The Geological Map of Mimas v1.0-2023
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1. Introduction
Geological mapping is one of the most prolonged and fundamental methods in geological research. It is rooted in the studies of Martin Lister (1639–1712), a naturalist who suggested visualizing the soil type distribution on maps. Lister’s idea was realized by Luigi Ferdinando Marsili (1658–1730). Following his contribution to the military by creating detailed maps of the landscape, including the rock outcrops, he mapped the distribution of gypsum and sulfur deposits around Bologna (1717), and later he published a map of mining districts in Hungary (1726) [1]. Unfortunately, the name of the cartographer who created the actual first geological map (1757) containing four different rock types, including sandstone (Bunter Sandstein, Buntsandstein), limestone (Mushelkalkstein), chalk (Kreide), and coal beds (Kohle), was lost. However, the three former rock types later became the basis of Mesozoic litho- and chronostratigraphy (Friedrich August von Alberti, 1834) [2].
Geological maps not only provide information about the geological features of a surface or the even areas below, but they also reveal the geological evolution of the studied area via the chronological and tectonic information embedded in the map.
In contrast to geological mapping on Earth, such expectations brought new challenges to researchers mapping the surfaces of various planetary bodies. Besides the Moon, geologists could not conduct fieldwork on different planetary bodies and draw geological maps based on their field observations.
Planetary mapping is divided into two significant periods: the phase of early observations from Earth (visual era), followed by the photographic era, which is when planetary scientists based their mapping on photographs from various sources (spacecraft, orbiters, and so on) [3].
On-site automatic missions and later human missions to the Moon needed more accurate maps containing additional geological elements to determine landing sites. Such needs led to the geological and geomorphological mapping of specific areas on the surface of the Moon and the creation of the first photo-image-based “astrogeological” maps, which comprised surface morphologic–structural elements, various regions with surface materials with various characters, and the stratigraphic and consequently chronological relation of those elements [3]. Mapping these elements required the involvement of new fields concerned with the study of planetary bodies, such as geology and geomorphology. This resulted in the development of the multi- and interdisciplinary fields of planetology or planetary science. In recent decades, photo-image-based maps have been completed, with additional information provided by various sensors (e.g., laser and radar altimetry and reflectance spectroscopy), and paper maps and atlases have been replaced by GIS (geographical information system) databases, along with digitalized, renovated versions of old maps [3].
There has been considerable development in the geological mapping of the Moon and terrestrial planets, including Mercury, Mars, and even Venus, despite the difficulties caused by the planets’ massive atmospheres. In contrast to those planetary bodies, the mapping of outer-solar-system objects, including the moons (also called satellites) of gas (Jupiter and Saturn) and ice giants (Neptune and Uranus), only started in the 1980s and was most likely triggered by two factors. There was a technical factor. Namely, the first spacecraft arrived at Jupiter (e.g., Voyager 1, 1979) and Saturn (e.g., Voyager 2, 1981) and sent the first images back to Earth around the end of the 1970s, allowing researchers to interpret various surface features and create the first maps of multiple satellites. An additional factor to be considered, which most likely triggered the renaissance of the study of outer-solar-system satellites, was the discovery of some subsurface water reservoirs and subsurface oceans on, e.g., Enceladus [4] and Europa [5,6], and the potential of life under the ice shell [7]. Along with the growing interest in the moons of the outer solar system, the list of potential icy satellites with subsurface oceans (harboring life) has been increasing exponentially [8,9].
Among the Jovian moons, Europa and Ganymede most likely have subsurface oceans hidden under their ice crusts [5,6,10,11,12]. The observed water plume on the images sent by Cassini’s spacecraft provided evidence of a subsurface reservoir and/or ocean under the frozen surface of Enceladus, the moon of Saturn [4,13]. Along with Enceladus, one of the moons in the outer solar system that has the most potential regarding the existence of, e.g., microbial life, is Titan, which also hosts a subsurface liquid layer between two icy units in its interior [14]. Based on calculations in very recent studies, numerous satellites of Uranus, including Ariel, Umbriel, Titania, and Oberon, may host liquid spheres as subsurface oceans under their surfaces [15,16]. Like Uranus, one moon of Neptune, Triton, may hide an ocean under its ice shell [17].
Like Earth, oxidants must be transported downward from the icy surface to keep those subsurface oceans oxygenated and keep the potential biosphere alive [18]. Two mechanisms that may be capable of executing such transport and material exchange between the surface and subsurface regions are cryotectonism and cryovolcanism. Based on our current knowledge and given the existing resources, the easiest way to identify and characterize these processes is via remote sensing-based (e.g., image-based mosaic maps and photogrammetry) geological and geomorphological mapping of the target icy satellites.
The growing number of candidates with subsurface oceans and the importance of geological mapping in the understanding of surface processes and the evolution of icy moons (along with the reconstruction of the planetary environment and knowledge of possible exobiological evolution) have triggered the geological mapping of icy satellites, such as Europa [19], Ganymede [20] (Jupiter), Enceladus [21], Dione [22], Titan [23] (Saturn), and Triton [24] (Neptune) (Table 1).
Despite numerous similarities, compared to most of the icy satellites listed in Table 1, Mimas has been considered an inactive moon, with no characteristic surface sign of global (cryo)tectonic activity and any significant, relatively young surface renewal processes. Based on the crater-counting method, the absolute age of the heavily cratered regions is around 4.3 Ga old, compared to the relatively young giant impact Herschel, which dates back roughly 4.1 Gyr ago [49].
Such common scientific knowledge about Mimas has been changing in the last decade, with a series of studies discussing the possibility of a subsurface “stealth” ocean below the frozen and ancient-looking surface of the satellite [36,37,38,39,40,41,42]. Although the appearance of a subsurface ocean looks plausible, there are still some controversies about the formation of a subsurface ocean and the lack of any characteristic marks of (cryo)tectonic processes on the surface, which may indicate indirectly the appearance of such a liquid layer below the ice crust. Based on various models, the lack of such surface features is explained in various ways, including a strong ice shell, which may withstand higher tidal stresses [39], or the young geological age of the subsurface ocean, which may explain the lack of tectonic features [42], i.e., the early tectonic evolution phase of the shell is “in progress” (e.g., even stagnant lid tectonism is still not recognizable [50]).
Along with the executed model studies, recognizing any surface mark of active or inactive stress fields feels crucial. This is not simply from the perspective of the existence of a subsurface ocean, which may harbor extraterrestrial life, but also for discerning the origin and evolution of the satellite itself. Observing potential marks of early-phase tectonism would support the theory about the “ring origin” of the moon instead of the primordial accretion theory [42]. Despite its time-consuming nature, global and detailed “fresh” Cassini image-based geological mapping of Mimas may solve some unanswered questions.
This study presents the first version of the semi-global geological map of Mimas, the icy satellite of Saturn, focusing on the first vital observations and raising concerns while interpreting the results.
2. Materials and Methods
During the global-scale geological mapping of the icy satellite, three base maps were used, namely Mimas Global Map—June 2017, Global 3-Color Map of Mimas (2014), and a supposedly earlier version of the former Mimas Global Map which appeared in the JMars version 5.3.15.2 (https://jmars.asu.edu/ (accessed on 23 December 2023)), named the Cassini ISS Cartographic Map of Mimas (Figure 1).
Despite the similarity between the former (Mimas Global Map—June 2017) and the Cassini ISS Cartographic Map, there are significant differences between the two image mosaic maps. Such differences were found during the mapping, e.g., in the shape of craters, and are discussed in Section 3 and Section 4. Both maps are mainly based on images taken during Cassini spacecraft flybys during the cooperative Cassini–Huygens mission between NASA, the European Space Agency, and the Italian Space Agency. While creating the Cassini ISS Cartographic Map of Mimas, Voyager 1 and 2 images were also used to fill some gaps. Compared to the mosaic that appears in Jmars, which looked very similar to the published initially Mimas global mosaic map, called Map of Mimas—June 2012 (https://photojournal.jpl.nasa.gov/catalog/PIA14926 (accessed on 23 December 2023)), the suggested 2012 version was updated with new images following the two most recent flybys in November 2016 and February 2017, and published as Map of Mimas—June 2017 (https://www.jpl.nasa.gov/images/pia17214-mimas-global-map-june-2017 (accessed on 23 December 2023)).
Along with the two versions of Cassini image mosaic maps, the Color Map of Mimas—2014 (https://www.jpl.nasa.gov/images/pia18437-color-maps-of-mimas-2014 (accessed on 23 December 2023)) was also used as a reference during the study. The image selection, radiometric calibration, geographic registration, and photometric correction, as well as mosaic selection and assembly, were performed at the Lunar and Planetary Institute (Houston, TX, USA).
The suggested update (June 2017) changed some of the map’s content, which potentially affected the interpretation of certain features. It resulted in the use and comparison of the three maps listed above during the geological mapping. The bias triggered by the differences between the maps is discussed in a later section of the study.
Along with the Cassini image-based global mosaic maps, maps and descriptions from the earliest publications about Mimas geology were also used [51,52].
Geological mapping and some related GIS research were performed using QGIS 3.22 software, followed by statistical analysis, which was executed with various Python 3.10.4 software packages, including NumPy 1.22.012, matplotlib, pandas, SciPy 1.8.0, and seaborn. All features, craters, and structural elements were interpreted visually and digitized interactively from the high-resolution and georeferenced images in QGIS 3.22. Basic information about the craters and lineaments was collected in a QGIS database and was used as a primary source for the analysis in Python 3.10.4 software. Besides the two basic geological maps published in the early 1990s [51,52], no datasets were used during the geological mapping. All presented results are connected to the executed research; no other database was used.
Considering the goal of this study, i.e., introducing the preliminary results of the mapping, the study does not contain the results of the in-progress stratigraphic (morphostratigraphy) and chronologic analysis to avoid jumping to too-early conclusions about Mimas’ surface evolution.
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