Near-ultraviolet to near-infrared spectral properties of hollows on Mercury: Implications for origin and formation process Océane Barraud, A. Doressoundiram, S. Besse, J. Sunshine To cite this version: Océane Barraud, A. Doressoundiram, S. Besse, J. Sunshine. Near-ultraviolet to near-infrared spectral properties of hollows on Mercury: Implications for origin and formation process. Journal of Geophys- ical Research. Planets, Wiley-Blackwell, 2020, 125 (12), 10.1029/2020JE006497. hal-03181555 HAL Id: hal-03181555 https://hal.sorbonne-universite.fr/hal-03181555 Submitted on 25 Mar 2021 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. manuscript accepted JGR: Planets 1 Near-ultraviolet to near-infrared spectral properties of 2 hollows on Mercury: Implications for origin and 3 formation process 1 1 2 3 4 O. Barraud , A. Doressoundiram , S. Besse and J. M. Sunshine 1 5 LESIA, Observatoire de Paris, Universit´ePSL, CNRS, Sorbonne Universit´e,Universit´ede Paris, 92195 6 Meudon, France 2 7 European Space Astronomy Centre, Camino Bajo del Castillo s/n, 28692 Villanueva de la Canada, 8 Madrid, Spain 3 9 Department of Astronomy, University of Maryland, College Park, MD 20742-2421, USA 10 Key Points: 11 • Hollows have unique spectral properties in the near-ultraviolet. 12 • The reflectance spectra of hollows have a pronounced curvature between 300 and 13 600 nm. 14 • Eminescu impact crater hollows are seen to grow via scarp retreat. Corresponding author: Oc´eaneBarraud, [email protected] {1{ manuscript accepted JGR: Planets 15 Abstract 16 [Among the geological features revealed by the MESSENGER (MErcury Surface, Space 17 ENvironment, GEochemistry and Ranging) mission on the surface of Mercury, hollows 18 are the most surprising and least understood. Possibly related to volatile components, 19 hollows are small depressions, surrounded by bright halos and are not observed on any 20 other surfaces in our Solar System. Previous analysis of multi-spectral data obtained by 21 Mercury Dual Imaging System (MDIS) has shown that some hollows have weak spec- 22 tral absorption features centered at around 600 nm. In this work, we analyzed four hol- 23 lows with observations acquired by the Mercury Atmospheric and Surface Composition 24 Spectrometer (MASCS) on board MESSENGER with more than 230 spectral channels 25 from the near-ultraviolet to the near-infrared. Unlike previous MDIS multispectral data, 26 the MASCS reflectance spectra exhibit no absorption features in the MDIS wavelength 27 range. However, we found that hollows have unique spectral properties in the near-ultraviolet, 28 with a spectral curvature between 300 and 600 nm that is distinctly different from other 29 geological units. Moreover, we used MASCS observations with the best available spa- 30 tial resolution (less than 0.5 km/pixel) to analyze both parts of a hollow: the flat floor 31 and the surrounding halo. Our results support the hypothesis that hollows form by a sub- 32 limation process and scarp retreat.] 33 Plain Language Summary 34 [The MESSENGER (MErcury Surface, Space ENvironment, GEochemistry and Rang- 35 ing) mission highlighted several geological terrains on Mercury's surface. Among the un- 36 expected discoveries are small bright depressions, named hollows. Often found within 37 impact craters, hollows have irregular shapes, flat floors and are surrounded by bright 38 halos. Hollows are still poorly understood and their formation remains a mystery for the 39 scientific community. Using observations made by the spectrometer on board the MES- 40 SENGER probe, we show for the first time that the spectra of hollows exhibit unique 41 reflectance properties at near-ultraviolet to visible wavelengths, which spectrally distin- 42 guishes them from other geological terrains. Moreover, the most detailed observations 43 show that their flat floors have different spectral characteristics from the bright halos. 44 Our results provide new knowledge about their formation, nature and differences from 45 other geological terrains and is consistent with them forming from sublimation processes.] {2{ manuscript accepted JGR: Planets 46 1 Introduction 47 The MESSENGER (MErcury Surface, Space ENvironment, GEochemistry and Rang- 48 ing; Solomon et al., 2007) missions highlighted the complex geological history of Mer- 49 cury's crust (Denevi et al., 2009). Multi-spectral images returned by the Mercury Dual 50 Imaging System (MDIS, Hawkins et al., 2007) and spectral observations obtained with 51 the Mercury Atmospheric and Surface Composition Spectrometer (MASCS, McClintock 52 & Lankton, 2007) show that Mercury's surface has steeper spectral slopes and lower re- 53 flectance than the Moon (Robinson et al., 2008; McClintock et al., 2008; Denevi et al., 54 2009). Mercury's reflectance spectra exhibit a red spectral slope (i.e the reflectance in- 55 creases with increasing wavelength) without silicates absorption features (Robinson et 56 al., 2008; Izenberg et al., 2014). Although the spectral signatures are few, the surface 57 of Mercury is spectrally and morphologically diverse. Based on spectral variation and 58 morphology, two main terrain types and several smaller units have been identified at the 59 surface of Mercury (Robinson et al., 2008; Denevi et al., 2009; Izenberg et al., 2014). 60 Low Reflectance Materials (LRM) covers approximately 15 % of the planet's sur- 61 face. They are among the darkest features on Mercury with an absolute reflectance 30 62 % below the average surface (Robinson et al., 2008). LRM have a shallower spectral slope 63 than Mercury's mean spectrum. They have diffuse margins and are generally associated 64 with impact structures, suggesting that this material has been excavated by impact cra- 65 tering. The smooth plains represent 27 % of Mercury's surface (Denevi et al., 2013). They 66 exhibit a large range of spectral properties: from high-reflectance red plains to low-reflectance 67 "blue" plains (blue corresponding to a less steep spectral slope than the average surface, 68 but still with an increasing reflectance with increasing wavelength) (Denevi et al., 2009, 69 2013). Smooth plains on Mercury could result from effusive volcanism, impact melting 70 or basin ejecta (Denevi et al., 2009, 2013). The majority of them share the spectral prop- 71 erties of the Northern Smooth Plains (NSP), the more largest (occupying 7% of the Mer- 72 cury's surface) and continuous area of smooth plains on Mercury (Denevi et al., 2013; 73 Byrne et al., 2018). 74 Besides LRM and smooth plains, which are considered as large units, there are also 75 small units that can be darkest features or high reflectance material. Dark spots are sub- 76 categories of LRM (Xiao et al., 2013), with diffuse edges but covering smaller areas (less 2 6 2 77 than 100 km by dark spot) than LRM (often larger than 10 km ). They have spec- 78 tral slopes similar to the LRM but a lower reflectance (Xiao et al., 2013). Another spec- 79 tral units are faculae which appear brighter and redder than the Mercury's average sur- 80 face. Most of faculae are accepted to be pyroclastic deposits emplaced by explosive erup- 81 tion. Brighter than faculae, small depressions named hollows have a reflectance approx- 82 imately twice the average reflectance of Mercury (Blewett et al., 2013). Hollows-like for- 83 mations have never been observed on other airless silicate bodies, which makes them an 84 interesting case study. 85 Hollows are irregularly shaped, shallow, with flat floors. Their edges are usually 86 diffused and brighter. They most often form near or directly within impact craters (Blewett 87 et al., 2011, 2013). Some appear on crater rims, others around central peaks/rings and 88 sometimes even in ejecta (Blewett et al., 2011, 2013). They have been often associated 89 with LRM, Low-reflectance Blue Plains (LBP) or dark spots (Blewett et al., 2013; Xiao 90 et al., 2013; Thomas et al., 2014a). The formation mechanism of hollows is widely dis- 91 cussed in literature (Blewett et al., 2013, 2018), and the loss of a volatile component is 92 the most commonly proposed process. The arguments favoring volatile loss are i) the im- 93 portant abundance of volatile species in Mercury's crust as measured by X-Ray spectroscopy 94 (Nittler et al., 2011), and ii) the preferable distribution of hollows on Sun-facing-slopes 95 (Blewett et al., 2011, 2013; Thomas et al., 2014a). Blewett et al. (2013) proposed a for- 96 mation scenario based on a vertical growth by loss of a volatile compound to a certain 97 depth, and then an extension of the hollows by scarp-retreat. {3{ manuscript accepted JGR: Planets 98 Due to the high abundance of sulfur in the sub-surface of Mercury (Nittler et al., 99 2011), CaS and MgS are commonly proposed as the volatile species (Blewett et al., 2011, 100 2013; Vilas et al., 2016; Lucchetti et al., 2018). This hypothesis is supported by the high 101 concentration of exospheric calcium (Bennett et al., 2016) above the Tyagaraja impact 102 crater in which there is a large field of hollows (Blewett et al., 2011). Vilas et al. (2016) 103 studied in detail the Dominici and Hopper crater hollows and showed a possible absorp- 104 tion band centered around 630 nm in MDIS multispectral observations. This feature was 105 also found in hollows from Canova and Velazquez impact craters (Lucchetti et al., 2018) 106 and Raditladi Basin (Thomas et al., 2016).