Mercury Hollows as Remnants of Original Bedrock Materials and Devolatilization Processes: A Spectral Clustering and Geomorphological Analysis

Abstract

We perform a detailed geomorphological and compositional analysis on three craters hosting hollows located in the Victoria and Kuiper Quadrangles of Mercury. Based on Mercury Dual Imaging System data, high‐resolution detailed geomorphological mapping is provided in order to fully characterize the geological framework where hollows formed. In addition, we apply an unsupervised spectral clustering, based on a K‐mean algorithm, to separate in clusters our data sets. The comparison between the spectral and the well‐defined geomorphological units reveals a spatially coherent distribution. In particular, all hollows are uniquely identified by a well‐defined spectrum showing a wide absorption band between 0.558 and 0.828, with a possible hint of absorption toward 1 μm. Our analysis suggests that the composition of hollows may be characterized by a mixture of different minerals contributing to the absorptions found in our spectra. Indeed, sulfides alone (CaS, MnS, and MgS) cannot explain the spectrum behavior of hollows, even if the mechanism forming hollows likely involves the loss of volatiles from the surface. Hence, we have to consider bedrock‐forming material as partial responsible of the absorptions. For the studied hollows we suggest that the bedrock‐forming minerals are pyroxenes presenting transitional elements, like Cr, Ti, and Ni in substitution of Mg and/or Fe. Therefore, we suggest that the spectral characteristics of hollows are related to both remnant material produced by devolatilization process and to bedrock in which the hollows formed.

Global Spectral Properties and Lithology of Mercury: The Example of the Shakespeare (H‐03) Quadrangle

“…Vilas et al () and Lucchetti et al () showed that a weak, broadband around 600 nm can be detected on the floor of some craters containing hollows, which contain fresh material according to (Blewett et al, ). This band is expected to be associated to a material present in the darkest regions of Mercury, that is, those that may contain LRM.…”

“…Ratios are thus also useful to enhance the bluest and brightest, so freshest material like ejecta and rays, what was not possible with the spectral slope alone. Vilas et al (2016) and Lucchetti et al (2018) showed that a weak, broadband around 600 nm can be detected on the floor of some craters containing hollows, which contain fresh material according to . This band is expected to be associated to a material present in the darkest regions of Mercury, that is, those that may contain LRM.…”

“…However, Blewett et al () suggested that microsized and nanosized particules of sulfides (e.g., FeS and CaS) can also be involved in the darkening agent responsible for the low reflectance of LRM terrains. The concentration of hollows on LRM terrains (Blewett et al, , ; Thomas et al, ) and the first detection ever in hollows of a weak mineral absorption band at 600 nm attributed to sulfides (Vilas et al, ) mixed with bedrock‐forming minerals like pyroxenes including elements such as chromium, titanium, and nickel (Lucchetti et al, ) strengthened this hypothesis. Moreover, the sublimation of the volatile sulfides would be related to the formation of hollows (Blewett et al, ).…”

Investigating Mercury’s Environment with the Two-Spacecraft BepiColombo Mission

“…BepiColombo observations will investigate correlations between the surface composition and any discernible patterns in the ratios of species released from the surface thus, having an effect on the exosphere (see also Rothery et al 2020, this issue). A new target, not anticipated until they were discovered by MESSENGER, is hollows (Blewett et al 2018 and references therein;Lucchetti et al 2018). These are steep-sided, flat-bottomed depressions where the upper 10-20 m of surface material has been somehow lost.…”

Rationale for BepiColombo Studies of Mercury’s Surface and Composition

“…6;Whitten et al 2014). There has also been some localized geomorphological mapping at up to about 1:300,000 scale in association with spectral studies of hollows (Lucchetti et al 2018).…”