiod boundaries are dashed; all boundaries are as revised by Werner and Tanaka (2011); red line is the best-fit. All five craters appear to be Late Hesperian or Early Amazonian in age. Crater 4 has a higher than expected crater density which we attribute to secondaries from the nearby Bakhuysen crater.

surrounding terrains mapped as Hesperian-ridged plains, which lead us to interpret it as having an age of Hesperian or younger, although crater counts published by Tornabene et al. (2012) seemingly disagree and are more consistent with a Noachian age. Regardless, the age of crater 4 is constrained by stratigraphy to be older than Bakhuysen. 4.6. Crater 5 Crater 5 is 39 km in diameter (Fig. 7) and is the most spectacular of the pit craters we examined in detail. Numerous valleys are located both on the crater’s interior and exterior (surrounding area and neighboring craters). Alluvial fans are located at the base of its

walls and prominent sedimentary deposits are also observed at the termini of its pit valleys (Fig. 9d). Many of the pit valleys are already well-developed and nearly at their full width where they emerge from the toes of the alluvial fans. Prominent tributaries for the central pit enter from the northeast, south, and southwest. The central pit of crater 5 has a valley we interpret as an outlet valley that incised the rim of the central pit and debouched onto the northwestern area of the crater floor (Fig. 10). The geometry of this outlet valley is similar to numerous larger, older candidate open-basin paleo-lakes seen elsewhere on Mars (e.g., Fassett and Head, 2008b). The presence of this outlet valley requires that the central pit was once a lake that overtopped, breached, and drained into the northern area of the crater floor. The area where the outlet

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Fig. 9. Details of central pits and pit fans. (a) Poorly preserved/mantled sedimentary fans are visible in crater 1’s central pit from valleys leading into the pit from the west, south, and east. (CTX image B16_015952_1386_XI_41S188W). (b) Sedimentary fans are visible in crater 2’s central pit from valleys leading into the pit from the west. (CTX image B19_017185_1542_XN_25S012W). (c) Sedimentary fans are visible in crater 4’s central pit. Note the distinct morphology of the deposits from the west, south, and northeast (CTX image B21_017751_1609_XN_19S347W). (d) Sedimentary deposits in the central pit of crater 5; see also the topography of this area in Fig. 11. (CTX image B19_016981_1432_XN_36S201W).

valley terminates is a particularly low-lying portion of the crater floor that has a mottled texture distinct from other parts of the crater floor (Fig. 10b). We speculate that this texture difference may be attributable to the transient filling of this low-lying area on the outside of the pit with water, which then froze and underwent sublimation. Regardless of these details, however, to allow the outlet to form, the pit crater must have been a standing body of water, at least transiently, as it is unlikely that the topography of the crater floor changed during or after valley incision. To our knowledge, the lacustrine environment that existed within this crater is among the youngest proposed locations where this likely occurred on Mars. Another feature of note in Crater 5 is a small, 800-m-diameter crater located at the apex of an alluvial fan northeast of its central pit. This small crater must substantially post-date the pit crater that it is superposed upon; it also has valleys leading into it and an outlet valley that drains it (Fig. 11a). The existence of this small crater suggests that fluvial activity within crater 5 initiated well after the pit crater itself formed, was recurrent, or persisted for a long enough duration of time to form complex superposition relationships. 4.7. Valley characteristics In the valleys we focus on in detail, craters have both sinuous and linear planform patterns (see Kernitz, 1932; Howard, 1967). The drainage pattern for most of the valleys in the craters we examine is quasi-dendritic (Fig. 2), and some show valleys that have tight meanders (e.g., eastern valley in crater 1; Fig. 3), although more rectilinear valleys with few tributaries are also

observed (crater 4; Fig. 6). These rectilinear valleys may be structurally controlled; alternatively, they may be a result of headward erosion or knick point propagation extending away from the pit (e.g., Foster and Kelsey, 2012). Tectonism alone is not likely to be the sole forming mechanism for these valleys, because the pit fans found at their termini (Fig. 9) implies transport of material. Typical pit valley widths are 100–450 m and depths are 15– 20 m; typical widths of internal channels are 10–200 m. Crosssectional profiles were taken of the valleys on the pit crater floors using MOLA or stereo DTM data (whichever had the highest resolution for each individual crater). These profiles indicate that valleys have V-shaped cross-sections or are V-shaped with flat floors, which may reflect aeolian infilling or mass-wasting from the valley side slopes (see also Williams and Phillips, 2001). The topography of the craters appears to have dictated the drainage patterns of the valleys. For example, the presence of terraces influenced the drainage pattern in crater 5 (Fig. 7). Generally, tributaries from the crater walls run radially towards the center of the crater due to the crater’s interior slopes. There are several observations that suggest that valley formation and fluvial activity did not take place in a single phase within the craters we examine. For example, in crater 5, the alluvial fans along the walls appear to be at least partially superposed on the larger pit valleys on the crater floor (Fig. 7). It is unclear if the transition between activity being concentrated in the pit valleys to being concentrated on the alluvial fans was gradual or rapid, or if any hiatuses occurred. In other examples besides crater 5, the alluvial fans along the crater walls and pit valleys on the crater floor appear more likely to have formed contemporaneously. For example, in crater 4 (Fig. 6) distributaries on the alluvial fans connect

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Fig. 10. (a) MOLA topography of the crater 5 pit, with PEDR data overlain on the gridded DTM. The white box outlines the location of (b), and the profile A–A0 is shown in (c). (b) Image of the northern portion of crater 5’s floor (CTX image B19_016981_1432_XN_36S201W). (c) Topographic profile A–A0 of the crater 5 pit. These data imply that the valley in subpart b formed as a breach in the rim of the central pit on its northern margin. Because the valley we interpret as an outlet begins at a drainage divide where it is downhill both into the pit and towards the pit’s exterior, this valley cannot be an input to the pit. Note that where the outlet valley terminates in the low-lying areas on the crater floor in b, there is a distinct texture from the surroundings (less smooth). The existence of this outlet is strong evidence for the central pit having been filled to near the 400 m contour.

Fig. 11. (a) Image of alluvial fan distributaries on a bajada on the eastern wall of crater 5. In addition, a valley (large white arrow) is the outlet from a small <800 m crater at the apex of the fan. This valley diminishes in width and branches as it continues across the fan surface to the southwest. (b) Additional examples of distributary channels on an alluvial fan surface along the southwestern wall of crater 5. Arrows highlight the distributary channels in both images. All channels on the fan surfaces are <100 m wide. (CTX images B19_016981_1432_XN_36S201W and B10_013658_1434_XI_36S202W).

directly with the main pit valley within the crater. Thus, the erosional and depositional history within the craters we examined appears to have been dependent on local conditions, which implies that there was no single evolutionary path for the fluvial modification of pit craters after their formation.

Valleys are observed outside the craters for three of the five pit craters we studied in detail (craters 1, 3 and 5). The mechanism for forming valleys outside these pit craters is unclear, although they may have formed concurrently with the valleys on the pit crater interiors.

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4.8. Depositional features Alluvial fans have been identified in at least three out of the five craters along the base of the crater walls (e.g., Figs. 5–7 and 11). They are found both as stand-alone features and in regions where they merge together to form bajadas. The interpretation of these features as alluvial fans is based on their geologic settings and morphological characteristics (e.g., fan-shape; slope diminishing towards their toes; non-lobate nature; tributaries leading to them and distributaries on the surfaces of well-preserved examples; apices located at the termini of parent valleys) (see, e.g., Moore and Howard, 2005). The surfaces of some fans on the northern floor of crater 4 are dissected by valleys, while others are transected by high-standing ridges that are likely channels preserved as inverted relief. Some of the alluvial fans in the northeastern region of crater 4 are incised by wide valleys that may have had higher characteristic discharges than the activity that led to the initial deposition of the fans. Another set of sedimentary deposits (‘‘pit fans’’) are located at the termini of the valleys that flow along the crater floors and drain into the central pits (Fig. 9). The pit fans have average slopes of 2– 11° where they were measureable. Even within a given pit, variation in the morphology of these pit fans is observed (e.g., especially, Fig. 9c). For example in crater 4, the northeast pit fan has a gentle, 2° slope (in MOLA data), and extends across almost half of the pit floor, while the southern pit fan has a much more limited extent. It is unclear whether this difference is due to distinct depositional conditions or to differences in post-depositional modification of these features. Given the presence of these fan-shaped deposits at the end of many of the valleys, their morphology, and their location within a significant depression where water may have ponded, these features are interpreted to be probable deltas. In the case of crater 5, the existence of an outlet valley in the northern rim of the pit (Fig. 10) is direct evidence that water must have pounded at least to the level of the outlet, which provides additional evidence that the pit fans observed in that location are deltaic. In all instances the elevation of the fans in crater 5 are below what was once the minimum lake level. 4.9. Hydrology and mass balance of fluvial activity within the pit craters The morphometry of the valleys and pits can provide insight into the surface hydrology required for their formation. We estimate the flux of water through the valleys based on the channel dimensions using three different techniques: the Manning–Chezy equation corrected for martian gravity with roughness coefficient n = 0.0545 (Wilson et al., 2004), the Darcy-Weisbach equation for both gravel and sand corrected for Mars (Wilson et al., 2004), and an empirical relationship from Irwin et al. (2005) that connects formative discharge to channel widths. The derived discharges in the pit valleys range from 10 to 104 m3/s, which reflects both uncertainty in the reconstruction of formative discharge and the variation in size of the channels that fed the pits (Table 2). This range is based on likely channels widths where they were directly measurable and estimated channel widths on the basis of the valley size when they were unclear. Where the channel width was estimated, we assumed a channel to valley ratio of 0.14 (Penido et al., 2013). Given the watershed areas that fed the valleys, these discharges would imply runoff production rates of between 4 and 60 cm/day. Even the most minimal of these runoff production rates are hard to reconcile with our current understanding of the post-Noachian martian climate. These fluxes also would quickly lead to the flooding of the central pit. Using Irwin et al.’s (2005) relationship of channel width to discharge leads to the smallest discharges and the longest time to fill the central pits; however, this still predicts

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that filling of the pit could occur in a few martian years if only a single feeder channel was active, or as quickly as a few days if all channels were active. Substantial intermittency between events would obviously lengthen these inferred filling times, but these results suggest that flooding of the central pit would have happened relatively rapidly at even the lowest estimated flux. This is also consistent with the interpretation of the pit fan deposits as deltas and the existence of an outlet valley in crater 5’s central pit that requires that it did in fact overtop. The inferred discharges are also sufficient to infer that inflow to the pits would have overwhelmed loss mechanisms from groundwater or evaporation. The intensity of fluvial activity within the pit craters may also have varied substantially over time. A wide range of valley widths is observed within the craters, from small, filamentous channels on the surface of alluvial fans that are <100 m wide, to channels of up to hundreds of meters in width. The range of these channel widths, suggests formative discharges that were quite variable. Although both the eroded volumes of the valleys and the volume of corresponding fans are uncertain (see Appendix A), we find that they are generally of the same order of magnitude as each other (Table 1). This suggests that most of the transported sediment remains trapped in the central pit. The volume of the pits compared to either the volume of the fans or pit valleys also suggests that substantially more water was transported into the pits than sediment, which implies that the fluid was dilute (a high water to sediment ratio). For example, crater 5 must have filled to its outlet contour with water, which is a much greater volume than either pit valleys or the sediment deposited within the central pit. For this crater, this implies a minimum sediment-to-water ratio of at least 1:25 (Table 1).

5. Discussion: valley formation mechanisms and possible sources of water Two possible sources of water that lead to fluvial activity in these craters are springs (groundwater or hydrothermal activity) and precipitation (snow or rain). The valleys that lead into the pits in craters 1 and 5 extend back to the crater rim, which is the local drainage divide in each instance. Because the valleys are perched high on the crater rim, a spring source would require a perched and confined aquifer. No clear recharge mechanism exists at the high elevations of these crater rims, nor are tributary valleys restricted to a particular elevation or strata. This suggests that groundwater is unlikely to be the source of water for the observed valleys. Published models of post-impact hydrothermal activity (e.g., Abramov and Kring, 2005) also suggest that outflow is focused in the center of craters of this size, rather than near the walls. Additionally, in craters 1, 4 and 5, valleys extend around the entire circumference of the crater and are also observed on the craters’ exteriors. For this reason, precipitation or some other distributed source of water is the most likely formation mechanisms for the pit valleys, particularly in craters 1, 4, and 5. Precipitation may have been in the form of rain or snow, although if snow was the source for runoff, a mechanism for melting is also required. Rain-on-snow or rain-on-ice may also have helped enhance the discharge to the valleys (e.g., McCabe et al., 2007). Melting of ground ice alone seems less likely given that valleys do not begin at point sources (e.g., at the margin of a single glacial stand). There are no obvious trends in the orientation of the valleys or preferential orientations for their source regions, unlike what is observed for gullies, at least at latitudes less than 40° (Balme et al., 2006; Dickson et al., 2007). This slope orientation preference of gullies has been attributed to differential insolation conditions on pole- and equator-facing slopes which then controlled the accumulation and melting of ice.

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Table 2 Estimated formative discharges (in m3/s) for channels in the pit valleys using a variety of different techniques described in the text. As is clear from the spread of these values that for a given pit valley, the discharge is uncertain by at least an order of magnitude. In addition, these estimates are sensitive to slope and channel widths, which are challenging to measure. Nonetheless, the discharges that formed the observed valleys were likely appreciable. Crater #

Valley location (relative to pit)

Manning discharge

Darcy-Weisbach discharge (gravel)

Darcy-Weisbach discharge (sand)

Irwin et al. discharge

1

West East South (western of two) South (eastern branch)

2.1  103 1.0  103 0.7  103 0.4  103

2.7  103 1.2  103 0.9  103 0.6  103

1.6  103 0.8  103 0.6  103 0.4  103

0.2  103 0.1  103 0.08  103 0.05  103

2 3

Northwest (largest) South

4.0  103 0.4  103

5.0  103 0.5  103

3.0  103 0.3  103

0.2  103 0.08  103

4

Northeast South

24  103 5.2  103

30  103 6.7  103

16  103 3.8  103

0.4  103 0.2  103

5

Northeast (with fan) Southwest (largest)

2.1  103 2.5  103

2.7  103 3.2  103

1.6  103 1.9  103

0.2  103 0.2  103

One mechanism for providing water to these pit craters is if the impact that formed the pit crater itself mobilized ground ice or water and triggered valley formation (e.g., Segura et al., 2002, 2008; Toon et al., 2010). However, in at least crater 5, valley formation seems temporally separable from the pit crater formation itself (e.g., Fig. 11a). In this instance, if this scenario is to work, valley formation following impact must persist for a non-negligible period of time. In most other examples, there are no direct constraints on the length of time between crater formation and the initiation of valley activity, and it is hard to rule out a direct relationship between crater and valley formation. However, geologic relationships do not demand this relationship. It seems equally likely that fluvial activity is unrelated to the formation of the pit crater itself, except insomuch as the pit provided a place for the valleys to drain, established local baselevel, and helped influence the morphology of the associated depositional features. Finally, an alternative hypothesis is that the valleys and their associated deposits are not related to water at all, but instead are a result of fluid impact melt (see, e.g., Morris et al., 2010; Jones et al., 2011; Tornabene et al., 2012; Mouginis-Mark and Boyce, 2012). We discount this hypothesis for the pit craters’ valleys and fans because the morphologies produced by flowing impact melt process appear quite distinct (e.g., Mouginis-Mark et al., 2007) from what we observe here. 6. Conclusion In this study we describe the distribution and characteristics of central pit craters modified by valleys. The pit craters that show evidence for fluvial modification are found preferentially, but not exclusively, in the mid-latitudes of Mars. Crater counting results on the five pit craters we examine in detail imply that activity in these craters is Hesperian or Amazonian. Although the pit valleys have a wide range of morphology and incisional and depositional history, their scale requires substantial discharges (10–104 m3/s) that could rapidly fill the pit where they terminate. In at least one instance, there is strong evidence that this resulted in a lacustrine environment. This evidence for fluvial activity is interesting given the global climate generally thought to exist on Mars in post-Noachian times, both in the volume of water required and the likely extended period of activity that our observations suggest. Further research into the post-emplacement modification of these craters is planned. Acknowledgments We acknowledge the CTX, HRSC, and MOLA science and engineering teams for acquiring the data that made this work possible. Detailed and helpful comments by Nadine Barlow and an

anonymous reviewer improved this manuscript. We also thank Tim Goudge for helpful discussion, and Stuart Robbins and Brian Hynek for releasing their catalog and crater classifications. Finally, SEP’s participation in this study was supported by a grant from the Massachusetts Space Grant Consortium.

Appendix A. Calculation of fan and valley morphometry Fan volumes were estimated with three methods. The first estimate of fan volume was based on the approximation of the fan shape as an isosceles triangle with a constant assumed thickness, emplaced into a pit that was initially a cylinder with a flat floor and vertical walls. This leads to a calculation of the form Vpit fan = l  (w/2)  hmax, where Vpit fan is the volume of the fan, l is the maximum length of the fan from apex to toe, w is the maximum width of the fan and hmax is the maximum height of the fan. The second estimate used the same assumptions with the exception that the shape of the fan was assumed to be a sector of a circle rather than a triangle. The equation used for this method is Vpit fan = hmax  p  r2  h/360 where Vpit fan is the calculated volume of the pit fan, r equals the maximum length of the fan, h equals the angle at the apex of the fan formed by its two opposing sides, and hmax is equal to the maximum thickness of the fan. A similar method was used by Weitz et al. (2006). A third volume estimate for the fans was calculated as Vpit fan = A  hmax where Vpit fan is the calculated volume of the pit fan, A equals the measured area of the fan, and hmax is equal to the maximum thickness of the fan. Each of these calculations will likely lead to a modest overestimate of the fan volume because the fan thickness may be overestimated and the original pit geometry is unknown. The valley volumes were assessed assuming a triangular crosssection for the valleys. These valleys are most likely V-shaped valleys with variable amounts of infilling based on cross-section profiles taken across a number of examples (see also Williams and Phillips, 2001). The valleys were assumed to have a constant depth and width throughout their length. Because the extent of the drainage network for each valley system was determined based on clear connections between tributaries with the trunk valley, the full length of the valleys may be modestly underestimated. The abandoning of older channels over time, observational limitations such as shadows, and/or later geologic features such as alluvial fans obscuring upstream tributaries may account for the lack of connectivity. In the worst case scenario, valley length measurements are believed to be accurate to a factor of 5, but most of the calculations are good to a factor of 2, taking into account levels of erosion and shadow that obscure their visible extent, as well as the probability that the valleys feeding the alluvial fans were once tributaries of the pit valleys. Shadows in a number of the CTX images, which obscure the extent of the valleys upstream, are the main source of

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