Geology of the Debussy quadrangle (H14), Mercury

ABSTRACT Mercury’s Debussy Quadrangle (H14) lies between 0–90° E and 22.5–65° S. Here we use MESSENGER data to produce the first geological map of this quadrangle at a scale of 1:3,000,000, based on linework completed at a scale of 1:300,000. We distinguish crater units and plains units. For compatibility with historic and recent maps of other Mercury quadrangles, and with the first global geological map (Main Map), we have made two versions of the map, with craters classified according to a 3-class and a 5-class degradation system. We distinguish additional units for the materials related to the Rembrandt impact basin. We subdivide the plains between the craters into three units: Smooth, Intermediate and Intercrater Plains, which represent different generations of plains formation. At least some of the Smooth Plains postdate the Rembrandt impact event.

NASA's MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) mission provided complete coverage of the planet (Solomon et al., 2007). A global 1:15 million scale map of Mercury is being produced (Kinczyk et al., 2018;Prockter et al., 2010). In addition, each quadrangle is being mapped at a scale of 1:3,000,000. Geological maps of quadrangles H-02 Victoria (Galluzzi et al., 2016), H-03 Shakespeare (Guzzetta et al., 2017), H-04 Raditaladi (Mancinelli et al., 2016), and H-05 Hokusai (Wright et al., 2019) have been published. The map of H-10 Derain was producedin parallel with ours (Malliband et al., 2020) and others such as H-13 Neruda are in progress (Man et al., 2020). Here we present the map of the H-14 Debussy quadrangle (Main Map), which covers 0 o E to 90 o E, and 22.5°to 65°S. The quadrangle includes part of the Rembrandt impact basin, which has been mapped separately by Hynek et al. (2017) and Semenzato et al. (2020).
Both the Mariner 10 maps and the post-MESSEN-GER maps show three separate groups of plains materials, which we continue in this paper. The original Mariner 10 maps used a 5-class system for recording crater degradation, as does the 1:15 million scale project, whereas previous post-MESSENGER 1:3 million scale quadrangle maps use only 3 classes to categorise impact craters, with the exception of Wright et al. (2019) who produced versions with 3 and 5 classes, we also use both classifications (Main Map). We divided the Rembrandt impact basin into multiple units because of its size and complexity.

Data
We used eight basemaps to produce the map. These are global mosaics made by the MESSENGER team. The image mosaics were produced from the Mercury Dual Imaging System, MDIS (Hawkins et al., 2007) data . Multiple mosaic datasets with different incidence angles were necessary to overcome shadows obscuring areas.

Bulk data record (BDR) basemaps
The BDR with 166 m/pixel resolution is the highest resolution globally available and the main mosaic used for mapping (Figure 1(a)). The BDR is constructed from images with an incidence angle close to 74°. A 250 m/pixel BDR mosaic produced during MESSENGER's mission was also used for certain areas where the mosaic is more consistent, or has different illumination geometries, than the 166 m/ pixel mosaic (Figure 1(b)).

Low incidence angle basemap
This mosaic at 166 m/pixel resolution is composed images captured close to solar noon. It was useful to identify albedo features and ejecta (Figure 1(c)).

Digital terrain model basemap
The digital terrain model helped to identify tectonic features. This has a resolution of 665 m/pixel and was derived from stereo images (Becker et al., 2016). It was displayed as colour-keyed elevation overlain by hill shaded relief during mapping (Figure 1(d)).

High incidence angle basemaps
High Incidence West/East mosaics with a resolution of 166 m/pixel were useful for identifying structures (Figure 1(e, f)).

Colour and enhanced colour basemap
We used two colour 665 m/pixel mosaics derived from the MDIS Wide Angle Camera. The colour base map (Figure 1(g)) uses 1000, 750, and 430 nm narrowband filters in the red, green, and blue channels . The enhanced colour basemap (Figure 1(h)) uses the 430, 750, and 1000 nm bands.  (Becker et al., 2016), e. High incidence west basemap, f. High incidence east basemap, g. colour basemap, and h. enhanced colour basemap .
It places the second principal component in the red, the first principal component in the green, and the ratio of 430/1000 nm bands in the blue channel . We used it to map plains and surficial features.

Projection
The map uses a spheroid radius of 2,439,400 m, with a Lambert conformal conic projection (Lambert, 1772) with standard parallels at 30°and 58°S and a central meridian of 45°E.

Scale
The publication scale is 1:3 million, consistent with other post MESSENGER quadrangle maps features needed to be at least 3 km wide to be mappable; this forms an element at least 1 mm wide on the map. We used a mapping scale 2,000 times the basemap pixel scale (Tobler, 1987) so 1: ∼300,000 for the 166 m/pixel basemap and a streaming length (distance between vertices) of 900 m as this representing 0.3 mm at mapping scale (Tanaka et al., 2011).

Mapping strategy
We followed the mapping standards of the US Geological Survey (Tanaka et al., 2011), Planmap (Rothery & Balme, 2018), and those set out by the other 1:3 million scale quadrangle maps (e.g. Galluzzi et al., 2016;Wright et al., 2019). We first produced the linework for tectonic structures and crater rims, and then contacts between units. We converted the contacts into polygons, with attribute information applied relating to interpretation. Line styles represent both the type of feature and the relative certainty of the line's location.

Contacts
The interface between units is known as a 'contact'. In some cases, this occurs at a lobate scarp, but the majority of the boundaries are stratigraphic contacts. The type of boundary and the confidence in its location dictate the style of the line.
'Certain' contacts are unbroken lines, indicating clear, abrupt boundary and representing confidence in the precise location (±1 km) of the contact at the scale mapped and most commonly found at the edge of Smooth Plains, or at the boundary between crater floor material and its wall/central peak. 'Approximate' contacts are where we could not identify the exact location due to data quality or a gradual transition between units. Such contacts are usually found between Intercrater Plains and Intermediate Plains. Internal contacts occur within both Smooth plains and Intercrater Plains where the unit morphologies match the description but there is a distinct internal boundary, likely due to different generations of the same morphology.

Tectonic features
3.5.1 Lobate scarps Thrusts faults typically manifest on the surface of Mercury as asymmetric lobate scarps (e.g. Watters et al., 1998). We drew linework at the break of slope at the base of the steeper side of the feature. The triangular teeth in the line symbol point towards the hanging wall. Levels of certainty reflect how clear the break in slope is; solid lines show clearly defined examples. Where the identification is uncertain, a dashed line is used ( Figure 2). Faults without a clear direction are shown as a dashed line with no teeth.

Wrinkle ridges
Wrinkle ridges are identified by their usually symmetrical profile and lower relief than lobate scarps (e.g. Watters & Nimmo, 2010). We mapped these as a single continuous pink line placed over the crest. Due to their subtle appearance, they are represented by a solid line with no certainty levels applied (Figure 2).

Grabens
Grabens are the only extensional structures identified. They are manifest as negative relief (shown by shadows), linear structures. Whilst generally the width of a single line, some grabens in the Rembrandt impact basin are wider; here the linework traces the middle of the graben (Figure 2). A solid line is used where the location is confident, and dashed where uncertain.

Geological units
We divided the map into geological units. We continue to use the main units identified in the Mariner 10 and previous 1:3 million scale maps as they help understand the geological history within the quadrangle and allow compatibility with the other quadrangle maps. There are two principal types of unit on Mercury: crater materials and plains materials.

Craters
We mapped and classified craters within the quadrangle based on their size and degradation.

Crater outlines
We mapped rims of craters >5 km but <20 km with a single black outline with no unit assigned to them. We showed rims buried by ejecta from subsequent nearby impactors with a dot-dashed symbol. We mapped the rims of craters >20 km diameter with a solid line with double inwards ticks and their associated geologic materials were distinguished into units.

Degradation
In the majority of cases crater degradation reflects how long crater features (rim, ejecta, walls, floor, terraces) have been modified by subsequent smaller impacts and space weathering. The maps produced using Mariner 10 images and the global map use a 5-class system (Kinczyk et al., 2018(Kinczyk et al., , 2020Spudis & Guest, 1988,   We use the same convention as Wright et al. (2019), with a capitalised 'C' for the 3-class system (e.g. C1, C2, C3) and a lower case 'c' for the 5-class system (e.g. c1, c2, c3, c4, c5). Increasing degradation state usually corresponds to older craters in the stratigraphic order, but for both systems we found rare cases of more degraded craters superposed on (and hence younger than) less degraded craters. We attribute this to either: smaller craters degrading more rapidly than larger craters, or proximity to a younger impact whose ejecta has degraded the morphology of the nearest craters. Smooth material confined to crater floors. In fresh craters this is interpreted as representing ponding of impact melt (Daniels, 2018). In older craters this may be subsequent volcanic plains.

Plains units
Outside of craters, we have followed previous maps of Mercury, (e.g. Galluzzi et al., 2016;Schaber & McCauley, 1980) by dividing the surface into plains with different textural features: Intercrater, Intermediate and Smooth Plains units.

Smooth plains (sp)
Smooth Plains are relatively flat, with few superimposed impact craters (Denevi et al., 2013). This is the youngest plains unit. It occurs as discrete patches with generally distinct boundaries and usually occupying low-lying areas such as within the Rembrandt impact basin. Smooth Plains tend to have a higher albedo and are spectrally redder in colour than Intercrater Plains (icp), however, this is not always the case. Older craters within Smooth Plains are infilled and/or embayed, whereas superposed craters retain a highly textured ejecta blanket. These plains are probably lava flows that have not been significantly degraded by subsequent cratering Head et al., 2008).

Intercrater plains (icp)
These plains gently undulating on scales of 10s to 100s of kilometres (Trask & Guest, 1975). Small (5-15 km) craters dominate their surfaces ( Figure  5). The craters' shallowness suggests that many are secondaries, although generally they cannot be linked to parent craters (Spudis & Guest, 1988;Whitten et al., 2014). Intercrater Plains are the most widespread plains on Mercury (Kinczyk et al., 2020;Murray et al., 1975;Strom et al., 2011;Whitten et al., 2014), and they have highly variable spectral properties and a wide range of craters sizes and all degradation stages. These are Table 2. Description of 5 Class crater system based on (Kinczyk et al., 2020 interpreted to be ancient lava flow fields significantly modified by subsequent impacts (Whitten et al., 2014).

Intermediate plains (ip)
Intermediate Plains contrast with Smooth Plains by having a more undulating texture and more   Figure 4. Examples of the 3-class system crater types: red arrows point at example craters. On the map interpretation panels, Yellow = C3 (least degraded), Green = C2 (moderately degraded), and Red = C1 (most degraded). The background is the BDR basemap, with 30% transparency in line interpretation. The geology map has 40% transparency.

Rembrandt-specific units
The Rembrandt impact basin has some noteworthy features within it and several different styles of ejecta. We elected to map these as basin-specific units. This is an approach adopted in maps of the Caloris basin McCauley et al., 1981;Guest & Greeley, 1983;Guzzetta et al., 2017;Mancinelli et al., 2016;Solomon et al., 2007;Trask & Guest, 1975) and also in previous maps of Rembrandt (Hynek et al., 2017;Semenzato et al., 2020). We preface Rembrandt-specific units with 'Re'.

Hummocky unit (Reh)
Part of the basin floor of Rembrandt is uneven and mapped as a hummocky unit (Hynek et al., 2017;Semenzato et al., 2020;Watters et al., 2009). The edges of this unit are gradational. The unit is morphologically different from typical crater floor due to smooth undulating terrain between the discrete hummocks, which are 15-50 m high. The hummocky unit has a lower albedo than most of the Smooth Plains that cover much of the rest of the floor of Rembrandt. (Figure 6). This unit is interpreted to be part of the basin floor not covered over by subsequent lavas ).

Rembrandt massifs (Rem)
These are blocky hills (up to 1 km high) protruding above the basin floor units Reh and sp ( Figure 6). They lack strong fabric and we interpret these to be blocks of impact ejecta.

Rembrandt rim material (Rer)
This unit was first identified by Hynek et al. (2017). It comprises a series of massifs that make up part of the

Rembrandt linear unit (Rel)
This unit is distinguished by surface texture radiating away from the basin in the form of ridges and troughs (Hynek et al., 2017;Watters et al., 2009;Whitten & Head, 2015). It includes blocky areas and smoother patches too small to map individually ( Figure 6). We interpret this to be radiating ejecta, similar, and probably analogous, to the Van Eyck formation at Caloris ).

Rembrandt ejecta (Ree)
This unit comprises hills undulating at scales of tens of kilometres. It is smoother than icp, has a lower density of craters, and often contains flat 'pools' that look like filled craters (Figure 6).

Correlation of map units
The stratigraphic column (Figure 7) summarises the inferred geological history of the quadrangle using the 5-class crater system. The formation ages of the plains units are based on global estimates (Byrne et al., 2016;Strom et al., 2011;Whitten et al., 2014

Summary
We have used data collected by the MESSENGER spacecraft to make the first geological map of the H-14 Debussy quadrangle on Mercury. We have mapped crater degradation using two schemes. The map is dominated by Intercrater plains and terrains related to the Rembrandt impact basin. We have distinguished an Intermediate plains unit, in agreement with other quadrangle maps.

Software
The basemaps were processed using USGS ISIS3, we used ESRI ArcMap 10.5 to produce the map, and the Map sheet was produced using CorelDraw.  (Spudis & Guest, 1988;Banks et al., 2017).

Disclosure statement
No potential conflict of interest was reported by the author(s).

Data availability statement
Digital copies of the shape files and basemaps can be found here: https://ordo.open.ac.uk/articles/dataset/Geological_ map_data_for_the_H14_Debussy_quadrangle/14207174