Research blames low sulfide concentration for Mercury's lack of hollows in smooth plains

Data obtained from the MESSENGER mission reasons why Mercury lacks hollows. Could it be sulfides? Or is it graphite?
Amal Jos Chacko
Stock photo: 3D rendering of Mercury as it would appear in color.
Stock photo: 3D rendering of Mercury as it would appear in color.


A “surface-space-environment-geochemistry-and-ranging” probe- as part of the MESSENGER (Mercury, Surface, Space Environment, Geochemistry, and Ranging) mission- has resulted in the discovery of hollows on Mercury.

Hollows are irregularly shaped shallow depressions that have halos and interiors with high reflectance and are uncommon in the smooth plains of Mercury.

A new report in Science Advances reveals the detection of these geological landforms by multispectral images and geochemical measurements and how they were formed.

Océane Barraud and a research team at the Sorbonne University, Paris, and the European Space Astronomy Center, Spain, studied these hollows by analyzing data collected by a Mercury atmospheric and surface composition spectrometer and the visible, infrared spectroscopy reflectance spectra of hollows in the impact craters of Eminescu, Hopper, Tyagraja, and Warhol.

These hollows were found to have a higher concentration of sulfides than Mercury’s high-reflectance smooth plains.

Could graphite have caused these hollows?

The research team observed thermal modeling that suggested the thermal decomposition of sulfides to have induced the hollow-forming phase. Composition and spectroscopic data revealed the enrichment of low-reflectance material with carbon, leading the team to propose graphite to be responsible for these hollow formations.

Analysis of multispectral data suggests the presence of magnesium chloride, sodium chloride, and calcium chloride to be representative of hollow material. These hypotheses were tested by comparing the hollow spectra with laboratory spectra obtained from sulfide, graphite, and chloride samples which led to the observation of several compounds with concave curvature.

Research blames low sulfide concentration for Mercury's lack of hollows in smooth plains
Average spectra obtained for each group of hollows

These spectral matches were incomparable, however, leading the researchers to use graphite samples of a convex curvature instead, reports

Further research on the curvature parameter values obtained from the laboratory spectra indicated that the materials forming these hollows are not pure compounds. 

The team then compared laboratory spectra to Mercury’s spectra by considering the spectrum of materials obtained from the floor of the crater as the basis for spectral modeling. Although a good match between chlorides and a notable match with labradorite were seen, these were less than that of sulfides which appeared to be best suited to reproduce the curvature of hollow spectra.

Observations within the Eminescu crater revealed a decrease in sulfite abundance when mixed with aluminum biminerals and groups of silicates and an average spectrum for the two hollow faces of the floor and bright halo. This prompted the team to explore matches within the bright halo for sulfides and chlorides, which resulted in finding the best matches with calcium chloride and magnesium chloride sediments. 

However, the match between surface composition spectra and modeled spectra was observed to be lower in the hollow floor than in the bright halo, proving that none of these volatile species proposed caused the observed outcomes.

The comparison of spectra corresponding to the hollows when compared to the host material of the crater floor indicated sulfide enrichment, leading Océane Barraud and colleagues to the conclusion that sulfides are involved in crater formation.

The team observed spectrophotometric properties of hollows in the craters to be consistent with hollow formation, and spectral and quantitative data showed the absence of hollows in the high-reflectance smooth plains of Mercury.

Study abstract:

MESSENGER (Mercury, Surface, Space Environment, Geochemistry, and Ranging) mission to Mercury led to the discovery of hollows. These geological landforms have no close counterpart on other airless silicate bodies. Multispectral images and geochemical measurements by MESSENGER suggest that hollows are formed by the loss of volatile-bearing minerals. We investigated the mineralogical composition of the hollows using near-ultraviolet to near-infrared spectra obtained by MESSENGER. We compared reflectance spectra of hollows with laboratory spectra of Mercury’s analogs: sulfides, chlorides, silicates, and graphite. The best candidates to reproduce the curvature of the hollow spectra are calcium sulfide, magnesium sulfide, and sodium sulfide. In addition, we performed spectral modeling with spectra obtained at the highest spectral and spatial resolution within the hollows. Our results show that the enrichment of sulfides in hollow material is up to two times higher than the sulfide concentration derived from chemical measurements of Mercury’s high-reflectance smooth plains. This result explains the small percentage of hollows found within these plains.

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