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The Raditladi impact basin (~265 km in diameter), imaged by MESSENGER as it flew past Mercury for the first time. (a) Monochrome image. (b) Principal-component enhanced-color image. The peak-ring mountains have an unusually high reflectance and relatively blue color. The dashed line marks the main basin rim. The arrow in Figure 2b indicates the bright, blue inner peak ring. The box in Figure 2a shows the approximate location of the view in Figure 7. Color assignments in Figure 2b are the same as in Figure 1b. Images are centered at 27.1 N, 119.2 E.
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unique to Mercury, hollows are shallow, flat-floored irregular
depressions notable for their relatively high reflectance and
characteristic color. Here we document the range of geological settings
in which hollows occur. Most are associated with impact structures
(simple bowl-shaped craters to multiring basins, and ranging from
Kuiperian to Caloria...
Contexts in source publication
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... Impact crater central peaks and basin peak rings host some of the most spectacular hol- lows on Mercury. The Raditladi basin (~265 km in diameter) [Strom et al., 2008;Prockter et al., 2010] exhibits the albedo, color, and morphological characteristics typical of peak-ring hollows (Figures 2 and 7) , Figure 1C]. The area of the peak ring shown in Figure 7 is distinctive for the striking flat-topped sections of the moun- tains and for the apron at the base. ...
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... craters in Figures 10 and 11 are not rayed but have well-defined rims and hence are interpreted to be Mansurian. Figure 2b indicates the bright, blue inner peak ring. The box in Figure 2a shows the approximate location of the view in Figure 7. Color assignments in Figure 2b are the same as in Figure 1b. ...
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... 2b indicates the bright, blue inner peak ring. The box in Figure 2a shows the approximate location of the view in Figure 7. Color assignments in Figure 2b are the same as in Figure 1b. Images are centered at 27.1 N, 119.2 E. ...
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... 2b indicates the bright, blue inner peak ring. The box in Figure 2a shows the approximate location of the view in Figure 7. Color assignments in Figure 2b are the same as in Figure 1b. Images are centered at 27.1 N, 119.2 E. ...
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... present, there is not sufficient evidence to discriminate between these hypotheses. Figure 2 indicates the location of this image. Note the smooth apron at the base of the peak mountain, possibly formed by downslope movement of material released by the hollows-forming process taking place above. ...
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... Sputtering by energetic ions and/or micrometeoroid impact melting and vaporization are other processes that could contribute to the destruction of volatile-bearing phases ( Figure 20). A depletion of sulfur on the surface of asteroid Eros relative to the element's abundance in ordinary chondrites was discovered by the Near Earth Asteroid Rendezvous X-ray spectrometer [Trombka et al., 2000;Nittler et al., 2001]. ...
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... ultimate depth of hollows could be controlled either by the thickness of the layer containing the volatile-bearing phase, or by development of a thermally insulating and mechanically resistant lag that prevents further loss of volatiles. These two scenarios are depicted in Figure 20, panel 4. ...
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... Reflectance spectra for a number of areas of interest ( Figure 21) are presented in Figure 22a. Figure 22b shows the spectra divided by the spectrum of the intermediate terrain (IT), a widespread spectral unit on Mercury , emphasizing the differences in albedo and slope among the different types of material in this area. ...
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... Reflectance spectra for a number of areas of interest ( Figure 21) are presented in Figure 22a. Figure 22b shows the spectra divided by the spectrum of the intermediate terrain (IT), a widespread spectral unit on Mercury , emphasizing the differences in albedo and slope among the different types of material in this area. ...
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... The spectra in and around de Graft include most of the major and minor color types recognized in the flyby data: IT, LRM, reddish units, fresh ray material, and hollows Blewett et al., 2009;Denevi et al., 2009]. The hollows are nearly twice as reflective at 749 nm as is Mercury on average (represented in Figure 22 by the IT). All Mercury surfaces have reflectances that increase to- ward longer wavelengths. ...
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... Mercury surfaces have reflectances that increase to- ward longer wavelengths. The relative reflectance plot (Figure 22b) illustrates that the spectra of hollows are much less steeply sloped than other types of material, resulting in the negative slope of relative reflectance spectra for the hollows. ...
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... The lack of strong absorption bands at visible to near- infrared wavelengths makes identification of the composition of the major phases in Mercury's regolith difficult. Figure 23 shows the de Graft spectra of Figure 22a plotted together with laboratory spectra for a variety of potential analog mate- rials. The strikingly low albedo of Mercury is apparent when MDIS spectra are plotted on the same scale as the analogs. ...
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... The lack of strong absorption bands at visible to near- infrared wavelengths makes identification of the composition of the major phases in Mercury's regolith difficult. Figure 23 shows the de Graft spectra of Figure 22a plotted together with laboratory spectra for a variety of potential analog mate- rials. The strikingly low albedo of Mercury is apparent when MDIS spectra are plotted on the same scale as the analogs. ...
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... Because overall reflectance and spectral slope are the chief distinguishing factors of Mercury's surface in the MDIS wavelength range, we can conveniently condense the major spectral variations into two parameters: reflectance at 749 nm and the ratio of reflectance at 433 nm to that at 749 nm. Figure 24 is a ratio-reflectance plot for the Mercury spectra and the laboratory analog spectra. The lack of an obvious absorption feature near a wavelength of 1000 nm (1 mm) has long suggested that Mercury's surface is domi- nated by silicates with low ferrous iron content [e.g., Vilas, 1988;Blewett et al., 1997Blewett et al., , 2002Warell, 2003;Warell and Blewett, 2004;Warell et al., 2006;Robinson et al., 2008;Blewett et al., 2009]. ...
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... lack of an obvious absorption feature near a wavelength of 1000 nm (1 mm) has long suggested that Mercury's surface is domi- nated by silicates with low ferrous iron content [e.g., Vilas, 1988;Blewett et al., 1997Blewett et al., , 2002Warell, 2003;Warell and Blewett, 2004;Warell et al., 2006;Robinson et al., 2008;Blewett et al., 2009]. The two low-iron silicates in Figure 24, enstatite from the Peña Blanca Spring (PBS) aubrite [Burbine et al., 2002] and anorthite, have higher reflectances and higher 433 nm/749 nm ratios than the Mercury spectra. ...
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... (cal- cium sulfide, CaS) is found in meteorites with chemically reduced compositions [Burbine et al., 2002]. Oldhamite has the same color ratio as the Mercury spectra in Figure 24 but by itself is too bright and in addition has an absorption feature near 500 nm and a weaker band at ~950 nm. Other sulfides (MgS, MnS) also display absorptions in the wave- length range 500-600 nm , although there is evidence that heating to Mercury daytime tempera- tures causes loss of the band and a decrease in spectral slope . ...
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... Mercury's degree of spectral variation is rather limited, as evidenced by the clustering of the de Graft spec- tra (diamonds) in Figure 24. The range of variation is less than that from an immature lunar highland soil to a mature one. ...
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... subtractive process could account for the high reflectance of active hollows. Sulfides (e.g., FeS and CaS, Figure 24) are red relative to iron-free silicates, so the loss of sulfides from a mineral assemblage would cause a decrease in spec- tral slope. Likewise, nanophase opaque minerals also cause reddening. ...
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... -red rim. Spectra are plotted in Figure 22 and represent 5 Â 5, 9 Â 9, or 11 Â 11 pixel averages. The approximately parallel ray seg- ments that cross the scene originated at Hokusai crater to the north. ...
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... suggest that finely disseminated sulfides (e.g., CaS, MgS, or FeS) could contribute to the low reflectance of the LRM relative to Mercury's average surface material [see also Vaughan et al. [2012] and Helbert et al. [2012]). The (relatively) high reflectance and characteristic blue color of hollows could be a consequence of the destruction of the darkening agent, of compositional differences related to altered minerals or vapor deposits, or of a physical state (grain size, texture, Figure 22. MDIS eight-color spectra for surfaces in and around de Graft crater. ...
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... Spectra relative to the intermediate terrain (IT) emphasize differences in spectral slope. Figure 23. Laboratory reflectance spectra for analog minerals and two lunar soils, together with the Mercury spectra from Figure 22a. ...
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... 23. Laboratory reflectance spectra for analog minerals and two lunar soils, together with the Mercury spectra from Figure 22a. The following spectra are from RELAB: enstatite from Peña Blanca Spring (PBS) aubrite (TB-TJM-045/ C1TB45), oldhamite (TB-TJM-038/C1TB38), troilite (TB- RPB-005/C1TB05), and lunar sample 61221 (LS-CMP-065- A/CALS65), and lunar sample 62231 (LS-CMP-030/ CALS30). ...
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Dark spots on Mercury are recently-formed thin and low-reflectance materials that are related with volatile activity and supposed to be much shorter-lived than their central hollows. Containing unique information about the possible building blocks of Mercury, dark spots have unresolved darkening phases, formation mechanism, and lifetime. Here we in...
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Citations
... However, both of these features are relatively small (∼meters to hundreds of meters), especially in comparison to the 1°× 1°binning performed on the MLA data, and thus would need to be highly abundant in order to influence the MLA signal. Surveys of the equatorial region do not reveal an anomalous abundance of these features (Kerber et al. 2009(Kerber et al. , 2011Blewett et al. 2011Blewett et al. , 2013Goudge et al. 2014); thus, they are unlikely to be responsible for the equatorial brightening we observe. ...
It has been predicted that high equatorial temperatures on Mercury could promote thermal annealing by Ostwald ripening, where nanophase metal particles (a product of space weathering) coalesce and grow into larger, microphase particles, resulting in lower albedo. Here, we test this prediction by studying the correlation between albedo and temperature in 1° spatial bins using newly recalibrated 1064 nm reflectance data acquired by the Mercury Laser Altimeter (MLA), low incidence angle data from the Mercury Dual Imaging System (MDIS), and newly modeled maximum surface temperatures (MSTs). Accounting for local geology and latitude, we compare the reflectance values of surfaces with MSTs >675 K (where Ostwald ripening is predicted to be most effective) to surfaces with MSTs <473 K (where ripening is predicted not to be effective). Smooth plain surfaces >675 K are 10% and 12% darker than surfaces <473 K in MLA and MDIS data, respectively, and nonsmooth plain surfaces >675 K are 8% and 7% darker than surfaces <473 K. However, open questions remain regarding the causation of this darkening; statistical tests cannot distinguish whether the reflectance differences are systematic or the result of compositional variations that happen to correlate with MST. Along Mercury’s thermal longitudes, we find that reflectance is typically lower along hot poles than along the 90°E cold pole in the low-to-midlatitudes, especially in the smooth plains, consistent with previous work identifying a decrease in optical maturity along the 90°E cold pole. Longitudinal reflectance variations correlate with temperature variations, rather than variations in micrometeoroid or solar wind fluxes.
... The hollows were morphologically attributed to degassing processes (e.g., [12,13]); relatively unexpected high S/Si has been measured on Mercury's surface (e.g., [1,14]); the surface of Mercury has undergone extensive pyroclastic activity (e.g., [15][16][17][18]) which indicates high levels of volatile components in the magma reservoir [19]. ...
Exploration of Mercury will continue in the near future with ESA/JAXA’s BepiColombo mission, which will increase the number and the type of datasets, and it will take advantage of the results from NASA’s MESSENGER (MErcury Surface, Space ENviroment, GEochemistry and Ranging) mission. One of the main discoveries from MESSENGER was the finding of a relatively high abundance of volatiles, and in particular of sulphur, on the surface. This discovery correlates well with the morphological evidence of pyroclastic activity and with features attributable to degassing processes like the hollows. BepiColombo will return compositional results from different spectral ranges and instruments, and, in particular, among them the first results from the orbit of emissivity in the thermal infrared. Here, we investigate the results from the emissivity spectra of different samples between a binary mixture of a volcanic regolith-like for Mercury and oldhamite (CaS). The acquisitions are taken at different temperatures in order to highlight potential shifts due to both mineral variation and temperature dependence on these materials that potentially could be present in hollows. Different absorption features are present for the two endmembers, making it possible to distinguish the oldhamite with respect to the regolith bulk analogue. We show how, in the mixtures, the Christiansen feature is strongly driven by the oldhamite, whereas the Reststrahlen minima are mainly dominated by mafic composition. The spectral contrast is strongly reduced in the mixtures with respect to the endmembers. The variations of spectral features are strong enough to be measured via MERTIS, and the spectral variations are stronger in relation to the mineralogy with respect to temperature dependence.
... One of the surprises from the elemental chemistry measurements from the MESSENGER mission, in conjunction with the discovery of the unique hollows morphological features (e.g., Blewett et al. 2011Blewett et al. , 2013Blewett et al. , 2018 and the confirmation of water trapped within the PSRs (e.g., Lawrence et al. 2013;Chabot et al. 2018), is the high abundance of volatiles (e.g., Nittler et al. 2011). There is a suite of sulfides that have been postulated to be present, especially associated with the hollows regions. ...
... Hollows are irregularly shaped, flat-floored, shallow depressions that occur predominantly within impact crater structures (crater floors, walls and rims, central peaks, and ejecta) and are considered very young in age (Blewett et al. 2011(Blewett et al. , 2013(Blewett et al. , 2018Thomas et al. 2014Thomas et al. , 2016. They are often, but not always, found in association with low-reflectance materials (Thomas et al. 2014(Thomas et al. , 2016, a unit considered rich in graphite . ...
... They are often, but not always, found in association with low-reflectance materials (Thomas et al. 2014(Thomas et al. , 2016, a unit considered rich in graphite . Their morphology suggests that they formed via sublimation of volatiles through a number of mechanisms, including solar heating and heating via contact with magmatic materials or impact melt (Blewett et al. 2011(Blewett et al. , 2013(Blewett et al. , 2018Thomas et al. 2014Thomas et al. , 2016Phillipps et al. 2021). Examination and modeling of the MESSENGER MDIS color imaging and MASCS Vis-NIR observations, coupled with comparisons with laboratory spectral measurements, indicate that the volatile species involved in hollows formation are predominately sulfides, such as CaS, MgS, and NaS (e.g., Barraud et al. 2023), though chlorides have also been considered (e.g., Lucchetti et al. 2021). ...
The fate of Mercury’s exospheric volatiles and, in a lesser way, of the refractory particles absorbed in the first few centimeters of the surface both depend highly on the temperature profile with depth and its diurnal variation. In this paper, we review several mechanisms by which the surface temperature might control the surface/exosphere interface. The day/night cycle of the surface temperature and its orbital variation, the temperature in the permanent shadow regions, and the subsurface temperature profiles are key thermal properties that control the fate of the exospheric volatiles through the volatile ejection mechanisms, the thermal accommodation, and the subsurface diffusion. Such properties depend on the solar illumination from large to small scales but also on the regolith structure. The regolith is also space-weathered by the thermal forcing and by the thermal-mechanical processing. Its composition is changed by the thermal conditions. We conclude by discussing key characteristics that need to be investigated theoretically and/or in the laboratory: the dependency of the surface spectra with respect to temperature, the typical diffusion timescale of the volatile species, and the thermal dependency of their ejection mechanisms.
... While these elements were detected in a superficial layer roughly between 10 cm and 100 μm thick (Lawrence et al. 2013;Neumann et al. 2013;Deutsch et al. 2017;Chabot et al. 2018), the finding of potential sublimation pits, also known as hollows ( Figure A1), sheds light on the distribution of materials within Mercury's volatile-rich subsurface. These hollows, identified as flat-floored, rimless depressions featuring bright interiors and halos (Blewett et al. 2011(Blewett et al. , 2013(Blewett et al. , 2016Helbert et al. 2013), have an average depth of 24 ± 16 m, interpreted by Blewett et al. (2016) to indicate that volatiles exist as a pervasive constituent extending well beneath the planet's surface. ...
... The Raditladi impact basin is thought to have formed ∼1 Ga (Prockter et al. 2009), and, according to our results, the development of its glacier-like features likely occurred within ∼1 Myr of the impact (Figures 9(B) and (C)). However, the hollows that have been identified in the region are believed to be relatively recent, perhaps currently active geological features, according to several studies (Blewett et al. 2011(Blewett et al. , 2013(Blewett et al. , 2016Helbert et al. 2013). ...
The discovery of global elemental volatile compositions, sublimation hollows, and chaotic terrains has significantly reshaped our understanding of Mercury's geology. These findings suggest the existence of volatile-rich layers (VRLs) extending several kilometers in depth, challenging the traditionally held view of a predominantly volatile-devoid Mercury crust. However, the precise nature and origin of these VRLs remain to be elucidated. The Raditladi basin exhibits morphologies analogous to terrestrial and Martian glaciers. These geomorphological features are potentially derived from impact-exposed VRLs, likely constituted of halite, other semivolatile salts, or organic volatiles. The distinctive rheological traits of substances such as halite substantiate this hypothesis. The inference posits a potential ubiquity of VRLs on a planetary scale, albeit potentially ensconced at considerable depth in specific regions. North polar chaotic terrains elucidate the VRLs' genesis and temporal evolution. The intense fragmentation of heavily cratered landscapes during their formation indicates a composition dominated by volatiles. This finding postulates a phase of volatile-enriched crustal accretion predating the Late Heavy Bombardment (∼3.9 Ga). Regardless of lost mass, the unaltered basal elevation post-collapse signals a transition to a volatile-free stratum. The exposure of an exhumed lithological substrate within Mercury's stratigraphy, identifiable in gravimetry as an impacted paleosurface, contests the magma ocean differentiation concept for VRL formation. It infers a grand-scale construct originating from depositional processes, possibly due to the collapse of a transient, hot primordial atmosphere. Unified Astronomy Thesaurus concepts: Mercury (planet) (1024); Solar system terrestrial planets (797)
... Results from the MESSENGER mission have identified different spectral color units (Denevi et al., 2009;Ernst et al., 2010;Murchie et al., 2015;Robinson et al., 2008) that are not systematically associated with geological units Murray et al., 1975;Spudis and Guest., 1988;Trask and Guest., 1975) or compositional units (Namur & Charlier, 2017;Nittler et al., 2020;Peplowski et al., 2015;Vander Kaaden et al., 2017;Weider et al., 2015). Among the numerous spectral color units (see Table 1 of Murchie et al., 2015 andD'Incecco et al., 2015) are: fresh crater materials characterized by brighter reflectance and slightly bluer spectral slopes than the average (Ernst et al., 2010), bright hollow materials showing in a significant way a higher reflectance and bluer slopes than surroundings Blewett et al., 2013), the red unit interpreted as pyroclastic vents with a relatively red spectra and high reflectance (Barraud et al., 2021;Ernst et al., 2010;Robinson et al., 2008), intermediate plains (Denevi et al., 2009), HRP, low-reflectance blue plains (LBP) and low reflectance material (LRM). HRP have a steeper reflectance spectrum slope in the visible to near-infrared domain than the planet's average and a high reflectance at 750 nm (20% higher than the global average). ...
Results from the NASA/MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission showed that 27% of Mercury's surface is covered by smooth plains mostly produced by extensive effusive volcanism. The Caloris impact basin is associated with two, mostly volcanic, smooth plains, one in its interior and one surrounding the basin. Previous studies have shown that it is difficult to estimate the relative ages of the interior and exterior plains and to explain their spectral and compositional differences. We perform an exhaustive spectral analysis of the basin based on data obtained by the Mercury Atmospheric and Surface Composition Spectrometer spectrometer onboard MESSENGER between 300 and 1,450 nm with a 5 nm resolution. Unlike previous results based on crater counts, we found that the exterior plains were emplaced before the interior plains although both postdate the formation of the basin. We propose a decrease in the partial melting degree and/or partial melting depth over time to explain the spectral and compositional differences. The study highlights spectrally heterogeneous exterior plains associated with a compositional heterogeneity and the presence of low reflectance material (LRM) deposits. This heterogeneity in the exterior plains could be related to the formation of the basin and asymmetry of the ejecta deposits or to a heterogeneous distribution of LRM present at depths before the formation of the basin.
... Recently it was also reported tha S reached the surface of Mercury through volcanic activity (in form of sulfides) as slag deposits in the hollows and as pyroclastic materials (Varatharajan et al., 2019;Namur et al. 2016;Cartier and Wood, 2019;Renggli et al. 2022). Hollows are depressions often, however not always, associated with impact craters on Mercury and display rounded edges (Blewett et al. 2013) and are a typical occurrence on its surface. They are rimless and shallow, and many have high-reflectance and diffuse bright halos (Blewett et al., 2011). ...
... They are rimless and shallow, and many have high-reflectance and diffuse bright halos (Blewett et al., 2011). The data from MESSENGER's Mercury flybys and especially the orbital phase of the mission revealed that hollows occur in a variety of settings and around craters as well as in locations not directly linked with impact structures (e.g., Blewett et al., 2013;Thomas et al. 2014). Here, we Dominici crater (20 km diameter, 1°N 323°E, see the inset of Fig. 1) (Vilas et al., 2016;Blewett et al., 2013;Thomas et al., 2016). ...
... The data from MESSENGER's Mercury flybys and especially the orbital phase of the mission revealed that hollows occur in a variety of settings and around craters as well as in locations not directly linked with impact structures (e.g., Blewett et al., 2013;Thomas et al. 2014). Here, we Dominici crater (20 km diameter, 1°N 323°E, see the inset of Fig. 1) (Vilas et al., 2016;Blewett et al., 2013;Thomas et al., 2016). In particular, Vilas et al. (2014) reported that reflectance spectra of a synthetic mixture composed of 20 vol% MgS and 80 vol % CaS are very similar to the reflectance spectra obtained by MESSENGER on Dominici crater. ...
The temperature excursion variation of Mercury's surface may significantly change crystal structure of surfacecomprising
minerals. The thermal stability of oldhamite (CaS) was investigated to validate its presence on the
Mercury's surface. In particular, X-Ray Powder Diffraction (XRPD) and Thermogravimetric Analyses (TGA) on
synthetic powder calcium sulfide (Alfa Aesar) were performed with the aim of confirming its stability up to
723.15 K, the highest temperature that is recorded for the surface of Mercury.
Our results by XRPD and TGA results confirmed that CaS phase is stable within the daily temperature excursion
on Mercury surface. Thermal expansion analyses determined the thermal expansion volume coefficient of
αV = 4.03 × 10−5 K−1. The results of this work support the presence of Ca-sulfide phases on Mercury's surface
and provide valid tools for interpreting the data that will be collected by the BepiColombo space mission (European
Space Agency and Japanese Aerospace Exploration Agency) to Mercury.
... Hollows are irregularly shaped, flat-floored, and shallow depressions with high-reflectance interiors and halos (1,2). Their pristine morphology and lack of superimposed craters are suggestive of freshly exposed material on the planet's surface (2)(3)(4). ...
... Hollows are irregularly shaped, flat-floored, and shallow depressions with high-reflectance interiors and halos (1,2). Their pristine morphology and lack of superimposed craters are suggestive of freshly exposed material on the planet's surface (2)(3)(4). However, hollows are uncommon in the smooth plains of Mercury (4), which are among the youngest global geological units (5,6). ...
... Ninety-six percent of the total hollowed area occurs associated with either regional or localized LRM (4); however, 79% of the hollows are not associated with this spectral unit [figure 12.11 in (3)]. The texture and geological setting of hollows are suggestive of formation by loss of a volatile-bearing phase through space-weathering processes (including solar heating) or sublimation by heating on contact with, or in proximity to, impact melt or magmatic materials (2)(3)(4)7). Multispectral observations obtained by the Mercury Dual Imaging System (MDIS) onboard NASA's MESSENGER (Mercury, Surface, Space Environment, Geochemistry, and Ranging) spacecraft exhibit a weak absorption band around 630 nm within the hollows located in the wall/rim of the Dominici impact crater (9). ...
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.
... The resulting volcanic gas in the system C-O-S was likely dominated by the species CO, S 2 , CS 2 , and COS (Zolotov et al., 2013;Zolotov, 2011), which is sim-ilar to a lunar volcanic gas (Renggli et al., 2017). Volcanogenic S may also be associated with the formation process of the Mercury hollows, which are currently active geological features (Blewett et al., 2018(Blewett et al., , 2013Thomas et al., 2014). A number of processes have been proposed for the formation of these depressions. ...
... A number of processes have been proposed for the formation of these depressions. They include mobilization of volatiles (sulfides or graphite) by space weathering and thermal decomposition (Blewett et al., 2018;Thomas et al., 2016;Vilas et al., 2016), solar heating in highly radiation exposed areas (Blewett et al., 2013;Thomas et al., 2014), or the oxidation and volatilization of graphite . ...
... We propose that this is a pathway to the formation of the Mercury hollows (Fig. 8). The hollows are surface features unique to Mercury and their origin remains enigmatic (Blewett et al., 2018(Blewett et al., , 2013Thomas et al., 2014). They are irregularly shaped depressions with depths averaging around 24 m, most commonly found in low reflectance areas, that appear bright and bluish with diffuse borders (Blewett et al., 2018). ...
The surface of Mercury is enriched in sulfur, with up to 4 wt.% detected by the NASA MESSENGER mission, and has been challenging to understand in the context of other terrestrial planets. We posit, that magmatic S was mobilized as a gas phase in volcanic and impact processes near the surface, exposing silicates to a hot S-rich gas at reducing conditions and allowing conditions for rapid gas-solid reactions. Here, we present novel experiments on the reaction of Mercury composition glasses with reduced S-rich gas, forming Ca- and Mg-sulfides. The reaction products provide porous and fragile materials that create previously enigmatic hollows on Mercury. Our model predicts that the gas-solid reaction forms Ca-Mg-Fe-Ti-sulfide assemblages with SiO2 and aluminosilicates, distinct from formation as magmatic minerals. The ESA/JAXA BepiColombo mission to Mercury will allow this hypothesis to be tested.
... Hollows preferentially occur in low reflectance material (LRM; ∼30% lower than the already dark global average reflectance) (Robinson et al., 2008) and on the floor, central peak, rim, or the ejecta deposit of large craters or basins. Previous studies suggested that hollows could result from the sublimation of volatile-bearing materials such as sulfides, space weathering, outgassing, and pyroclastic volcanism (Barraud et al., 2020;Blewett et al., 2011Blewett et al., , 2013. Additionally, their crisp features and lack of superimposed craters indicate that they are the youngest non-impact features on Mercury (Blewett et al., 2018). ...
Plain Language Summary
We use before and after (temporal) image pairs collected by the Mercury Dual Imaging System to identify 20 surface changes on Mercury that formed between 2011 and 2015. We identified at least one change likely resulting from a newly formed impact crater with bright rays that extend away from the site. If all the changes result from impact events, then the present‐day rate of impactors striking the innermost planet is 1,000 times higher than models predict. Therefore, we investigate other sources for these detected changes. We located several changes on steep slopes near tectonic landforms, consistent with ongoing tectonic activity. Additionally, we identified several changes in areas adjacent to hollow formations, consistent with present‐day activity. These detected changes will be critical targets for the upcoming BepiColombo mission. This work also provides a framework for detecting future changes using cross‐mission image comparisons to constrain present‐day surface processes further.
... Blewett et al. [8,10,44,45] characterized distinctive features called "hollows" composed of relatively blue (433 nm to 559 nm ratio) and high-reflectance material. Hollows are shallow, rimless, irregularly shaped depressions, are distributed globally, and appear to form preferentially on crater floors, central peaks, and peak rings. ...
... Vaughan et al. [46] suggested that sulfides or chlorides in differentiated impact melt could sublimate at surface temperatures. Blewett et al. [8,44] proposed that space weathering (sputtering by solarwind ions, bombardment by micrometeoroids, exposure to solar heating, and ultraviolet fluxes) could cause the destruction and loss of volatile-bearing phases, such as sulfides. Alternately, Blewett et al. [8,44] noted that sequestered magmatic volatiles or fumarolic minerals from volcanic activity could later be exposed by impacts and lost by solar heating. ...
... Blewett et al. [8,44] proposed that space weathering (sputtering by solarwind ions, bombardment by micrometeoroids, exposure to solar heating, and ultraviolet fluxes) could cause the destruction and loss of volatile-bearing phases, such as sulfides. Alternately, Blewett et al. [8,44] noted that sequestered magmatic volatiles or fumarolic minerals from volcanic activity could later be exposed by impacts and lost by solar heating. Thomas et al. [9] suggested that endogenic heating of volatiles in upper crustal rocks could lead to sublimation. ...
The Mercury Dual Imaging System (MDIS) on the Mercury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft has provided global images of Mercury’s surface. A subset of off-nadir observations acquired at different times resulted in near-global stereo coverage and enabled the creation of local area digital elevation models (DEMs). We derived fifty-seven DEMs covering nine sites of scientific interest and tied each to a geodetic reference derived from Mercury Laser Altimeter (MLA) profiles. DEMs created as part of this study have pixel scales ranging from 78 m/px to 500 m/px, and have vertical precisions less than the DEM pixel scale. These DEMs allow detailed characterizations of key Mercurian features. We present a preliminary examination of small features called “hollows” in three DEM sites. Depth measurements from the new DEMs are consistent with previous shadow and stereo measurements.
