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date: 21 August 2019

Petrology and Geochemistry of Mercury

Abstract and Keywords

Having knowledge of a terrestrial planet’s chemistry is fundamental to understanding the origin and composition of its rocks. Until recently, however, the geochemistry of Mercury—the Solar System’s innermost planet—was largely unconstrained. Without the availability of geological specimens from Mercury, studying the planet’s surface and bulk composition relies on remote sensing techniques. Moreover, Mercury’s proximity to the Sun makes it difficult to study with Earth/space-based telescopes, or with planetary probes. Indeed, to date, only NASA’s Mariner 10 and MESSENGER missions have visited Mercury. The former made three “flyby” encounters of Mercury between 1974 and 1975, but did not carry any instrument to make geochemical or mineralogical measurements of the surface. Until the MESSENGER flyby and orbital campaigns (2008–2015), therefore, knowledge of Mercury’s chemical composition was severely limited and consisted of only a few facts. For example, it has long been known that Mercury has the highest uncompressed density (i.e., density with the effect of gravity removed) of all the terrestrial planets, and thus a disproportionately large Fe core. In addition, Earth-based spectral reflectance observations indicated a dark surface, largely devoid of Fe within silicate minerals.

To improve understanding of Mercury’s geochemistry, the MESSENGER scientific payload included a suite of geochemical sensing instruments: in particular, an X-Ray spectrometer and a gamma-ray and neutron spectrometer. The datasets obtained from these instruments (as well as from other complementary instruments) during MESSENGER’s 3.5-year orbital mission allow a much more complete picture of Mercury’s geochemistry to be drawn, and quantitative abundance estimates for several major rock-forming elements in Mercury’s crust are now available. Overall, the MESSENGER data reveal a surface that is rich in Mg, but poor in Al and Ca, compared with typical terrestrial and lunar crustal materials. Mercury’s surface also contains high concentrations of the volatile elements Na, S, K, and Cl. Furthermore, the total surface Fe abundance is now known to be <2 wt%, and the planet’s low-reflectance is thought to be primarily caused by the presence of C (in the form of graphite) at a level of >1 wt%. Such data are key to constraining models of Mercury’s formation and early evolution. Large-scale spatial variations in the MESSENGER geochemical datasets have also led to the designation of several geochemical “terranes,” which do not always align with otherwise mapped geological regions.

Based on the MESSENGER geochemical results, petrological experiments and calculations have been, and continue to be, performed to study Mercury’s surface mineralogy and petrology. The results show that there are likely to be substantial differences in the precise mineral compositions and abundances amongst the different terranes, but Mercury’s surface appears to be dominated by Mg-rich olivine and pyroxene, as well as plagioclase and sulfide phases. Depending on the classification scheme used, Mercury’s ultramafic surface rocks can thus be described as similar in nature to terrestrial boninites, andesites, norites, or gabbros.

Keywords: gamma-ray spectroscopy, geochemistry, Mercury, MESSENGER, mineralogy, neutron spectroscopy, petrology, radar, reflectance spectroscopy, X-ray spectroscopy

Introduction

Although it was known to the ancients, and is one of Earth’s closest neighbors, Mercury was historically one of the least studied planets. Yet gaining an in-depth knowledge of Mercury—the innermost and end-member planet of our Solar System—is critical to obtaining a more thorough understanding of terrestrial planet evolution and of Earth’s immediate surroundings. Determining the surface composition, geochemistry, and petrology of Mercury informs models of its accretion, differentiation, and geological history, as well as interactions with its space environment (i.e., exosphere and magnetosphere). Such information is becoming ever more important as context for the increasing number of known extrasolar planets.

In general, however, studying Mercury is a difficult task. Its geochemistry and petrology cannot be studied via examination of easily accessible lithological specimens, as no Mercury meteorites (i.e., with Mercury as their parent body) have yet been discovered. Instead, the study of Mercury’s surface (and bulk) chemistry relies primarily on data obtained via a variety of remote sensing techniques. Mercury’s proximity to the Sun makes it difficult to observe from Earth, despite the relatively small (~92 million km, on average) Earth–Mercury distance. In addition, Mercury is a forbidden target for space-based telescopes because of the risk of exposing optical systems to direct sunlight. Nonetheless, over time, a picture of Mercury’s surface chemistry has been constructed from ground-based spectral reflectance measurements (with coarse spatial resolution) across a wide range of wavelengths—encompassing the visible, near-infrared, mid-infrared (mid-IR), microwave, and radio parts of the electromagnetic spectrum (Boynton et al., 2007).

Mercury’s proximity to the Sun also makes it a challenging environment (in terms of temperature, gravity, and space-weather conditions) to send spacecraft, and only two probes—NASA’s Mariner 10 and MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) missions—have visited Mercury thus far. In addition, the joint European Space Agency (ESA)–Japanese Aerospace Exploration Agency (JAXA) BepiColombo mission was launched in late 2018 and is due to arrive at Mercury in 2025 (ESA, 2018). Mariner 10’s three “flyby” encounters of Mercury provided the first close-up measurements of the planet. Less than half of the surface, however, was observed and the probe did not carry any instrument capable of obtaining direct geochemical or mineralogical data from the planet’s surface. Limited and somewhat speculative geochemical information, however, was garnered indirectly from visible-wavelength images (Robinson & Lucey, 1997) and from observations of the planet’s exosphere (e.g., Hunten, Morgan, & Shemansky, 1988; Kumar, 1976; McGrath, Johnson, & Lanzerotti, 1986).

The MESSENGER mission—the three flybys of Mercury, followed by a 3.5-year orbital campaign—presented the first opportunity to study the entire planet and its immediate surroundings in detail. The spacecraft payload included several instruments to directly measure the geochemistry and composition of Mercury’s surface. Principally these were the X-Ray spectrometer (XRS; Schlemm et al., 2007), Gamma-Ray and Neutron Spectrometer (GRNS; Goldsten et al. 2007), Mercury Atmospheric and Surface Composition Spectrometer (MASCS; McClintock & Lankton, 2007), and Mercury Dual Imaging System (MDIS; Hawkins et al., 2007).

Pre-MESSENGER Geochemical View of Mercury

Mercury Formation Theories

It has long been known that Mercury’s bulk density—compared with the other terrestrial planets—is peculiarly high. From the results of Earth-based radar measurements and radio-frequency tracking of Mariner 10’s trajectory, the bulk density of Mercury was estimated as ~5.44 g cm−3 (equivalent to an uncompressed density of ~5.3 g/cm−3; e.g., Solomon, 2003). Furthermore, the planet’s high bulk density was used to infer (on the basis of thermal evolution models; Solomon, 1976) an abnormally high metal/silicate ratio (i.e., a disproportionately large core). Several Mercury formation and early evolution theories were therefore proposed to explain this geochemical constraint.

In the “chemical equilibrium model” (Lewis, 1972, 1973, 1974), the compositions of the planets—including Mercury—were derived by considering condensation of the primitive solar nebula. Lewis proposed that heliocentric gradients in the temperature, pressure, and density of the nebula (Cameron, 1969) gave rise to gradients in the composition of condensed material and, in turn, these gradients were preserved in the present-day bulk composition of the planets. At Mercury’s position in the solar nebula, the predicted pressure (10−5–10−3 bar) would have caused metallic Fe to condense at a slightly higher temperature than Mg-rich silicates. This temperature gap, therefore, could be responsible for Mercury’s Fe enrichment. The model also predicts that Mercury has a massive Fe-Ni core (containing no S, Si, or O) surrounded by a Mg-rich silicate portion, in which the mantle consists mostly of enstatite and refractory components (e.g., Al2O3, CaO, and TiO2), and is essentially devoid of FeO and volatile species. Subsequent studies, however, highlighted the need to invoke an additional mechanism to cause preferential removal of silicates (or to concentrate the metal) and to thus match Mercury’s measured bulk density.

To that end, Weidenschilling (1978) proposed the “aerodynamic depletion model.” In this scenario, smaller silicate particles that originally condensed in Mercury’s zone of the solar nebula were entrained within the gas cloud more easily than larger metal-rich particles. Weidenschilling argued that the preferential removal (via gas drag) of silicate material—compared with metal particles—into the Sun could account for Mercury’s high bulk density. According to this model, Mercury’s bulk composition would be similar to that predicted by the chemical equilibrium model, but with a higher metal content.

On the basis of the available meteorite collection, Wasson (1988) argued that Mercury’s high density is the direct result of the planet’s accretion from highly reduced chondritic materials. Such precursors would have been similar to the most-reduced known chondrites (high enstatite, EH, chondrites), but with an Fe : Si ratio that was four to seven times greater. A clan of metal-rich, FeO-poor chondrites (i.e., the CR, CH, and CB groups) have also been suggested as possible Mercury building blocks (Taylor & Scott, 2003) as their high concentration of metallic Fe and low inventory of volatiles are compatible with formation in the hot, inner Solar System.

The models of Lewis, Weidenschilling, and Wasson frame Mercury’s high metal/silicate ratio as a function of a pre-accretion process. In another set of theories, however, Mercury’s high bulk density is seen as the result of a post-accretion episode. For example, it has been suggested that an original “protomercury” had a typical (approximately chondritic) bulk density, but most of its silicate portion (mantle and crust) was lost at some point after the body’s differentiation. Indeed, according to the “vaporization model” (Cameron, 1985; Fegley & Cameron, 1987), the silicate portion was lost during a very-hot, late stage of the solar nebula when the temperature at the position of Mercury was ~2500–3500 K. In this model, it is predicted that 70–80% of the protoplanet’s mantle (assumed to consist mostly of enstatite) could have been removed over the time such temperatures persisted (3 × 104 years). The resultant, Fe-rich body would thus have an uncompressed density equal to that of the current planet. The model also indicates that Mercury’s remaining silicates should be depleted in alkalis, FeO, SiO2, and volatiles, but enriched in CaO, MgO, Al2O3, and TiO2 (all relative to chondritic material).

A giant impact event (or events) is another post-differentiation mechanism that has been presented as a way to remove the majority of protomercury’s original silicate portion. After Wetherill (1986) suggested that large, high-velocity projectiles within Mercury’s accretion zone would have been likely, several authors (e.g., Benz, Slattery, & Cameron, 1988; Cameron, Fegley, Benz, & Slattery, 1988) investigated the idea that a roughly chondritic protomercury (about 2.25 times the mass of the current planet) was hit by at least one of these projectiles (with a mass about one-sixth of the protoplanet) after its initial differentiation. Simulations show that this kind of impact could eject a large fraction of the planet’s mantle and leave behind an Fe-rich body. Moreover, it has been shown (Benz, Anic, Horner, & Whitby, 2007) that it is just as likely for the ejected material to be lost into the Sun (through Poynting–Robertson drag) as to be re-accreted to Mercury. It is therefore possible for the bulk density and composition resulting from a giant impact to have been retained.

Surface Composition and Mineralogy

Reflectance spectroscopy is an important methodology for investigating the chemical and mineralogical composition of planetary surfaces. Ground-based telescopic observations of Mercury’s surface reflectance were first obtained in the 1960s (e.g., Harris, 1961; Irvine, Simon, Menzel, Pikoos, & Young, 1968) and have since spanned the ultraviolet (UV) to thermal infrared (IR) wavelengths. At the shorter wavelengths (UV to visible range), an absorption feature at ~1 μ‎m is commonly found in spectra from many Solar System bodies (e.g., the Moon and asteroids) and is indicative of Fe2+ in olivine and/or pyroxene phases (Burns, 1970). Mercury reflectance spectra with sufficient spectral resolution to resolve this, and other, mineralogical features first became available in the late 1960s (e.g., McCord & Adams, 1972a, 1972b; Vilas, Leake, & Mendell, 1984), yet robust observations of the 1 μ‎m absorption feature were not forthcoming.

Several studies reported the existence of a very shallow absorption feature close to 1 μ‎m, which was attributed to the presence of Fe2+ in pyroxenes (e.g., McCord & Adams, 1972b; McCord & Clark, 1979; Vilas, 1988; Vilas et al., 1984; Warell, Sprague, Emery, Kozlowski, & Long, 2006). These spectra, however, were plagued by telluric H2O absorptions (caused by the Earth’s atmosphere) that are notoriously difficult to correct (Vilas, 1988). The majority of reported 1 μ‎m absorption detections were thus discounted, and several other studies reported no evidence for the 1 μ‎m absorption band (e.g., Vilas, 1988; Vilas & McCord, 1976).

Petrology and Geochemistry of MercuryClick to view larger

Figure 1. Ground-based telescopic spectra of Mercury at a range of wavelengths. A: Ultraviolet–visible wavelength reflectance spectra (McCord & Adams, 1972a). The two spectra (scaled to 1.0 at 0.55 μ‎m and offset for clarity) are red-sloped and relatively featureless. B: Mid-infrared emissivity spectrum (Sprague et al., 2009). A peak associated with the presence of pyroxene (e.g., diopside or augite), at ~5 μ‎m, is labeled.

Overall, it was found that Mercury’s UV–visible reflectance spectra (Figure 1a) were dark (low reflectance) and mostly featureless (e.g., McCord & Adams, 1972b). Furthermore, the “redness” (positive gradient) of the spectra—similar to lunar spectra—was interpreted as evidence for Fe- and Ti-bearing agglutinates (products of space weathering, i.e., alteration caused by the space environment) in the planet’s regolith (Vilas, 1988). The scant 1 μ‎m absorption observations meant that the maximum FeO content of Mercury’s surface was calculated as 2–5 wt% (e.g., Blewett, Lucey, Hawke, Ling, & Robinson, 1997; Hapke, 1977; McCord & Adams, 1972a; McCord & Clark, 1979; Warell et al., 2006; Warell & Blewett, 2004). The relative lack of FeO was also thought to be evidence of a highly reduced surface that contained few volatile species (Vilas, 1985, 1988).

Earth-based observations of Mercury’s mid-IR spectra (~3–13.5 μ‎m) have also been obtained since the 1990s. At these wavelengths, the spectra (Figure 1b) contain several transparency and emissivity features that are diagnostic of major rock-forming minerals and silica content. The various observation campaigns (e.g., Cooper, Potter, Killen, & Morgan, 2001; Emery et al., 1998; Sprague et al., 2002; Sprague, Kozlowski, Witteborn, Cruikshank, & Wooden, 1994; Sprague & Roush, 1998) indicated considerable chemical heterogeneity across Mercury’s surface. The spectra were interpreted as emanating from intermediate, mafic, and ultramafic components on Mercury’s surface (i.e., from rocks dominated by plagioclase feldspar, Mg-rich pyroxenes and olivines, as well as some feldspathoids).

At even longer wavelengths, Mercury’s microwave emissivity (~0.3–20.5 cm) was determined to be more transparent than the Moon and terrestrial basalts (Mitchell & de Pater, 1994). Jeanloz, Mitchell, Sprague, and de Pater (1995) modeled such spectra to derive a total FeO and TiO2 content of ~1.2 wt% for Mercury’s surface, which was consistent with the shorter-wavelength reflectance spectroscopy results. Jeanloz et al. (1995) also suggested, by analogy with the Moon, that Mercury’s surface is predominantly anorthosite and contains no major basalt deposits (note that this view of Mercury’s geology has since been overhauled; see “Mercury’s Geochemical Terranes” for more information).

Mariner 10 “color” images—obtained at UV (375 nm) and orange (575 nm) wavelengths—also shed some light on the mineralogical diversity of Mercury’s surface. The original analyses of these images (Hapke, Christman, Rava, & Mosher, 1980; Hapke, Danielson, Klaasen, & Wilson, 1975; Rava & Hapke, 1987) showed that color variations across the planet did not match photogeologically mapped units (Spudis & Guest, 1988). Color units identified from a subsequent recalibration and analysis of the data, however, coincided with mapped geologic units (Robinson & Lucey, 1997). That is, the color variations matched boundaries between weathered and fresh surfaces, and between relatively young smooth plains and older, more-heavily-cratered, surrounding terrains. Although it was not possible to make definitive mineralogical interpretations from the Mariner 10 data, Robinson and Lucey (1997) suggested that the color variations could be caused by space weathering, grain size, or compositional differences, and that the spectral parameters of the data were consistent with low-Fe basaltic material.

Radar-Bright Polar Deposits

Earth-based observations at radio (centimeter to meter) wavelengths have been another important method for studying Mercury’s surface characteristics (e.g., topography and surface roughness; Clark, Leake, & Jurgens, 1988; Goldstein, 1970; Harmon & Campbell, 1988). Radar experiments in the early 1990s also provided one of the most curious pre-MESSENGER observations of Mercury’s geochemistry. Radar (3.5 cm) mapping of Mercury, conducted with the use of the Goldstone 70 m antenna and the Very Large Array, led to the discovery of a “radar bright” region over the planet’s north pole (Slade, Butler, & Muhleman, 1992). This observation was soon corroborated by 12.6 cm-wavelength results from the Arecibo observatory, which revealed a similar region at Mercury’s south pole (Harmon & Slade, 1992).

Petrology and Geochemistry of MercuryClick to view larger

Figure 2. Image of Mercury’s north pole (down to a latitude of 82°N) at radar wavelengths, obtained at the Arecibo Observatory (Harmon, Slade, & Rice, 2011). Yellow areas are the radar-bright deposits, within permanently shadowed craters. Credit: National Astronomy and Ionosphere Center, Arecibo Observatory.

The coherent backscatter signature of Mercury’s polar regions measured during these campaigns indicated a highly reflective surface at radar wavelengths. That is, the circular polarization ratios of Mercury’s polar regions are much higher (1.0–1.4) than for typical terrestrial surfaces (<0.1). Mercury’s values are instead similar to those of the ice cap at the Martian south pole and to icy Galilean satellites. By analogy, the Mercury radar-bright observations were therefore originally presented as evidence for near-surface water ice (Slade et al., 1992). Moreover, thermal modeling results showed that areas of permanent shadow within polar craters should be cold enough to harbor meter-scale-thick deposits of water ice (beneath a thin layer of insulating regolith) for billions of years (Paige, Wood, & Vasavada, 1992; Vasavada, Paige, & Wood, 1999). Subsequent higher-resolution radar observations (Figure 2) of Mercury’s poles (Harmon, Perillat, & Slade, 2001; Harmon, Slade, & Rice, 2011) also confirmed that the radar-bright features were confined to the floors of permanently shadowed craters.

According to Barlow, Allen, and Vilas (1999), Mercury’s putative polar ice deposits were unlikely to be exposures of subsurface ice caps within impact craters because polar and equatorial craters exhibit similar morphologies (indicating substrates of similar strength). Instead, several possible sources of ice for the deposits were proposed. The water, for example, may have an endogenic origin and be brought to Mercury’s surface during volcanic eruptions and outgassing (Butler, Muhleman, & Slade, 1993). Alternatively, the water may be exogenic and be delivered by micrometeoroids or volatile-rich comets and asteroids (Butler et al., 1993; Crider & Killen, 2005; Moses, Rawlins, Zahnle, & Dones, 1999). Potter (1995) also posited that chemical sputtering of Mercury’s surface rocks (by protons in its magnetosphere) could provide water vapor in sufficient quantities to account for the presumed ice deposits. No matter which option is correct, the delivered water molecules must undergo transport to, and accumulation in, the polar traps. This is thought to occur via random migration of individual molecules until they reach a site of thermal stability (Butler, 1997; Moses et al., 1999).

The more recent Arecibo radar observations (Harmon et al., 2001), however, revealed many additional radar-bright features at relatively low northern latitudes (down to ~72°N). Although it is possible for small areas of permanent shadow to exist at such latitudes, it is harder to maintain a sufficiently cool thermal regime that can support water ice over billions of years (because of increased indirect heating). Doubt was therefore cast over the interpretation of Mercury’s radar-bright features as deposits of near-surface water ice. Elemental S was another leading candidate to explain the radar-bright signals (Sprague, Hunten, & Lodders, 1995). S is less volatile than water, and is thus stable at warmer temperatures (therefore not requiring permanently shadow). Sprague et al. (1995) also noted that abundant S should be available from infalling micrometeorites containing S-bearing minerals (e.g., troilite, pyrrhotite, and sphalerite). Other authors suggested that the radar-bright signatures could arise from anomalous physical characteristics of Mercury’s surface materials, such as low-Fe and low-Ti silicates at very cold temperatures (Starukhina, 2001) or unusually high surface roughness (Weidenschilling, 1998).

Mercury’s Exosphere

Mercury, like the Moon, is surrounded by a tenuous atmosphere known as an exosphere—in which atoms collide with the surface more often than each other. Historically, neutral species in Mercury’s exosphere were discovered via observations of resonantly scattered sunlight (Hunten et al., 1988). For example, He was the first element to be discovered in Mercury’s exosphere—from Mariner 10 UV airglow spectrometer data (Broadfoot, Kumar, Belton, & McElroy, 1974a, 1974b). Subsequent analyses of the Mariner 10 data also led to the discovery of H, and to a tentative detection of O in Mercury’s exosphere (Broadfoot, Shemansky, & Kumar, 1976). Na (Potter & Morgan, 1985), K (Potter & Morgan, 1986), and Ca (Bida, Killen, & Morgan, 2000) were discovered in the planet’s exosphere from later, ground-based spectroscopic observations.

It was proposed that the measured abundances of Mercury’s exospheric species (and some spatial heterogeneities) were indicative of surface composition. The relatively high concentration of Na and K, and the relatively low concentration of Ca, in the exosphere were thought to arise via sputtering from volatile-rich and refractory-poor crustal material (Killen & Morgan, 1993; Morgan & Killen, 1997; Sprague, Nash, Witteborn, & Cruikshank, 1997). With the data available, however, the exact influence of other potential exospheric source mechanisms (e.g., photon-stimulated desorption and impact vaporization of infalling meteoritic material) was unclear. The exospheric measurements could not, therefore, be used to provide insight into Mercury’s surface composition.

Next-Generation, Question-Led Exploration

The discovery of multiple Venus and Mercury gravity-assist braking combinations in the 1980s (Yen 1986, 1989) made Mercury-orbit mission scenarios feasible for the first time. As a result, Mercury orbiter missions could potentially be achieved within sensible limits for chemical propulsion and cost (Balogh et al., 2007). This advance led to several new Mercury orbiter studies and, eventually, to the selection of the MESSENGER and BepiColombo missions by NASA (in 1999) and ESA (in 2000; JAXA’s BepiColombo involvement was confirmed later), respectively. The two missions were developed almost in parallel, primarily to answer a number of key scientific questions (McNutt, Solomon, Grard, Novara, & Mukai, 2004) that had arisen from the Mariner 10 and ground-based observations (Table 1).

Table 1. The Guiding Scientific Questions for the NASA MESSENGER and European Space Agency (ESA)/Japan Aerospace Exploration Agency (JAXA) BepiColombo Missions to Mercury (Grard & Balogh, 2001; Solomon et al., 2001).

MESSENGER Questions

BepiColombo Questions

What planetary formational processes led to the high metal/silicate ratio in Mercury?*

Why is Mercury’s density so high?*

What is the geological history of Mercury?*

How has Mercury evolved geologically?*

What are the nature and origin of Mercury’s magnetic field?

What is the origin of Mercury’s magnetic field?

What are the structure and state of Mercury’s core?

Is there water ice in the polar regions?*

What are the radar-reflective materials at Mercury’s poles?*

What are the constituents of Mercury’s exosphere?*

What are the important volatile species and their sources and sinks on and near Mercury?*

How does the planetary magnetic field interact with the solar wind in the absence of any ionosphere?

Can we take advantage of the Sun’s proximity to test general relativity with improved accuracy?

Note: (*) Questions that relate to, or involve, an understanding of Mercury’s geochemistry and petrology.

Post-MESSENGER Geochemical View of Mercury

The success of MESSENGER’s three Mercury-flyby encounters and its orbital mission gave rise to a wealth of new data that continue to feed a renaissance in Mercury science. The average composition of Mercury’s surface (in terms of 15 elements, or elemental ratios), as determined from MESSENGER XRS and GRS measurements is given in Table 2. Some of the major geochemical and petrological findings that have stemmed from the MESSENGER results are summarized in the following sections (see Solomon, Nittler, & Anderson, 2018 for a more comprehensive review).

Table 2. The Average Chemical Composition of Mercury’s Surface*, as Determined From MESSENGER X-Ray Spectrometer (XRS) and Gamma-Ray Spectrometer (GRS) Data

Element*

XRS

GRS

K

1288 ± 234 ppm1

Th

0.155 ± 0.054 ppm1

U

90 ± 20 ppb2

Mg/Si

0.436 (0.106)3

Al/Si

0.268 (0.048)3

0.029 + 0.05/ − 0.134

S/Si

0.076 (0.019)3

0.092 ± 0.0155

Ca/Si

0.165 (0.030)3

0.24 ± 0.055

Ti/Si

0.012 (0.003)6

Cr/Si

0.003 (0.001)7

Mn/Si

0.004 (0.001)6

Fe/Si

0.053 (0.013)3

0.077 ± 0.0135

Na/Si

0.12 ± 0.0135*

Cl/Si

0.0057 ± 0.00108*

O/Si

1.2 ± 0.19

C

1.4 ± 0.9 wt%10

Notes: (*) Most XRS and GRS data (except absolute abundances of the radioactive elements K, Th, and U) are usually reported as elemental ratios to Si (Nittler, Chabot, Grove, & Peplowski, 2018). This is because elemental ratios are more readily obtained than absolute abundances, and because ratios reduce the effect of some systematic uncertainties. Normalizing to Si is possible because its abundance typically varies less than other major elements in common rock types, and because GRS Si measurements vary by only ~15% across Mercury’s surface (Peplowski et al., 2012a). Ratios given are by mass.

() GRS measurements are for Mercury’s northern hemisphere because MESSENGER’s highly eccentric orbit allowed useful gamma-ray signal to be obtained only from latitudes north of −20ºS (e.g., Peplowski et al., 2011).

Data sources: (1) Peplowski et al. (2012a);

(2) Peplowski et al. (2011);

(3) Frank et al. (2017);

(4) Peplowski et al. (2012b);

(5) Evans et al. (2012);

(6) Weider, Nittler, Starr, McCoy, & Solomon (2014);

(7) Nittler et al. (2018);

(8) Evans et al. (2015);

(9) McCubbin et al. (2017);

(10) Peplowski et al. (2015a).

A Volatile-Rich Planet

Almost immediately after entering into Mercury orbit, MESSENGER’s geochemistry instruments began returning data that helped to constrain theories of the planet’s formation and early history (see “Constraining Mercury’s Formation and Early Evolution”). Through the technique of planetary X-ray fluorescence spectroscopy (Yin, Trombka, Adler, & Bielefeld, 1993), XRS data obtained during solar flares were used to derive quantitative abundance estimates for major rock-forming elements over large swaths of Mercury’s surface (Nittler et al., 2011). These early results showed that Mercury has a higher Mg/Si ratio, as well as lower Al/Si and Ca/Si ratios (Table 2) than typical terrestrial and lunar crustal materials. The XRS data also revealed low Ti and Fe surface abundances (see Table 2). Most noteworthy, perhaps, was the detection of S at an abundance of ~4 wt% (assuming a Si abundance of ~25 wt%), i.e., about ten times greater than typical values for other terrestrial bodies.

Petrology and Geochemistry of MercuryClick to view larger

Figure 3. The K/Th ratio of the terrestrial planets and the Moon. Mercury’s ratio, determined from MESSENGER Gamma-Ray Spectrometer (GRS) measurements (Peplowski et al., 2011, 2012a) is similar to that of the other planets, and an order of magnitude higher than that of the volatile-depleted Moon. Adapted from the original image, provided by NASA/The Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.

GRS measurements made during the first weeks of MESSENGER’s orbital mission were used to calculate absolute abundances of naturally radioactive elements on Mercury’s surface: K, Th, and U, abundances of 1150 ± 220 ppm, 220 ± 60 ppb, and 90 ± 20 ppb, respectively, were reported by Peplowski et al. (2011). Given that K is a moderately volatile incompatible element, whereas Th and U are more refractory incompatible elements, it is primarily the relative proportions of these three elements (rather than their absolute abundances) that provides important insights into the volatile inventory of the planet. For instance, the measured K/Th ratio for Mercury is similar to the ratios for the other terrestrial planets (Figure 3)—and the most like that of Mars. Moreover, Mercury’s K/Th ratio is an order of magnitude higher than that of the Moon, which is thought to be volatile-depleted because of the putative Moon-forming giant impact.

Overall, the early MESSENGER geochemistry results (since corroborated by subsequent, more-substantial datasets) indicated that Mercury has a refractory-poor (low Al/Si and Ca/Si ratios) and volatile-rich (high S/Si and “normal” K/Th ratio) composition. Mercury’s designation as a volatile-rich planet, however, was soon questioned by McCubbin, Riner, Vander Kaaden, and Burkemper (2012) who argued that Mercury’s relatively high K/Th ratio could reflect substantial partitioning of Th into the planet’s core rather than purely an abundance of K on the surface.

The low Fe and high S concentrations of the planet indicate that Mercury formed under highly reducing chemical conditions. Estimates of the planet’s interior O fugacity (amount of O available for chemical reactions) range from ~3 to ~7 log units below the iron–wüstite buffer (McCubbin et al., 2012; Namur, Charlier, Holtz, Cartier, & McCammon, 2016; Zolotov, 2011; Zolotov et al., 2013). At such redox conditions, McCubbin et al. (2012) proposed that elements normally considered lithophilic (affinity for O), such as K, Th, and U, may exhibit siderophilic or chalcophilic (an affinity for Fe or S, respectively) characteristics and thus partition more readily into the planet’s core. There is still, however, only limited metal/silicate partitioning data for Th and U at conditions relevant to Mercury’s core formation (Malavergne et al., 2007; Malavergne, Toplis, Berthet, & Jones, 2010) and an accurate interpretation of the planet’s K/Th and K/U ratios (i.e., whether they reflect Mercury’s bulk volatile content) is still required.

Nonetheless, several other MESSENGER results are indicative of Mercury’s high volatile content. For example, surface abundances of other moderately volatile elements, i.e., Na and Cl (see Table 2), are similar to those of Mars (and greater than lunar abundances by an order of magnitude or more). Mercury’s Cl content is a particularly good measure of the planet’s bulk volatile inventory because it is even more susceptible to loss during planetary accretion than other moderately volatile elements (Sharp & Draper, 2013). Moreover, the similar nature of Cl and K (both incompatible, moderately volatile lithophile elements; Lodders, 2003) means they behave similarly during geological processes. The Cl/K ratio of Mercury’s surface—similar to that of Mars and carbonaceous chondrites—is thus a robust indication of the planet’s substantial bulk volatile content (Evans et al., 2015).

The presence of about 150 pyroclastic deposits (Thomas, Rothery, Conway, & Anand, 2014a), as well as “hollows,” are further evidence of Mercury’s substantial volatile inventory. The pyroclastic deposits—products of explosive volcanism—were confirmed from MESSENGER multispectral images (Goudge et al., 2014; Head et al., 2008, 2009; Kerber et al., 2009, 2011; Murchie et al., 2008; Prockter et al., 2010) and formed separately from the vast effusive lavas that largely shaped Mercury’s surface (e.g., Denevi et al., 2013; Head et al., 2011; Whitten, Head, Denevi, & Solomon, 2014). As on Earth, Mercury’s pyroclastic volcanism was driven by the exsolution of magmatic volatiles. On the basis of MESSENGER XRS, Neutron Spectrometer (NS), MASCS, and MDIS data, the loss of S- and C-bearing species from erupting magmas is thought to be responsible for Mercury’s pyroclastic activity—at least for the case of Mercury’s largest pyroclastic deposit (Weider et al., 2016).

Similarly, the discovery of Mercury’s hollows—irregular, flat-floored depressions, with high-reflectance interiors and haloes—from MESSENGER high-resolution images (Blewett et al., 2011) has been interpreted as evidence of geologically recent volatile-driven activity on Mercury. These landforms are unique to Mercury, but comparisons with morphological features on ice-bearing surfaces of Mars and icy satellites give clues to their formation. According to current consensus (Blewett, Ernst, Murchie, & Vilas, 2018), hollows form when a volatile-bearing phase (likely graphite or a sulfide) sublimates or is destroyed (e.g., via solar heating, exposure to UV radiation or the solar wind, ion sputtering, or micrometeorite bombardment) and causes the host rock to weaken and collapse. The hundreds of hollows identified across Mercury’s surface (Blewett et al., 2013, 2016; Thomas, Hynek, Rothery, & Conway, 2016; Thomas, Rothery, Conway, & Anand, 2014b) are thus another manifestation of Mercury’s volatile-rich composition.

Constraining Mercury’s Formation and Early Evolution

Mercury’s relatively low Al/Si and Ca/Si ratios (Evans et al., 2012; Nittler et al., 2011; Peplowski et al., 2012b; Weider et al., 2012) are evidence that the present-day surface did not form as a lunar-like plagioclase-rich flotation crust (Warren, 1985), i.e., from a global magma ocean. Although one of the post-accretion episodes invoked to explain Mercury’s high density (i.e., the giant impact or vaporization models; see “Mercury Formation Theories”) may have removed such a crust, Mercury’s abundance of volatiles seems to indicate otherwise. It is still uncertain, however, what the final fate of vaporized mantle material (including volatile species) would be (Ebel & Stewart, 2018). It is possible that volatiles re-condensed onto the planet’s surface after an impact event, but this re-accretion of material (after a single impact event) would limit the magnitude of core fractionation and it would therefore be difficult to attain Mercury’s high metal/silicate ratio in this way (Ebel & Stewart, 2018).

Lewis’s chemical equilibrium model for Mercury’s formation can also be ruled out on the basis of the MESSENGER-derived surface compositions. This model predicts low concentrations of Fe, Fe-free silicates, as well as chondritic Th and U abundances—all consistent with MESSENGER results. The relatively low abundances of Al and Ca—and high abundances of S and K—on Mercury’s surface, however, directly contradict the model’s predictions.

Given the new MESSENGER geochemistry datasets, it is thought that Mercury’s high density probably arises because of the materials from which the planet accreted. That is, because of Mercury’s unique position close to the Sun, Mercury formed from highly reduced precursor materials (along the lines of the Weidenschilling and/or Wasson models). At present, the best candidates for Mercury’s building blocks are materials akin to the metal- and sulfide-rich enstatite chondrites (Burbine et al., 2002; McCoy, Dickinson, & Lofgren, 1999; Nittler et al., 2011), metal-rich (e.g., CB) chondrites (e.g., Brown & Elkins-Tanton, 2009; Krot et al., 2001; Nittler et al., 2011; Peplowski et al. 2011), or anhydrous interplanetary dust particles (Ebel & Alexander, 2011; Nittler et al., 2011). In their recent review of Mercury’s formation theories, however, Ebel and Stewart (2018) emphasize that none of Mercury’s extant formation models are consistent with all the available observations and data. More than one of the proposed processes may therefore have occurred to produce Mercury’s composition and density, and further studies are required to make headway on this issue.

Mercury’s Geochemical Terranes

Petrology and Geochemistry of MercuryClick to view larger

Figure 4. Maps of Mercury’s chemical composition. The K abundance map (measured in parts per million), for the northern hemisphere, is derived from MESSENGER GRS data (Peplowski et al., 2012a). The Mg/Si, Al/Si, S/Si, Ca/Si, and Fe/Si elemental weight ratio maps are derived from MESSENGER X-Ray Spectrometer data (Nittler et al., 2016; Weider et al., 2015). Coverage is complete for the Mg/Si and Al/Si maps, whereas portions of the S/Si, Ca/Si, and Fe/Si maps (mostly in the northern hemisphere) were not mapped during the MESSENGER campaign. All maps are shown in a Mollweide projection, centered on 0°N, 0°E.

Variations in the composition of Mercury’s surface were revealed as the MESSENGER geochemistry datasets grew (Evans et al., 2015; Peplowski et al., 2014; Weider et al., 2012). It was eventually possible to produce maps (Figure 4) for some elements (and elemental ratios) measured by the XRS and GRS, including K (Peplowski et al., 2012a), Mg/Si, Al/Si, S/Si, Ca/Si, and Fe/Si (Nittler et al., 2016; Weider et al., 2014, 2015). The maps, as well as observed variations in Mercury’s neutron absorption—as measured by the GRS (Peplowski et al., 2015b) and NS (Lawrence et al., 2017)—clearly show that the geochemical distinctions are often spatially consistent. The geochemical variations, however, do not always match geomorphologic units (e.g., Mercury’s smooth plains deposits; Denevi et al., 2013) and the concept of geochemical “terranes” (i.e., regions defined by distinctive geochemistry) on Mercury has emerged (Peplowski et al., 2015b; Weider et al., 2015). Such terranes have been defined in several ways by different authors, and a definitive number (ranging between three and nine) has yet to be determined (Lawrence et al., 2017; Peplowski et al., 2015b; Vander Kaaden et al., 2017; Weider et al., 2015).

Petrology and Geochemistry of MercuryClick to view larger

Figure 5. Map of Mercury showing the locations of the four geochemical terranes (Northern, Low-Fast, High-Mg, and Caloris Interior) identified by McCoy, Peplowski, McCubbin, and Weider (2018). Major smooth plains deposits (Denevi et al., 2013) are outlined.

In their recent review, McCoy, Peplowski, McCubbin, & Weider (2018) chose three criteria to define Mercury’s geochemical terranes: the region must be (1) spatially continuous; (2) geochemically distinct from Mercury’s average crustal composition (represented by an average of the southern hemisphere); and (3) spatially extensive (>1000 km in its minimum lateral dimension). With these criteria, four terranes were identified (in addition to the average, southern hemisphere, composition): the Northern, Caloris Interior, High-Mg, and Low-Fast Terranes (Figure 5).

Petrology and Geochemistry of MercuryClick to view larger

Figure 6. Classification, according to standard schemes, of the oxide abundances and calculated mineralogies for the four geochemical terranes identified by McCoy et al. (2018), as well as for a southern hemisphere (average Mercury) composition. A: Total alkalis versus silicate diagram. The field for high-Mg rocks (Le Bas, 2000) is shown in green, indicating that all of the Mercury compositions can also be classified as boninites. The terrane mineralogies are also shown on International Union of Geological Sciences diagrams for plutonic rocks, in terms of (B) plagioclase–orthopyroxene–clinopyroxene and (C) plagioclase–pyroxene–olivine.

Building upon earlier petrological (experimental and theoretical) studies in which MESSENGER-derived surface compositions were used to constrain Mercury’s surface lithologies and mantle-melting conditions (Charlier, Grove, & Zuber, 2013; Namur et al., 2016; Stockstill-Cahill, McCoy, Nittler, Weider, & Hauck, 2012; Vander Kaaden et al., 2017), McCoy et al. (2018) used an adapted CIPW-normative calculation to derive the mineralogy for each of their five terranes (Table 3). The terrane mineralogies were then classified according to a number of standard schemes (Figure 6), which showed that the terranes range from basaltic andesites to trachytes (in terms of their total alkali versus silica content). The high Mg (>8 wt%) and low Ti (<0.5 wt%) contents of the terranes, however, means that they can all also be defined as boninites. If classified as plutonic rocks, the terrane compositions would span norite, anorthositic norite, and anorthositic gabbro compositions—reflecting variations in plagioclase abundance and the relative amount of high- and low-Ca pyroxenes.

The abundant S on Mercury’s surface is thought to reside within a variety of sulfide phases. On the basis of XRS data, it has been suggested that such phases may include oldhamite, niningerite, daubréelite, troilite, or djerfisherite (Nittler et al., 2011; Weider et al., 2012, 2014). The results of several petrological studies, however, indicate that Mercury’s sulfide phases are likely to be exotic and complex, i.e., containing various amounts of Fe, Mg, Ti, Cr, Mn, and Ca (McCoy et al., 2018; Stockstill-Cahill et al., 2012; Vander Kaaden et al., 2017; Vander Kaaden & McCubbin, 2016). Variations in the precise nature of the sulfide phases may also contribute to the compositional differences observed between Mercury’s geochemical terranes. For instance, McCoy et al. (2018) find that the southern hemisphere and the High-Mg Terrane have Fe-dominated sulfides, whereas the Northern, Caloris Interior, and Low-Fast Terranes exhibit Mg-dominated sulfide phases.

Table 3. Chemical Composition (in wt%) of the Average Mercury Surface (Southern Hemisphere) and the Four Geochemical Terranes Identified by McCoy et al. (2018). The calculated liquidus temperature (Tliq), viscosity (η‎), and CIPW-derived mineralogy (in modal abundance) for each composition are also given. All data are taken from McCoy et al. (2018)

Average Mercury (Southern Hemisphere)

Northern

Low-Fast

High-Mg

Caloris Interior

O

39.65

42.27

41.13

37.21

41.31

Na

2.83

5.74

2.94

2.66

2.95

Mg

12.44

7.55

12.34

16.48

9.15

Al

7.79

6.04

7.05

5.32

9.44

Si

28.32

30.19

29.38

26.58

29.51

S

2.07

2.11

1.76

2.92

1.77

Cl

0.14

0.45

0.24

0.13

0.15

K

0.13

0.20

0.15

0.10

0.08

Ca

4.55

4.23

3.82

5.58

4.43

Ti

0.34

0.36

0.35

0.32

0.35

Cr

0.14

0.15

0.15

0.13

0.15

Mn

0.11

0.12

0.12

0.11

0.12

Fe

1.48

0.60

0.59

2.44

0.59

Total

100.02

100.01

100.01

99.98

100.00

Tliq (°C)

1460

1350

1476

1542

1365

η‎ (Pa s)

3.0

37.8

3.6

0.4

28.5

Plagioclase

50.4

55.7

48.2

37.4

57.7

Orthoclase

0.8

1.3

1.0

0.6

0.5

Albite

29.2

54.3

30.7

27.0

30.7

Anorthite

20.4

0.0

16.5

9.8

26.5

Pyroxene

37.3

29.0

47.5

26.2

32.8

Diopside

5.4

20.0

4.8

17.8

0.0

Hypersthene

31.9

9.0

42.7

8.4

32.8

Olivine

7.5

7.5

0.0

29.5

0.0

Sulfides

4.7

4.3

3.7

6.8

3.7

Accessory

None

None

Quartz

None

Quartz, corundum

McCoy et al. (2018) also used the MELTS program (Gualda & Ghiorso, 2015) to estimate the liquidus temperature of lavas from the composition of each terrane (Table 3). The results show that the Mercury lavas would have substantially higher liquidus temperatures than terrestrial oceanic basalts, which is consistent with Mercury’s higher Mg abundance. In addition, McCoy et al. (2018) followed the method of Shaw (1972) to show that the magma viscosities for each of Mercury’s terranes (Table 3) are substantially lower than for typical terrestrial basaltic magmas (Basaltic Volcanism Study Project, 1981) and that the melts would have produced thin, laterally extensive deposits. This is compatible with the morphologic evidence for large amounts of flood volcanism on Mercury (e.g., Denevi et al., 2013; Head et al., 2011).

Based on the geochemical results and their petrologic findings, McCoy et al. (2018) suggested a sequence of mantle and crustal events to explain the geochemical and mineralogical variations among the terranes. In their model, Mercury’s surface consists of a primary crust that formed during an early magma ocean phase (the ocean would have been ~400 km thick; Hauck et al., 2013), and the products of subsequent volcanic activity. It has been shown that graphite is the only phase that could have formed a flotation crust from Mercury’s low-FeO mantle (Vander Kaaden & McCubbin, 2015). As the magma ocean crystallized, the mantle became stratified, with ultramafic lithologies (dunites, harzburgites, and wehrlites) at the base grading to incompatible- and volatile-rich (Vander Kaaden & McCubbin, 2016) gabbroic material near the surface. The low density of the thin mantle, meant that partial melts from all mantle depths would have been buoyant and able to reach the surface (Vander Kaaden & McCubbin, 2015). Indeed, different-degree partial melts from separate source regions in the stratified mantle (melting caused by adiabatic decompression and/or heating from the decay of radioactive elements) can explain much of the chemical heterogeneity on Mercury’s surface. This scenario is most likely for the three terranes—the Northern, Caloris Interior, and Low-Fast Terranes—that are clearly volcanic in origin (McCoy et al., 2018).

Large, basin-forming impacts may also have played a key role in shaping Mercury’s geochemical terranes. For example, the Caloris-forming impact would have excavated through the planet’s crust and into the upper layers of the mantle (McCoy et al., 2018), and would have induced substantial amounts of post-impact volcanism (Roberts & Barnouin, 2012). The composition of the smooth plains deposits within the Caloris basin can be derived from high-temperature partial melting of the mantle’s harzburgite layer (Namur et al., 2016; McCoy et al., 2018). In contrast, lava flows within the High-Mg Terrane (with an unusually high olivine concentration of ~30 wt%) likely originated from partial melting of a deeper lherzolitic layer, and may have sampled materials from the base of Mercury’s mantle (Frank et al., 2017; Namur et al., 2016). It has been suggested that the geochemical signature of the High-Mg Terrane is also evidence of a very large (~3000 km in diameter), ancient, and degraded impact basin (Weider et al., 2015). Although Frank et al. (2017) showed that a basin of this size could easily exhume Mg-rich mantle material, they concluded that the level of basin-modification needed to hide such a basin’s physical structure is unfeasible and would have also obscured the terrane’s distinctive chemical signature.

Graphite: Mercury’s Darkening Phase?

Petrology and Geochemistry of MercuryClick to view larger

Figure 7. Representative Mercury Dual Imaging System (MDIS) reflectance spectra (on a logarithmic scale) for Mercury’s northern plains and low-reflectance material (LRM), as well as an average MDIS spectrum for the planet’s northern hemisphere. Laboratory spectra for three “darkening agent” candidates are also shown. Troilite and ilmenite were ruled out as major phases in Mercury’s surface (and as the phase responsible for Mercury’s low reflectance) because of the low Fe and Ti contents measured by MESSENGER’s geochemistry instruments. Graphite—with a dark, red-sloped, and featureless spectrum—is thought to be responsible for Mercury’s low reflectance. Modeling by Murchie et al. (2015) shows that it may be present at ~1 wt% globally, and up to ~5 wt% in Mercury’s darkest color unit (the LRM).

The reflectance datasets (UV to short-wavelength IR) obtained by MESSENGER’s MDIS Wide-Angle Camera and by the MASCS Visible and Infrared Spectrograph, and Ultraviolet and Visible Spectrometer, present measurements with substantially improved resolution and fidelity compared with the pre-MESSENGER equivalents. The new datasets (Figure 7) confirm the pre-MESSENGER observations of Mercury’s red-sloped and relatively featureless short-wavelength reflectance spectrum. The MESSENGER data also confirm that Mercury’s overall reflectance is substantially lower than that of the Moon, and that there is no evidence of a FeO crystal absorption band at 1 μ‎m (e.g., Murchie, Klima, Izenberg, Domingue, & Blewett, 2018).

Even the freshest of Mercury’s surface materials are up to ~50% darker than fresh lunar highlands (Braden & Robinson, 2013; Denevi & Robinson, 2008). Space weathering therefore cannot be invoked as the main cause of Mercury’s darkness. Instead, a dark, red-sloped “opaque” phase is thought to be the primary cause. Variations in the concentration of this darkening agent are the primary cause of small differences in the reflectance and spectral slope of Mercury’s color units, and variations in space weathering are a secondary cause (Denevi et al., 2009; Murchie et al., 2015; Robinson et al., 2008). The opaque phase is most concentrated in Mercury’s darkest spectral unit (Robinson et al., 2008)—known as the low-reflectance material (LRM)—which tends to be found within impact craters and their ejecta deposits (e.g., Denevi et al., 2009).

Through the years, several phases (Figure 7) have been suggested—and later ruled out—as Mercury’s darkening agent. Ilmenite (the main opaque phase on the Moon) was seen as a likely option (Denevi et al., 2009; Riner, Lucey, Desch, & McCubbin, 2009; Riner, McCubbin, Lucey, Taylor, & Gillis-Davis, 2010), but the MESSENGER-derived low Fe and Ti abundance estimates (Evans et al., 2012; Nittler et al., 2011; Weider et al., 2014) mean that this mineral has to be discounted as a major phase in Mercury’s surface (troilite is also ruled out for the same reason). Moreover, the absence of a correlation between Fe/Si and spectral reflectance characteristics (Weider et al., 2014) is further evidence that Mercury’s opaque phase does not contain substantial Fe.

Several studies published in 2015, however, led to a major change in the Mercury surface reflectance paradigm. First, spectral modeling of MDIS reflectance data indicated that C—in the form of fine-grained graphite—could produce a good match for Mercury’s reflectance, if present at ~1 wt% globally and ~5 wt% in the LRM (Murchie et al., 2015). Second, GRS data were used to obtain a mean C abundance estimate (for the northern hemisphere) of <4.1 wt% (Peplowski et al., 2015a). Third, Vander Kaaden and McCubbin (2015) showed experimentally that C—in the form of graphite—would have been the only phase buoyant enough to have formed a flotation crust from Mercury’s early magma ocean. Given that scenario, Mercury’s LRM deposits would be exposures of the primary graphite-rich flotation crust (i.e., that have survived impact disruption and burial from volcanism). Spatially resolved NS data—obtained during the latter stages of MESSENGER’s mission—for three LRM deposits provide further evidence of C-enrichment in the LRM (Peplowski et al., 2016) and of endogenic graphite as Mercury’s opaque phase. All three deposits exhibit enhanced neutron signals that correspond to elevated C abundances of between ~1 and ~3 wt%, compared with non-LRM deposits (Peplowski et al., 2016).

New Evidence for Water Ice at the Poles

Petrology and Geochemistry of MercuryClick to view larger

Figure 8. The flux of (A) epithermal (0.4 eV–0.5 MeV) and (B) fast (>0.5 MeV) neutrons, as measured by the MESSENGER Neutron Spectrometer (orange data). There is a clear decrease in the flux of both epithermal and fast neutrons at high northern latitudes. For comparison, simulated model count rates are shown for the case where (i) no H is present (gray data) and (ii) where there is a thick layer of 100 wt% water ice at the surface of all radar-bright regions (blue data). Data are normalized to the 0 wt% H case in A.

Adapted from Lawrence et al. (2013).

The NS component of the GRNS instrument was specifically included as part of MESSENGER’s payload to help distinguish between the competing pre-MESSENGER explanations for Mercury’s “Radar-Bright Polar Deposits.” By measuring the flux of neutrons from Mercury’s surface (created via nuclear spallation reactions that occur when galactic cosmic rays hit the surface), the NS was particularly sensitive to the presence of H (and H2O). Indeed, planetary neutron spectroscopy has previously been demonstrated as a reliable technique for measuring the surface H concentration of airless (or nearly airless) bodies and for constraining the nature of radar-bright materials (Feldman, Barraclough, Hanesen, & Sprague, 1997; Prettyman, 2007). NS data collected during the first year of MESSENGER’s orbital mission (Figure 8) showed a clear decrease in the flux of epithermal (with energies of 0.4 eV to 0.5 MeV) and fast (energies >0.5 MeV) neutrons at regions close to Mercury’s north pole (Lawrence et al., 2013). Models indicate that these results are consistent with the presence of nearly pure water ice in the permanently shadowed regions, and that the deposits typically contain an H-rich layer (>10 cm thick) beneath a surficial layer (10–30 cm thick) that is less rich in H (Lawrence et al., 2013).

Complementary studies by Neumann et al. (2013) and Paige et al. (2013) presented further compelling evidence for the presence of water ice at, or near, the surface of the polar permanently shadowed regions. Surface reflectance measurements at 1064 nm, obtained with MESSENGER’s Mercury Laser Altimeter (MLA) instrument (Cavanaugh et al., 2007), reveal areas of anomalous darkness and brightness that occur on poleward-facing slopes and that coincide with the radar-bright regions (Neumann et al., 2013). In addition, MLA topography measurements were used to create new thermal models (Paige et al., 2013). The models show that there is a good correlation between the radar-bright regions and areas where water ice is predicted to be thermally stable at, or close, to the surface. The coldest areas—where ice can be stable at the surface—coincide with the brightest surfaces; the darker regions occur in and around warmer areas, where ice may only be stable in the near subsurface (Paige et al., 2013). Whereas water ice is likely to be present at the surface of the bright regions, it is thought that an organic-rich sublimation lag deposit accounts for the darker areas and acts as a thermally insulating layer for the ice below (Neumann et al., 2013; Paige et al., 2013).

Petrology and Geochemistry of MercuryClick to view larger

Figure 9. A: MDIS image of the Prokofiev crater (86°N, 296.3°W), which has a diameter of ~112 km. The dark area in the bottom half of the crater contains a region of persistent shadow, within which a radar-bright signal (yellow in B) has been observed (Harmon et al., 2011). The radar-bright signal from within the crater closely matches an area that has higher reflectance than its surroundings. The high-reflectance area is observed at both 600 nm in the MDIS Wide-Angle Camera filter image (Chabot et al., 2014), shown in C, and at 1064 nm in the Mercury Laser Altimeter (MLA) data (Neumann et al., 2013), shown in D. Red and blue colors in the overlain MLA data represent high and low reflectance, respectively.

During the later (including the low-altitude) stages of the MESSENGER mission, images of several permanently shadowed regions near to the north pole were obtained with the MDIS Wide-Angle Camera filter (600 nm). By using backscattered light for illumination, it was possible to peer inside the permanently shadowed regions (Chabot et al., 2014, 2016). Images, for example of the 112‑km-diameter crater Prokofiev (Figure 9), clearly show that areas of higher reflectance are collocated with areas of high radar backscatter (Chabot et al., 2014). The high-resolution images of the polar craters also illustrate that the low-reflectance (lag) deposits have well-defined boundaries and do not have a uniform brightness, even within a single deposit. The presumed volatile deposits are therefore thought to be geologically young and consist of multiple organic compounds (Chabot et al., 2016).

Several potential sources for Mercury’s polar ice (and other volatiles) have been suggested, including solar wind interactions, planetary outgassing, or exogenic delivery. Although solar-wind interactions with Mercury’s regolith can produce water molecules, it is more difficult to explain the production of other volatile materials in this manner (Chabot, Lawrence, Neumann, Feldman, & Paige, 2018). Planetary outgassing is also unlikely to be the main source of the volatile deposits because Mercury’s interior is thought to be depleted in H and H2O (Chabot et al., 2018). It is therefore more likely that the volatiles were (or continue to be) delivered via micrometeoroid bombardment, or via the impacts of large comets or volatile-rich asteroids. Bruck Syal and Schultz (2015) showed that >99% of Mercury’s ice inventory can be delivered via micrometeoroid impacts, but it may be difficult to produce relatively pure ice deposits this way. A relatively recent large impact event (e.g., that which created the Hokusai crater; Ernst, Chabot, & Barnouin, 2018), may be the most likely source of Mercury’s polar volatiles. A comet or volatile-rich impact would provide a substantial amount of relatively pure ice along with organic-rich material and would produce deposits with sharp boundaries.

Detections of New Exospheric Species

Results from MASCS observations (made during MESSENGER’s flyby and orbital missions), together with recent ground-based measurements, have brought the total number of detected species in Mercury’s exosphere to ten. In addition to the pre-MESSENGER detections of H, He, Na, K, and Ca, observations of neutral Mg (McClintock et al., 2009), Al (Bida & Killen, 2017; Doressoundiram, Leblanc, Foellmi, & Erard, 2009; Vervack et al., 2016), Fe (Bida & Killen, 2017), and Mn (Vervack et al., 2016), as well as ionized Ca (Vervack et al., 2010, 2016) have now been reported. The tentative Mariner 10 detection of exospheric O (Broadfoot et al., 1976), however, has not been confirmed with the MESSENGER data (McClintock et al., 2018). Despite the wealth of new exospheric observations, a direct link between Mercury’s exospheric and surface compositions remained elusive until recently. Merkel et al. (2018), however, have now demonstrated that there is a clear enhancement in exospheric Mg over Mercury’s High-Mg Terrane. This finding provides strong evidence that impact vaporization of crustal material is a major source of the Mg in Mercury’s exosphere.

Conclusions and Remaining Questions

Before NASA’s MESSENGER mission there was a severe lack of information regarding Mercury’s chemical composition. MESSENGER, however, provided a rich dataset with which to study Mercury’s geochemistry and petrology. Data obtained from MESSENGER’s suite of geochemical sensing instruments have revealed that Mercury is a volatile-rich and chemically reduced planet, and that the heterogeneous surface can be divided into geochemical terranes. The nature of these terranes (i.e., their petrology, origin, and relationships with Mercury’s geology and tectonics) continues to be investigated. In addition, a definitive understanding of Mercury’s original formation—and the reason for its anomalously high metal/silicate ratio—has yet to be determined and continues to be an active area of research. MESSENGER data also show that fine-grained graphite is the major darkening agent on Mercury’s surface, and that the radar-bright signals from polar craters are caused by the presence of relatively pure water ice at, or near, the surface. Although thought to be delivered exogenically, the precise source of the water ice (and associated organic-rich deposits) is an open question.

Several petrological studies (experimental and theoretical) have been conducted since the MESSENGER geochemical results became available. In general, these investigations show that Mercury’s surface is dominated by Mg-rich mafic rocks (e.g., anorthositic norites or gabbros)—with significant amounts of sulfides. Given the recent discovery of graphite as Mercury’s darkening phase and a major component of the surface, it will be important to incorporate relevant C concentrations into future petrological models.

Despite the outstanding successes of the MESSENGER mission, its highly eccentric orbit meant that its perspective of Mercury was skewed heavily towards the northern hemisphere. The BepiColombo spacecraft, however, will orbit Mercury in a circular orbit and will provide a more balanced view of the planet (and will thus complement the MESSENGER datasets). It will therefore be interesting to compare BepiColombo’s high-fidelity geochemical measurements (see Table 4) from both the northern and southern hemispheres in the coming years.

Table 4. Scientific Instruments that Comprise the Payloads of the Mercury Planetary Orbiter (MPO) and Mercury Magnetospheric Orbiter (MMO) Parts of the BepiColombo Mission (Benkhoff et al., 2010)

MPO Instruments

MMO Instruments

BELA (BepiColombo Laser Altimeter)

MERMAG-M/MGF (Mercury Magnetometer)

ISA (Italian Spring Accelerometer)

MPPE (Mercury Plasma Particle Experiment)

MPO-MAG (Magnetic Field Investigation)

PWI (Plasma Wave Instrument)

MERTIS* (Mercury Radiometer and Thermal Imaging Spectrometer)

MSASI (Mercury Sodium Atmospheric Spectral Imager)

MGNS* (Mercury Gamma-Ray and Neutron Spectrometer)

MDM (Mercury Dust Monitor)

MIXS* (Mercury Imaging X-ray Spectrometer)

MORE (Mercury Orbiter Radio Science Experiment)

PHEBUS* (Probing of Hermean Exosphere by UV Spectroscopy)

SERENA* (Search for Exospheric Refilling and Emitted Natural Abundances)

SIXS (Solar Intensity X-ray and Particle Spectrometer)

SIMBIO-SYS* (Spectrometers and Imagers for MPO BepiColombo Integrated Observatory)

Note: (*) Instruments that will provide compositional measurements of Mercury’s surface and/or exosphere.

Studies of Mercury’s geochemistry and petrology would benefit tremendously from a future lander mission. In situ compositional, mineralogical, and isotopic measurements from Mercury’s surface would provide important ground-truth information for the calibration of remote sensing datasets, and would provide a level of detail far beyond the possibilities of remote sensing. Alternatively, a meteorite sample from the planet (or, indeed, a sample return Mercury mission) would be a major boon to Mercury science. Although it is dynamically possible for such samples to reach Earth’s surface, the probability is very low, and it has been predicted that only a few Mercury meteorites may be present in our collection (Gladman, Burns, Duncan, Lee, & Levison., 1996). With the new MESSENGER geochemical information available as clues to the potential characteristics of a Mercury meteorite, a re-examination of anomalous and ungrouped meteorites (to search for a Mercury sample) may be warranted.

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