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remote sensing Article Morphometric Analysis of Pluto’s Impact Craters Caio Vidaurre Nassif Villaça 1 , Alvaro Penteado Crósta 1,* and Carlos Henrique Grohmann 2 1 Institute of Geosciences, State University of Campinas, R. Carlos Gomes 250, Campinas 13083-855, Brazil; [email protected] 2 Institute of Energy and Environment, University of São Paulo, Av.Prof. Luciano Gualberto 1289, São Paulo 05508-010, Brazil; [email protected] * Correspondence: [email protected] Abstract: The scope of this work is to carry out a morphometric analysis of Pluto’s impact craters. A global Pluto digital elevation model (DEM) with a resolution of 300 m/px, created from stereoscopic pairs obtained by the New Horizons Mission, was used to extract the morphometric data of craters. Pluto’s surface was divided according to different morphometric characteristics in order to analyze possible differences in the impact dynamics and modification rate in each region. A Python code was developed, within the QGIS 3× software environment, to automate the process of crater outlining and collection of morphometric data: diameter (D), depth (d), depth variation, slope of the inner wall (Sw), diameter of the base (Db), and the width of the wall (Ww). Data have been successfully obtained for 237 impact craters on five distinct terrains over the west side of Sputnik Planitia on Pluto. With the collected data, it was possible to observe that craters near the equator (areas 3 and 4) are deeper than craters above 35◦N (areas 1 and 2). Craters on the western regions (areas 2 and 3) contain the lowest depth values for a given diameter. The transition diameter from simple to complex crater morphology was found to change throughout the areas of study. Craters within areas 1 and 4 exhibit a transition diameter (Dt) of approximately 10 km, while Dt for craters within areas 3 and 5 the transitions occurs at 15 km approximately. The presence of volatile ices in the north and north-west regions may be the reason for the difference of morphometry between these two terrains of Pluto. Two hypotheses are presented to explain these differences: (1) The presence of volatile ices Citation: Villaça, C.V.N.; Crósta, A.P.; can affect the formation of craters by making the target surface weaker and more susceptible to major Grohmann, C.H. Morphometric changes (e.g., mass waste and collapse of the walls) during the formation process until its final stage; Analysis of Pluto’s Impact Craters. (2) The high concentration of volatiles can affect the depth of the craters by atmospheric decantation, Remote Sens. 2021, 13, 377. https:// considering that these elements undergo seasonal decantation and sublimation cycles. doi.org/10.3390/rs13030377 Keywords: New Horizons; Pluto; impact crater; morphometry Received: 17 November 2020 Accepted: 15 January 2021 Published: 22 January 2021 1. Introduction Publisher’s Note: MDPI stays neutral Impact cratering is an active and fundamental phenomenon from the initial accretion with regard to jurisdictional claims in of bodies on the protoplanetary disk to the present day. It is the main process by which published maps and institutional affil- iations. planetary surfaces are modified. Morphometric data of impact craters are fundamental for understanding the evolution of the Solar System and its components [1]. The study of impact craters can result in a vast amount of information, such as the ages, rheology properties, erosion rate of planetary surfaces, as well as constrain the cratering history (impact flux) on terrestrial planets and other solid planetary bodies [2]. Copyright: © 2021 by the authors. Robbins et al. [3] reviewed the main methods for morphometric analysis of impact Licensee MDPI, Basel, Switzerland. craters using remote sensing data. Since digital elevation models (DEMs) allow the analysis This article is an open access article of these structures in three dimensions, Robbins et al. [3] favor the use of DEMs to obtain distributed under the terms and conditions of the Creative Commons morphometric data. Robbins and Hynek [4] analyzed the morphometry of Mars’ impact Attribution (CC BY) license (https:// craters in order to determine the planet’s global and regional transition diameter (Dt). creativecommons.org/licenses/by/ Dt represents the threshold that defines the formation of simple craters (formed below 4.0/). Dt) and complex craters (formed above Dt). This transition between simple and complex Remote Sens. 2021, 13, 377. https://doi.org/10.3390/rs13030377 https://www.mdpi.com/journal/remotesensing Remote Sens. 2021, 13, x FOR PEER REVIEW 2 of 18 Remote Sens. 2021, 13, 377 2 of 17 (Dt). Dt represents the threshold that defines the formation of simple craters (formed be- low Dt) and complex craters (formed above Dt). This transition between simple and com- cratersplex craters occurs occurs due due to a to change a change in thein the regime regime that that predominates predominates in in the the process process ofof cratercrater formation [[5].5]. Robbins and Hynek [[4]4] found that, near Mars’ equatorial regionregion (Dt(Dt == 6 km), km), the craters are deeper than those nearnear thethe polespoles (Dt(Dt == 11km), probably due to the presence of volatile elements. Watters et al. [[6]6] observedobserved that Martian craters that were formedformed inin regions of more resistant rheology (e.g., basalt)basalt) are deeperdeeper thanthan craterscraters formedformed inin weakerweaker material (e.g., ejecta deposits). In July July 2015, 2015, the the New New Horizons Horizons spacecraft spacecraft imaged imaged approximately approximately 30% 30% of Pluto’s of Pluto’s sur- surfaceface [7]. [ 7Since]. Since then, then, some some preliminary preliminary studies studies regarding regarding the the morphometry morphometry of of its impact craters have been carried out, and the subject needs to be further investigated.investigated. According to Moore et al.al. [[8],8], thethe NewNew HorizonsHorizons missionmission found out that Pluto’sPluto’s surface is mainlymainly composed ofof H22O, CH4,, CO, CO, and and N 2 ices.ices. The The dwarf dwarf planet planet has has active active geological geological processes and atmospheric cycles which still modify its surfacesurface inin thethe present.present. Robbins et al. [[9]9] identifiedidentified approximately 52005200 circularcircular structuresstructures onon Pluto’sPluto’s surfacesurface andand divided themthem into a five-point five-point scale system, where one means little chance of being an impact crater and fivefive indicates a certain impact crater. Robbins et al.al. [[9]9] measuredmeasured thethe diameterdiameter ofof eacheach impactimpact crater on Pluto to create a globalglobal distribution of impactimpact frequency byby size.size. A preliminary division of Pluto’s surface into morphologica morphologicall domains was presented by Moore et al. [8]. [8]. Robbins et al. [[10]10] measured the depth and diameter of 113 craters on Pluto’s surface and deduced a global transition diameter of approximately 1010 km.km. Singer et al. [[7]7] observed a deficitdeficit of smallsmall craterscraters (less(less thanthan 1515 km)km) overover thethe entireentire surface of the dwarf planet by analyzinganalyzing their size frequencyfrequency distributiondistribution (SFD). As itit would be unlikely that the possible processes of erosion and modificationmodification present in icy bodies couldcould modifymodify only only small small craters, craters, this this observation observation may may also also reflect reflect the deficit the deficit of small of objectssmall objects in the in Kuiper the Kuiper belt. It belt. is still It is necessary still necessary to analyze to analyze in more in more detail detail the morphometry the morphom- of Pluto’setry of Pluto’s impact cratersimpact incraters order in to order help decipher to help decipher their formation their formation dynamics dynamics and the processes and the thatprocesses could that modify could them modify throughout them thro theughout dwarf planet’sthe dwarf surface. planet’s surface. Since Pluto exhibits a complex geology (Figure 11),), thethe scopescope ofof thisthis workwork isis toto carrycarry out a detailed analysis of the morphometry of the impact craters larger than 5 km and up to 60 km inin diameter,diameter, withinwithin differentdifferent terrainsterrains acrossacross Pluto’sPluto’s surface.surface. ThisThis isis intendedintended toto assess if there is any correlation between the differences of morphometry and the different assess if there is any correlation between the differences of morphometry and the different compositions of Pluto’s surface. In addition, we investigate the transition diameter between compositions of Pluto’s surface. In addition, we investigate the transition diameter be- simple and complex craters of Pluto in these different terrains. tween simple and complex craters of Pluto in these different terrains. Figure 1. Global view of Pluto obtained by the combination of blue, red and infrared images taken by the Ralph/Multispectral Visual Imaging Camera (MVIC) (Image modified from Moore et al. [8] and Schenk et al. [11]; www.solarsystem.nasa.gov). Remote Sens. 2021, 13, x FOR PEER REVIEW 3 of 18 Figure 1. Global view of Pluto obtained by the combination of blue, red and infrared images taken by the Ralph/Multi- spectral Visual Imaging Camera (MVIC) (Image modified from Moore et al. [8] and Schenk et al. [11]; www.solarsys- tem.nasa.gov). 2. Materials and Methods 2.1. Study Area Pluto, currently designated as a dwarf planet, is 5.9 billion kilometers from the Sun and is the main object of the Kuiper Belt. The study area comprises the region west of Sputnik Planitia (SP), between 0° and 180° longitude (Figure 2). In this area, images with the best spatial resolution acquired by New Horizons are found in a variety of morpho- Remote Sens. 2021, 13, 377 logically distinct domains containing the majority of impact structures of Pluto’s surface 3 of 17 [7,9]. Pluto’s surface is composed mainly of H2O, CH4, N2, and CO ices [8]. H2O ice is known to be a strong material, capable of forming mountains with high slopes [8].
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