Geologic Landforms and Chronostratigraphic History of Charon as Revealed by a Hemispheric Geologic Map

Abstract

Geologic mapping has been used for over 200 years as a technique to synthesize a complicated surface into a more simplified product, identifying similar types of surface features, and placing them into a relative stratigraphy. Geomorphologic mapping has applied those principles to other terrestrial bodies throughout the solar system and has formed an important product set to understand these surfaces, plan future exploration, and conduct different scientific endeavors. We created a geomorphologic map of the New Horizons encounter hemisphere of Pluto's binary companion, Charon. Ten primary geomorphologic unit categories were identified, covering approximately 35% of Charon's surface, and we used lower resolution data to speculate about other regions of Charon. Over 1,000 linear features were mapped, nearly 90% of them are tectonic in nature, and we use these to provide evidence of Charon being active in its past. Additionally, we placed the mapped features into a chronostratigraphic sequence, and we present a possible surface history for the body. The northern terrain typified by large crustal blocks is the oldest, having fractured early in Charon's history, and potentially similar blocks were submerged in a cryoflow of which the now solid surface of Vulcan Planitia is the remnant today.

Plain Language Summary

Planetary scientists will create geologic maps of a surface to perform different kinds of studies. Geologic maps identify features of different types and help to distill many different kinds of data into an easier to use format from which scientific investigations can be done. We have created such a map for Pluto's largest companion, Charon, imaged during the flyby of New Horizons in July 2015. This geologic map covers approximately 35% of the best‐imaged region of Charon, and we mapped 16 different geologic units, over 1,000 linear features, and a variety of albedo features. We developed a chronology that places these features in time‐order, and we provide potential interpretations of the different features that we mapped. We found that Charon has potentially one of the most convincing examples of possible large cryoflow(s) of any heretofore imaged body in the solar system.

1 Introduction

Charon, the smaller companion in the binary Pluto‐Charon system, is a 606.0 ± 3.0‐km‐radius body (Nimmo et al., 2017) that NASA's New Horizons spacecraft revealed to be geologically diverse when it flew by in July 2015. Numerous researchers have summarized some of its diverse landforms (e.g., Grundy, Binzel, et al., 2016; Moore et al., 2016; Stern et al., 2015) including its large, red polar deposit (Grundy, Cruikshank, et al., 2016); global tectonics system (Beyer et al., 2017); features of its equatorial plains (Beyer et al., 2019); crater population (Robbins et al., 2017); and craters with abrupt terminus ejecta (Robbins, Runyon, et al., 2018).

However, it remains for Charon's landforms to be placed within the context of a hemispheric geomorphologic map so they might be used to piece together Charon's chronostratigraphic history. We have now filled that gap. Importantly, this work seeks to describe Charon's current geologic surface features and provide plausible scenarios for their origin; this work does not seek to model the processes or potential interpretations, for much of those are described in companion work (the above references, especially Beyer et al., 2017, 2019). In section 2, we describe the data and methods used in constructing this geologic map. Section 3 discusses an overview of the body vis‐à‐vis map units (effectively, a Description of Map Units (DOMU) and Explanation of Map Symbols (EOMS)), and section 4 describes our interpretation of the units. Section 5 brings those together into a chronostratigraphy (Correlation of Map Units (COMU)). Section 6 uses that material to hypothesize about the nonencounter hemisphere of Charon, while section 7 summarizes our work and compares our findings with other bodies in the solar system.

Important Note: This work uses both formal and informal nomenclature, the latter not yet having been approved by the International Astronomical Union (IAU); see Figure 1. The New Horizons team intends to submit all or most of these preliminary names to the IAU for approval, but this process is lengthy and only a few names can be submitted at one time.

Figure 1

Formal and informal nomenclature for regions and features of Charon used in this work.

2 Data and Methods

2.1 Panchromatic Image Base

The panchromatic basemap (Schenk et al., 2018) was constructed using images from the LOng‐Range Reconnaissance Imager (LORRI; Cheng et al., 2008) and Multi‐spectral Visible Imaging Camera (MVIC; Howett et al., 2017; Reuter et al., 2008). The basemap is rendered at 300 m/pixel. Individual images were also used in some regions for mapping due to different viewing geometry, signal‐to‐noise, and, in some cases, higher resolution (see Figures 2 and 3). In cases where the controlled MVIC and LORRI did not match, if the feature was resolvable in both sets, images taken at lower emission angle were used; for example, MVIC was favored over LORRI near Charon's southern terminator. New Horizons returned coverage of ≈80% of Charon's surface because south of ≈30–40°S is currently in shadow and will not be completely illuminated until 2114 C.E. (Zangari, 2015). Charon rotates with a 6.8‐day period, and due to the fast 14 km/s flyby, only images obtained within a few hours of closest approach—and thus, only a single hemisphere—are appropriate for a geologic mapping effort, hereafter referred to as the “encounter hemisphere”.

Figure 2

Mollweide projection of data used in construction and interpretation of the geologic map. Boundaries of the geologic map are regions not masked by red hashing. (a) Panchromatic basemap from LOng‐Range Reconnaissance Imager (LORRI) and Multi‐spectral Visible Imaging Camera (MVIC), (b) color basemap from MVIC, and (c) topographic map from LORRI and MVIC pairs draped over (a) (all basemaps: Schenk et al., 2018).

Figure 3

Parameters of the best images used in construction of the panchromatic basemaps. Projection is simple cylindrical. Southern boundary is ideal, and different images were cropped slightly differently for Figure 2's basemaps such that they do not exactly match. (a) Solar incidence angle, (b) pixel scale, and (c) emission angle.

2.2 Color Images

The MVIC instrument can take both panchromatic and four‐color data (blue, red, near‐IR, and CH₄). The best MVIC color mosaic was obtained at 650 m/pixel and used for mapping as an ancillary data product.

2.3 Derived Composition Data

Data from the Linear Etalon Imaging Spectral Array (LEISA), a short‐wavelength IR spectral imager (Reuter et al., 2008), were processed to produce mineralogy maps (Dalle Ore et al., 2018; Grundy, Binzel et al., 2016), which were used for mapping as an ancillary data product.

2.4 Derived Topographic Data

A hemispheric digital elevation model (DEM) was derived from several different combinations of LORRI and MVIC stereopairs to provide elevation data in the mapping region (Schenk et al., 2018). The DEM has a vertical accuracy of anywhere from 0.1 to 1.4 km, depending on the pixel scale and convergence angle of the stereopairs used (Cook et al., 1996), but the relative accuracy of any one pixel to the next is better than this limit.

2.5 Mapping Methods

Standard geomorphologic methods were used as described in the United States Geologic Mappers' guide (Skinner et al., 2018). After a final control network was created from which basemaps and topography were derived (Schenk et al., 2018), the final geomorphologic map was drafted, which is the product shown in Figures 4 and 5; much higher resolution versions of these figures are available as supporting information. Early in the mapping process it was determined that a cohesive, encounter hemisphere map product would be difficult if not impossible to construct due to the limited nature of the New Horizons data: The only useful data for mapping were returned at, effectively, a single moment in time during Charon's day such that the lighting angle went from 0° (noon sun) to below 90° (terminator). Geomorphologic mapping is best done at sun angles near to or lower than 45°, and the transition between >45° and <45° sun angle approximately coincides with the boundary between the two primary physiographic provinces on Charon: Oz Terra and Vulcan Planitia. This transition both in lighting and province makes matching what may be similar geologic units in each section uncertain, so, with very few exceptions, the only common units between the two are impact craters.

Figure 4

Geomorphologic unit map of Charon's encounter hemisphere in (a) cropped Molleweide and (b) polar stereographic cropped to the map region and north of −10°N. Parenthetical numbers in the legend indicate number of units or linear features of each type. See section 3.1 and Table 2 for a description of the map units. Examples of some units are labeled in (a). See the supporting information for a higher resolution version of this figure with more labels.

Figure 5

Albedo‐based map of Charon's encounter hemisphere, same layout as Figure 4. See section 3.3 for a description of the features. See supporting information for a higher resolution version of this figure.

We have mitigated this issue by creating three types of geomorphologic maps: geologic unit map (Figure 4), which is most complete for Vulcan Planitia due to the lower Sun; linear features map (Figure 4), which is also most complete for Vulcan Planitia; and albedo patterns map (Figure 5), which is most complete for Oz Terra due to the higher Sun. The synergy between these maps forms the basis of the analysis and discussion in later sections.

3 Mapping Investigation

This Charon geomorphologic mapping investigation covers approximately 35% of Charon's disk. The northern border of the unmapped region is as far north as +67°N and as far south as −37°N. The southern margin is based on the terminator at the time of closest approach, and the eastern and western map boundary is based on Charon's limb at the time of closest approach, minus a margin due to the high emission angle. This section is separated by the three primary types of mapping we performed: Geomorphologic units, linear features, and albedo features. See Table 1 for scale‐based limits on unit and feature sizes.

Table 1. Map Scale and Relevant Scale Information

Category	Map information
Final Map Scale	1:3,000,000
Mapping Scale	1:1,000,000
Smallest Mapped Feature	3 km (1 mm on printed map)
Vertex Spacing	1 km (0.3 mm on printed map)
Minimum Crater Mapped	15 km diameter (5 mm on printed map)
Minimum Linear Feature Mapped	9 km length (3 mm on printed map)
Minimum Area of Mons/Outcrop Mapped	30 km^2 (~6 km across if circular; ~2 mm on map)
Minimum Area of Albedo Feature Mapped	50 km^2 (~8 km across if circular; ~3 mm on map)a^a Four exceptions: Dark‐toned ejecta mapped within light‐toned ejecta were <50 km^2.

• ^a Four exceptions: Dark‐toned ejecta mapped within light‐toned ejecta were <50 km².

3.1 Overview of Charon Geomorphologic Units

We identified 10 primary geologic units, subdivided to form a total of 16 units, and these are shown in Figure 4, and a brief DOMU table is included as Table 2. Figure 6 contains numerous higher resolution examples of several of the units discussed in this and the next sections, and superposition relationships in support of section 5.

Table 2. Brief Description of Map Units (DOMU) With Areas to 10³‐km² Precision

Name	Symbol	No. of features	Area (10^3 km^2)	Stratigraphic position	Brief interpretation
Map Area		1	1,598	N/A	N/A
Blocky Terrain	Bl	1	997	Ozian	original crust material that has been fractured due to planetary expansion
Smooth Terrain	Sm1,2	2	306	Ozian/Vulcanian	current expression of a large‐area cryoflow that has solidified
Rough Terrain	Rt	1	39	Ozian/Vulcanian	see section 5 for speculation
Smooth Terrain, Elevated	Sme	2	1	Vulcanian/Spockian	unknown; potentially constructional from upwelling
Mottled Terrain	Mt	26	78	Vulcanian/Spockian	see section 4.6
Lobate Apron	La1,2	13	4	Spockian	landslides, likely from general material failure and mass wasting (La1) or caused by impact craters (La2)
Mons	Moa,b,c	55	18	Ozian/Vulcanian	various stages of Bl material sinking in Sm material, or preexisting high‐elevation features embayed by Sm
Depressed Material	Dm1,2,3	8	7	Vulcanian	Sm cryoflow which froze before it reached an equipotential surface, or unknown
Crater	Cr	127	128	Ozian/Vulcanian/Spockian	exogenic impact crater
Crater Ejecta	Ej	12	23	Vulcanian/Spockian	ejecta from exogenic impact craters

Figure 6

Several regions of interest discussed in the text; colors correspond to units in Figure 4. Parenthetical question marks indicate uncertainty in interpretation. (a) Mt texture superposed on many different features, indicating that it is young at least in some areas. (b) Craters and linear features indicating extended formation period: Large crater (far left) has ejecta that appears to superpose grooves, but the ejecta itself is superposed by a smaller crater. Another groove might crosscut that crater's ejecta. Other craters are clearly superposed on other grooves. (c) Catena. (d) Dm₃. (e) One of the few degraded craters, next to a similarly sized crater with a central peak (left of it). (f) Degraded crater with large tectonic disruption. (g) Focus on Serenity Chasma showing several different features discussed in the text. (h) Focus north of Clarke Montes showing several different features discussed in the text.

3.1.1 Blocky Terrain (Bl)

Most mapping in the northern region of Charon was based on linear and albedo features visible in topography and panchromatic imaging. The vast majority of this province was mapped as a Blocky Terrain (Bl) unit. The primary characteristic is at least 10 large (hundreds of kilometers across), high‐standing plateaus separated by scarps with troughs tens of kilometers wide between them (Figure 2c). The topography indicates that many of the blocks have raised rims. Bl is bounded by the map boundary on all sides except to the south, which is characterized by a large series of scarps and ridges, which are included in the Bl unit based on our interpretation of how it formed (section 5).

Mapping the entire region as Bl diverges from precedent set by the published Ganymede geologic map (Collins et al., 2013) and the in‐progress Europa map (Senske et al., 2018). The former has a grooved material unit with three subdivisions, and the latter has band material of two different types. The decision to simplify this for Charon was made for two reasons: First, the available data in Oz do not support a full mapping of individual troughs, and they were mainly identified using topography instead of visible imagery; second, unlike both Ganymede and Europa, where the grooves and ridges are specifically textural with some relatively minor elevation differences, the large blocks on Charon are very distinct topographic features with troughs up to several kilometers deep.

3.1.2 Vulcan Planitia Smooth Terrain (Sm₁, Sm₂, and Sme)

Vulcan Planitia is a province that is mostly composed of a single geologic unit. At approximately tens to hundreds of kilometer scales, it is significantly smoother than Bl, and it is lower by ~1 km in elevation relative to Bl (Schenk et al., 2018). It was imaged at similar lighting throughout the region, spanning incidence angles ~60° through >90° (terminator). The terrain also appears relatively smooth compared with cratered surfaces elsewhere in the solar system so is designated a Smooth Terrain unit (Sm₁). The DEM indicates many broad (hundreds of kilometers) and gently sloping swells. There is an additional region mapped as Sm₂ southeast of Ripley crater. Sm₂ extends to the north of the fractures that roughly cross near Charon's equator. Sm₂ was identified due to its significantly more gradual margins relative to other blocks in Oz Terra, the absence of marginal ridges, the absence of throughgoing tectonics, and it looks compositionally distinct from Bl due to the presence of NH₃ and the size of H₂O grains (Dalle Ore et al., 2018). Therefore, it might be an extension of Vulcan Planitia despite being topographically higher, and it was designated Sm₂ instead of Bl₂. In addition to these background units, two small regions within Vulcan Planitia, up to 30 km across, were mapped as Smooth Terrain Elevated (Sme). Both have distinct margins that visually appear to elevate them slightly above the surrounding terrain, though the available DEM can only distinctly show that the southeastern of the pair is vertically elevated. Additionally, the southeastern one has a distinct, smoother texture than the tectonically disrupted surrounding material.

3.1.3 Rough Terrain (Rt)

Near the eastern edge of the map region, spanning from ~7° to ~34°N is a Rough Terrain (Rt) unit. On the scale of approximately tens of kilometers, it is qualitatively the roughest terrain in the map area (Figure 2). The region is bound to the north by a scarp; to the west by montes, scarps, and impact craters; and to the south and east by the terminator. It is characterized by large ridges and blocks at several scales, undulating topography at a variety of wavelengths, and impact craters. It is elevated relative to the material to its north.

3.1.4 Mottled Terrain (Mt)

Irregularly shaped regions within Vulcan Planitia show a textured surface with mottling at the approximately hundreds of meter scale (Figures 6a, 6b, and 6d), mapped as Mottled Terrain (Mt). In some locations it has a distinct boundary, but in others it is approximate, appearing to grade into the surrounding smoother surface (or vice versa). The scale varies across the planitia from as large as ~1 km to at least as small as ~500 m (a few pixels in the best images).

3.1.5 Lobate Apron (La₁ and La₂)

Charon shows numerous lobate aprons draped across large topographic slopes, and they are mostly confined to Serenity Chasma (Figure 6g). Those that emanate from scarps form the La₁ unit, and those with up‐slope points immediately adjacent to a crater rim form the La₂ unit. The La_1,2 units are smooth at available pixel scales. No La units are within impact craters.

3.1.6 Mons (Mo_a, Mo_b, and Mo_c)

Numerous montes are on Charon, identified as high‐standing, short‐wavelength features that rise prominently above the surrounding surface. We take the rare step of using alphabetic subscripts instead of numerical to emphasize that these may not be sequential in evolution. Almost all were mapped in Vulcan Planitia, and it is possible that this is not a bias in the data based on the reconstructed history (section 5). Montes were divided into three subunits based on interpretation (section 4.8): Mo_a indicates a general mons or montes cluster, Mo_b indicates mons or montes within a Dm₂ unit (Figure 6h), and Mo_c indicates the single, large block that forms the southern rim of Serenity Chasma (Figure 6g). (Why Mo_c is mapped separately from Bl is based on interpretation; see sections 4.8 and 5.) There is no characteristic size to these features, but they follow a power law distribution with differential slope of −2.0 ± 0.2.

3.1.7 Depressed Material (Dm₁, Dm₂, and Dm₃)

Charon has several areas of deep (generally >1 km), non‐impact‐crater‐related depressions across its surface (Figures 6a, 6d, and 6h). Seven were mapped in Vulcan Planitia and one in the Rt unit. Subscripts were used to indicate if they are very deep in the center (Dm₁), whether they contain a “mountain in a moat” (Dm₂; discussed in more detail in sections 4.8 and 4.9) or form a somewhat shallower, circular depression (Dm₃), of which there is one example resolved at the map scale (described in more detail in section 4.3). While Oz Terra contains numerous regions that are topographically low, the lack of sufficiently low sun angle images led us to exclude these as separate units and instead either indicate them with scarp crests or, if they appeared enclosed, as a depression margin linear feature.

3.1.8 Impact Craters (Cr)

Impact craters pockmark the surface of Charon. A thorough discussion of these features is found in Robbins et al. (2017). We used that database as a starting point and remapped all distinct impact craters with diameters D ≥ 15 km that form the “Cr” unit. The largest crater is Dorothy Gale, D ≈ 220 km.

3.1.9 Impact Crater Ejecta (Ej)

Charonian impact craters display ejecta mapped as both geologic units and as albedo features. As a geologic unit, a single Crater Ejecta (Ej) unit type was mapped. See sections 3.3.1 and 3.3.2 for a discussion of the albedo features.

3.2 Overview of Charon Linear Features

Nine types of linear features were mapped and are shown in Figure 4.

3.2.1 Crest of Crater Rim

Charon's craters' rims are included in the geologic units map but are also bound by the crater rim crest linear feature symbol. One exception is an apparent crater rim that superposes the heavily tectonized southern margin of Oz, but it is so disrupted that it could not be designated as a specific Cr subunit (Figure 6f).

3.2.2 Depression Margin

Depression margins where there was favorable lighting (Sm₁ and Rt) are included both as linear features and as geologic units, but where there was not favorable lighting (Bl), they were only included as linear features. This distinction was mostly used for features that appeared to be self‐contained and somewhat irregular in shape. Topography data were relied on almost exclusively for their identification, and due to the noise level of the topography in Oz Terra, only larger features could be reliably identified (perimeters ranged 171–30,200 km; median 1,601 km). Additionally, several depression margin linear features were demarked interior and parallel to the western and northern boundary of Vulcan Planitia, where there is a lower elevation region several kilometers wide.

3.2.3 Groove

Hundreds of tectonic features cover the map area, and many of them are small, quasi‐linear troughs that were mapped as a generic groove feature. These features span lengths smaller than the map scale to as large as nearly 220 km. The grooves are at least as thin as ~3 pixels (155 m/pixel) in the best pixel scale images, and they are up to a few kilometers across. As is typical with planetary landforms, their distribution in length and width is roughly Pareto‐distributed, with many more smaller, thinner features. There is no overwhelming preferential direction to these features (discussed in Beyer et al., 2017, 2019), but there is almost certainly a bias to detect them parallel to the local sun direction (e.g., Watters et al., 2009).

3.2.4 Scarp Crest

Scarp crests were mapped for quasi‐linear topographic highs that were bordered by the high on one side and a low on the other. These were sometimes paired with scarp bases where such bases were distinct. Similar to grooves, the scarp crests form a Pareto distribution in length and were mapped from the minimum scale length up to 2,412 km. The extreme topography of Charon is reflected in these scarps that tend to be >1 km high.

3.2.5 Scarp Base

Scarp bases were mapped for quasi‐linear topographic lows that were bordered by the low on one side and an upslope to a high on the other. The position of bases is somewhat uncertain due to the non‐nadir pointing and parallax issues of the imagery used to map them. Scarp bases were always mapped with a scarp crest to form a pair, though the lengths of these were more uniformly distributed than scarp crests. This was in part because a single scarp crest could be matched to multiple scarp bases due to disruption of the downslope material.

3.2.6 Graben Trace

Graben traces were used between pairs of scarp crests to indicate multikilometer‐wide grabens between larger, topographically high features in the Bl unit. These sets sometimes included scarp bases to form a set of up to five linear features. Grabens were typically marked for features that were several kilometers wide.

3.2.7 Ridge Crest

Ridge crests were mapped for quasi‐linear topographic highs that had downslopes perpendicular to the crest in both directions. These are distributed in sizes more similarly to scarp bases and are as small as the map scale to as long as 85 km.

3.2.8 Catena

One catena was identified in the mapping area (Figure 6c), manifest as a long, linear topographic trough with craters throughout it (see section 4.1).

3.2.9 Broad Warp

The “broad warp” linear feature was used in Vulcan Planitia on the crests of numerous smooth, undulating linear and arcuate warps on the scale of tens of kilometers (Figure 6a). Vulcan Planitia has several other, much longer wavelength undulations (hundreds of kilometers) that were not denoted in the map. The warps were always associated with the Dm units or wide grooves.

3.3 Overview of Charon Albedo Features

Albedo features were identified across the mapping area based on panchromatic imaging. However, due to low sun in Vulcan Planitia, except in the case of much lower resolution approach imagery, albedo features are mostly limited to the Oz Terra region. Five types of albedo features were mapped and are shown in Figure 5.

3.3.1 Dark‐Toned Ejecta

Charon has many impact craters that display a dark ejecta deposit that extends from the edge of the rim. If mapped, this was always paired with light‐toned ejecta (Figure 6g).

3.3.2 Light‐Toned Ejecta

Charon has many impact craters that display a light ejecta deposit (Figure 6g). This deposit was often found extending from the edge of a dark ejecta deposit, but it was occasionally seen extending approximately from the edge of the crater rim without dark ejecta. The absence of a dark inner component exists in Sm₁, but the absence in Bl could be a resolution effect.

3.3.3 Dark‐Toned Halo

The dark‐toned halo unit was used only once in the case of a crater where dark material was seen to extend beyond the light ejecta of a crater; the dark material was approximately centered on the crater, in contrast with the dark‐toned mantling unit described below.

3.3.4 Light‐Toned Halo

The light‐toned halo was used for material that was lighter than the surrounding surface and which may or may not be impact crater‐related. By area, it was mostly used in Sm₁ and did not correlate with the locations of impact craters.

3.3.5 Dark‐Toned Mantling

Dark‐toned mantling was used to define regions of dark material that had no apparent link to any crater or other topographic feature expression. These occurred across the mapping area, the most prominent examples being Mordor and Gallifrey Maculae (see section 4.5).

4 Unit Interpretation and Discussion of Interesting Features

This section begins with several self‐contained interpretations of smaller features and units, and then it ends with a discussion of some of the larger features that are used to build a chronostratigraphy in the next section. This section specifically does not address broad regional interpretations of Oz Terra and Vulcan Planitia; those are addressed in the next section on chronostratigraphy and in more detail in Beyer et al. (2017, 2019).

4.1 Catena

The one observed catena (Figure 6c) is ≈72 km long and averages 2–3 km wide, though the width is variable and appears as several craters linked together. A possible explanation is it is a secondary impact crater chain. However, we consider that unlikely because there are no similar features observed (and singular secondary crater chains have never been observed elsewhere in the solar system); there is no obvious source primary crater; and it is extraordinarily linear, unlike many secondary crater chains on other bodies in the solar system. A second possible exogenic explanation is an impact from a body that broke up in orbit or en route, impacting the surface. This is frequently seen on moons of Jupiter, especially Callisto (e.g., Melosh & Schenk, 1993), but we consider this explanation similarly unlikely because of its unique nature over the ~35% of Charon mapped and the significantly less gravity of the Pluto‐Charon system. A third possible explanation is endogenic, that it is a collapse structure (e.g., Wyrick et al., 2004). A typical scenario would be a volcanic lava tube that was evacuated, and the crust collapsed to form a pit crater chain, but there is no obvious volcanic construct anywhere in the vicinity (though hectometer‐scale pixels could easily mask any such feature), and it is as‐yet unknown whether pit craters can happen with cryovolcanism. An alternative endogenic explanation is that the catena formed over a tectonic fracture, and given the hundreds of tectonic features mapped, this is not unreasonable. Therefore, at this time, we consider an endogenic explanation to be most likely.

4.2 Lobate Apron (La₁ and La₂)

Thirteen distinct lobate aprons were mapped on Charon, though several smaller ones were visible but below the minimum size for map inclusion (Table 1). Two subgroups are based on different interpretations. The more numerous group is La₁, and those are interpreted as common mass wasting, typical of landslides observed on many bodies throughout the solar system. Preliminary investigation of the features by Beddingfield et al. (2018) and some follow‐up work indicates that the runout length versus drop heights is large compared to landslides seen on Callisto and Rhea and some landslides seen on Earth and Mars. But, they are comparable to some terrestrial and Martian debris flows, rock avalanches, and pyroclastic flows, and they are similar to some landslides observed on Ceres (Schmidt et al., 2017), Iapetus (Singer et al., 2012), and Tethys, with relatively low friction that is consistent with cold water‐ice near the base of the landslide material at Charonian strain rates and overburden pressures (e.g., Schulson & Fortt, 2012). The mineralogy analyses in Dalle Ore et al. (2018) neither indicate nor preclude other mineralogy present, though this does not rule out buried volatiles that could have contributed to the flow. Unit La₂ has four members that we interpret as landslides caused by an impact crater. It is notable that unlike on some icy bodies (e.g., Iapetus (Singer et al., 2012), Ceres (Schmidt et al., 2017), and other bodies listed above), there are no distinct lobate debris aprons from crater walls onto the crater floors.

4.3 Craters (Cr), Crater Ejecta (Ej), and a Possible Buried Crater (Dm₃)

Detailed discussion about Charon's crater population is found in Robbins et al. (2017), and interpretation of the ejecta units is in Robbins, Runyon, et al. (2018). Charon's craters span diameters from as large as ≈220 km (Dorothy Gale) to as small as could be resolved (approximately hundreds of meters). Due to the available images and an absence of many modified craters, the Cr unit is not subdivided into preservation classes, as is often done in geologic mapping. Large craters have been used to demonstrate that Oz Terra is older than Vulcan Planitia (Moore et al., 2016; Robbins et al., 2017) and so they can be used to help develop a chronostratigraphy (section 5).

The continuous ejecta surrounding several of the craters appears to have a distinct, well‐defined boundary, dissimilar from typical lunar ejecta, and these were determined to be similar to other ejecta observed on approximately eight other solar system bodies (Robbins, Runyon, et al., 2018). Robbins, Runyon, et al. (2018) interpreted this to mean that the subsurface contains volatile material that helped lubricate a ground‐hugging flow that formed this morphologic type of continuous ejecta on Charon.

The Dm₃ unit has a single example, centered at −12°N, −1°E (Figure 6d). It is a roughly 14 × 20 km ellipsoidal depression that is distinct from all other depressed material in the map. Its formation is uncertain, but it is the most likely candidate observed for a buried or extremely modified impact crater. Otherwise, many impact craters on Charon retain a well‐preserved morphology indicated by relatively sharp (at available pixel scales) rims and some preserved ejecta. The interiors of many of the larger craters D > 50 km (e.g., Sundiata, Alice) have hummocky floors, similar to what is generally agreed to represent pristine crater morphology on icy Saturnian satellites, so it is likely original morphology. The best example of a smaller modified crater is ≈26 km across at 36°N, 24°E, and it is the only clear example of a heavily modified crater that is typical of such features seen on most other solar system bodies: It is large enough that it should be a complex crater (Robbins, Watters, et al., 2018; Schenk et al., 2018), and it is adjacent to a similarly sized crater with a central peak (Figure 6e). However, it is significantly shallower than a crater should be for that size, has a smooth floor, numerous multikilometer superposed craters, and some of the rim appears to be flush with the surrounding surface. The only clear examples of large craters disrupted by tectonics occur on the large tectonic southern margin of Bl, such as the ≈35 km diameter Rosencrantz crater centered at 27°N, 20°E. It shows potential disruption from the tectonized terrain, as does the crater mentioned earlier that could only be mapped as a linear feature due to tectonic disruption (Figure 6f).

4.4 Dark and Light Crater Ejecta

Dark and light ejecta deposits were mapped as albedo features rather than separate geomorphologic units (Figure 5). Of ejecta that had a distinct albedo, only light deposits were observed in Vulcan Planitia, while dark and light deposits from the same craters were observed in Oz Terra. In craters with both the light and dark deposits, the ratio of the extent of bright ejecta versus the dark ejecta from the crater center was 3.3 ± 1.2. In Oz Terra, dark ejecta were observed around craters with a large range of diameters, so this is interpreted to be endogenic—that is, impactor velocities at Charon are so low (~1–2 km) that material from the impactor could be incorporated into the ejecta, but there is no reason why this should be limited to a single geologic region. Therefore, assuming that it is endogenic, the basic premise of inverted stratigraphy in crater ejecta would imply a layering of light material overtop dark material throughout Oz Terra, but that layering is not present to at least the excavation depth of D ~ 50 km craters in Vulcan Planitia.

Spatially, the light and dark deposits are randomly distributed throughout Oz Terra, showing no preference for any particular latitude or longitude band or region. The dark ejecta is found around craters at least as small as 3 km across, indicating that craters at least as deep as is typical for a D = 3 km crater (≈0.4 km) are excavating to the dark layer (Robbins, Watters, et al., 2018; Schenk et al., 2018). Several of the larger light ejecta correlate well with localized larger band depths corresponding to 1.5 and 2.0 μm water‐ice features observed with LEISA, and some—especially Nasreddin's ejecta—correlate with the 2.01 and 2.21 μm NH₃·H₂O bands (Dalle Ore et al., 2018), suggesting that these craters are bringing fresh ices to the surface. The spectra also indicate larger grain sizes, typical of ejecta, and seen on Saturnian satellites (Stephan et al., 2012).

4.5 Dark and Light Halos, Dark Mantling

In the near‐IR (LEISA wavelengths), Charon's surface is remarkably homogeneous, the majority of terrains appearing very similar. Therefore, those data can only be used to make very broad statements, such as the above about water‐ice generally correlating with crater ejecta. The larger albedo and color features identified in much broader band‐pass MVIC data did not tend to correlate with any specific features in LEISA data.

One exception is large dark mantling deposits, Mordor and Gallifrey maculae, which appear red and appear similar to each other in LEISA data, and may represent the same veneer of material. Mordor is the largest observed dark deposit on Charon, and Gallifrey the second‐largest on the encounter hemisphere. However, they might have different deposition mechanisms, for Grundy, Cruikshank, et al. (2016) found that Mordor could form from seasonally cold‐trapped volatiles; given that Gallifrey is equatorial, the same argument could not be made. Further hindering a straightforward interpretation, in LEISA spectra, deposits coincident with Ripley crater—which are just dark and not red—are similar to both Mordor and Gallifrey.

Finally, there are some additional, broad correlations between LEISA spectral clusters and the dark and light deposits on Charon (Figure 3 and Table 3 from Dalle Ore et al., 2018), though these differences are subtle, roughly 10 ± 4%. The light areas (regardless of being ejecta or halos) tend to correlate with a spectral cluster that is rich in NH₃ and characterized by larger H₂O ice grains. Darker deposits tended to have a redder spectral slope, smaller grains, little NH₃, and more amorphous H₂O ice that could be an indication of an older surface (though the chronostratigraphy developed in section 5 is not fine enough to verify or refute this).

4.6 Vulcan Planitia's Smooth (Sm₁) Versus Mottled Terrain (Mt)

The distinction between Sm and Mt in Vulcan Planitia was perhaps the most uncertain in our mapping, for all image parameters in Figure 3 factored into whether these two units could be distinguished or not. We do not know if it is a coincidence that the vast majority of Mt is centered in the middle of Vulcan Planitia because that is its only location or because that is where imaging was best suited to identify it. Therefore, we restrict our interpretation to not make assumptions that require either to be true. In describing the unit, we also do not know whether the mottled terrain is a positive or negative relief feature: In the closest approach LORRI sequence that provided images at up to 155 m/pixel, it appears to be a positive relief feature; however, whether that is true and whether it would hold for all areas mapped as Mt is unknown (it could be an optical illusion, and/or it could be coated with bright or dark material on one side). The stereogrammetry‐based topography data are neither high enough resolution nor accurate enough to assist in this distinction. While preliminary photoclinometric models (unpublished) indicate on the order of ~50–100 m relief of the features, it cannot distinguish whether the pits or peaks are at the level of the surrounding terrain, and elevation histograms of the surrounding terrain versus Mt are indistinguishable. Interpretation is also difficult because the margins of the unit are not always distinct. In some cases, there appears to be an obvious border, but in others the pattern grades into the general background Sm₁ unit of Vulcan Planitia (Figures 6b, 6d, and 6h). However, this could be a feature of the unit that should factor into any formation explanation. Additionally, the scale of the mottled terrain varies from just a few pixels in the highest resolution imaging to many pixels at lower image scales, thus spanning at least hundreds to thousands of meters, with no apparent pattern in scale versus location nor proximity to other physiographic units. Finally, the largest light‐toned halo units in Sm₁ are approximately anticorrelated with Mt, where the largest two light‐toned halos' southern borders approximately coincide with the northern border of the largest Mt unit that extends north from the encounter terminator.

There are no distinct stratigraphic relations between Mt and other units, with two possible exceptions. First, Mt does not occur in any Dm₂ nor Mo_a,b,c units (e.g., Figure 6h). Second, the Mt texture is possibly seen to carry onto several of the Ej units (Figure 6a), though this is ambiguous. If it does overprint some Ej, then at least some of its formation postdates crater ejecta formation, which also must postdate the formation of Sm₁. Impact crater statistics are not precise enough to give a relative age in specific locations in Vulcan Planitia, but Robbins et al. (2017) noted that the N(5 km) crater spatial density does vary in Vulcan Planitia: The lower crater spatial density areas tend to correlate with both Mt and tectonic features. Additionally, if albedo features are assumed to be young (a not uncommon assumption), one might infer the Mt formed recently, after the light‐toned halos, thus erasing the albedo difference. These findings together reinforce each other and suggest that at least some areas of Mt postdate Sm₁ by a nontrivial time margin, meaning that Mt is either younger material or it formed from Sm₁ that was modified after emplacement.

We have several potential explanations for this unit, but they are speculation at this point. They lack rigorous testing, and some lack data fidelity to reliably test them; it is beyond the scope of this work to perform detailed tests of these different ideas so we leave that to future efforts. One possibility, if they are topographic highs, is they are constructional, such as either a vast series of small eruptions of material or some sort of patchy, folded, or crumpled ductile layer over a softer layer (e.g., a flow texture); we consider these unlikely because there is no good analog for this in silicate volcanism and cryovolcansim is even more poorly understood (former), and it would require some sort of compressive force across the area that we have no evidence for (latter). Related, it does not fit the possible diapiric model of cantaloupe terrain on Triton due to that terrain's larger cell structure (Schenk & Jackson, 1993). Another possibility is that they are postemplacement features formed from a long‐timescale release of volatiles. We consider this unlikely because no outgassing has been detected on Charon (Gladstone et al., 2016) and the surface ices should be stable at current temperatures over the lifetime of the solar system (Fray & Schmitt, 2009). A related idea is that they represent release of volatiles coincident with or very soon after the emplacement of Vulcan Planitia itself, rather than after: Volatiles trapped in a cryoflow (see later sections) may have been at a high enough temperature to escape the material, leaving behind a cavity that we see as the mottled terrain today. Of these four possibilities, we consider this the most likely.

4.7 Tectonic Features

Figure 4 shows that the vast majority of tectonic features by number were mapped in Vulcan Planitia, as grooves. These grooves are broadly parallel to the nearest boundary of Vulcan Planitia, indicating a response to large, regional stress fields, but there are many exceptions, including a long groove that trends north‐south centered at ≈−8°E, almost in the middle of Vulcan Planitia. These exceptions indicate a likely response to more local stress fields: For example, grooves are concentric to the Dm₂ unit around Clarke Montes to the west (including the afore‐mentioned long groove at ≈−8°E), north, and east (Figure 4).

The number of tectonic features mapped in Vulcan Planitia is misleading with respect to Charon's tectonic past (Beyer et al., 2017): The length and vertical scale of the tectonic features mapped through Oz Terra dwarf those in Vulcan Planitia. This is also reflected in the second‐most‐common linear feature mapped, which comprised nearly 200 scarp crests, sometimes paired with a base (bases were more difficult to map in the available data and have gaps where there was no distinct, continuous base). In contrast with grooves, no scarps were mapped in Vulcan Planitia; this is another way that the Rt unit differs from Sm₁, for we mapped several scarps within Rt. The majority of scarps were mapped on and near the tectonically disrupted southern margin of Oz Terra. It is unclear from the lighting whether this margin is truly more heavily tectonized than the Bl region north of it or whether it is just an observational bias. The scarps that form the Oz‐Vulcan boundary are several kilometers high, similar to many others mapped throughout Bl to delineate large crustal blocks. We incorporate these observations and interpretation in section 4.9 so refrain from further discussion of them here.

4.8 Mons and Montes (Mo_a, Mo_b, and Mo_c)

We identified a general Mons unit that we subdivided into three subunits. First, Mo_a is used for general, short‐wavelength topographic highs that are seen throughout Vulcan Planitia. They were not identified over approximately the western third of the province, but this could be due to emission and incidence angle limitations. There was a small preference for them to be observed near the northeastern portion of Vulcan Planitia. We have no preferred interpretation of these features, though an important issue arises of how mountains can form where there are no obvious convergent plates, volcanic vents, nor impact crater rims to act as a formation mechanism. It is therefore possible that these scattered mountains are end‐members of a sequence that includes Mo_b and Mo_c in one of our formation models (see next paragraph and section 5), and/or it is possible that they are remnants of a more original crust that has been embayed with margins below the available pixel scales.

Six features comprise the Mo_b unit, defined as mountains surrounded by a depression. Importantly, three of the Mo_b form Clarke Montes (Figure 6h), but the southwesternmost component has an impact crater and ejecta emplaced on top; due to the map scale (Table 1), the crater was not mapped, so this occurrence of Mo_b is mapped as an approximate boundary through the crater and ejecta. Mo_c is a single feature, a large mountain located at ~16°N, ~0°E (Figure 6g). Under one formation model for Sm₁, Mo_c is interpreted to be the other end‐member to Mo_a with Mo_b as the intermediate: Mo_c is a block of crust that was originally a part of Oz Terra that partially foundered (in this case, both translating and rotating) into the material that later solidified to be Sm₁. The surrounding material froze before this block could sink further. This hypothesis is described in quantitative detail in Beyer et al. (2019), but the basic structural justification is that the block appears tilted, striking east‐west, and dipping south. The north face of the block is a similar elevation to the scarp north of it, together forming Serenity Chasma. The block also contains what we interpret as a shelf of material that slipped off the north end and fell ≈1.5 km before stopping; we interpret this shelf as material failure that occurred as the large, >200‐km‐long block was sinking. In this possible scenario, Mo_b and Dm₂ are a later evolution of Mo_c, where the large blocks have sunk further and Sm₁ was embaying the space left behind when it froze. Further in this evolution, Mo_a could be the final peaks of such blocks, either stable (à la rockbergs, the same as tips of icebergs) or frozen in place just before sinking below the surface.

4.9 Depressed Material (Dm₁and Dm₂) and Broad Warps in and Near Vulcan Planitia

The Dm₁ unit is characterized by a gradually sloping depression along the outer margins that quickly steepens to a much deeper center, and these are found in the Mt and Rt units (Figure 6a). The DEM indicates slopes in excess of 11°. The Dm₂ materials' inner margins resemble flow margins of a viscous flow that encountered a boundary layer that retarded it, similar to—but much different rheology than—a wave approaching a shoreline (Beyer et al., 2019). Six of the 15 linear “broad warp” features are associated with either Dm₁ (4) or Dm₂ (2), used to indicate very wide (multiple kilometer) quasi‐linear topographic highs.

The warps near Dm₁ and the existence of the Dm₁ unit itself could be interpreted in two different ways. The first is that these depressions are intermediate between the Dm₂‐Mo_b features and the general Sm₁ of Vulcan Planitia: The foundering crustal blocks are completely submerged, and the viscous material was in the process of completely filling the void when it froze (similar to mud filling a void left by a rock that has just submerged). This works in concert with a possible explanation for the Mo_a blocks where the tips of former crustal blocks remained above the equipotential flow surface. The above scenario involving Sm₁, Dm₂, and Mo_a,b is complicated by the fact that one of these depressions is in the texturally different and topographically higher Rt unit to the east of Vulcan Planitia. If all Dm₁ units have the same formation mechanism and the above scenario is true, then this Rt must form in the same or similar way as Vulcan Planitia, which means that its chaotic surface postdates Sm₁ emplacement.

Alternatively, it is possible that Dm₁ are constructional features, such as cryovolcanic source vents. There are no obvious flow fronts, nor is there any indication of a broad topographic high or circumferential fractures around any Dm₁—except potentially for the Rt example; that is, the regions are absent obvious volcanic constructs. However, it is possible that a low viscosity material was sourced from them such that any topographic mound has relaxed, at least to the vertical accuracy of the DEM. An alternative scenario for Dm₂ is that they represent loading and flexure of the lithosphere (Moore et al., 2016). We elaborate on both hypotheses for Dm₁ and the former hypothesis for Dm₂ in the next section.

5 Chronostratigraphy and Resurfacing History

With the above discussion and interpretation of the various geologic units, we have developed the Correlation of Map Units (COMU) that includes linear features, shown in Figure 7. In developing this relative stratigraphy, we separated the surface history into three broad chronostratigraphic epochs, named for the primary or a type feature: Ozian, Vulcanian, and Spockian. Often, epochs are named after a prominent crater that often defines the base of the unit. However, this could not be done for Charon because of a lack of a clear impact crater that forms the base of those units; additionally, the Spock crater defines not the base of the sequence, but a typical and prominent feature within it. Epochs are also usually described additionally by an N(D) crater retention, where N is the spatial density of craters ≥D. It is only possible to do this for the largest of craters due to image limitations: Robbins et al. (2017) gave N(50 km) for Oz Terra as 15.4 ± 4.1 · 10⁶ km⁻² and Vulcan Planitia as 9.3 ± 4.6 · 10⁶ km⁻². This makes the ages similar or Vulcan Planitia is slightly younger, depending upon how one interprets the N^1/2 uncertainty; stratigraphically, Vulcan Planitia is younger. The chronostratigraphic epochs are defined in the following way, which works with either of our two possible models, described in brief below:

• Ozian: Characterized by formation of the crust that is Oz Terra (Bl), early in Charon's history. Based on the crater spatial density, we estimate that this epoch is ancient, >4 Ga (Moore et al., 2016; Singer et al., 2016).
• Vulcanian: Characterized by the formation of Vulcan Planitia, the cryoflow processes that created its smooth surfaces, and potentially the mountains and depressions. Based on the crater spatial density, we estimate this epoch started >4 Ga, but it may have continued for an extended period of time, potentially active in isolated areas as heat was lost (Moore et al., 2016; Robbins et al., 2017).
• Spockian: Characterized by a time after the surface of Vulcan Planitia had solidified whereon topographic relief could form and persist to the present day. Possibly ongoing groove formation concurrent with crater formation, landslides, and possibly the mottled terrain.

Figure 7

Correlation of Map Units (COMU) and linear features based on the interpretation described in sections 4 and 5. Question marks indicate uncertainty in temporal boundary.

The stratigraphy observed suggests that the blocky unit of Oz Terra is the oldest in the map area, but that cratered material formed throughout Charon's history. We hypothesize that the pattern of scarps and ridges began to form soon after the body's formation, driven by global expansion of up to 1%, likely due to temperature and phase changes (detailed in Beyer et al., 2017). Tidal energy may have contributed (Barr & Collins, 2015) to the formation of an early ocean that froze later (likely into the Vulcanian epoch). The expansion caused Oz Terra to fragment into large crustal blocks, hundreds of kilometers on a side, forming the current Bl unit and associated scarps and graben. The bases of these blocks extended into the surface below the current scarp bases and were denser than the warmer material below.

Vulcan Planitia's history is more uncertain, and several potential models to explain it are presented in detail in Beyer et al. (2019) and briefly described in Moore et al. (2016). The first hypothesis is advocated for and described quantitatively in Beyer et al. (2019). In brief, the most likely scenarios presented in Beyer et al. (2019) rely on the fractional crystallization of a cooling subsurface ocean that is enriched in ammonia hydrates. When the overlying lithosphere is fractured during the last stages of global expansion, this cryo‐fluid buoyantly rises to the surface, enveloping any existing topography and creating the new Vulcan Planitia surface. The dark‐light ejecta from craters in Oz but not in Vulcan further support the idea that Vulcan Planitia has had a different history. Given this general mechanism that could manifest in a range of scenarios, the formation of several units that manifest within the broad Vulcan Planum region can be explained:

• Sm₁: The frozen surface left by a viscous cryomaterial, either from a subsurface ocean or cryovolcanism. This is consistent with our preferred model of Mt formation (section 4.6).
• Sm₂: This could be a region of cryoflow that either spilled into the northern latitudes or was potentially sourced there.
• Mo_a: These could be remnants of an original fractured crust from before Sm₁ formed, or they could be topography associated with the cryo‐flow itself.
• Dm₁: These could be regions where large crustal blocks sunk, the cryoflow converged on that area, but it froze before an equipotential surface was reached. Alternatively, it is possible that they could be cryovolcanic source vents where the cryo‐material was withdrawn in the late stages of the flow
• Dm₂ and Mo_b: We propose that these two units are linked. The Mo_b units represent large, mountainous structures that preexisted the cryo‐flow with potentially wide (now submerged) bases. When the viscous cryo‐flow embayed these broad, topographic highs, the cryo‐flow was losing heat and beginning to freeze. The Dm₂ units represent the flow front as it froze before it could relax into an equipotential surface.
• Mo_c: This unit is likely a large, rotated, and tilted block. It is tilted like the blocks that are still part of Bl to the southwest of Alice crater, but this block is either subsided more or translated farther away from its neighboring blocks to the north, enough that Sm₁ material separated it from the Bl unit that makes up the north rim of Serenity Chasma.
• Rt: Could be evidence of a viscous cryovolcanic eruption, while Sm₁ was warmer or had slightly different chemistry that lowered the viscosity. Alternatively, Rt could be a manifestation of a buoyancy process: a large crust block sank and the current Rt material rose buoyantly, similar to a large rock sinking in viscous mud. Or, Rt could be a combination of either of these scenarios, or something different.
• Broad Warps: Could represent convergent flows in any of the above scenarios, or a linear source vent in the latter.

Stratigraphically, because the Rt unit has a Dm₁ in its center, we place this unit in the same formation epoch as Vulcan Planitia. We hypothesize that the Dm₃ unit formed in the midst of (but not necessarily the middle point of) the Vulcanian epoch, potentially as (a) an impact crater that was heavily modified by the cryo‐material or (b) as a flowfront or lobate scarp. The smooth elevated terrain patches of Vulcan Planitia (Sme) formed toward the end of the Vulcanian epoch, potentially into the Spockian epoch: They are elevated and within Vulcan Planitia, so they must have formed after the flow had cooled enough that the rheology could support a broad topographic feature. They potentially formed through a constructional process of an upwelling of underlying material. Some Mt may have formed before the bulk of Sm₁ cooled, but the relative timing is difficult to constrain with current data.

The geology of the Spockian epoch is characterized by more muted topography that could imply a general absence of surface heat, such that more passive processes modified the surface geology. It is likely that the Mt unit formed after the Sm₁ unit as justified above, and the possible emplacement of it on Ej material (Figure 6a). Alternatively, it is possible that Mt on Ej is an illusion, in which case Mt could have formed earlier than Sm_1,2 and was embayed, though other arguments presented in section 4.6 we think indicate otherwise. Impact craters and their ejecta continued to form and the tectonic features throughout Sm₁ formed, as well. The relative timing of the tectonics in Vulcan Planum is uncertain: The majority formed in the early Spockian epoch, but whether they continued to more recent times is ambiguous based on cross‐cutting relationships with impact craters and ejecta (Figure 6b). Because the Sm₁ unit may have cooled at different times in different areas, we have the grooves in Figure 7 forming back into the Vulcanian epoch. Finally, we hypothesize that the lobate aprons must have formed after the region had solidified, and they could be very recent features, potentially still active to the present day.

6 Speculation About Nonencounter Hemisphere

As Charon rotated as New Horizons approached, the lowest resolution images incorporated into the global basemap to the west of the geologic map area are ≈35 km/pixel, while those outside the geologic map area to the east are >1 km/pixel. Preencounter limb images also help derive some information about the nonmapped area from a geologic mapping perspective. Here we speculate about the nonmapped region based on the mapped units.

• Blocky Terrain: This material certainly extends beyond the mapped area. If Charon had preferential melting near its equatorial region (Beyer et al., 2019), Bl likely exists at approximately similar latitudes in the nonencounter northern regions, and the unseen high southern latitudes may also mirror the blocky textures of the high northern latitudes.
• Smooth Plains: The southern margin of Sm₁ is inferred in the map and almost certainly extends beyond the terminator from closest approach, likely at least several degrees (tens of kilometers) south based on the presence of Butler Mons. Argo Chasma potentially bounds another Sm unit to the east, and there is no reason to suspect that Sm_1,2 are unique.
• Rough Terrain: The single identified Rt unit visibly extends beyond the map boundary, approximately to Argo Chasma. As our speculation about its source is vague and uncertain, it is also uncertain whether there should be additional Rt units elsewhere; certainly, it cannot be ruled out, but there is nothing diagnostic about it that we could point specifically to nonencounter‐hemisphere features and state that they are additional Rt material.
• Mons/Montes: These also certainly exist elsewhere, and at least one other Mo_b–Dm₂ unit pair is visible in the data as Butler Mons.
• Depressed Material: As heavily tectonized a surface as Charon is, the body certainly contains additional regions of nonimpact depressions. As with Butler Mons, we can see the additional Dm₂ unit surrounding it, just off the map area because it had rotated beyond the terminator for closest approach.
• Impact Craters and Ejecta: Other than Charon's dark north pole, the first feature visible in approach imagery was the light‐ejecta crater at ~6°N, ~105°E. The diameter of this crater is highly uncertain because of the possible dark‐light ejecta pattern potentially giving the impression that the crater is up to ~2× as large as it truly is (Robbins et al., 2017). It is the largest crater visible in the nonencounter hemisphere, perhaps ~60 km across, and progressively smaller craters are visible west of it as pixel scales improve. A Dorothy‐sized impact or larger cannot be seen, if it exists, due to its much subtler topography and lack of albedo cues. However, based on the maximum size of impact craters relative to the size of the body across the solar system, it is not unlikely that there is at least one crater larger than Dorothy.
• Tectonics: Vast chasmata were some of the earliest features observed on approach limb images, and it is certain that other, smaller tectonic features exist, likely across Charon's entire surface.
• Albedo and Color Variations: Figure 2b clearly shows color variations across Charon's surface, and Figure 2a shows additional albedo variations, also described by Buratti et al. (2017). Consequently, it is difficult to speculate about large depressions, tectonics, and craters from the nonencounter imagery: What may appear to be shadows and Sun‐highlighted slopes may instead be albedo markings.

7 Summary

Far from some expectations, New Horizons' encounter with Charon showed that the body is incredibly diverse with a variety of different landforms, which we have now parameterized in a new geomorphologic map. The map illustrates 16 geologic units and subunits, over 1,000 linear features, and numerous albedo patterns in an attempt to synthesize the varied landscape and use them to construct a chronostratigraphy. As with most solar system bodies, the perhaps most interesting part of Charon's history is dictated by its internal heat, and how that heat manifests in surface expression. On Charon, unraveling the history of Vulcan Planitia is likely a key component of understanding that history, and our mapping supports interpretations that it is a viscous flow of material in that region of Charon (and, potentially other regions off‐map). This would not necessarily be unique: Many other large, icy bodies in the outer solar system show evidence of at least some localized resurfacing by a fluidized flow (Moore & Pappalardo, 2011). Regardless of the exact nature of this history, it is clear that Charon is a diverse world, and any future mission to the body will have a wealth of new data to add to the complicated story of Charon's past.

Acknowledgments

The authors acknowledge constructive feedback from the Planetary Mappers' Meeting attendees in 2016, and specifically D. A. Williams. T. M. Hare provided the ArcMap map package used in this work. Support for the authors was made possible through NASA's New Horizons mission within the New Frontiers program. This manuscript benefited from reviews by B. Schmidt and one anonymous person. All image and topographic data used in this work are available via NASA's PDS' Small Bodies Node (https://pds‐smallbodies.astro.umd.edu/data_sb/missions/newhorizons/index.shtml).