2012 Annual Science Report
Carnegie Institution of Washington Reporting | SEP 2011 – AUG 2012
Project 3: The Origin, Evolution, and Volatile Inventories of Terrestrial Planets
Project 3 focuses on understanding the nature of volatiles (principally water and gase like carbon dioxide and methane) in planetary interiors. The origin of Earth’s oceans and the initiation of plate tectonics may have related through the retention of water deep in Earth’s mantle. In this project scientists study how volatiles behave in silicate melts and Earth’s deep interior. They also study other rock planets, e.g. Mars and Mercury to understand how the presence or absence of volatiles may have lead to such disparate outcomes relative to Earth.
Project 3: Origin, evolution, and volatile inventories of terrestrial planets
3.3 Chambers-Models of planet formation, initial inventory of the planets
Chambers has examined the role of giant impacts in the final stage of planetary accretion. This stage begins with tens or hundreds of lunar-to-Mars-sized planetary embryos and ends with a handful of rocky planets that move on stable orbits. Previous studies of this stage of growth have typically assumed that all collisions between planetary embryos lead to a merger. Chambers has carried out more realistic simulations that include fragmentation during impacts, and “hit and run” collisions in which embryos collide and escape from each other.
These new simulations show that nearly half of collisions are of the hit and run kind, with no net growth. However, the timescale required for an Earth-like planet to acquire most of its mass is essentially unchanged. Collision rates are higher as a result. Only as planets sweep up the last 10% of their mass do growth rates slow down compared to the case in which perfect mergers are assumed. Fragmentation and hit and run collisions both allow fractionation of rocky mantle material from elements in iron-rich cores since mantle material preferentially escapes during collisions. Small fragments tend to be silicate-rich as a result, while large planetary embryos can become somewhat enriched in siderophile (core residing) elements. However, these differences are largely erased by the end of planetary growth as most silicate-rich fragments are reaccreted by the final planets.
3.2 Inner Solar System: constraints from Mercury and Mars
Co-I Solomon is the Principal Investigator and Co-I Nittler the Deputy Principal Investigator of the MESSENGER mission to Mercury. As part of this NAI project, Solomon and Nittler are integrating the information derived from MESSENGER into a better understanding of the processes that led to the formation of the small, embryo-sized inner planets, including Mercury at about 5% of Earth’s mass and Mars at about 10%. That the bulk compositions, volatile abundances, magmatic histories, and magnetic field histories differ so strongly on these two bodies demonstrates the strongly stochastic nature of the planet-building process and probably some dependence on solar distance. Because all of these aspects of planetary evolution affect the spatial extent and temporal duration of zones of habitability at the planetary surface and within the shallow planetary subsurface, an improved understanding of the profound differences in the make-up and evolution of these two similar-size planets holds the promise of illuminating the general nature of planetary habitability on smaller Earth-like planets, including those in other planetary systems. For both Mars and Mercury, recent spacecraft observations make such a comparison particularly timely. The ongoing Mars Odyssey, Mars Express, Mars Exploration Rover, Mars Reconnaissance Orbiter, and Mars Science Laboratory missions continue to build spectacular data sets from imaging and geochemical and geophysical remote sensing, and the recently completed Phoenix mission augmented our understanding of water and other volatiles at high Martian latitudes. The MESSENGER mission completed its three flybys of Mercury in 2008 and 2009 and was successfully inserted into orbit about Mercury on 18 March 2011. The broad goal of this task is a comparative evaluation of bulk composition, volatile inventory, magmatic history, and core dynamo history on Mars and Mercury, with a focus on aspects of those processes (water availability and circulation, organic material inventory, internal energy, magnetospheric shielding) most strongly relevant to habitability in space and time.
MESSENGER’s orbital observations of Mercury have provided the first direct measurements of Mercury’s surface composition by X-ray and gamma-ray spectrometry. These measurements indicate that the surface of Mercury is depleted in Al and Ca and enriched in Mg relative to the terrestrial and lunar crusts, has relatively low total iron and titanium concentrations (< 4 wt% and < 0.8 wt%, respectively), and is surprisingly rich in the volatile elements S, K, and Na. These compositional data suggest that Mercury formed from a similar mix of precursor materials to those that formed the other terrestrial planets, but substantially more chemically reduced. Planetary accretion models indicate that although there was radial mixing of planetesimals in the inner solar system, any radial gradients in chemical composition should have been partially preserved in the final compositions of the accreted planets. One possible explanation for the chemically reduced nature of Mercury suggested by Co-I Alexander is that Mercury may have formed from precursors enriched in anhydrous, C-rich solids analogous to cometary dust particles. Under this scenario, the other terrestrial planets would have formed from similar solids but with substantially higher abundances of water ice, resulting in more oxidizing conditions.
MESSENGER imaging has revealed that a large contiguous expanse of smooth plains, occupying more than 6% of Mercury’s surface area, covers much of Mercury high northern latitudes. These plains are clearly volcanic in origin and formed approximately contemporaneously with the volcanically emplaced smooth plains that lie within and exterior to the 1500-km-diameter Caloris basin, a result confirming that volcanism was a globally extensive process in the era immediately following the late heavy bombardment of the inner solar system. Spatially resolved composition measurements have shown that these plains are more Al-, and K-rich and Mg-, Ca-, and S-poor than the surrounding older material. This result indicates that the plains formed from chemically more evolved and cooler materials than those that formed the older terrain, consistent with the thermal history inferred from topographic and gravity-field measurements. Orbital imaging of bright deposits within impact craters on Mercury has also revealed numerous fresh-appearing, irregular, shallow, rimless depressions, known as hollows. The most likely formation mechanisms for the hollows involve recent loss of volatiles through some combination of sublimation, space weathering, outgassing, or pyroclastic volcanism. These features support the inference from compositional measurements and from observations of pyroclastic volcanic deposits elsewhere on the planet that Mercury’s interior contains higher abundances of volatile materials than predicted by most scenarios for the planet’s formation.
MESSENGER observations show that Mercury’s internal magnetic field is dominantly dipolar, has a vector moment closely aligned with the spin axis, and displays scant evidence for crustal magnetic anomalies. These results support the inference that Mercury’s magnetic field is the product of a dynamo in the planet’s fluid outer core. Mercury’s magnetic equator is located 484±11 km north of the geographic equator, i.e., the best fitting dipole is offset northward from the center of the planet by about 0.2 planetary radii. This offset leads to a substantial north-south asymmetry in the strength of the surface field and in the surface area at high latitudes with open magnetic field lines along which charged particles may readily gain access to Mercury’s surface. The high axisymmetry and strong equatorial asymmetry of the internal field point to a dynamo with characteristics different from those of Earth and other solar system planets.
Radio tracking of the MESSENGER spacecraft has allowed the gravity field of Mercury to be determined with high accuracy and together with knowledge of the spin state, this result has been used to place constraints on the density distribution within the planet. The models that best match the data include a layer of high-density material, possibly FeS, lying between the outer Fe-rich liquid core and the thin silicate mantle and crust. This interior structure is quite different from that of the other terrestrial planets and its origin is not yet well understood.
3.3 CHON in planetary interiors
In the last year, a major emphasis of CoI’s Mysen and Fogel’s efforts working with collaborator Foustoukos was on the behavior of silicate-saturated fluids coexisting with fluid-saturated silicate melts. Their physicochemical behavior was determined in-situ with the materials at temperature (up to 900˚C) and pressure (up to 2250 MPa) with samples contained in an Ir-gasketed hydrothermal diamond anvil cell (HDAC).
In these studies, Raman vibrational spectroscopy is used to determine at in-situ appropriate temperatures, pressures and redox conditions the bulk D/H molar ratio and the relative distribution of H/D isotopologues of dissolved H2, CH4 and H2O, while the samples are at the conditions of the Earth’s interior. These experiments address the role of supercritical water and hydrogen-bonding on the distribution of H/D isotopologues of H2 and CH4 (e.g. CH3D, CH2D2, CHD3), provide a way to describe in a mechanistic view the equilibrium and the kinetics of D/H exchange reactions, features that are severely lacking of theoretical or experimental measurements. Experimental results indicate that theoretical isotope fractionation models need to be revised to address thermodynamic contributions from the solvation (excess energy/entropy) of H/D isotopologues in supercritical aqueous solutions.
In systems with coexisting hydrous silicate melt and silicate-saturated aqueous fluid silicate solute in aqueous solutions governs stable isotope fraction because of silicate speciation changes. In Al-free Na-silicate systems, the enthalpy change of the (D/H) equilibrium of fluid is 3.1±0.7 kJ/mol, whereas for coexisting melt, ∆H=0 kJ/mol within error. With Al/(Al+Si)=0.1, ∆H=5.2±0.9 kJ/mol for fluid and near 0 within error for coexisting melt melt. For the exchange equilibrium between melt and fluid, H2O(melt)+D2O(fluid)=H2O(fluid)+D2O(melt), the ∆H=4.6±0.7 and 6.5±0.7 kJ/mol for the two Al-free and Al-bearing compositions, respectively, respectively. The D/H equilibration within fluids and melts and, therefore, D/H partitioning between coexisting fluid and melt reflect the influence of dissolved H2O(D2O) in melts and dissolved silicate components in H2O(D2O) fluid on their structure. The positive temperature- and pressure-dependence of silicate solubility and on silicate structure in silicate-saturated aqueous fluid governs the D/H fractionation in the fluid because increasing silicate solute concentration in fluid results in silicate polymerization. Aqueous fluid coexisting with aluminosilicate contain up to several mol of dissolved silicate components and with an equation-of-state, therefore, that differs from that of pure H2O. Structural units of Q3, Q2, Q1, and Qo type occur together in fluids, in melts, and, when outside the two-phase melt+fluid boundary, in single-phase liquids. The ∆H for the equilibrium between the Qn-species and dissolved H2O is 400±50 kJ/mol. In fluids and melts, hydrogen bonding becomes undetectable at T>500˚C. The ∆H is ~ 22 kJ/mol for aqueous fluid and ~10 kJ/mol for H2O dissolved in silicate melts.
Solubility and speciation of COHN volatiles in silicate melts and fluids at mantle pressures and temperature govern melt properties because the nature and bond energy involving these species. Methane solubility in melts, calculated as CH4, increases from 0.2 wt% to ≤0.5 wt% in the melt composition range between haploandesite and haplobasalt. The solubility increases by ~150% between the IW and MH oxygen buffers at constant temperature and pressure. Nitrogen solubility decreases from 0.98 to 0.28 wt% in the melt NBO/Si-range from 0.4 to 1.18 at fH2(IW) and decreases by about 50% between fH2(IW) and fH2(MH). Oxidized COH species tend to be more soluble than reduced species (e.g., CO2 and CH4 solubility in haplobasalt melt differs by >100% in the 1-3 GPa pressure range and N2 and NH3 by similar percentages), whereas reduced NOH species are more soluble than oxidized species at the same temperature and pressure.
The solution mechanisms of oxidized and reduced COH and NOH species, determined both on quenched samples and in-situ, at high temperature and pressure, in silicate melts also differ. In the C-O-H-silicate system at redox conditions of the NNO oxygen buffer and more oxidizing, carbon exists in melts as carbonate complexes and in the fluid as CO2. From diamond cell (HDAC) experiments conducted in-situ from ambient temperature and pressure to 800˚C and 1435 MPa under redox conditions near those of the IW buffer, the dominant fluid species in the fluid are CH4, H2, and H2O. In coexisting melt, CH3 – groups linked to the silicate melt structure via Si-CH3 bonding exist together with molecular CH4. There is no evidence of changes in hydrocarbon species or polymerization with temperature and pressure. Oxidized nitrogen is dissolved as molecular N2, whereas under reducing conditions (near the IW buffer), molecular NH3 and NH2-groups bonded to Si coexist. Their abundance ratio is silicate melt composition dependent.
Abundance ratios of C, H, and N-species in melts and coexisting fluids govern abundance ratio variations.
Changing abundance of volatiles dissolved in silicate melts in molecular form and as structural complexes that form bonds with the silicate melt structure is an important factor that can affect stable isotope fractionation during melting and crystallization at high pressure and temperature. These variations also govern other melt properties that depend on silicate melt structure.
PI Cody working with Post Doctoral Fellow Wang and CoI Mysen have been applying D and H solid state NMR to study intramolecular isotope fractionation in silicate glasses quenched from melts. We find very large differences in the affinity of D or H in different molecular environments within the quenched melts. This leads to enormous intramolecular isotope fractionation. Such fractionation arises not due to any thermal equilibrium effects or kinetic isotope effects, but rather due to partial molar volume differences between OH and OD species. This work was presented at the American Geophysical Union Fall meeting and is being written up for publication.
CoI’s Goncharov and Hemeley used Raman and visible transmission spectroscopy to investigate dense hydrogen (deuterium) up to 315 (275) GPa at 300 K. At around 200 GPa, we observe the phase transformation, which we attribute to phase III, previously observed only at low temperatures. This is succeeded at 220 GPa by a reversible transformation to a new phase, IV, characterized by the simultaneous appearance of the second vibrational fundamental and new low-frequency phonon excitations and a dramatic softening and broadening of the first vibrational fundamental mode. The optical transmission spectra of phase IV show an overall increase of absorption and a closing band gap which reaches 1.8 eV at 315 GPa. Analysis of the Raman spectra suggests that phase IV is a mixture of graphene-like layers, consisting of elongated H2 dimers experiencing large pairing fluctuations, and unbound H2 molecules. In brief, a new phase (IV) of solid dense hydrogen was discovered (phase III was reported 24 years ago, in 1988, Hemley & Mao, PRL). This phase has mixed interatomic bonding scheme.
3.4 Martian Organic Carbon
This year CoI Steele along with Mysen, Fogel and a number of collaborators have had two important papers published on reduced organic carbon compounds on Mars.
In the first study we use confocal Raman imaging spectroscopy and transmission electron microscopy to study the martian meteorite Allan Hills (ALH) 84001, which is an ancient sample (4.1 Ga) of the martian crust previously reported to contain mineral assemblages within carbonate globules (carbonate + magnetite), previously interpreted as potential relict signatures of ancient martian biota. Models for an abiologic origin for these assemblages required the presence of graphite, and this study is the first report of graphite within ALH84001. The graphite occurs as hollow spheres, filaments, and highly crystalline particles in intimate association with magnetite in the carbonate globules. In addition to supporting an abiologic origin for the carbonate globule assemblages in ALH84001, this work proves that there is an inventory of reduced carbon phases on Mars that has not yet been thoroughly investigated. ALH84001 hosts an indigenous reduced carbon component composed of polymorphs of graphite and polyaromatic platelets constituting the first concrete evidence of a reduced carbon phase on Mars. It is not possible to infer whether these formed during the initial formation event of the carbonate globules or from a later impact related event; however it is clear that the co-existence of magnetite with graphite at highly spatially resolved scales is consistent with the formation of these minerals through abiotic processes. Understanding the textural and morphological context of any reduced martian carbon is fundamental to interpreting the petrologic or biologic history of that carbon. Given that the Mars Science Laboratory mission is targeted at Gale crater, the questions raised as to the formation of reduced carbon species from ground water interactions or from impact processes should be measurable by the Sample Analysis at Mars (SAM) instrument.
In the second study we report the results of confocal Raman imaging spectroscopy on eleven Martian meteorites, spanning ~4.2 Ga of Martian history. Ten of the meteorites contain abiotic macromolecular carbon (MMC) phases detected in association with small oxide grains included within high temperature minerals. Polycyclic aromatic hydrocarbons were detected along with MMC phases in Dar al Gani 476. The association of organic carbon within magmatic minerals indicates Martian magmas favored precipitation of reduced carbon species during crystallization. Our results imply that primary organic carbon is nearly ubiquitous in Martian basaltic rocks. It formed through igneous, not biological, processes and was delivered over most of Martian geologic history to the surface as recently as the late Amazonian. The ubiquitous distribution of abiotic organic carbon in Martian igneous rocks is important for understanding the Martian carbon cycle and has implications for future missions to detect possible past Martian life. Fore instance, a positive detection of organics (especially PAHs) on Mars by Mars Science Laboratory, even if coupled with isotopically “light” δ13C values, may be detecting this abtiotic reservoir. Furthermore, the origin of the carbon in mantle rocks is strong evidence that this carbon was indigenous to the Martian interior because the absence of extensive plate tectonics would have prevented exchange between surface and near surface carbon reservoirs. Consequently, the storage of carbon within Mars occurred very early in its history, at the time of planet-wide differentiation, which has also been suggested for hydrogen storage on Mars. This process is likely not unique to Mars and could have been widely responsible for the production and delivery of abiotic organic carbon to the surfaces of the other terrestrial planets including the early Earth.
3.5 Early Earth recycling processes
The goal of this task, undertaken by Co-investigator Shirey, is to extend the understanding surface geological processes seen on Earth’s continents through the 150 km depths of their continental lithospheric mantle keels into the convecting mantle below and back in time. Such recycling is a fundamental geodynamic process that has occurred on Earth in some form since it accreted. A major advance made previously was the recognition of the onset of eclogite capture in the Archean cratonic keels and its recognition as a marker for tectonic change in geodynamic style on the Earth. Since then, with non-NAI collaborators M VanKranendonk and S H Richardson, Shirey has been investigating the implications of this geodynamic shift for the composition, structure and heat budget of the Earth and how continental crust is constructed during the transition to more traditional plate tectonics.
This endeavor requires data and insights from the convecting mantle, the subcontinental lithospheric mantle (SCLM) and the crust. Geochemical studies of silicate and sulfide inclusions in cratonic macrodiamonds characterize the SCLM through time. Diamonds >3.2 Ga contain exclusively peridotitic (harzburgitic) silicate and sulfide inclusions whereas diamonds <3.0 Ga contain inclusions that are predominantly eclogitic and to a lesser extent lherzolitic. Similarly, >3.0 Ga kimberlite-borne eclogite xenoliths are largely absent in the SCLM rock record, whereas they are common thereafter. Therefore, significant differences exist globally between >3.2 Ga versus <3.0 Ga crust and SCLM. This age has been proposed as a boundary between different geodynamic regimes on Earth, marking the onset of plate tectonics. The lack of early eclogite implies an absence of steep slab subduction. Archean crust also records major differences across the 3.0–3.2 Ga interval. Prior to 3.2 Ga, crust grew by vertical accretion over upwelling mantle in long-lived plateaux floored by extremely depleted residual harzburgitic SCLM or via slab melting and crustal imbrication over shallow subduction zones (e.g. West Greenland), whereas lateral accretion, allochthonous greenstone belt growth and calcalkaline magmatic products of mantle wedge melting emerge only after 3.2 Ga. This temporal and geochemical change can be explained as the result of the mantle temperature reaching a maximum at about 3 Ga with a step-wise shift in tectonic style from rapid mantle convection, small plates, shallow subduction, and localized recycling >3.2 Ga, followed by large plates, steep subduction, and full upper mantle recycling <3.0 Ga. These geodynamic changes had profound effects on mantle evolution, crustal growth, and geochemical cycles of heat-producing elements.
3.6 Hazen-Diversification of continental mineralogy
The Hadean Eon, encompassing Earth’s first 700 million years, was a time of significant planetary evolution. Nevertheless, CoI Hazen and collaborators estimate that prebiotic Earth’s near-surface environment held no more than about 410 different rock-forming or accessory mineral species that were widely distributed and/or volumetrically significant. This Hadean mineralogical parsimony, perhaps comparable to the mineral diversity of the near-surface environment of Mars today, is a consequence of the relatively limited modes of mineral paragenesis prior to 3.85 Ga compared to the last 3.0 billion years. Dominant Hadean Eon mineralizing processes include the evolution of a diverse suite of intrusive and extrusive igneous lithologies; hydrothermal alteration over a wide temperature range, notably serpentinization; authigenesis in marine sediments; and impact-related processes, including shock mineralization, creation of marginal hydrothermal zones, and excavation of deep metamorphosed terrains. On the other hand, the Hadean Eon may have been lacking in mineralization associated with plate tectonic processes, such as subduction zone volcanism and associated fluid-rock interactions, which result in massive sulfide deposition; convergent boundary orogenesis and consequent extensive granitoid-rooted continental landmasses; and the selection and concentration of pegmatophile elements in complex pegmatites with hundreds of accompanying minerals. The dramatic mineralogical consequences of life are reflected in the absence of Hadean biomineralization; for example, the lack of extensive carbonate deposits restricted the development of skarn and cave minerals prior to 3.85 Ga. Most importantly, it was not until after the establishment via photosynthesis of significant near-surface redox gradients that supergene alteration, redox-controlled ore deposition, and subaerial weathering in an oxidizing environment could diversify Earth’s near-surface mineralogy. These post-Hadean processes may be responsible for more than 4000 of the >4600 known mineral species. Any scenario for life’s origins that invokes minerals as agents of molecular synthesis, selection, protection or organization must take into account the limited mineralogical repertoire of the time.
PROJECT INVESTIGATORS:George Cody
Project InvestigatorJohn Chambers
PROJECT MEMBERS:Marilyn Fogel
RELATED OBJECTIVES:Objective 1.1
Formation and evolution of habitable planets.
Sources of prebiotic materials and catalysts
Earth's early biosphere.