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Tianrui Bai, , Linhua Liu
Monthly Notices of the Royal Astronomical Society, Volume 500, pp 2496-2502; https://doi.org/10.1093/mnras/staa3392

Abstract:
The radiative association process for the formation of magnesium oxide (MgO) may be of great importance due to its frequent occurrence in the low-density and dust-poor astrochemical environments. In this work, the cross-sections and rate coefficients for the A1Π → X1Σ+, ${\rm X}^1\Sigma ^+\, \rightarrow \, {\rm A}^1\Pi$, D1Δ → A1Π, a3Π → e3Σ−, ${\rm X}^1\Sigma ^+\, \rightarrow \, {\rm X}^1\Sigma ^+$, and A1Π → A1Π radiative association processes of forming MgO are theoretically estimated. The cross-sections for the transitions between the different states are obtained by using the semiclassical method for direct contributions and the Breit–Wigner theory as a complement for resonance contributions. For the transitions between the same states, the quantum mechanical method is used. The rate coefficients are then obtained from the cross-sections for the temperatures in the range of 10–10 000 K and the results are found to vary from 4.69 $\times \, 10^{-16}$ to 6.27 $\times \, 10^{-14}$ cm3 s−1. For temperatures lower than around 693 K, the rate coefficients for the A1Π → X1Σ+ process are dominant, which indicates this process is the most efficient way of producing MgO at low temperatures. However, the rate coefficients for the D1Δ → A1Π process go through a rapid increase with increasing temperature and become dominant at higher temperatures. For other processes, their rate coefficients are several orders of magnitude lower than those for the two processes mentioned above. The results can be used to further investigate the formation and evolution of MgO in low density and hot gas close to the photosphere of evolved oxygen-rich stars.
, Elizabeth A. Frank, Shoshana Z. Weider, Ellen Crapster-Pregont, Audrey Vorburger, Richard D. Starr, Sean C. Solomon
Published: 28 February 2020
The publisher has not yet granted permission to display this abstract.
Journal of Geophysical Research: Planets, Volume 124, pp 2414-2429; https://doi.org/10.1029/2019je005997

Abstract:
Principal component analysis of Mercury chemical composition maps provides analytically robust identification of distinct geochemical terranes on Mercury. The results reveal a wide diversity of terrane types, including previously identified terranes like the high‐Mg terrane and northern terranes, as well as a new terrane not identified in prior studies, the intermediate composition, high‐K (IHK) terrane. We propose that the IHK is material excavated from beneath the Borealis planitia by the Rustaveli‐basin‐forming impact. In addition to the new IHK terrane, Caloris‐basin‐like materials are recognized outside of the basin rim for the first time. This indicates that the terrane type found within the Caloris basin is not a unique consequence of the Caloris impact event. Geochemical modeling of terrane compositions facilitates interpretation of the principal components in terms of mineral end‐members, making the individual PC maps also a measure of the relative variability of pyroxenes, olivine, and plagioclase. Our mineralogical modeling also confirms the low‐viscosity nature of the lava's that flooded Mercury's surface to produce the present‐day crust, consistent with the geomorphology of Mercury's extensive volcanic plains units.
Published: 7 August 2019
Abstract:
We obtained mid-infrared spectra and major-element analyses of glasses produced in pulsed laser experiments of basalt. Materials from pits excavated in a basalt slab, as well as of a larger, separated melt droplet were studied. The results of this study show that these glasses exhibits spectral features clearly distinguishable from the unprocessed starting material. Spectra and chemistry show changes, which could be the result of not only melting but also vaporization. Christiansen Features (CF) for the melt glass in the laser-excavated pits are at 8.3–8.5 μm, and a dominating Reststrahlen Band (RB) at 10.1–10.5 μm in wavelength. The spectra of the powdered glass droplet has a CF at 8.8–8.9 μm and a RB at 10.3–10.5 μm. The spectra are clearly different from the spectra of the surrounding starting material, which shows CF between 8.0 and 8.3 μm, and ample RBs between 9.3 μm and 14.7 μm, typical olivine, plagioclase and pyroxene features. The results reflect the chemical composition, which shows significant losses of volatiles like K2O and Na2O, as well as of moderate volatiles like FeO, SiO2, and MgO. Refractories TiO2, Al2O3, and CaO tend to be enriched compared to the bulk starting composition. This indicates loss of material through evaporation. While the spectra of size fractions of the powdered bulk melt glass droplet follow this trend in general, but, because of contamination by the experimental set-up, CaO was found to be strongly enriched in contrast to the other refractories TiO2 and Al2O3. At least the composition of the glasses in the laser-excavated pits could serve as an ‘endmember’ for the sequence of glassy materials expected to be produced in high energy impact processes involving a basaltic target. Correlation of CF with SiO2 contents and the SCFM (SiO2/(SiO2 + CaO + FeO + MgO)) index show similar behaviour of the pit melts like found in earlier studies. However, when the position of the RB in the pit glass is correlated with the SiO2 content, the result shows a different trend compared with earlier studies. Consequently, the data presented in this study could help distinguishing between surface regions formed by volcanic processes and such modified by high-velocity impacts, where evaporation could play a central role. This is of high interest for remote sensing studies of Mercury, which, because of its proximity to the Sun, was probably affected by high-velocity impacts to a very high degree.
, Zoë E. Wilbur, Rachel R. Rahib, Francis M. McCubbin, Kathleen E. Vander Kaaden, , Karen Ziegler, Juliane Gross, Christopher DeFelice, Logan Combs, et al.
Published: 22 January 2019
Meteoritics & Planetary Science, Volume 54, pp 785-810; https://doi.org/10.1111/maps.13252

The publisher has not yet granted permission to display this abstract.
Journal of Geophysical Research: Planets, Volume 123, pp 952-971; https://doi.org/10.1002/2017je005450

Abstract:
Mercury, a planet with a predominantly volcanic crust, has perplexingly few, if any, constructional volcanic edifices, despite their common occurrence on other solar system bodies with volcanic histories. Using image and topographical data from the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft, we describe two small (< 15 km‐diameter) prominences with shallow summit depressions associated with volcanically flooded impact features. We offer both volcanic and impact‐related interpretations for their formation, and then compare these landforms with volcanic features on Earth and the Moon. Though we cannot definitively conclude that these landforms are volcanic, the paucity of constructional volcanic edifices on Mercury is intriguing in itself. We suggest that this lack is because volcanic eruptions with sufficiently low eruption volumes, rates, and flow lengths, suitable for edifice construction, were highly spatiotemporally restricted during Mercury's geological history. We suggest that volcanic edifices may preferentially occur in association with late‐stage, post‐impact effusive volcanic deposits. The ESA/JAXA BepiColombo mission to Mercury will be able to investigate further our candidate volcanic edifices, search for other, as‐yet unrecognized edifices beneath the detection limits of MESSENGER data, and test our hypothesis that edifice construction is favored by late‐stage, low‐volume effusive eruptions.
, O. Abramov, E.A. Frank, R. Brasser
Published: 1 January 2018
Earth and Planetary Science Letters, Volume 482, pp 536-544; https://doi.org/10.1016/j.epsl.2017.11.023

The publisher has not yet granted permission to display this abstract.
Journal of Geophysical Research: Planets, Volume 122, pp 2702-2718; https://doi.org/10.1002/2017je005390

Abstract:
We report electrical conductivity measurements on metal-olivine systems at about 5 and 6 GPa and up to 1,675°C in order to investigate the electrical properties of core-mantle boundary (CMB) systems. Electrical experiments were conducted in the multianvil apparatus using the impedance spectroscopy technique. The samples are composed of one metal layer (Fe, FeS, FeSi2, or Fe-Ni-S-Si) and one polycrystalline olivine layer, with the metal:olivine ratio ranging from 1:0.7 to 1:9.2. For all samples, we observe that the bulk electrical conductivity increases with temperature from 10−2.5 to 101.8 S/m, which is higher than the conductivity of polycrystalline olivine but lower than the conductivity of the pure metal phase at similar conditions. In some experiments, a conductivity jump is observed at the temperature corresponding to the melting temperature of the metallic phase. Both the metal:olivine ratio and the metal phase geometry control the electrical conductivity of the two-layer samples. By combining electrical results, textural analyses of the samples, and previous studies of the structure and composition of Mercury's interior, we propose an electrical profile of the deep interior of the planet that accounts for a layered CMB-outer core structure. The electrical model agrees with existing conductivity estimates of Mercury's lower mantle and CMB using magnetic observations and thermodynamic calculations, and thus, supports the hypothesis of a layered CMB-outermost core structure in the present-day interior of Mercury. We propose that the layered CMB-outer core structure is possibly electrically insulating, which may influence the planet's structure and cooling history.
, Kathleen E. Vander Kaaden, , Aaron S. Bell, , , Larry G. Evans, Lindsay P. Keller, , Timothy J. McCoy
Journal of Geophysical Research: Planets, Volume 122, pp 2053-2076; https://doi.org/10.1002/2017je005367

The publisher has not yet granted permission to display this abstract.
, , Natalie M. Curran, , Amy F. Fagan, David A. Kring
Published: 19 October 2016
Earth, Moon, and Planets, Volume 118, pp 133-158; https://doi.org/10.1007/s11038-016-9495-0

Abstract:
The Moon is an archive of impact cratering in the Solar System throughout the past 4.5 billion years. It preserves this record better than larger, more complex planets like the Earth, Mars and Venus, which have largely lost their ancient crusts through geological reprocessing and hydrospheric/atmospheric weathering. Identifying the parent bodies of impactors (i.e. asteroid bodies, comets from the Kuiper belt or the Oort Cloud) provides geochemical and chronological constraints for models of Solar System dynamics, helping to better inform our wider understanding of the evolution of the Solar System and the transfer of small bodies between planets. In this review article, we discuss the evidence for populations of impactors delivered to the Moon at different times in the past. We also propose approaches to the identification and characterisation of meteoritic material on the Moon in the context of future lunar exploration efforts.
, Patrick N. Peplowski, , William C. Feldman, Elizabeth A. Frank, Timothy J. McCoy, Larry R. Nittler, Sean C. Solomon
Published: 2 August 2016
Icarus, Volume 281, pp 32-45; https://doi.org/10.1016/j.icarus.2016.07.018

Abstract:
We report measurements of the flux of fast neutrons at Mercury from 20ºS to the north pole. On the basis of neutron transport simulations and remotely sensed elemental compositions, cosmic-ray-induced fast neutrons are shown to provide a measure of average atomic mass, <A>, a result consistent with earlier studies of the Moon and Vesta. The dynamic range of fast neutron flux at Mercury is 3%, which is smaller than the fast-neutron dynamic ranges of 30% and 6% at the Moon and Vesta, respectively. Fast-neutron data delineate compositional terranes on Mercury that are complementary to those identified with X-ray, gamma-ray, and slow-neutron data. Fast neutron measurements confirm the presence of a region with high <A>, relative to the mean for the planet, that coincides with the previously identified high-Mg region and reveal the existence of at least two additional compositional terranes: a low-<A> region within the northern smooth plains and a high-<A> region near the equator centered near 90ºE longitude. Comparison of the fast-neutron map with elemental composition maps show that variations predicted from the combined element maps are not consistent with the measured variations in fast-neutron flux. This lack of consistency could be due to incomplete coverage for some elements or uncertainties in the interpretations of compositional and neutron data. Currently available data and analyses do not provide sufficient constraints to resolve these differences.
, , Francois Holtz, Camille Cartier, Catherine McCammon
Earth and Planetary Science Letters, Volume 448, pp 102-114; https://doi.org/10.1016/j.epsl.2016.05.024

The publisher has not yet granted permission to display this abstract.
, Deborah L. Domingue, , , Alessandro Maturilli, , , , , David T. Blewett, et al.
Geophysical Research Letters, Volume 43, pp 1450-1456; https://doi.org/10.1002/2015gl067515

Abstract:
Early during MErcury Surface Space ENvironment GEochemistry, and Ranging (MESSENGER)'s orbital mission, the Mercury Dual-Imaging System imaged the landform called hollows on the two craters Dominici and Hopper, using its Wide-Angle Camera with eight narrowband color filters ranging from 433 to 996 nm. An absorption feature centered in the MDIS 629 nm filter is evident in reflectance spectra for Dominici's south wall/rim hollows. A different absorption feature found in photometry of Dominici's center hollows extends through the MDIS 828 nm filter. Hollows in Hopper exhibit a weaker spectral absorption feature than that observed in Dominici's center. At Dominici, we postulate that fresher hollows-hosting material in the wall/rim was exposed to the space environment through a process of slumping of the overlying material. With time, local and global processes darken the hollows and change or mix the surface mineralogy, so that the spectral signature evolves. The hollows could contain low-density MgS and an opaque component, potentially derived from background material.
Journal of Geophysical Research: Planets, Volume 121, pp 118-136; https://doi.org/10.1002/2015je004832

Abstract:
To discuss mantle evolution in Mercury, I present two-dimensional numerical models of magmatism in a convecting mantle. Thermal, compositional, and magmatic buoyancy drives convection of temperature-dependent viscosity fluid in a rectangular box placed on the top of the core that is modeled as a heat bath of uniform temperature. Magmatism occurs as a permeable flow of basaltic magma generated by decompression melting through a matrix. Widespread magmatism caused by high initial temperature of the mantle and the core makes the mantle compositionally stratified within the first several hundred million years of the 4.5 Gyr calculated history. The stratified structure persists for 4.5 Gyr, when the reference mantle viscosity at 1573 K is higher than around 1020 Pa s. The planet thermally contracts by an amount comparable to the one suggested for Mercury over the past 4 Gyr. Mantle upwelling, however, generates magma only for the first 0.1–0.3 Gyr. At lower mantle viscosity, in contrast, a positive feedback between magmatism and mantle upwelling operates to cause episodic magmatism that continues for the first 0.3–0.8 Gyr. Convective current stirs the mantle and eventually dissolves its stratified structure to enhance heat flow from the core and temporarily resurrect magmatism depending on the core size. These models, however, predict larger contraction of the planet. Coupling between magmatism and mantle convection plays key roles in mantle evolution, and the difficulty in numerically reproducing the history of magmatism of Mercury without causing too large radial contraction of the planet warrants further exploration of this coupling.
Andreas Morlok, , Isabelle Dittmar, Harald Hiesinger, Manuel Tiedeken, Martin Sohn, ,
Published: 1 January 2016
Icarus, Volume 264, pp 352-368; https://doi.org/10.1016/j.icarus.2015.10.003

, A. Morlok, A. Bischoff, H. Hiesinger, D. Ward, , , N. D. Jastrzebski, , , et al.
Published: 20 December 2015
Meteoritics & Planetary Science, Volume 51, pp 3-30; https://doi.org/10.1111/maps.12586

Abstract:
This work is part of a project to build an infrared database in order to link IR data of planetary materials (and therefore possible Mercury material) with remote sensing observations of Mercury, which will probably be obtained by the MERTIS instrument on the forthcoming BepiColombo mission. The unique achondrite Northwest Africa (NWA) 7325, which has previously been suggested to represent the first sample from Mercury, was investigated by optical and electron microscopy, and infrared and Raman spectroscopy. In addition, the oxygen, strontium, xenon, and argon isotopes were measured and the abundance of selected trace elements determined. The meteorite is a cumulate rock with subchondritic abundances of HFSE and REE and elevated Sr contents, which underwent a second heating and partial remelting process. Oxygen isotope measurements show that NWA 7325 plots in the ureilite field, close to the ALM‐A trachyandesitic fragment found in the unique Almahata Sitta meteorite breccia. On the other hand, mineralogical investigations of the pyroxenes in NWA 7325 provide evidence for similarities to the lodranites and acapulcoites. Furthermore, the rock is weakly shocked and argon isotope data record ancient (~4.5 Ga) plateau ages that have not been reset. The sample records a cosmogenic exposure age of ~19 Ma. Systematics of Rb‐Sr indicate an extreme early volatile depletion of the precursor material, similar to many other achondrite groups. However, despite its compositional similarities to other meteorite groups, our results suggest that this meteorite is unique and unrelated to any other known achondrite group. An origin for NWA 7325 as a sample from the planet Mercury is not supported by the results of our investigation. In particular, the evidence from infrared spectroscopy indicates that a direct relationship between NWA 7325 and the planet Mercury can be ruled out: no acceptable spectral match between laboratory analyses and remote sensing observations from Mercury has been obtained. However, we demonstrate that infrared spectroscopy is a rapid and nondestructive method to characterize mineral phases and thus an excellent tool for planetary surface characterization in space missions.
Journal of Geophysical Research: Planets, Volume 120, pp 1924-1955; https://doi.org/10.1002/2015je004792

Abstract:
We experimentally determined the rheological evolution of three basaltic analog compositions appropriate to Mercury's surface, during cooling, and crystallization. Investigated compositions are an enstatite basalt, and two magnesian basalts representing the compositional end-members of the northern volcanic plains with 0.19 wt % (NVP) and 6.26 wt % Na2O (NVP-Na). The viscosity-strain rate dependence of lava was quantified using concentric cylinder viscometry. We measured the viscosities of the crystal-free liquids from 1600°C down to the first detection of crystals. Liquidus temperatures of the three compositions studied are around 1360°C, and all three compositions are more viscous than Hawaiian basalt at the same temperature. The onset of pseudoplastic behavior was observed at crystal fractions ~0.05 to 0.10, which is consistent with previous studies on mafic lavas. We show that all lavas develop detectable yield strengths at crystal fractions around 0.20, beyond which the two-phase suspensions are better described as Herschel-Bulkley fluids. By analogy with the viscosity-strain rate conditions at which the pahoehoe to `a`a transition occurs in Kilauea basalt, this transition is predicted to occur at ~1260 ± 10°C for the enstatite basalt, at ~1285 ± 20°C for the NVP, and at ~1240 ± 40°C for the NVP-Na lavas. Our results indicate that Mercury lavas are broadly similar to terrestrial ones, which suggests that the extensive smooth lava plains of Mercury could be due to large effusion rates (flood basalts) and not to unusually fluid lavas.
Journal Of Petrology, Volume 56, pp 1407-1424; https://doi.org/10.1093/petrology/egv041

Abstract:
The concentration of sulfur in basalt-like silicate melts as S2– is limited to the amount at which the melt becomes saturated with a sulfide phase, such as an immiscible sulfide melt. The limiting solubility is called the ‘sulfur content at sulfide saturation’ (SCSS). Thermodynamic modelling shows that the SCSS depends on the FeO content of the silicate melt from two terms, one with a negative dependence that comes from the activity of FeO in the silicate melt, and the other with a positive dependence that comes from the strong dependence of the sulfide capacity of the melt (CS) on FeO content. The interaction between these two terms should yield a net SCSS that has an asymmetric U-shaped dependence on the FeO content of the melt, if other variables are kept constant. We have tested this thermodynamic model in a series of experiments at 1400°C and 1·5 GPa to determine the sulfur contents at saturation with liquid FeS in melt compositions along the binary join between a haplobasaltic composition and FeO. The SCSS is confirmed to have the asymmetric U-shaped dependence, with a minimum at ∼5 wt % FeO. The effect of FeO on the selenide content at selenide saturation (SeCSeS) was investigated in an analogous fashion. SeCSeS shows a similar, though not identical, U-shaped dependence, implying that the solubility mechanism of selenide in basalt-like silicate melts is similar to that of sulfide. The observation of increasing SCSS with decreasing FeO in hydrous silicic melts was explored by inverse modelling of datasets from pyrrhotite-saturated hydrous silicic liquids, revealing that high SCSS at low FeO can be explained in terms of the low-FeO limb of the ‘U’, rather than dissolution of sulfur as hydrous species such as H2S or HS. Recent measurements of the composition of the surface of Mercury prompted examination of the high-SCSS, low-FeO limb of the ‘U’ as a potential explanation for the sulfur-rich but Fe-poor surface of Mercury.
Larry G. Evans, , Francis M. McCubbin, Timothy J. McCoy, Larry R. Nittler, Mikhail Yu. Zolotov, Denton S. Ebel, David J. Lawrence, Richard D. Starr, , et al.
Published: 7 May 2015
Icarus, Volume 257, pp 417-427; https://doi.org/10.1016/j.icarus.2015.04.039

The publisher has not yet granted permission to display this abstract.
, Larry R. Nittler, Richard D. Starr, Ellen J. Crapster-Pregont, , , James W. Head, Paul K. Byrne, , Denton S. Ebel, et al.
Earth and Planetary Science Letters, Volume 416, pp 109-120; https://doi.org/10.1016/j.epsl.2015.01.023

The publisher has not yet granted permission to display this abstract.
Numerical Modelling of Astrophysical Turbulence pp 9-32; https://doi.org/10.1007/978-1-4939-2244-4_2

The publisher has not yet granted permission to display this abstract.
, David J. Lawrence, William C. Feldman, John O. Goldsten, David Bazell, Larry G. Evans, James W. Head, Larry R. Nittler, Sean C. Solomon,
Published: 13 February 2015
Icarus, Volume 253, pp 346-363; https://doi.org/10.1016/j.icarus.2015.02.002

The publisher has not yet granted permission to display this abstract.
, David J. Lawrence, Larry G. Evans, , , John O. Goldsten, , Timothy J. McCoy, Larry R. Nittler, Sean C. Solomon, et al.
Published: 30 January 2015
Planetary and Space Science, Volume 108, pp 98-107; https://doi.org/10.1016/j.pss.2015.01.008

The publisher has not yet granted permission to display this abstract.
Published: 8 January 2015
Geophysical Research Letters, Volume 42, pp 1029-1038; https://doi.org/10.1002/2014gl062487

Abstract:
To gain insight into the thickness of the crust of Mercury, we use gravity and topography data acquired by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging spacecraft to calculate geoid‐to‐topography ratios over the northern hemisphere of the planet. For an Airy model for isostatic compensation of variations in topography, we infer an average crustal thickness of 35 ± 18 km. Combined with the value of the radius of the core of Mercury, this crustal thickness implies that Mercury had the highest efficiency of crustal production among the terrestrial planets. From the measured abundance of heat‐producing elements on the surface, we calculate that the heat production in the mantle from long‐lived radioactive elements at 4.45 Ga was greater than 5.4 ×10−12W/kg. By analogy with the Moon, the relatively thin crust of Mercury allows for the possibility that major impact events, such as the one that formed the Caloris basin, excavated material from Mercury's mantle.
Journal of Geophysical Research: Planets, Volume 120, pp 287-310; https://doi.org/10.1002/2014je004713

Abstract:
To explore the mechanisms of support of surface topography on Mercury, we have determined the admittances and correlations of topography and gravity in Mercury's northern hemisphere from measurements obtained by NASA's MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft. These admittances and correlations can be interpreted in the context of a number of theoretical scenarios, including flexural loading and dynamic flow. We find that long‐wavelength (spherical harmonic degree l< 15) surface topography on Mercury is primarily supported through a combination of crustal thickness variations and deep mass anomalies. The deep mass anomalies may be interpreted either as lateral variations in mantle density or as relief on compositional interfaces. Domical topographic swells are associated with high admittances and are compensated at 300–400 km depth in the lower reaches of Mercury's mantle. Quasi‐linear topographic rises are primarily associated with shallow crustal compensation and are weakly correlated with positive mass anomalies in the mantle. The center of the Caloris basin features some of the thinnest crust on the planet, and the basin is underlain by a large negative mass anomaly. We also explore models of dynamic flow in the presence of compositional stratification above the liquid core. If there is substantial compositional stratification in Mercury's solid outer shell, relaxation of perturbed compositional interfaces may be capable of creating and sustaining long‐wavelength topography.
, Mark S. Robinson, Jennifer L. Whitten, Caleb I. Fassett, Robert G. Strom, James W. Head, Sean C. Solomon
Published: 17 November 2014
Icarus, Volume 250, pp 602-622; https://doi.org/10.1016/j.icarus.2014.11.010

The publisher has not yet granted permission to display this abstract.
, Kevin J. Zahnle, Roxana E. Lupu
Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, Volume 372; https://doi.org/10.1098/rsta.2013.0172

Abstract:
Much of the Earth's mantle was melted in the Moon-forming impact. Gases that were not partially soluble in the melt, such as water and CO 2 , formed a thick, deep atmosphere surrounding the post-impact Earth. This atmosphere was opaque to thermal radiation, allowing heat to escape to space only at the runaway greenhouse threshold of approximately 100 W m −2 . The duration of this runaway greenhouse stage was limited to approximately 10 Myr by the internal energy and tidal heating, ending with a partially crystalline uppermost mantle and a solid deep mantle. At this point, the crust was able to cool efficiently and solidified at the surface. After the condensation of the water ocean, approximately 100 bar of CO 2 remained in the atmosphere, creating a solar-heated greenhouse, while the surface cooled to approximately 500 K. Almost all this CO 2 had to be sequestered by subduction into the mantle by 3.8 Ga, when the geological record indicates the presence of life and hence a habitable environment. The deep CO 2 sequestration into the mantle could be explained by a rapid subduction of the old oceanic crust, such that the top of the crust would remain cold and retain its CO 2 . Kinematically, these episodes would be required to have both fast subduction (and hence seafloor spreading) and old crust. Hadean oceanic crust that formed from hot mantle would have been thicker than modern crust, and therefore only old crust underlain by cool mantle lithosphere could subduct. Once subduction started, the basaltic crust would turn into dense eclogite, increasing the rate of subduction. The rapid subduction would stop when the young partially frozen crust from the rapidly spreading ridge entered the subduction zone.
, , James M. St. John, James W. Head, William M. Vaughan, , Matthias Gottschalk, Sabrina Ferrari
Earth and Planetary Science Letters, Volume 398, pp 58-65; https://doi.org/10.1016/j.epsl.2014.04.035

The publisher has not yet granted permission to display this abstract.
, , Steven A. Hauck Ii, William B. Moore, Sean C. Solomon
Journal of Geophysical Research: Planets, Volume 119, pp 850-866; https://doi.org/10.1002/2013je004459

Abstract:
The combination of the radio tracking of the MErcury Surface, Space ENvironment, GEochemistry, and Ranging spacecraft and Earth‐based radar measurements of the planet's spin state gives three fundamental quantities for the determination of the interior structure of Mercury: mean density ρ, moment of inertia C, and moment of inertia of the outer solid shell Cm. This work focuses on the additional information that can be gained by a determination of the change in gravitational potential due to planetary tides, as parameterized by the tidal potential Love number k2. We investigate the tidal response for sets of interior models that are compatible with the available constraints (ρ, C, and Cm). We show that the tidal response correlates with the size of the liquid core and the mean density of material below the outer solid shell and that it is affected by the rheology of the outer solid shell of the planet, which depends on its temperature and mineralogy. For a mantle grain size of 1 cm, we calculate that the tidal k2 of Mercury is in the range 0.45 to 0.52. Some of the current models for the interior structure of Mercury are compatible with the existence of a solid FeS layer at the top of the core. Such a layer, if present, would increase the tidal response of the planet.
, Francesco Minelli, , Cristina Pauselli, Costanzo Federico
Geological Society, London, Special Publications, Volume 401, pp 57-75; https://doi.org/10.1144/sp401.10

Abstract:
In this work, we combined multi-scale geological maps of Mercury to produce a new global map where geological units are classified based on albedo, crater density and morphological relationships with other units. To create this map, we used the 250 m/pixel mosaic of images acquired by the narrow- and wide-angle cameras onboard the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft during its orbital phase. The geological mapping is supported by digital terrain model data and surface mineralogical variation from the global mosaic of MESSENGER Mercury Atmospheric and Surface Composition Spectrometer observations. This map comprises the global-scale intercrater plains, smooth plains and Odin-type units as reported in previous studies, as well as units we term bright intercrater plains, Caloris rough ejecta and dark material deposits. We mapped a portion of the Raditladi quadrangle (19–35°N, 106–133°E) at a regional scale at a resolution of 166 m/pixel. We characterized the geological context of the area and evaluated the stratigraphic relationships between the units. To obtain a representative geological section, we analysed and corrected available topographical data. The geological cross-section derived from our regional mapping suggests that volcanic emplacement of Raditladi's inner plains followed the topography of the basin after the deposition of impact-related units (i.e. melts, breccias and rim collapse) and was driven by low-viscosity flows. Hollows that appear on Raditladi's peak ring were possibly formed from low-reflectance intercrater plains materials exposed through the peak ring unit. Supplementary material: Cleaner, larger version of the global-scale geological map and a local-scale map for comparison are available at http://www.geolsoc.org.uk/SUP18741.
, Larry R. Nittler, Richard D. Starr, Timothy J. McCoy, Sean C. Solomon
Published: 11 March 2014
Icarus, Volume 235, pp 170-186; https://doi.org/10.1016/j.icarus.2014.03.002

The publisher has not yet granted permission to display this abstract.
, Larry G. Evans, , David J. Lawrence, John O. Goldsten, Timothy J. McCoy, Larry R. Nittler, Sean C. Solomon, Ann L. Sprague, Richard D. Starr, et al.
Published: 21 September 2013
Icarus, Volume 228, pp 86-95; https://doi.org/10.1016/j.icarus.2013.09.007

The publisher has not yet granted permission to display this abstract.
Journal of Geophysical Research: Planets, Volume 118, pp 1013-1032; https://doi.org/10.1029/2012je004174

Abstract:
[1] Landforms 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 Calorian in age). Hollows are found in the low‐reflectance material global color unit and in low‐reflectance blue plains, but they appear to be absent from high‐reflectance red plains. Hollows may occur preferentially on equator‐ or hot‐pole‐facing slopes, implying that their formation is linked to solar heating. Evidence suggests that hollows form because of loss of volatile material. We describe hypotheses for the origin of the volatiles and for how such loss proceeds. Intense space weathering and solar heating are likely contributors to the loss of volatiles; contact heating by melts could promote the formation of hollows in some locations. Lunar Ina‐type depressions differ from hollows on Mercury in a number of characteristics, so it is unclear if they represent a good analog. We also use MESSENGER multispectral images to characterize a variety of surfaces on Mercury, including hollows, within a framework defined by laboratory spectra for analog minerals and lunar samples. Data from MESSENGER's X‐Ray Spectrometer indicate that the planet's surface contains up to 4% sulfur. We conclude that nanophase or microphase sulfide minerals could contribute to the low reflectance of the low‐reflectance material relative to average surface material. Hollows may owe their relatively high reflectance to destruction of the darkening agent (sulfides), the presence of alteration minerals, and/or physical differences in particle size, texture, or scattering behavior.
, , Sean C. Solomon, , , , , Timothy J. McCoy, , Stanton J. Peale, et al.
Journal of Geophysical Research: Planets, Volume 118, pp 1204-1220; https://doi.org/10.1002/jgre.20091

Abstract:
[1] The recent determination of the gravity field of Mercury and new Earth‐based radar observations of the planet's spin state afford the opportunity to explore Mercury's internal structure. These observations provide estimates of two measures of the radial mass distribution of Mercury: the normalized polar moment of inertia and the fractional polar moment of inertia of the solid portion of the planet overlying the liquid core. Employing Monte Carlo techniques, we calculate several million models of the radial density structure of Mercury consistent with its radius and bulk density and constrained by these moment of inertia parameters. We estimate that the top of the liquid core is at a radius of 2020 ± 30 km, the mean density above this boundary is 3380 ± 200 kg m−3, and the density below the boundary is 6980 ± 280 kg m−3. We find that these internal structure parameters are robust across a broad range of compositional models for the core and planet as a whole. Geochemical observations of Mercury's surface by MESSENGER indicate a chemically reducing environment that would favor the partitioning of silicon or both silicon and sulfur into the metallic core during core‐mantle differentiation. For a core composed of Fe–S–Si materials, the thermodynamic properties at elevated pressures and temperatures suggest that an FeS‐rich layer could form at the top of the core and that a portion of it may be presently solid.
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