Hollows on Mercury: A Comprehensive Analysis of Spatial Patterns and Their Relationship to Craters and Structures
Authors: De Toffoli Barbara , Galluzzi Valentina , Massironi Matteo , Besse Sebastien , Schmidt Gene Walter , Barraud Oceane , Buoninfante Salvatore , Palumbo Pasquale 
Published: July 2024

IntroductionMercury's hollows are small (tens of meters to few kilometers), localized, shallow depressions found on the surface of the planet. They are unique features distinct from their surroundings, and characterized by their appearance as irregular, bright spots, often found clustered in groups called hollow fields. Their origin is still not fully understood, but they are believed to be related to sublimation processes, where volatile materials [1; 2] close to the surface directly transition from solid to gas due to exposure to high temperatures due for example to intense sunlight or volcanic activity [3]. However, as the ESA/JAXA BepiColombo spacecraft is approaching Mercury [4], understanding the specific geological and environmental factors influencing the formation and distribution of hollows remains a key research objective [5].&#160;&#160;MethodsTo improve our grasp on this topic, we herein renew the previous hollows dataset provided by [7] by updating the database and its degree of detail. Indeed, in this work we make use of MESSENGER end of mission mosaic datasets [6] in order to exploit the most updated data that were not yet available at the time when the previous database [7] was released. We provide GIS-ready polygonal features to encompass areas where fields of hollows are present on the surface and present a statistical analysis on their occurrence across the surface. We also provide statistical information on the craters hosting hollows and compare them to the global crater database [8]. This study aims at understanding the global stratigraphic occurrence of hollows through a statistical approach, setting a foundation for future multidisciplinary research, rather than focusing solely on their intrinsic morphology or composition. By compiling this updated global database of hollows, we can preliminarily explore their stratigraphic occurrence to investigate their debated formation origins. Since most of the population of hollows is contained within impact craters, which by definition excavate the crust of the planet exposing the underlying stratigraphy, it is possible to investigate whether relationships exist between the presence of hollows and specific crater populations and, in turn, constrain whether there is one or more identifiable source layers. Specifically, to profitably test these hypotheses, we compared the population of craters containing hollows (c. 430 in total) and an existing dataset of craters on Mercury [8]. This database, that we will refer to as the global population, is based on a thorough mapping effort that provided a broad and nearly global coverage of Mercury craters by classifying them into 5 morphological classes [8]. Although other datasets exist, and this one includes only craters with diameters larger than 40 km, it represents one of the most recent and complete datasets available that also considers morphological classification for a statistically significant number of craters. By comparing diameter, depth, and degradation between the two crater datasets, it is possible to reveal differences between the global crater population and the subpopulation of hollow containing craters. This helps understanding whether the hollow containing craters are a random subset of the global population, and thus closely replicates its main characteristics (i.e. diameter, elevation, degradation), or not.&#160;ResultsHollows occurrence seems to be ubiquitous and variously spatially correlated with multiple surface morphologies, foremost among them impact craters. In conclusion, the nuanced relationship between hollows and Mercury's geological history unveils the intricate interplay of endogenous and exogenous processes. The dynamic nature of hollows, with their occurrence in association with both impact and volcano-tectonic events, hints at a complex history of material redistribution on the planet's surface. Overall, we provide quantitative evidence supporting: 1- the lack of a single (or a limited and measurable number) planet-wide unit bearing the volatile materials necessary for hollows formation due to: (i) widespread depth range of hollows appearance in the crust; (ii) lack of correlation between hollow emergence and one or more geological units (including color units). 2- the short-lived nature of hollows due to: (i) a significant lack, or total absence, of hollows in older and more degraded craters compared to younger, fresher craters; (ii) a higher abundance of hollows correlated to tectonic structures or pits when located within older and degraded craters. Hence, from a stratigraphic point of view, this work highlights the likely existence of widespread presence of a both horizontally and vertically discontinuous volatile-bearing formation.&#160;References[1] Blewett, D., et al. (2013). JGR-Planets, 118, 1013&#8211;1032. [2] Barraud, O., et al., (2020). JGR-Planets, 125, e2020JE006497. [3] Blewett, D., et al. (2011). JGR-Planets, 116(E12). [4] Benkhoff, J., et al., (2021). Space Sci Rev, 217(8), 90. [5] Rothery, D., et al., (2020). Space Sci Rev, 216, 1-46. [6] Denevi, B.W., et al., (2018). Space Sci. Rev. 214, 2. [7] Thomas, R. J., et al., (2014), Icarus, 229, 221&#8211;235. [8] Kinczyk, M. J., et al., (2020). Icarus, 341, 113637.&#160;AcknowledgementAuthors from Italian Institutes acknowledge Italian Space Agency (ASI) support within SIMBIOS-SYS project under ASI-INAF agreement 2017-47-H.0. Some of the authors gratefully thank the European Union &#8211; NextGenerationEU and the 2023 STARS Grants@Unipd programme &#8211; &#8220;HECATE project&#8221; support.