Hollows-like materials, what are their spectral properties telling us?
Authors: Carli Cristian , Zambon Francesca , Giacomini Lorenza , De Toffoli Barbara , Galiano Anna , Massironi Matteo , Galluzzi Valentina , Capaccioni Fabrizio , Palumbo Pasquale 
Published: July 2024

IntroductionHollows are one of the peculiar surface features that was highlighted by the MESSENGER mission [e.g. 1]. They have morphological evidence showing flat-floored and rimless depressions often clustered in fields and relatively bright. They are also associated with a peculiar cyan color within the RGB of the enhanced color mosaic [2]. This color variation is mainly indicating a higher reflectance, in particular within the visible, and a consequent bluer slope within the 400-1000 nm wavelength range [3]. In a few cases, such as the hollow field in the Dominici crater, literature has reported a potential absorption band in that spectral range [e.g. 4,5] with an inflection around 1000 nm, which could indicate an absorption [5]. The putative absorption has been attributed to some sulfides (e.g. CaS, [4]) or to the presence of transitional elements, different from iron, on silicates [5]. Here we investigate different hollow fields and other bright features with hollow-like (i.e. Dominici-like) spectral properties, present in the Kuiper quadrangle.Data and Analytical ApproachMDIS 8 color mosaics have been produced with the same process described in [6] at different spatial resolutions taking care of the original image resolution, avoiding those frames with extreme geometrical values and pixel resolution larger than 3.6 km (see also [7]). We investigate the spectral properties of the hollow fields mapped by [8], as well as other bright features with cyan colors in the enhanced color mosaics.ResultsThe spectral properties of the investigated features show in general a bluer slope and higher reflectance with respect to those of the Hermean average spectrum (see fig.1). Interestingly, the brighter features show the same spectral properties of Dominici hollow field present on the central peak: a potential absorption around 630-750 nm and an inflation towards 1000 nm. Hollow fields with these properties (first group) are, for instance, in Homer, Warhol, and Abu Navas craters. A second set of features show similar spectral properties with a slightly lower reflectance, and weaker, but still recognizable, absorption and inflation. This behavior is present for example in hollow fields of Chaickowski, Yets, Veronese craters, as well as some small bright features (probably small craters) like those closer to Yets and Beck craters. A third group with reflectance variable between the previous two groups shows the same spectral shapes but the absorption around the 630-750 nm is absent whereas the inflation towards the longer wavelengths is still present. In this case we have, for example, the second hollow field in Dominici and Abu Navas craters, with other small craters within the quadrangle. Further cases, in general those with a lower reflectance, show an intermediate slope, no absorption band, and a reduced, in some cases absent, inflection.DiscussionAll morphologically recognizable hollow fields present in the Kuiper quadrangle, maintain the same peculiar spectral properties reported in literature [4,5]. Spectra are dominated by a bluer spectral slope, they are relatively high in reflectance, but they can span from very bright up to an intermediate reflectance. Some of them clearly show an absorption like the one observed in the hollow field on the central peak of Dominici (first group); others show weaker absorption or inflection, up to spectra with a less blue slope and absence of the band and inflection. In general, those spectral properties peculiar to the hollow fields decrease with the reflectance of those regions. Where spectral properties of hollows become less evident, we find features like hollows but also like bright spots and small, fresh craters.Hollow-like spectral properties seem to suggest that they are compositionally different from the surrounding and several cases, non-only in Dominici crater, since to have the same spectral evidence. Nevertheless, a transition between the materials present in the hollow fields and the cases where spectral properties become closer to the average spectrum of a hermean terrain seems to be present. Interestingly, this hollows-like material seems to be present in regions where we cannot clearly identify the presence of hollow fields by means of photo-interpretation yet.&#160;ImplicationsThe variation of spectra within the different cases could be probably related to 1) a possible variation of the composition of the hollows, 2) mixing of the hollow-like constituent with different material from the surrounding terrains, 3) a different number density of hollows within the hollow fields, or 4) an increasing effect of space weathering (i.e. aging) or physical degradation of the hollow fields.Moreover, there is evidence that similar spectral properties are present both in regions where hollow fields can be clearly identified, and in other small bright regions, often clearly associated with craters, where hollows have not been identified yet. This could indicate that the image resolution does not permit identifying hollow fields in those areas. Conversely, the similitude of spectral properties between hollow fields and other features could indicate the composition of a material forming part of the crust of Mercury, only exposed on relatively fresher regions.Our future activity will focus on understanding better if the hollows-like material can be present not only in hollow fields and why we see a variation in its spectral properties. Moreover, we will suggest which of those could be the most interesting targets for the BepiColombo mission.AcknowledgmentThis research is funded from the Italian Space Agency (ASI) within SIMBIOS-SYS project under ASI-INAF agreement 2017-47-H.0. CC, FZ, GL, MM, VG were also supported by Europlanet RI20-24 research grant agreement No. 871149-GMAP.References[1] Blewett et al. (2011) Science 333. [2] Denevi et al. (2011) Science 333.&#160; [3] Blewett et al. (2013) JGR 118. [4] Vilas et al. (2016) GRL 43.[5] Lucchetti et al. (2018) JGR 123. [6] Zambon et al. (2022) JGR 127. [7] Carli et al. 2022, CNSP XVII, Abstract#. [8] Giacomini et al. 2023, Journal of Maps 18.