# Spectral Study of Five Major Impact Basins and Their Volcanic Infills on Mercury: A Window Into the Mantle's Properties

E Caminiti, S Besse, A Doressoundiram, J Wright

► To cite this version:

E Caminiti, S Besse, A Doressoundiram, J Wright. Spectral Study of Five Major Impact Basins and Their Volcanic Infills on Mercury: A Window Into the Mantle's Properties. Journal of Geophysical Research. Planets, 2025, 130 (12), pp.e2025JE009359. 10.1029/2025je009359. hal-05431457

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# JGR Planets

## RESEARCH ARTICLE

10.1029/2025JE009359

**Key Points:**

* The so-called Young High-reflectance Red Plains (YHRP) is a major and widespread spectral unit

* There is a common sequence of volcanic episodes that infilled major impact basins

* The basin's size and the age of its volcanic infill are likely key factors in the formation of YHRP

**Correspondence to:**

E. Caminiti,
caminiti.emma@jaxa.jp

**Citation:**

Caminiti, E., Besse, S., Doressoundiram, A., & Wright, J. (2025). Spectral study of five major impact basins and their volcanic infills on Mercury: A window into the mantle's properties. *Journal of Geophysical Research: Planets*, 130, e2025JE009359. https://doi.org/10.1029/2025JE009359

Received 31 JUL 2025

Accepted 30 NOV 2025

**Author Contributions:**

**Conceptualization:** E. Caminiti

**Formal analysis:** E. Caminiti, J. Wright

**Funding acquisition:** A. Doressoundiram

**Investigation:** E. Caminiti, S. Besse, J. Wright

**Methodology:** E. Caminiti

**Supervision:** S. Besse, A. Doressoundiram

**Validation:** S. Besse, A. Doressoundiram, J. Wright

**Visualization:** E. Caminiti

**Writing – original draft:** E. Caminiti

**Writing – review & editing:** E. Caminiti, S. Besse, A. Doressoundiram, J. Wright

© 2025. The Author(s). This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

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# Spectral Study of Five Major Impact Basins and Their Volcanic Infills on Mercury: A Window Into the Mantle's Properties

E. Caminiti<fnref n="1" /> ID, S. Besse<fnref n="2" /> ID, A. Doressoundiram³ ID, and J. Wright⁴,⁵ ID

**Abstract** NASA's MErcury Surface, Space ENvironment, GEochemistry, and Ranging mission has revealed that about 27% of the surface of Mercury is covered by smooth plains, which are mostly volcanic in origin. These plains are mainly located in the northern hemisphere, as well as within and around major impact basins. We used Mercury Atmospheric and Surface Composition Spectrometer data to perform an exhaustive spectral analysis of five major impact basins: Caloris, Rembrandt, Beethoven, Tolstoj, and Rachmaninoff. We highlighted the existence of a new high-reflectance spectral unit, that had previously only been identified within the Rembrandt basin, as a major unit being more widespread. We named this new unit Young High-reflectance Red Plains. We found a common sequence of volcanic episodes that infilled the basins and shaped their current surface spectral properties. We have shown that the size of the basin and the age of the volcanic infills are likely important parameters for the layering of different volcanic plains, defining the surface spectral units. Our study gives access to mantle properties, and we suggest that heterogeneity in the mantle is certainly not necessary to explain the spectral properties of effusive volcanism associated with impact basins. Future observations by the ESA-JAXA-BepiColombo mission are eagerly awaited to better constrain the planet's spectral, compositional, morphological, and geophysical surface properties.

**Plain Language Summary** Mercury's smooth plains have relatively few impact craters and were mostly formed by eruptions of fluid lavas. These plains are mainly located in the northern hemisphere, as well as within and around major impact basins. Differences in how the smooth plains reflect sunlight ("spectral units") reveal information about their formation. We used spectral reflectance data from NASA's MErcury Surface, Space ENvironment, GEochemistry, and Ranging mission to reconstruct the geological evolution of five impact basins on Mercury. We found that a spectral unit previously only seen in the Rembrandt basin is more widespread on Mercury. We found that five basins across Mercury followed the same sequence of erupted smooth plains spectral units. We showed that the size of the basin and the age of the volcanic flows are important factors that could explain the different layers of volcanic plains. While the diversity of smooth plains spectral units might indicate different parent materials in magma source regions, our study shows that such primary differences are not necessary. Instead, the same magmas under different basins perhaps crystallized for different periods, leading to different spectral properties. Our method can be applied to other regions on Mercury. The upcoming ESA-JAXA BepiColombo mission will provide new data that will further improve our understanding of Mercury's surface and geological history.

## 1. Introduction

Results from the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission highlighted that Mercury's surface is mainly composed of lavas (Head et al., 2008). Widespread volcanism has shaped Mercury's surface over time by a series of volcanic flows, playing an important role in surface evolution (Denevi et al., 2009, 2013; Head et al., 2008, 2009, 2011; Murchie et al., 2008; Ostrach et al., 2015; Robinson et al., 2008). Almost the entire current crustal surface is volcanic and was battered by impacts that formed craters that were subsequently modified by additional volcanic events (Denevi et al., 2013).

<footnote n="1">¹Institute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA), Sagamihara, Japan,</footnote>

<footnote n="2">²European Space Agency (ESA), European Space Astronomy Center (ESAC), Madrid, Spain, ³LIRA, Observatoire de Paris, Université PSL, CNRS, Sorbone Université, Université de Paris, Meudon, France, ⁴School of Physics and Astronomy, University of Leicester, Leicester, UK, ⁵School of Physical Sciences, The Open University, Milton Keynes, UK</footnote>
Smooth plains, of which 65% are volcanic in origin, are commonly located within or around impact structures (Denevi et al., 2009, 2013; Head et al., 2008, 2009). Impacts represent fundamental structural features of Mercury's crust. They played a significant role in Mercury's early geological evolution, shaping the planet's topography, stratigraphy, and crustal structure (Buoninfante et al., 2023; Fassett et al., 2012). They facilitate magma extraction to the surface by: removing pressure, promoting subsurface uplift, generating fractures, introducing heat, and resetting the local stress regime (Byrne, 2020). Therefore, the connection between basin formation and post-basin volcanism provides a crucial basis for understanding the interactions between exogenic and endogenic processes on Mercury.

In the absence of in situ measurements, impacts provide a unique opportunity to study Mercury's crust and mantle (Ernst et al., 2010; D'Incecco et al., 2015, 2016; Klima et al., 2018). Large basins are of major interest because they can excavate material from depths of tens of kilometers and are associated with large volumes of volcanic material. Studying both surface and buried volcanic units provides valuable insights into the long-term evolution of magmas and helps define the extent and significance of volcanism in Mercury's geological history, contributing to the reconstruction of the planet's thermal and compositional evolution. Impact basins with different ages and sizes are spread on the surface. Their comparison is of great interest to better understand the volcanism globally on Mercury.

Major element chemistry of the surface lavas suggests that Mercury's mantle should be slightly heterogeneous in composition (Namur et al., 2016; Nittler et al., 2018). However, the only major difference in composition is in Ca and Na contents, which represent no more than 5 wt.% in total (Nittler et al., 2018). Thus, it is plausible to consider a homogeneous mantle while studying mantle melt production (see Section 2.3 in Beuthe et al. (2020)). Based on a homogeneous lherzolitic mantle, Beuthe et al. (2020) have shown that variations in surface composition could be explained by a heterogeneous mantle melt, driven by secular cooling of the planet. A comparative analysis of several basins' units interpreted as consistent with volcanic infills can therefore help to explore Mercury's internal structure and determine whether mantle heterogeneities are required to explain surface properties.

In this work, we compare the spectral properties of five major impact basins on Mercury: Caloris, Rembrandt, Beethoven, Tolstoj, and Rachmaninoff to better understand their geological evolution. We examine the units interpreted as consistent with volcanic infills within basins to investigate the possible heterogeneities of the mantle and the long-term evolution of effusive volcanism. In the following sections, we will use the short expression “volcanic infills,” commonly used in the literature, to refer to units interpreted as consistent with volcanic infill, even though no direct evidence is systematically observed.

## 2. Geological Properties of Mercury's Surface

Since the first Mariner 10 data, and especially with MESSENGER data, Mercury's surface has been divided according to its morphological, geochemical, and spectral properties. The defined units do not necessarily correlate, limiting the understanding of the geological history of the planet (Weider et al., 2014). It seems that the surface composition is not the only parameter that influences the spectral properties of the surface, nor is the morphology. There are several classifications based on broadly similar criteria (see Table 1 from D'Incecco et al. (2015)). In this study, we will use the main geomorphological units (Denevi et al., 2009, 2013; Trask & Guest, 1975) and spectral units defined as follow by Murchie et al. (2015).

The surface of Mercury is dominated by three major geomorphological units: intercrater plains, smooth plains, and the rims, walls, and ejecta of impact craters. The intercrater plains cover more than 70% of the surface (Denevi et al., 2013) and were globally formed during a short period of time, between ~4.1 and 3.75 Ga (Marchi et al., 2013), mostly by widespread effusive volcanism (Strom et al., 1975). The intercrater plains lie between the large craters and are gently rolling terrain with a high abundance of craters (Denevi et al., 2009). The smooth plains have a lower density of craters, typically fill low-lying areas such as impact craters and basins, and often have sharp color and morphological boundaries with surrounding terrains. Smooth plains of large volume were placed between ~3.7 and 3.5 Ga before major eruptive activity ended (Byrne et al., 2016; Marchi et al., 2013; Wang et al., 2021). Borealis Planitia (Head et al., 2011; Ostrach et al., 2015), Caloris Planitia (Ernst et al., 2015; Fassett et al., 2009; Hirata et al., 2022; Murchie et al., 2008; Rothery et al., 2017), and the circum-Caloris smooth plains are the largest volcanic smooth plains on Mercury (Denevi et al., 2013). Other major volcanic smooth plains are located within preexisting impact structures, especially the largest impact basins (e.g., Rembrandt,
<table><caption>**Table 1**

Properties of Studied Impact Basins From the Largest to the Smallest</caption><thead><tr><th>Basin</th><th>Diameter (km)</th><th>Center coordinates</th><th>Age estimation and periods (Ga)</th><th>Mg/Si ratio (Nittler et al., 2020)</th><th>Crustal thickness (km) (Watters et al., 2021)</th><th>Melting depth (km) (Ernst et al., 2015)</th><th>Detection of YHRP</th></tr></thead><tbody><tr><td>Caloris</td><td>1,550</td><td>31.5°N, 162.7°E</td><td>3.7–3.9 (Calorian)</td><td>0.21–0.38</td><td>7–57</td><td>&gt;44</td><td>Yes</td></tr><tr><td>Rembrandt</td><td>716</td><td>32.8°S, 87.5°E</td><td>3.8–3.9 (Tolstojan)</td><td>0.41–0.46</td><td>21–54</td><td>&gt;44</td><td>Yes</td></tr><tr><td>Beethoven</td><td>630</td><td>20.9°S, 235.8°E</td><td>3.8–3.9 (Tolstojan)</td><td>0.40–0.50</td><td>27–62</td><td>&gt;44</td><td>Yes</td></tr><tr><td>Tolstoj</td><td>355</td><td>16.2°S, 195.4°E</td><td>3.9–4.0 (Tolstojan)</td><td>0.38–0.40</td><td>24–34</td><td>&lt;44</td><td>No</td></tr><tr><td>Rachmaninoff</td><td>305</td><td>27.7°N, 57.4°E</td><td>3.6 (Calorian)</td><td>0.55–0.62</td><td>14–36</td><td>&lt;44</td><td>No</td></tr></tbody></table>

*Note.* Periods are taken from Marchi et al. (2009). We extracted the Mg/Si ratios from the smoothed map of Nittler et al. (2020) and they correspond to the minimum and maximum within each basin. The crustal thicknesses are extracted from Watters et al. (2021). They correspond to the minimum and maximum crustal thicknesses within each basin, including both models from Watters et al. (2021). The reader is invited to refer to the detailed description of each basin in this manuscript for additional references.

Rachmaninoff, Tolstoj, and Beethoven) (André et al., 2005; Denevi et al., 2013; Fassett et al., 2012; Watters, Head, et al., 2009).

The surface of Mercury is also classified according to its color properties (Murchie et al., 2015). In ascending order of reflectance the major spectral units are the Low Reflectance Material (LRM), the Low-reflectance Blue Plains (LBP), the Intermediate Plains (IP), and the High-reflectance Red Plains (HRP). LRM deposits have low reflectances, low spectral slopes, and cover around 15% of the surface (Denevi et al., 2009; Klima et al., 2018). They are hypothesized to be exhumed remnants of Mercury's putatively carbon-rich primary crust buried under the secondary volcanic crust (Denevi et al., 2009; Ernst et al., 2010; Klima et al., 2018; Peplowski et al., 2016; K. E. Vander Kaaden & McCubbin, 2015). LRM is mostly associated with craters, although also found within large areas (e.g., southern hemisphere (Klima et al., 2018)). LBP have reflectances and spectral slopes below the planet's average. The intercrater plains are typically associated with LBP, however the largest expanse of LBP is the circum-Caloris smooth plains (Robinson et al., 2008). Most of Mercury's surface consists of IP. IP are observed in intercrater plains, smooth plains, and in association with impact craters. They have intermediate reflectances and slopes between the LBP and HRP. HRP have a high reflectance and a steeper reflectance spectrum than the average and have the strongest affinity for smooth plains even if they are also associated with intercrater plains and craters.

Temperature variations across Mercury's surface can influence the properties of the surface (Deutsch et al., 2024; Leblanc et al., 2023) and the subsurface (Verkercke et al., 2024), highlighting the importance of considering such effects while analyzing the geological properties. The surface temperature may control the surface/exosphere interface and subsurface reservoirs as well as surface feature formation such as hollows. Mercury's maximum surface temperatures vary across the longitudes (Phillips et al., 2018). Due to the planet's spin-orbit resonance the same longitude repeatedly faces the Sun at aphelion and perihelion, leading to distinct “hot” (0°E and 180°E) and “warm” (90°E and 270°E) poles. Moreover, temperatures change (~150–300 K) with the latitudes with greater contrast in the hot-pole longitudes.

### 3. Geological Properties of Selected Impact Basins

In the present work, we propose to begin by studying five major basins that have marked the planet's geological evolution: Caloris, Rembrandt, Rachmaninoff, Beethoven, and Tolstoj (Table 1). These basins vary in size, latitude, longitude, age, surface composition, and maximum surface temperature, allowing for a comparative analysis of how these parameters influence spectral characteristics. Furthermore, their prior study in the scientific literature (e.g., geological maps) provides valuable context for interpreting our spectral data accurately.
**3.1. Rembrandt**

With a diameter of about 716 km, the Rembrandt basin is the largest, well-preserved, impact basin of the southern hemisphere and the second largest of the planet formed around 3.8–3.9 Ga (Ferrari et al., 2015; Hirata et al., 2022). Its interior is covered by volcanic smooth plains that postdate the basin formation by ~30–200 Myr (Ferrari et al., 2015; Hirata et al., 2022; Padovan et al., 2017; Watters, Head, et al., 2009). The estimated magma eruption flux in Rembrandt was 5.2–12.3 km/Gy (Hirata et al., 2022).

Geological analyzes of the basin have shown a prolonged period of plains formation with the most recent close to the center of the basin (Watters, Head, et al., 2009). Using crater size-frequency distributions, Ferrari et al. (2015) proposed that a resurfacing of an older unit by a younger one happened within the basin interior. They may be two geologically distinct units, one on top of the other in the interior of the basin. Even if morphologically similar and on a first order associated with HRP (Ferrari et al., 2015; Watters, Head, et al., 2009; Whitten & Head, 2015), there is a subtle color difference between the interior plains potentially associated with different volcanic flooding events (Semenzato et al., 2020). Close to the center of the basin, plains have a higher albedo compared to the surroundings. The brighter plains within the center of the basin have been identified as younger interior smooth plains. Both the interior smooth HRP and the brighter ones are in total about 1.0–1.5 km thick (around 0.5 and 0.5–1 km, respectively) (Semenzato et al., 2020). The spectrally distinct interior plains belong to two phases of the same infilling event. More than 1 Ga could separate the two plains with a last volcanic event at 2.3 Ga (Semenzato et al., 2020).

The exterior plains can be divided into different geomorphological and spectral units (Semenzato et al., 2020; Whitten & Head, 2015). Unlike the clear volcanic origin of the interior plains, the exterior plains origin is still debated. They could be partially produced by impact melt or effusive volcanic events (Ferrari et al., 2015; Semenzato et al., 2020; Whitten & Head, 2015). The exterior smooth LBP could be produced by impact melt however, the use of crater counting to determine the relative ages of the exterior and interior plains has proven intractable because the standard errors on the counts overlap and significantly change from one sampled area to another (Whitten & Head, 2015). In this work we will consider that the majority of the exterior smooth plains are volcanic in origin. In fact, Ferrari et al. (2015) dated a south-west part of the exterior plains as coeval with the interior plains, suggesting a possible correlation between the two events.

**3.2. Caloris**

Located at 31.5°N–162.7°E with an elliptical shape of 1,525 km by 1,315 km (Fassett et al., 2009), Caloris is the largest, well-preserved impact basin known on Mercury (Murchie et al., 2008) formed about 3.7–3.9 Ga (Fassett et al., 2009; Head et al., 2011; Hirata et al., 2022; Massironi et al., 2009). Caloris exposes layering of the planet's crust and contains volcanic features. Two spectrally different types of volcanic smooth plains are identified in association with the basin: Caloris Planitia within the interior of the basin (Ernst et al., 2015; Fassett et al., 2009; Hirata et al., 2022; Murchie et al., 2008), post-dating the Caloris impact event by 60–200 Myr (Fassett et al., 2009; Hirata et al., 2022; Massironi et al., 2009; Padovan et al., 2017), and the circum-Caloris plains surrounding it (Denevi et al., 2013). Both have been identified as mostly volcanic in origin with a comparable age (Fassett et al., 2009; Head et al., 2008; Rothery et al., 2017; Watters, Murchie, et al., 2009). However, Caminiti et al. (2023) and Hirata et al. (2025) have argued that the circum-Caloris plains were emplaced slightly before Caloris Planitia. The estimated magma eruption flux within Caloris was from 9.3 to 15.1 km/Gy (Hirata et al., 2022).

Caloris Planitia is identified as HRP and is heterogeneous with a slightly redder, higher albedo part in the northwestern half of the interior plains in comparison with the southeastern half (Murchie et al., 2008). The observed crater size-frequency distribution shows multiple resurfacing events caused by episodic eruptions (Hirata et al., 2022). The exterior plains are spectrally associated with LBP and are relatively homogeneous around the basin, even though there is an east-west LRM asymmetry (Caminiti et al., 2023).

**3.3. Rachmaninoff**

Discovered during the third MESSENGER flyby, Rachmaninoff is a mid-sized, ~305-km-diameter peak ring (double ring) basin centered at 27.7°N–57.4°E (Prockter et al., 2010). The projectile responsible for the basin formation impacted the surface with a downrange direction toward the south (Gault & Wedekind, 1978; Wright
et al., 2024). The impact may have excavated the already depressed interior of an older basin (Orgel et al., 2020; Wright et al., 2024), explaining the lowest surface elevation on Mercury contained in Rachmaninoff's center (Becker et al., 2016). Rachmaninoff has a fresh appearance and its ejecta superpose Borealis Planitia (~3.7 Ga), showing that the impact is younger than 3.7 Ga (Wright et al., 2021). The basin formation is estimated at around 3.6 Ga even though still uncertain (Marchi et al., 2011; Wang et al., 2021).

The basin floor is partially covered by smooth plains, with those located within the peak ring being spectrally brighter and redder than most Rachmaninoff ejecta. This has been interpreted as evidence that at least some smooth plains within Rachmaninoff are later volcanic plains, rather than impact melt (Blair et al., 2013; Chapman et al., 2012; Marchi et al., 2011; Prockter et al., 2010), and must be either younger than, or contemporaneous with, the impact. Moreover, concentric grabens on the inner floor are consistent with subsidence after lava emplacement (Blair et al., 2013). The small number of superposed craters indicates that they may be among the youngest volcanic deposits on Mercury (Prockter et al., 2010). The study of Rachmaninoff's interior volcanic plains is particularly interesting as they possibly formed well after the presumed cessation of effusive volcanism around 3.5 Ga, at times as recent as 1 Ga (Chapman et al., 2012; Marchi et al., 2011; Prockter et al., 2010; Wang et al., 2021). Smooth plains are also found on a portion of the southernmost part of the peak ring (Blair et al., 2013; Prockter et al., 2010; Wright et al., 2024). Between the peak ring and the crater rim is an annulus of well-preserved LRM composed of around 4.5 wt.% carbon (Klima et al., 2018; Wright et al., 2024; Zambon et al., 2022).

### 3.4. Beethoven and Tolstoj

Beethoven (20.9°S, 235.8°E) and Tolstoj (16.2°S, 195.4°E) are older and currently less studied than Caloris, Rembrandt, and Rachmaninoff. They are flat-floored and shallow basins (~2.5 and 2 km deep, André et al., 2005), with diameters of ~630 and 355 km, respectively (Fassett et al., 2012), and formed ~3.8 Ga and 3.9 Ga (André et al., 2005; Fassett et al., 2012; Massironi et al., 2009). Beethoven was resurfaced with about 1.5–3 km thick volcanic smooth plains material (André et al., 2005; Denevi et al., 2013). Tolstoj is partially flooded by smooth plains material of ~1.7 to 3.2 km thickness formed after the Caloris basin (André et al., 2005). For both basins, embayment relations were observed within their smooth plains suggesting a volcanic origin (Byrne et al., 2016; Denevi et al., 2013). Beethoven's volcanic infills were estimated ~3.7–3.8 Ga similar to the major volcanic smooth plains on Mercury, while smooth plains within Tolstoj, as an exception, formed ~3.5 Ga (Byrne et al., 2016; Wang et al., 2021). Tolstoj has an annulus of LRM material, exterior to the basin, containing around 2 wt.% carbon content in comparison with >4 wt.% around Rachmaninoff (Denevi et al., 2009, 2013; Klima et al., 2018; Peplowski et al., 2016).

## 4. Data Set and Spectral Parameters

In our previous study (Caminiti et al., 2023), we developed a spectral classification which served as a tool to identify the local stratigraphy beneath the Caloris basin, reconstruct the relative timing of volcanic smooth plains, and retrace the geological evolution of the area. However, our study focused on spectral units associated with the Caloris basin (i.e., LRM, LBP, and HRP) without extending the spectral classification to IP. Moreover, other volcanic units associated with impact basins have been identified (e.g., the brighter interior smooth plains within Rembrandt (Semenzato et al., 2020)) and must be considered to extend the spectral study to other areas. Here, we extended our spectral classification to take into account IP and the brighter interior plains of Rembrandt. In this work, we will call the latter Young High-reflectance Red Plains (YHRP).

### 4.1. MESSENGER-MASCS-VIRS Data

In this work, we superimpose data from the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) (McClintock & Lankton, 2007) on Mercury Dual Imaging System (DIS) images (Hawkins et al., 2007). MASCS was composed of both an Ultraviolet and Visible Spectrometer (UVVS) and a Visible and InfraRed Spectrograph (VIRS) designed to map the surface reflectance with a 5-nm spectral resolution. VIRS was composed of two detectors: VIS (300–1,050 nm) and NIR (850–1,450 nm). MASCS-VIRS data are available on the Planetary Data System (PDS) (Washington University, St. Louis). In this work, we used the radiometrically calibrated and photometrically normalized Derived Data Record. To combine the VIS and NIR data into continuous spectra from 300 to 1,450 nm, we used the method developed and explained in detail by Besse et al. (2015). The correction process consists of four steps: removing outlier data, smoothing the spectra to reduce
<table><caption>**Table 2**
Spectral Parameters Used to Define the Spectral Classification of the Mercury Atmospheric and Surface Composition Spectrometer Footprints</caption><thead><tr><th>Parameter</th><th>Spectral range (nm)</th><th>Mathematical definition</th></tr></thead><tbody><tr><td>VIS_Slope</td><td>445–750</td><td>R(λ) = VIS<sub>Slope</sub> × λ + b</td></tr><tr><td>R750</td><td>745–755</td><td>R(750 nm)</td></tr><tr><td>UV_Downturn</td><td>300–400</td><td>Depth<sub>303</sub> + Depth<sub>324</sub> + Depth<sub>350</sub> with Depth<sub>λ</sub> = (R401 – (401 – λ) × VIS<sub>Slope</sub>) / Rλ<br>and VIS<sub>Slope</sub> = (R550 – R750) / (550 – 750)</td></tr><tr><td>VISNIR_Slope</td><td>445–1,400</td><td>R(λ) = VISNIR<sub>Slope</sub> × λ + b</td></tr><tr><td>Color index 415/750</td><td>415–750</td><td>R415 / R750</td></tr></tbody></table>

Note. In the equations, b represents the y-intercept.

the noise especially for the NIR detector whose SNR is 3–5 times lower than the VIS detector (use of a three-point moving average), adjusting the NIR part to the VIS part (offset added or subtracted from the reflectance values in the NIR detector's wavelength range which is more sensitive to temperature variations), and calculating quality coefficients to validate the correction. The final reflectances are average values of three points due to the data correction. These data and correction processes were already used in numerous studies focused on explosive volcanism (Barraud et al., 2021; Besse et al., 2015, 2020; Leon-Dasi et al., 2023, 2025), effusive volcanism (Caminiti et al., 2023), and hollows (Barraud et al., 2020, 2023). For the rest of this work, when we talk about the data, we mean corrected spectra between 300 and 1,450 nm. Four points delimit MASCS-VIRS elliptical footprints with an additional one in the center of the footprint. One spectrum was obtained from the integrated signal received from the surface of each footprint.

## 4.2. Data Filtering

We applied a temperature filter because the detectors were sensitive to the instrumental temperature, which can reduce the signal-to-noise ratio. To maximize the amount of data while ensuring the quality of the data, we filtered out the data obtained with an instrumental temperature >40°C. This filter removed data mainly over Mercury's hot poles (0° and 180°E). Moreover, shadows, which become increasingly pervasive as the solar incidence angle increases, reduce the data quality. Therefore, we have decided to use only the data obtained with an incidence angle <75°. Despite this filtering, topography must still be considered in the data analysis. The footprints of the instruments can extend to several tens of kilometres, which does not allow for a precise study of the spectral properties of the surface. Thus, by comparing footprints, we estimated a size limit to avoid overlapping between spectral units. We filtered out footprints larger than 65 km². As we consider a homogeneous composition within the footprints, this last filter allows us to be confident about our data quality.

## 4.3. Spectral Parameters

Spectral parameters are used to study Mercury's generally featureless spectra and quantify the spectral variability. The overall shape of the spectra varies according to the location on the surface and the use of spectral parameters helps to constrain the surface properties (Zambon et al., 2022). Parameters used in this work are the same as those used by Caminiti et al. (2023) and are summarized in (Table 2).

We obtain the slope parameters using linear fits and normalize it to Mercury's average spectrum (Izenberg et al., 2014). We calculate the VISNIR_Slope by extending the VIS_Slope with a linear fit to 1,400 nm before noise becomes significant. R750 is the absolute reflectance at 750 nm. Reflectance and slope variations may indicate different compositions, physical properties (e.g., grain size), and space weathering exposures (Domingue et al., 2014; Pieters & Noble, 2016). The UV-Downturn (Goudge et al., 2014) characterizes the drop of reflectance in the UV. It measures the offset between measured reflectances and expected reflectances. The color index is a simple reflectance ratio.
<figure><img src="image_3.png" /><figcaption>**Figure 1.** Sampled area (black boxes) used to study the Young High-reflectance Red Plaines. (a) Mercury Dual Imaging System enhanced color map centered on the Rembrandt basin, showing the area of high reflectance from Semenzato et al. (2020). (b) Map of Mercury Atmospheric and Surface Composition Spectrometer footprints centered on the Rembrandt basin. Colors indicate reflectance measured at 750 nm.</figcaption></figure>

# 5. Spectral Units' Properties

## 5.1. The Young High-Reflectance Red Plains (YHRP) and the Intermediate Plains (IP)

Similarly to Caminiti et al. (2023), we sampled the spectral units, performed statistical analyses using spectral parameters, and defined spectral frameworks for each spectral unit and parameter with different levels of confidence. Thereafter, the frameworks can be used to classify MASCS-VIRS footprints according to a quality parameter.

We selected areas to sample the YHRP in the center of the Rembrandt basin as defined by Semenzato et al. (2020) (Figure 1). We sampled the IP within Sihtu planitia, commonly called the Rudaki plains (Figure 2). The Rudaki plains are volcanic smooth plains that lie within an unnamed, degraded basin ~370 km in diameter near the crater Rudaki (Denevi et al., 2009; Ernst et al., 2010).

We selected areas large enough to best represent the units of interest while avoiding topographic perturbations (e.g., craters, wrinkle ridges), other spectral units (e.g., fresh material), and areas where MASCS-VIRS spatial

<figure><img src="image_4.png" /><figcaption>**Figure 2.** Sampled area (white boxes) used to study the Intermediate Plaines. (a) Mercury Dual Imaging System enhanced color map centered on the Rudaki plains. (b) Map of Mercury Atmospheric and Surface Composition Spectrometer footprints centered on the Rudaki plains. Colors indicate reflectance measured at 750 nm.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>**Figure 3.** Spectral properties of sampled area for Low Reflectance Material, Low-reflectance Blue Plains, High-reflectance Red Plains as well as Young High-reflectance Red Plains. (a) For each unit, the mean value is represented and the errors show the standard deviation for R750 and VIS_Slope. The correlation coefficient is 0.99. (b) Spectra of each spectral unit. The gray line represents R750.</figcaption></figure>

coverage is insufficient. Around 390 and 790 footprints were sampled for IP and YHRP, respectively. To obtain the spectral properties of YHRP and IP, we applied a statistical analysis to the sampled footprints using spectral parameters (Table 2). For each spectral unit and each spectral parameter, we calculated the mean and standard deviation (Figure 3a). We found that the IP are within the error margin of the LBP and are not directly distinguishable (Figure 3a). The spectral differences between the two units are very small (Figure 3b) and there is no morphological difference (Murchie et al., 2015). Moreover, they are both mostly volcanic in origin. We have therefore decided to join the two units into a single LBP + IP spectral unit.

Using the statistical analysis, we defined unit-specific frameworks. Two confidence levels were established: level 1 (L1) and level 2 (L2). L1 is based on 100% of the sampled data remaining after the filters (temperature, incidence angles, and footprint size). L2 is based on 50% of the filtered sampled data between the first and third quartiles. To obtain a clear separation between each unit and avoid ambiguity for every parameter, if there is an overlap of frameworks between units for the same spectral parameter, the overlap range was cropped. This approach avoids any overlap between units, giving greater confidence in the established frameworks. The final frameworks used in this work are summarized in (Table 3).

Using the spectral classification, each spectrum (and thus footprint) was analyzed to determine whether the spectral properties belong to LRM, LBP + IP, HRP, or YHRP. Subsequently, each of the points defining the footprint will be mapped as LRM, LBP + IP, HRP, YHRP, or unclassified if none of these. For spectral classification, it is important to consider the varying abilities of spectral parameters to discriminate between units and to assign greater weight to those that most effectively distinguish them. To this purpose, Caminiti et al. (2023) created a quality parameter where the maximum mark is 16 points. This quality parameter highlights the strongest reliability of L2 (Table 3) and gives more importance to the VIS_Slope as this parameter allows for a better separation of units. Frameworks obtained with L2 (Table 3) easily separate the spectral units. The weight assigned to a footprint having one of the properties inside L2 is four. Among frameworks obtained with L1 (Table 3), the weight assigned to the VIS_Slope is three, two for R750, and one for the UV_Downturn and VISNIR_Slope. We used the thresholds defined by Caminiti et al. (2023) (9/16 for HRP and LBP + IP, and 6/16 for LRM) and applied a minimum of 5/12 for the YHRP. These thresholds provide an accurate spectral classification while providing sufficient footprints for a robust analysis. A threshold lower than 5/16 for the YHRP increases the possible ambiguity of classification, while a higher threshold removes the number of footprints, weakening the robustness of the analysis.

According to the high reflectance and slope associated with the YHRP spectral units, we observe some false positives. These false positives can be easily identified using the geological context, which means they did not influence our interpretations. Pyroclastic deposits can be mapped as YHRP. However, as the spectral properties of pyroclastic deposits are varied, they can't be isolated using only spectral data (Barraud et al., 2021; Besse et al., 2020; Leon-Dasi et al., 2023). In fact, rather than showing a unique spectral signature, pyroclastic deposits exhibit spectral parameters that span the range of most units on Mercury (Leon-Dasi et al., 2025).
<caption>**Table 3**</caption>

### Frameworks Obtained From the Statistical Analysis of the Sampled Areas for the Study of Low Reflectance Material (LRM), Low-Reflectance Blue Plains (LBP), Intermediate Plains (IP), High-Reflectance Red Plains (HRP), and Young High-Reflectance Red Plains (YHRP) Using Four Spectral Parameters

<table><thead><tr><th>Spectral parameters</th><th>LRM</th><th>LBP + IP</th><th>HRP</th><th>YHRP</th></tr></thead><tbody><tr><td colspan="5">Level 1: 100% of the sampled data</td></tr><tr><td>R750</td><td>0.026428 ≤ R750 &lt; 0.031190*</td><td>0.033036 &lt; R750 ≤ 0.041009*</td><td>0.041695 ≤ R750 &lt; 0.047107*</td><td>0.049263 &lt; R750 ≤ 0.053069*</td></tr><tr><td>VIS_Slope</td><td>0.45535 ≤ VIS_Slope ≤ 0.58875</td><td>0.62834 ≤ VIS_Slope ≤ 0.82735</td><td>0.87851 ≤ VIS_Slope ≤ 1.0143</td><td>1.0512 ≤ VIS_Slope ≤ 1.1578</td></tr><tr><td>UV_Downturn</td><td>2.9330 ≤ UV_Downturn &lt; 2.9828*</td><td>3.0502 ≤ UV_Downturn ≤ 3.1574</td><td>3.2008 &lt; UV_Downturn ≤ 3.2651*</td><td>—</td></tr><tr><td>VISNIR_Slope</td><td>0.47203 ≤ VISNIR_Slope &lt; 0.62447*</td><td>0.69185 ≤ VISNIR_Slope ≤ 0.93871</td><td>0.96957 &lt; VISNIR_Slope &lt; 1.0633*</td><td>1.1715 &lt; VISNIR_Slope ≤ 1.3591*</td></tr><tr><td colspan="5">Level 2: 50% of the sampled data between the first and third quartiles</td></tr><tr><td>R750</td><td>0.028663 ≤ R750 ≤ 0.030209</td><td>0.034903 ≤ R750 ≤ 0.039268</td><td>0.043686 ≤ R750 ≤ 0.046586</td><td>0.048757 ≤ R750 ≤ 0.050875</td></tr><tr><td>VIS_Slope</td><td>0.50592 ≤ VIS_Slope ≤ 0.54748</td><td>0.70260 ≤ VIS_Slope ≤ 0.79758</td><td>0.91430 ≤ VIS_Slope ≤ 0.97044</td><td>1.0816 ≤ VIS_Slope ≤ 1.1204</td></tr><tr><td>UV_Downturn</td><td>2.9959 ≤ UV_Downturn ≤ 3.0481</td><td>3.0724 ≤ UV_Downturn ≤ 3.1240</td><td>3.1902 ≤ UV_Downturn ≤ 3.2288</td><td>—</td></tr><tr><td>VISNIR_Slope</td><td>0.56128 ≤ VISNIR_Slope ≤ 0.64148</td><td>0.75108 ≤ VISNIR_Slope ≤ 0.85978</td><td>1.0193 ≤ VISNIR_Slope ≤ 1.0931</td><td>1.1719 ≤ VISNIR_Slope ≤ 1.2459</td></tr></tbody></table>

*Note.* The UV_Downturn parameter is not selective for YHRP. Frames identified by a \* were cropped because of an overlap between spectral units to avoid any ambiguity concerning the spectral distinction of units.

## 5.2. Comparison of Spectral Units

Figure 3a shows an evolution of spectral properties from older, darker, and “bluer” spectral units to younger, brighter, and redder units. As a reminder, the LRM characterizes the spectral properties of the primary crust (Klima et al., 2018) and therefore represents the oldest spectral unit. LBP, HRP, and YHRP are considered as secondary products due to volcanism. LBP has been argued to be older than HRP (Caminiti et al., 2023; Hirata et al., 2025), which is itself older than YHRP (Semenzato et al., 2020), reflecting a temporal evolution of magmas from LBP to YHRP at the origin of the evolution of spectral properties. Mercury's surface exhibits a spectral trend of decreasing reflectance with the decrease in the spectral slope (Ernst et al., 2010). This trend is in opposition with space weathering spectral alteration of silicates, where the surface darkens and reddens over time (Hapke et al., 1970; Pieters & Noble, 2016). Several hypotheses can be made to explain Mercury's trend.

First, it is possible that the spectral variations among spectral units can be attributed to compositional differences. The units can be considered to represent a compositional continuum (Ernst et al., 2010). However, compositional and spectral units are not necessarily correlated showing that the composition alone cannot justify all the spectral variations on Mercury (Murchie et al., 2015; K. Vander Kaaden et al., 2017).

Second, space weathering produces submicroscopic iron particles (SMFe) when an iron ion is freed from the silicate crystalline structure during solar wind or micrometeoroid bombardments. Optical changes are dependent on the size and amount of SMFe. Small SMFe cause systematic reddening and darkening while larger Britt-Pieters particles (>40 nm, Noble et al., 2007) cause darkening with no reddening (Lucey & Riner, 2011; Noble et al., 2007; Penttilä et al., 2020). Even though the iron content on Mercury's surface is very low (~<2 wt.% Fe according to Weider et al. (2014)), only a low concentration of SMFe (0.02 wt.% according to Noble et al. (2007)) is needed to influence the reflectance properties. The assumed 3.5 wt.% of SMFe on Mercury (Lucey & Riner, 2011) is enough to alter the spectral properties of the surface. On the 3.5 wt.% of predicted SMFe, 3 wt.% could be at the origin of a darkening with no reddening of the surface. Nevertheless, after a certain time exposure, the surface of Mercury could reach a saturation in space weathering limiting the spectral evolution of the surface (Caminiti et al., 2024). For units of several billion years, space weathering alone certainly cannot justify all the spectral variations on Mercury. In this work, the studied areas are globally older than 1 Ga, we expect the ground to be covered with optically mature regolith probably saturated by space weathering.

The spectral changes due to space weathering depend on the composition of the soil, so for two different compositions the effect of space weathering will be different, including the saturation effect (Caminiti et al., 2024; Riner & Lucey, 2012). The differences between spectral units are thus expected to reflect space-weathered compositionally different soil.
<figure><img src="image_6.png" /><figcaption>Figure 4.</figcaption></figure>

# 6. Results

## 6.1. Rembrandt

Spectral mapping was applied to the Rembrandt basin to better constrain the stratigraphy and geological evolution of the area since its formation. Our spectral classification identifies the interior YHRP and HRP, and exterior LBP + IP, HRP, and some YHRP (Figure 4a). The interior YHRP and HRP volcanic smooth plains are well identified confirming the observations of Semenzato et al. (2020) with MASCS-VIRS data (Figure 4b). These results emphasize that even locally, smooth plains are spectrally heterogeneous (Helbert et al., 2013). Moreover, some HRP and YHRP are found within the exterior plains, especially on topographic lows as expected (Whitten & Head, 2015). On the contrary, LBP + IP footprints are more widespread covering the surroundings of the basin (Figure 4c). The crater labeled “a” in Figure 4c is located on the interior plains and its ejecta deposits were previously mapped as LRM (Semenzato et al., 2020). However, in addition to LRM we noticed LBP + IP deposits around the crater, excavated by its formation. This crater is not the only example as the same observations can be done around craters labeled “b” and “c” (Figure 4c). Similarly to the Caloris basin (Caminiti et al., 2023; Hirata et al., 2025), these observations suggest the presence of an LBP + IP layer below the high-reflectance layers. This likely highlights a minimum of three effusive volcanic infills, although the volcanic character of the LBP + IP in this area has to be verified. Craters a, b, and c show HRP, suggesting that they formed before of during the HRP volcanic infill. Some YHRP footprints are identified within the interior of crater “b” (Figure 4b). These high-reflectance volcanic infills suggest that the crater possibly formed either before or during the HRP and YHRP events. Unlike “a” and “c,” this crater seems to have allowed YHRP to reach the surface (only a few footprints classified at YHRP are observed in “a” and “c”). Among the deposits of b and a several footprints are spectrally associated with LRM excavated from depth, showing the presence of LRM beneath Rembrandt. We then propose the following stratigraphy beneath Rembrandt from the oldest and deepest unit to the youngest and shallowest: impact melt and LRM, volcanic LBP + IP, volcanic HRP, volcanic YHRP.

## 6.2. Caloris

In order to further our understanding of the biggest well-preserved impact structure on Mercury, we applied the spectral mapping to the Caloris basin. We find similar results as in (Caminiti et al., 2023) (Figure 5a) with new details on the spectral properties of the interior plains. Our new spectral mapping allows for the classification of some previously mostly unclassified footprints as YHRP within Caloris interior plains (Figure 5b). This highlights

**Figure 4.** Spectral mapping of the Rembrandt basin. The base map is an Enhanced-color Global Mosaic (665 m/pixel) from MErcury Surface, Space ENVironment, GEochemistry, and Ranging-Mercury Dual Imaging System displaying its surface color variations. Mercury Atmospheric and Surface Composition Spectrometer footprints are over-plotted on the base map according to the spectral classification criteria. Footprints filtered out because of the instrumental temperature, incidence angle, or size are shown in gray. The filtered but unclassified footprints are in black. (a) All spectral units, (b) High-reflectance Red Plains and Young High-reflectance Red Plains, (c) Low-reflectance Blue Plains + Intermediate Plains and Low Reflectance Material. "a", "b", and "c" refer to unnamed craters located within Rembrandt.
<figure><img src="image_7.png" /><figcaption>**Figure 5.** Spectral mapping of the Caloris basin. The base map is an Enhanced-color Global Mosaic (665 m/pixel) from MErcury Surface, Space ENvironment, GEochemistry, and Ranging-Mercury Dual Imaging System displaying its surface color variations. Mercury Atmospheric and Surface Composition Spectrometer footprints are over-plotted to the base map according to the spectral classification criteria. Footprints filtered out because of the instrumental temperature, incidence angle, or size are shown in gray. The filtered but unclassified footprints are in black. (a) All spectral units, (b) High-reflectance Red Plains and Young High-reflectance Red Plains, (c) Low-reflectance Blue Plains + Intermediate Plains and Low Reflectance Material.</figcaption></figure>

the capacity of our new spectral classification to take into account smaller spectral contrasts than the one initially developed. This subtle spectral variation within Caloris interior plains was already observed by Murchie et al. (2008) using MDIS data without relating it to any specific volcanic event. By analogy with the Rembrandt basin, the YHRP within Caloris can be interpreted as the youngest spatially extensive effusive volcanic event within the basin. Based on this new result, we suggest that the Caloris basin was infilled by a minimum of three volcanic units, superimposed on a LRM and impact melt unit, from the oldest to the youngest, LBP + IP, HRP, and YHRP. The identification of YHRP within Caloris shows that this spectral unit, originally defined within Rembrandt, is not unique to that basin, but is instead part of the global evolution of volcanism on Mercury.

### 6.3. Rachmaninoff

The well-preserved LRM annulus between the peak-ring and the crater rim of Rachmaninoff (Klima et al., 2018; Zambon et al., 2022) is easily identifiable thanks to our spectral mapping (Figure 6A). With the exception of Suge Facula, there is no mix of spectral units within this annulus, perhaps indicative of the younger age of this basin in comparison with the other studied basins where spectral mixing is observed. As expected, HRP materials are distributed within the interior plains as well as in the southern part of the basin (Figure 6b) (Blair et al., 2013; Prockter et al., 2010). Our spectral mapping highlights some LBP + IP footprints within the interior of the basin (Figure 6c) that were previously identified as impact melt (Wright et al., 2024). This may indicate that the interior

<figure><img src="image_8.png" /><figcaption>**Figure 6.** Spectral mapping of the Rachmaninoff basin. The base map is an Enhanced-color Global Mosaic (665 m/pixel) from MErcury Surface, Space ENvironment, GEochemistry, and Ranging-Mercury Dual Imaging System displaying its surface color variations. Mercury Atmospheric and Surface Composition Spectrometer footprints are over-plotted to the base map according to the spectral classification criteria. Footprints filtered out because of the instrumental temperature, incidence angle, or size are shown in gray. The filtered but unclassified footprints are in black. (a) All spectral units, (b) High-reflectance Red Plains, and Young High-reflectance Red Plains, (c) Low-reflectance Blue Plains + Intermediate Plains and Low Reflectance Material.</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>**Figure 7.** Spectral mapping of the Beethoven basin. The base map is an Enhanced-color Global Mosaic (665 m/pixel) from MErury Surface, Space ENVironment, GEochemistry, and Ranging-Mercury Dual Imaging System displaying its surface color variations. Mercury Atmospheric and Surface Composition Spectrometer footprints are over-plotted to the base map according to the spectral classification criteria. Footprints filtered out because of the instrumental temperature, incidence angle, or size are shown in gray. The filtered but unclassified footprints are in black. (a) All spectral units, (b) High-reflectance Red Plains and Young High-reflectance Red Plains, (c) Low-reflectance Blue Plains + Intermediate Plains and Low Reflectance Material. "d", "e", and "f" refer to unnamed craters located within Beethoven.</figcaption></figure>

volcanic infill has not completely flooded the basin and is only a thin layer as proposed by Marchi et al. (2011). We possibly find some YHRP footprints within the southern volcanic unit south-west of the southern rim of the crater (Figure 6b). However, we observe only a few YHRP footprints, which are located on topographic features, within the interior plains (Figure 6b). Thus, there is a lack of extent YHRP among the interior plains. Wright et al. (2024) have proposed that if several generations of effusive volcanism did occur within the interior plains, they appear to have a relatively similar primary composition and probably occurred within a short span of geological time, unlike within Rembrandt (Semenzato et al., 2020). This hypothesis aligns with the absence of an extended spectrally distinct YHRP layer.

## 6.4. Beethoven

Figure 7a shows that the alteration of the area, because of Beethoven's older age, complicates the analysis of the basin due to the mixing of spectral units with time. The spectral mixing prevents the distinction between the interior and exterior volcanic spectral units in comparison with the younger basins. The interior plains (Figure 7b) are generally associated with HRP even though, certainly due to excavations by impact craters, footprints classified as LBP + IP are mapped (Figure 7c). Footprints classified as LBP + IP are mainly found on the walls, rims, and ejecta of craters and were previously mapped as well-preserved crater material areas (Lewang et al., 2018). This association of LPB + IP with impact material shows the same results as for Caloris and Rembrandt: an LBP + IP volcanic layer excavated by impact craters beneath the visible high-reflectance interior volcanic layers. Nevertheless, these LBP + IP ejecta are less preserved than those of Atget within the Caloris basin (Caminiti et al., 2023) (e.g., the deposits are heterogeneous around Bello and "e") (Figure 7c). We suggest that this is due to the HRP and YHRP infills of Beethoven that partially covered the LBP + IP deposits and/or mixed with them. This suggests that Bello and "e" craters formed before or at the same period as the HRP and YHRP volcanic infills leading to a mix of units. This is corroborated by the volcanic infills of Bello. Numerous footprints mapped as HRP and YHRP close to Bello's center are visible, showing an evolution of the volcanic properties of the crater's infills. We can see that footprints classified as YHRP are not found in fresh deposits in the center of the crater, which increases the reliability of their observations. We also suspect a possible volcanic infilling of "f" with HRP and YHRP material. However, the lack of classified footprints within "e" does not enable us to identify a volcanic infill with certainty. It is complicated to evaluate the reliability of footprints associated with YHRP at the south of Bello's rim due to the steep topography of the area. In comparison with Bello and "f," the crater named "d" in this study formed after the volcanic infills of Beethoven similarly to Atget within Caloris. The "d" crater excavated LRM and LBP + IP materials but did not show any HRP or YHRP infill. If LRM material was excavated by Bello, "e", or "f" the ejecta could have been buried, explaining the global absence of LRM deposits. The fresher "d" crater exposes LRM, indicating the presence of an LRM layer beneath the volcanic layers. Using these observations, we interpret the Beethoven impact basin to have the following stratigraphy: an impact melt and LRM layer buried beneath a volcanic LBP + IP layer, followed by another volcanic HRP layer and a final YHRP volcanic layer.
<figure><img src="image_10.png" /><figcaption>**Figure 8.** Spectral mapping of the Tolstoj basin. The base map is an Enhanced-color Global Mosaic (665 m/pixel) from MERcury Surface, Space ENvironment, GEochemistry, and Ranging-Mercury Dual Imaging System displaying its surface color variations. Mercury Atmospheric and Surface Composition Spectrometer footprints are over-plotted to the base map according to the spectral classification criteria. Footprints filtered out because of the instrumental temperature, incidence angle, or size are shown in gray. The filtered but unclassified footprints are in black. (a) All spectral units, (b) High-reflectance Red Plains and Young High-reflectance Red Plains, (c) Low-reflectance Blue Plains + Intermediate Plains and Low Reflectance Material.</figcaption></figure>

## 6.5. Tolstoj

The spectral mapping of Tolstoj partially highlights the well-known LRM annulus around the basin (Figures 8a and 8c) (Denevi et al., 2009; Klima et al., 2018). The annulus is less preserved than around the Rachmaninoff basin because Tolstoj is older and has undergone a mixing of its associated spectral units through time. On the eastern rim of the basin some fresh craters (in lighter blue on the MDIS enhanced color map) are responsible for spectral mixing that could explain the lack of LRM mapped on this part of the annulus. On the south-eastern part of the rim, some volcanic features and craters have also altered the LRM deposit. Impact structures like the Liszt crater (Figure 8c) are also responsible for the LRM annulus mixing on the western side of the crater. Focusing on the basin interior, it seems to have an asymmetric spectral unit distribution (Figure 8b). The MDIS enhanced color base map shows color variations between the eastern and western parts of the basin's interior, the eastern part being redder than the western part. Our spectral classification tends to show that the eastern and western parts are associated with HRP and LBP + IP, respectively. This observation possibly suggests that the western part of the interior plains did not experience an HRP volcanic infill. The study of Liszt crater, which formed after Tolstoj, corroborates this observation with LBP + IP in its center with no HRP. This spectral asymmetry could be induced by asymmetrical melting conditions. However, the lack of MASCS footprints in this area and the number of unclassified footprints prevent a clear analysis of this potential asymmetry. Similarly to the Rachmaninoff basin, no YHRP is observed on the spectral mapping of Tolstoj (Figure 8b). The only footprints mapped as YHRP are associated with crater rims or explosive volcanism and are considered false positives.

# 7. Discussion

## 7.1. Reconstruction of Basins' Stratigraphy

The number of volcanic events is one of the main points to take into account when reconstructing the geological and thermal evolution of Mercury. In this work, we show for the first time the presence of YHRP within impact basins other than Rembrandt (Semenzato et al., 2020). Our comparative study of Rembrandt, Caloris, Rachmaninoff, Beethoven, and Tolstoj reveals a similar stratigraphy and chronology of basin volcanic infills. After the formation of the basin, the first layer contains a mix of impact melt and LRM. Then, basins were infilled by an effusive volcanic layer with LBP spectral properties difficult to distinguish from the volcanic IP forming the secondary volcanic crust. Thereafter, the basins were infilled with more evolved magma leading to a second brighter and redder effusive volcanic layer with HRP spectral properties, covering the LBP + IP layer (Caminiti et al., 2023; Hirata et al., 2025). Finally, only in certain cases (within Rembrandt, Caloris, and Beethoven but not within Tolstoj and Rachmaninoff) a last effusive volcanic layer erupted. This last volcanic unit is redder and brighter than the HRP and spectrally associated with the YHRP. Hence, we propose that there exists a common spectral progression for impact basin infills on Mercury.
<figure><img src="image_11.png" /><figcaption>**Figure 9.** Relationship between the Young High-reflectance Red Plains (YHRP) spectral unit and topographic elevation. The base map is a colorized shaded relief map of the original digital elevation model (Becker et al., 2016). Mercury Atmospheric and Surface Composition Spectrometer/Visible and InfraRed Spectrograph footprints classified as YHRP are over-plotted.</figcaption></figure>

## 7.2. What Influences the Distribution of YHRP Within Impact Basins?

### 7.2.1. Effects of Basins Location

There is a non-uniform geographic distribution of basins on Mercury (Fassett et al., 2012; Orgel et al., 2020) that highlights the importance of comparing basins spread across the planet. The studied basins cover a wide range of latitudes and longitudes (Table 1). Rachmaninoff, located in the northern hemisphere, and Tolstoj, located in the southern hemisphere, lack an extended YRHP volcanic unit showing probably no N-S hemispherical dependence on the formation of YHRP. Moreover, Caloris in the northern hemisphere and Rembrandt in the southern hemisphere were both infilled by YHRP. There certainly is no hemisphere that prevents the production of YHRP compared to the other one.

Moreover, the formation of YHRP is probably not directly related to the surface temperature. Some YHRP are observed within Caloris, which is located on a “hot” pole, as well as within Rembrandt, which is located on a “warm” pole.

### 7.2.2. Effects of Topographic Elevation

We compared the YHRP locations with Mercury's original digital elevation model (Figure 9) (Becker et al., 2016) to study a potential correlation between the presence of YHRP and topographic elevation. On average, Rachmaninoff contains the lowest elevation, followed by Rembrandt, Beethoven, Tolstoj, and Caloris, suggesting no correlation between the presence of YHRP and the topographic elevation. The location of YHRP within Caloris covers several elevations from slightly below 0 to around 1,800 m above the reference radius, showing no specific correlation between the presence of YHRP and current topographic elevation. The lack of correlation between YHRP and variations in elevation is consistent with their emplacement predating most of the changes in long-wavelength topography that affected this basin (Ernst et al., 2015). An exception is Rembrandt, where most of the YHRP are found in a low-lying area.
### 7.2.3. Effects of Crustal Thickness

We investigated a possible correlation between YHRP and crustal thickness (Table 1). Crustal thickness maps have been obtained using different models (Beuthe et al., 2020; Buoninfante et al., 2023; Watters et al., 2021), showing a consistent co-location of smooth plains and thin crust (Wang et al., 2021). This association may be due to the easier ascent of the magma with less confining pressure. We observe that the maximum crustal thickness is lower for Rachmaninoff and Tolstoj than Rembrandt, Caloris and Beethoven (Table 1), suggesting a possible correlation between the presence of YHRP and the higher crustal thickness which might favor fractional crystallization during their ascent. However, the interior of the Caloris basin is divided into two parts, one in the center where the crustal thickness is <20 km and one surrounding this area where the crustal thickness is slightly higher: between about 20 and 40 km thick. The YHRP are distributed in both these areas, suggesting no correlation with the current crustal thickness. The lack of correlation is corroborated by other basins, especially Rembrandt for which the current data do not allow any distinction between HRP and YRHP using the crustal thickness. YHRP could flow into areas where the crust is thinner or thicker and drain at different locations depending on the paleotopography at the time of their emplacement. These observations on the location of YHRP have only been made on our five basins, and a systematic study of the YHRP will have to be carried out to consolidate a possible correlation between crustal thickness and YHRP.

### 7.2.4. Effects of Elemental Composition in the Impacted Area

Given the current resolution of elemental compositional data, the effect of the surface composition should be interpreted with caution, especially in the southern hemisphere. However, the formation of YHRP does not seem to be influenced by the elemental composition of the surrounding surface. Among our studied areas, Rachmaninoff has the richest Mg/Si content while Caloris has the lowest (Nittler et al., 2020). In between, the Mg/Si content decreases from Beethoven to Rembrandt and Tolstoj (Table 1). Within the Caloris basin, compositional heterogeneities have been highlighted, but we found no direct link with the YHRP distribution. All of this suggests that the YHRP emplacement has no relation to the overall elemental composition of the surface at the impact location.

### 7.2.5. Effects of Age

The age of the basin is an important parameter to take into account when analyzing the surface properties and reconstructing the geological evolution of the area. Comparison of basins of different ages allows the study of the effect of crater degradation on the identification of spectral units. The older basins are less well preserved than the more recent ones. This is illustrated by the mixing of spectral units and the degradation of the morphological properties that complicate the analysis. Comparing basins of different ages also allows us to study the evolution of effusive volcanic infills over time. The oldest basin, Tolstoj, and the youngest, Rachmaninoff, studied here (Table 1) are not infilled by YHRP. However, they are both infilled by volcanic HRP, showing that their respective ages are not a limitation to volcanic activities. This supports the hypothesis that the emplacement of YHRP does not depend on the age of the basin for our five basins. However, the age of the volcanic infill is a relevant parameter to investigate the origin of YHRP. Tolstoj and Rachmaninoff both have interior volcanic infills which formed respectively around 3.65 and 1.6 Ga: after the ones within Caloris, Rembrandt, and Beethoven (3.7–3.8 Ga) (Byrne et al., 2016; Wang et al., 2021). Basins showing no extensive YHRP unit have a more recent volcanic infill. This could be related to the secular cooling of the planet which would limit the formation and/or ascent of evolved magma, such as YHRP, to the surface (Byrne et al., 2016). Because of secular cooling, more evolved magma with spectral properties associated with YHRP may not be able to form, or may form but not reach the surface.

### 7.2.6. Effects of Crater Size

The crater's size is a reliable indicator of the maximum excavation depth and maximum depth of melting. For a given target material, craters of comparable size have comparable maximum excavation depth and maximum depth of melting (Ernst et al., 2010; D'Incecco et al., 2015, 2016). Impacts induce melting by different processes (e.g., Padovan et al., 2017). The release of shock-pressure due to the impact can increase the temperature around the impact location and create impact melt forming a melt pool, which rapidly solidifies. Moreover, the excava- tion of material by the impact formation can modify the lithostatic pressure under the crater and induce in situ
decompression melting (Elkins-Tanton et al., 2004). The interaction between the impact-induced thermal anomaly in the subsurface and the pre-existing temperature field triggers convective motions, which may subsequently lead to additional post-impact melting (Elkins-Tanton et al., 2004; Padovan et al., 2017; Roberts & Barnouin, 2012). For a given target material, the greater the energy released by the impact (i.e., the greater the impactor and thus the crater size), the greater the amount of melt produced by these processes. Looking at the crater diameters, Rachmaninoff and Tolstoj differ from the others for their smaller diameter <400 km (Table 1). The size of the crater could then be an important parameter for the formation of YHRP on Mercury. Below the smallest basins, evolved magma with spectral properties associated with YHRP may not be able to form. If the YHRP are a direct consequence of the basin formation, only basins of sufficient size to have either reached a specific layer at depth or generated a strong enough internal thermal anomaly could subsequently be filled with YHRP. However Hirata et al. (2022) compared Caloris and Rembrandt and concluded that the larger diameter of Caloris enables an easier magma ascent, while magma production in the mantle should be approximately similar. This suggests that there are no large spatial variations in the abundance of heat-producing elements in the mantle and lower crust of Mercury, and highlights the major role of the crater's size in associated volcanism. YHRP could then be formed at depth, but could not reach the surface if the impact basin and thus the induced fractures are not large enough to facilitate the ascent.

### 7.3. Access to the Mantle Properties

D'Incecco et al. (2016) showed that there are horizontal and vertical heterogeneities in the shallow crust on Mercury via the comparative analysis of several impact craters. Our study is different because we have used larger basins and are studying the volcanic infills, giving information about the mantle's properties in addition to the crust. The study of volcanic smooth plains within impact basins provides insight into their mantle magmatic sources. We have suggested that the age of the volcanic infills and/or the size of the craters likely determine the presence of YHRP. They can constrain the formation and/or ascent to the surface of the magma with YHRP spectral properties. In the following sections, we will discuss the formation of YHRP considering that they can reach the surface through the fractured lithosphere in the region below and around the impact.

#### 7.3.1. Formation of YHRP: Considering a Homogeneous Mantle Layer

##### 7.3.1.1. Partial Melting

Tolstoj and Rachmaninoff are smaller than the other basins studied, which shows that the energy of the impact that caused their formation was certainly less significant than for larger basins such as Caloris, Rembrandt, and Beethoven. This lower energy could explain why the partial melting in depth was less intense and shorter, possibly preventing the formation of sufficiently evolved lavas with the properties of YHRP (Figure 10a). The age of the volcanic infills is also crucial because the properties of the magmas will vary according to the secular cooling of the planet. The secular cooling of the mantle during the early history of Mercury can possibly explain the variation in surface properties due to a decrease of the average melting degree (Namur et al., 2016). The global thermal evolution of the planet may have contributed to the absence of YHRP within Tolstoj and Rachmaninoff due to lower overall temperatures and thus melting degree, compared with the infills of Caloris, Rembrandt and Beethoven.

Beuthe et al. (2020) have suggested that there is a correlation between the surface geochemical units, mantle melt production, and crustal thickness. The higher the degree of partial melting, the thicker and more Mg rich the crust. Following this logic, the YHRP being formed after the HRP, the YHRP should be associated with a thinner crust produced by a lower partial melting degree. However, we discussed the ambiguous link between YHRP and crustal thickness, showing that our study cannot conclude about a decrease in melting production between the HRP and YHRP. However, an important parameter to consider is that the YHRP are not an isolated volcanic layer but are superimposed on a minimum of two underlying volcanic layers (LBP + IP and HRP) in addition to the LRM and impact melt layer. The accumulation of several volcanic layers complicates the link between the melting rate and the overall crustal thickness.

##### 7.3.1.2. Melting Depth

Depending on the pressure conditions and therefore the depth, some minerals are more stable than others and will be more or less subject to partial melting. The composition of the magma will therefore vary according to the
<figure><img src="image_12.png" /><figcaption>**Figure 10.** Schematic diagram of the stratigraphy of volcanic events that contributed to the infilling of basins, including the Young High-reflectance Red Plains (YHRP). (a) Scenario of YHRP formation by partial melting. (b) Scenario of YHRP formation by variation in partial melting depth. (c) Scenario of YHRP formation by fractional crystallisation. Not to scale.</figcaption></figure>

depth of melting (Stockstill-Cahill et al., 2012) (Figure 10b). In fact, a stable mineral does not melt easily. Depending on the pressure and temperature conditions, the stability of mineral phases changes, and consequently so does the composition of the resulting lava. Between the oldest and most recent volcanic plains, there has been an evolution in mantle melting conditions from deeper and hotter to shallower and cooler. The rapid mantle convection induced by the high temperatures during the first hundreds of millions of years of Mercury's history led to a rapid cooling and change in temperature-pressure evolution (Namur et al., 2016). These changes led to the production of lavas with different compositions, reflecting the stability of the minerals (Namur & Charlier, 2017; Namur et al., 2016). Lava may have successively evolved from being forsterite-dominated, to forsterite + diopside, and finally forsterite + diopside + plagioclase (Namur & Charlier, 2017). These compositional differences could explain some spectral differences. Moreover, impact-induced changes in source depth have been modeled by Padovan et al. (2017) who showed that the source depth of the volcanic infilling of large impact basins becomes shallower with time. If the partial melting at depth at the origin of the volcanic infills of Tolstoj and Rachmaninoff did not last long enough, due to the weaker impact energy or secular cooling because of the
younger ages of their volcanic infills, it is possible that the infills do not show variations in the depth of melting in their lavas at the origin of YHRP (Figure 10B).

### 7.3.1.3. Fractional Crystallization

After its production by partial melting, a magma can evolve by fractional crystallization if early-formed crystals are left behind as the magma ascends. The physical and chemical properties of the crystallized minerals, as well as the residual melt, change temporally. Thus, fractional crystallization produces lavas with different compositions over time, which could give rise to different spectral properties (Figure 10c). Faster-cooling magma will have a final composition closer to that of the initial magma while slower-cooling magma will have a more differentiated composition. Namur and Charlier (2017) have experimentally shown that magma with Mercury's crustal composition undergoing fractional crystallization will have different composition according to the fractional crystallization. This hypothesis was recently corroborated by Hirata et al. (2025) who showed that fractional crystallization could be at the origin of the spectral and compositional differences between Caloris LBP and HRP. If the YHRP are obtained by the evolution of the HRP themselves from the differentiation of LBP, we expect a low Mg composition for the YHRP (Hirata et al., 2025). If the fractional crystallization of magma that infilled Tolstoj and Rachmaninoff did not last long enough due to faster cooling, because of a weaker impact energy or secular cooling due to later volcanic infills, it is possible that the infills do not show YHRP in their associated lavas (Figure 10c).

### 7.3.2. Formation of YHRP: Considering a Vertically Heterogeneous Mantle

Variations of Mercury's volcanic secondary crust properties may reflect potential vertical heterogeneity (Charlier et al., 2013). Large impacts dig deeper into the surface and allow the excavation and melt of deeper layers than smaller impacts. It is possible that a deep layer, sampled only by basins of diameter >400 km (Table 1), will be at the origin of the YHRP unit. Ernst et al. (2015) have shown that the melting depth of Caloris, Rembrandt, and Tolstoj are respectively around 220, 98, and 44 km. Beethoven, being bigger than Tolstoj but smaller than Rembrandt, ought to have a melting depth between 98 and 44 km. The smallest studied basin, Rachmaninoff, has the smallest melting depth (<44 km). Tolstoj and Rachmaninoff, the two smallest of the studied basins, would then have a melting depth lower (or equal) than 44 km. Based on the estimated crustal thicknesses (Beuthe et al., 2020), the inferred melting depths for all basins lie within the mantle. In the case of Rachmaninoff, our smallest basin, the crust is locally thinner, which allows the melting depth to reach the mantle. At a depth of around 44 km, there could be a change in mantle properties. This would lead to the production of YHRP from a deeper material.

Although local anomalies cannot be ruled out in this work, this scenario could suggest a vertically heterogeneous mantle. Namur et al. (2016) proposed several explanations at the origin of possible mantle heterogeneity. Crystallization of the magma ocean could have generated lateral and/or vertical compositional stratification. Mantle cell convection could have led to vertical migration of heterogeneous mantle parcels. Mantle sources could have progressively evolved by melt extraction through time. These phenomena can explain the potential current vertical heterogeneity of Mercury's mantle at the origin of the surface spectral properties.

However, we detailed other scenarios for the formation of YHRP, which do not need any heterogeneity in the mantle. This suggests that there is certainly no need for any heterogeneity in the mantle to explain the surface spectral properties of volcanic infills on Mercury.

## 8. Conclusions and Perspectives

### 8.1. Conclusion

In this work, we analyzed the spectral characteristics of five major impact structures on Mercury: Caloris, Rembrandt, Rachmaninoff, Beethoven, and Tolstoj. We extended the spectral classification developed by Caminiti et al. (2023) using suitable spectral parameters and taking into account the main properties of Mercury's surface. We investigated five spectral units using MESSENGER-MASCS-VIRS data: the Low-Reflectance Material (LRM), the Low-reflectance Blue Plains (LBP), the Intermediate Plains (IP), the High-reflectance Red Plains (HRP) and the so-called Young High-reflectance Red Plains (YHRP).
1. We have shown that the YHRP is a major spectral unit associated with effusive volcanism and impact basins on Mercury.

2. Our spectral mapping allows for the reconstruction of the stratigraphy and timing of effusive volcanism events associated with major impact basins on Mercury. We propose that there is a common sequence of volcanic episodes within impact basins covering the LRM and impact melt bottom layer. The first volcanic layer is spectrally associated with LBP + IP, while the second, more evolved, is associated with HRP. We propose a third, more recent, volcanic unit with spectral properties associated with YHRP.

3. We used surface spectral properties of basins' volcanic infills to explore Mercury's internal diversity and evolution. Our results suggest that the crater's size and the age of the volcanic infills could drive the magmatic evolution and thus the spectral properties of the surface.

4. Heterogeneity in the mantle is not necessary to explain the spectral properties of effusive volcanism associated with impact basins, but can not be excluded.

Our spectral classification is a powerful tool for comparing several basins and obtaining information about Mercury's interior. We recommend applying this spectral classification more globally on the planet, including other basins and plains (e.g., borealis planitia) to consolidate the observations of the present work. For a deeper analysis of each basin individually, we suggest using similar data fusion techniques as in Wright et al. (2024) and/or Hirata et al. (2025). This will provide the opportunity to study new geostratigraphic units that contain information on spectral properties, composition, and terrain morphology.

## 8.2. Future Contributions From BepiColombo

This work highlights some objectives and areas of interest for the upcoming ESA-JAXA-BepiColombo mission (Benkhoff et al., 2021). MESSENGER was placed in an elliptical polar orbit reducing the spatial measurement resolution of several instruments in the southern hemisphere. This limitation constrains our study, especially due to a lack of sufficiently resolved compositional data. BepiColombo will provide data with a better resolution allowing a more extensive study of the surface. The Spectrometers and Imagers for MPO BepiColombo Integrated Observatory SYStem (SIMBIO-SYS) (Cremonese et al., 2020), the MERcury Thermal Infrared Spectrometer (Hiesinger et al., 2020), Mercury Gamma-ray and Neutron Spectrometer (MGNS) (Mitrofanov et al., 2021), and the Mercury Imaging X-ray Spectrometer (Bunce et al., 2020) will provide morphological, spectral, and compositional maps of Mercury's surface. SIMBIO-SYS will analyze the surface including the geology, volcanism, surface age, geophysics, and composition. The instrument suite is composed of three channels: The Stereo Channel, The High spatial Resolution Imaging Channel (HRIC), and the Visible Infrared Hyperspectral Imager Channel (VIHI). STC will provide global color coverage of the surface in full stereo to define the main geological units and impact crater population. HRIC will characterize special surface targets with high-resolution images. It would be interesting to obtain HRIC images of YHRP and HRP (e.g., within Rembrandt) to compare their texture and further constrain their age difference, as well as to compare their probable volcanic origin. VIHI (Capaccioni et al., 2010) will operate in the visible to near-infrared (0.4–2 µm). The instrument will image the surface to provide a global mineralogical composition map, helping to define boundaries between different units. Through impact craters, VIHI will obtain a mineralogical characterization of different layers probing Mercury's interior and helping in the stratigraphy reconstruction. The instrument will also identify weathering products and effects on the surface's properties. MERTIS will obtain information about the mineralogical composition by a global mapping from 7 to 14 µm. MERTIS will also complement SIMBIO-SYS in the understanding of space weathering (Caminiti et al., 2024). MIXS and MGNS will produce elemental abundance maps of key rock-forming elements with a better resolution than MESSENGER. Additional instruments will contribute to a better understanding of impact basins' volcanic infills, such as BepiColombo Laser Altimeter (BELA) (Thomas et al., 2021) which will characterize the topography and surface morphology. The Mercury Orbiter Radio-science Experiment (Iess et al., 2009) will provide geophysical and geodesy data of major interest for the study of impact basins and their geological history. In addition, the Search for Exospheric Refilling and Emitted Natural Abundances (SERENA) instrument (Orsini et al., 2021) will also help to constrain space weathering and clarify our interpretations. This work shows the importance of future data processing and comparison between different instruments onboard BepiColombo. The methodology shown in this comparative study of major impact basins displays what can be done with the future global data set from BepiColombo and highlights specific spectral parameters crucial to distinguishing surface units.
## Conflict of Interest

The authors declare no conflicts of interest relevant to this study.

## Data Availability Statement

Data used in this work are publicly available on the Planetary Data System (PDS) (Washington University, St. Louis). The data set used contains the latest calibration provided by the MESSENGER science team. https://pds-geosciences.wustl.edu/missions/messenger/mascs.htm. To combine the VIS and NIR data into continuous spectra from 300 to 1,450 nm, we used the method developed by Besse et al. (2015). For easier access to the data, the filtered spectra used for the spectral classification shown in Figures 4–9 are publicly available in Caminiti (2025). All spectral parameters used in the present work can be calculated directly using this document.

## Acknowledgments

The authors acknowledge the Centre National d'Etudes Spatiales (CNES) for continuous and long-term support. We acknowledge the Domaine d'Intérêt Majeur en Astrophysique et Conditions d'Apparition de la Vie+île de France for their support. EC acknowledges the support of the European Space Astronomy Center (ESAC) faculty council for funding visits to ESAC as part of this work. JW acknowledges support from the European Space Agency (ESA) as an ESA Research Fellow and from the Horizon Europe program as a Marie Skłodowska-Curie Actions (MSCA) Postdoctoral Fellow, grant agreement no. 101208070, received during the write-up of this work. JW also acknowledges additional support from the Open University as a Visiting Fellow. The authors would like to sincerely thank Matteo Massironi and Piero D'Incecco for reviewing this manuscript. Funded by the European Union. Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or the European Commission. Neither the European Union nor the granting authority can be held responsible for them.

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