Comparison of extended hollow fields in Warhol, Hopper, Sander and Eminescu craters

Introduction

Hollows are peculiar surface features that were highlighted by the MESSENGER mission [e.g. 1]. They are small cavities 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 mainly indicates a higher reflectance and a bluer slope within the 400-1000nm wavelength range [3]. In literature, few cases have been deeply spectroscopically investigated at the best spatial resolution possible, and only for the hollow field in the Dominici crater was reported a potential absorption around 600-700nm [e.g. 4,5,6] with a subsequent inflection around 1000nm, which could indicate another absorption [5,6]. The putative absorption has been attributed to some sulfides (e.g. CaS, [4]) or with some chloride (e.g. NaCl, [6]). Whereas [5] argues to the possibility of attributing that absorption to rock forming minerals considering transitional elements, different from iron, on mafic silicates.

Here we investigated four different hollow fields widespread on the floor of Eminescu, Hopper, Sander and Warhol Craters, which shows a relatively high 600nm Band Depth parameters [7] and a high reflectance in correspondence of the hollow fields.

Data and Analytical Approach

MDIS 8 color mosaics have been produced with the same process described in [8] at different spatial resolutions taking care of the original image resolution from circa 385m/px up to 1600m/px. Global BDR, LOI and, preliminary, 8 color enhanced color mosaics, together with the best spatial resolution NAC and WAC images for each crater were used for morphological mapping. We used the ad-hoc color mosaics to stretch the enhanced color combination and better highlight the variability within the hollow fields and with respect to the surrounding terrains. Moreover, we investigate the spectral variability within the hollow fields of each crater, defining specific trends with respect to the different surrounding materials.

Preliminary Results

First, we analyzed the two different cases of Warhol and Hopper craters, where we have relatively high (385m/px) and low (1600m/px) color mosaics. We showed that the hollows are principally present on the crater floor, and partially on the exposed bedrock of the central pick, with a higher concentration of hollows around the central pick or on contact with the crater wall (in Figure1 an example for Warhol crater). Crater floors are mainly characterized by smooth material representing plains formed due to the refilling after the impact.

The Enhanced color mosaics of the selected images show a widespread distribution of bright and cyan deposits feeding almost all the crater floor of Warhol and Hopper (see figure1b for Warhol). If we stretched the enhanced color mosaics zooming on regions enriched on hollow fields, we highlighted a color variation emphasizing the fields with respect to the surrounding materials (e.g. Figure1c). We then investigate the spectral variation within different regions of interest (R.O.I.) on the hollow fields (see figure1b) showing that the spectral features are always consistent one to each other (Figure1d), with the presence of the putative band and the inflation towards the infrared. The main variation seems to be the relative reflectance values from 0.10 up to 0.15 at 560nm.

Moreover, if we zoom on specific R.O.I.s we can see how the stretched enhanced color mosaics match with the pixels showing morphological evidence of the hollows. As an example, here, we consider the area (figure2a) consistent with the brown R.O.I. on figure1b. Considering a specific transect that moves from the central area of an isolated hollow/small hollows field and the surrounding halo we can highlight a reduction of the absorptions where the morphological evidence indicating the hollow disappears. Thus suggesting a variation of composition with respect to the material deposited outside the hollows. The change in spectral slope seems to be the last to modify towards reddening and darkening spectra (Figure2b). Isolating the absorption band around 600-700nm shows how spectrally the main contribution of the material characterizing the composition of the hollows seems to involve the 630nm filter (Figure2c), which disappears on spectra outside the hollows, where the apparent absorption is mainly attributable to the 750nm filter.

Implications

The spectral variability within different regions of interest we investigated on Warhol crater, as well as in Hopper crater, and in the ongoing investigation on Eminescu and Sander craters, seems to highlight as the isolated hollows and hollow fields are characterized by the 600-700nm absorption and an inflation towards 1000nm. Stretching the enhanced color mosaics on hollow fields permits to better constrain the regions characterized by these features which match with the morphological counterpart of the hollows. Hollows spectral properties seem to suggest that they are compositionally different from the surrounding, as already shown for Dominici crater, taking also into account that the surrounding terrains on those craters look like to have different spectral properties. Moreover, it seems that a net change is present on the absorptions moving from inside to outside of a specific hollow/hollows field indicating that the halo could have spectral properties different from the hollow itself. This could highlight that the peculiar spectral properties of the hollows could be to assign to the material that characterized the region inside the hollows than those attributed to the halo, maybe revealing the evidence of the rock forming minerals instead of secondary alteration phases (e.g. sulfides or chlorides).

The spatial resolution will play an important role in understanding the possible nature of the material present in the hollows and to understand the hollows formation. Hollows can be considered as one of the most interesting targets for the BepiColombo mission, where we could combine the higher spectral range and the high spatial resolution from the SIMBIO-SYS instrument.

Acknowledgment

This research is funded from the Italian Space Agency (ASI) within SIMBIOS-SYS project under ASI-INAF agreement 2024-18-HH.0.

 References

[1] Blewett et al.(2011) Science 333. [2] Denevi et al.(2011) Science 333.  [3] Blewett et al.(2013) JGR 118. [4] Vilas et al.(2016) GRL 43.[5] Lucchetti et al.(2018) JGR 123. [6] Emran & Stack (2025) Icarus 435. [7] Klima et al.(2018) GRL, 45. [8] Zambon et al.(2022) JGR 127.