Asymmetric thermal evolution of the Moon Laneuville, M.; Wieczorek, M. A.; Breuer, D. ...
Journal of geophysical research. Planets,
July 2013, Letnik:
118, Številka:
7
Journal Article
Recenzirano
Odprti dostop
The Moon possesses a clear dichotomy in geological processes between the nearside and farside hemispheres. The most pronounced expressions of this dichotomy are the strong concentration of ...radioactive heat sources on the nearside in a region known as the Procellarum KREEP Terrane (PKT) and the mare basaltic lava flows that erupted in or adjacent to this terrane. We model the thermochemical evolution of the Moon using a 3‐D spherical thermochemical convection code in order to assess the consequences of a layer enriched in heat sources below the PKT on the Moon's global evolution. We find that in addition to localizing most of the melt production on the nearside, such an enriched concentration of heat sources in the PKT crust has an influence down to the core‐mantle boundary and leaves a present‐day temperature anomaly within the nearside mantle. Moderate gravitational and topographic anomalies that are predicted in the PKT, but not observed, may be masked either by crustal thinning or gravitational anomalies from dense material in the underlying mantle. Our models also predict crystallization of an inner core for sulfur concentrations less than 6 wt %.
Key Points
The thermochemical consequences of the PKT are consistent with the observations
A thermal anomaly is present today in the mantle below the PKT
Heat sources enrichment in the PKT has an influence down to the CMB
The present‐day thermal state, interior structure, composition, and rheology of Mars can be constrained by comparing the results of thermal history calculations with geophysical, petrological, and ...geological observations. Using the largest‐to‐date set of 3‐D thermal evolution models, we find that a limited set of models can satisfy all available constraints simultaneously. These models require a core radius strictly larger than 1,800 km, a crust with an average thickness between 48.8 and 87.1 km containing more than half of the planet's bulk abundance of heat producing elements, and a dry mantle rheology. A strong pressure dependence of the viscosity leads to the formation of prominent mantle plumes producing melt underneath Tharsis up to the present time. Heat flow and core size estimates derived from the InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) mission will increase the set of constraining data and help to confine the range of admissible models.
Plain Language Summary
We constrain the thermal state and interior structure of Mars by combining a large number of observations with thermal evolution models. Models that match the available observations require a core radius larger that half the planetary radius and a crust thicker than 48.8 km but thinner than 87.1 km on average. All best‐fit models suggest that more than half of the planet's bulk abundance of heat producing elements is located in the crust. Mantle plumes may still be active today in the interior of Mars and produce partial melt underneath the Tharsis volcanic province. Our results have important implications for the thermal evolution of Mars. Future data from the InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) mission can be used to validate our models and further improve our understanding of the thermal evolution of Mars.
Key Points
We combine the largest‐to‐date set of 3‐D dynamical models with observations to constrain the thermal state and interior structure of Mars
Best‐fit models suggest a core radius strictly larger than 1,800 km and an average crustal thickness 48.8 km < dc < 87.1 km
Models suggest a large pressure dependence of the viscosity and a crust containing 65‐70% of the total amount of heat producing elements
A number of observations performed by the MESSENGER spacecraft can now be employed to better understand the evolution of Mercury's interior. Using recent constraints on interior structure, surface ...composition, volcanic and tectonic histories, we modeled the thermal and magmatic evolution of the planet. We ran a large set of Monte Carlo simulations based on one‐dimensional parametrized models, spanning a wide range of parameters. We complemented these simulations with selected calculations in 2‐D cylindrical and 3‐D spherical geometry, which confirmed the validity of the parametrized approach and allowed us to gain additional insight into the spatiotemporal evolution of mantle convection. Core radii of 1940 km, 2040 km, and 2140 km have been considered, and while in the first two cases several models satisfy the observational constraints, no admissible models were found for a radius of 2140 km. A typical thermal evolution scenario consists of an initial phase of mantle heating accompanied by planetary expansion and the production of a substantial amount of partial melt. The evolution subsequent to 2 Gyr is characterized by secular cooling that proceeds approximately at a constant rate and implies that planetary contraction should be ongoing today. Most of the models predict mantle convection to cease after 3–4 Gyr, indicating that Mercury may be no longer dynamically active. Finally, assuming the observed surface abundance of radiogenic elements to be representative for the entire crust, we determined bulk silicate concentrations of 35–62 ppb Th, 20–36 ppb U, and 290–515 ppm K, similar to those of other terrestrial planets.
Key Points
Models predict initial heating phase followed by cooling and contraction
Stiff rheology, thin crust and present‐day conductive mode are preferred
1D parametrized models agree well with 2D and 3D dynamic simulations
► We model the thermal and crustal evolution of Mars. ► Crustal recycling is common due to low mantle viscosities and an insulating crust. ► Observations suggest a primordial crust and low initial ...mantle temperatures. ► Dehydration stiffening of the mantle favors recent volcanism. ► Water extraction from the mantle is efficient and can exceed 40%.
We have reinvestigated the coupled thermal and crustal evolution of Mars taking new laboratory data concerning the flow behavior of iron-rich olivine into account. The low mantle viscosities associated with the relatively higher iron content of the martian mantle as well as the observed high concentrations of heat producing elements in a crust with a reduced thermal conductivity were found to promote phases of crustal recycling in many models. As crustal recycling is incompatible with an early separation of geochemical reservoirs, models were required to show no episodes of crustal recycling. Furthermore, admissible models were required to reproduce the martian crust formation history, to allow for the formation of partial melt under present day mantle conditions and to reproduce the measured concentrations of potassium and thorium on the martian surface. Taking dehydration stiffening of the mantle viscosity by the extraction of water from the mantle into account, we found that admissible models have low initial upper mantle temperatures around 1650
K, preferably a primordial crustal thickness of 30
km, and an initially wet mantle rheology. The crust formation process on Mars would then be driven by the extraction of a primordial crust after core formation, cooling the mantle to temperatures close to the peridotite solidus. According to this scenario, the second stage of global crust formation took place over a more extended period of time, waning at around 3500
Myr b.p., and was driven by heat produced by the decay of radioactive elements. Present-day volcanism would then be driven by mantle plumes originating at the core–mantle boundary under regions of locally thickened, thermally insulating crust. Water extraction from the mantle was found to be relatively efficient and close to 40% of the total inventory was lost from the mantle in most models. Assuming an initial mantle water content of 100
ppm and that 10% of the extracted water is supplied to the surface, this amount is equivalent to a 14
m thick global surface layer, suggesting that volcanic outgassing of H
2O could have significantly influenced the early martian climate and increased the planet’s habitability.
Aims. The compositions of meteorites and the morphologies of asteroid surfaces provide strong evidence that partial melting and differentiation were widespread among the planetesimals of the early ...solar system. However, it is not easily understood how planetesimals can be differentiated. To account for significantly smaller radii, masses, gravity and accretion energies early, intense heat sources are required, e.g. the short-lived nuclides 26Al and 60Fe. Here, we investigate the process of differentiation and core formation in accreting planetesimals taking into account the effects of sintering, melt heat transport via porous flow and redistribution of the radiogenic heat sources. Methods. We use a spherically symmetric one-dimensional model of a partially molten planetesimal consisting of iron and silicates, which considers the accretion by radial growth. The common heat conduction equation has been modified to consider also melt segregation. In the initial state, the planetesimals are assumed to be highly porous and consist of a mixture of Fe,Ni-FeS and silicates consistent to an H-chondritic composition. The porosity change due to the so called hot pressing is simulated by solving a corresponding differential equation. Magma segregation of iron and silicate melt is treated according to the flow in porous media theory by using the Darcy flow equation and allowing a maximal melt fraction of 50%. Results. We show that the differentiation in planetesimals depends strongly on the formation time, accretion duration, and accretion law and cannot be assumed as instantaneous. Iron melt segregation starts almost simultaneously with silicate segregation and lasts between 0.4 and 10 Ma. The degree of differentiation varies significantly and the most evolved structure consists of an iron core, a silicate mantle, which are covered by an undifferentiated but sintered layer and an undifferentiated and unsintered regolith – suggesting that chondrites and achondrites can originate from the same parent body.
The Heat Flow and Physical Properties Package HP
3
for the InSight mission will attempt the first measurement of the planetary heat flow of Mars. The data will be taken at the InSight landing site in ...Elysium planitia (136
∘
E, 5
∘
N) and the uncertainty of the measurement aimed for shall be better than ±5 mW m
−2
. The package consists of a mechanical hammering device called the “Mole” for penetrating into the regolith, an instrumented tether which the Mole pulls into the ground, a fixed radiometer to determine the surface brightness temperature and an electronic box. The Mole and the tether are housed in a support structure before being deployed. The tether is equipped with 14 platinum resistance temperature sensors to measure temperature differences with a 1-
σ
uncertainty of 6.5 mK. Depth is determined by a tether length measurement device that monitors the amount of tether extracted from the support structure and a tiltmeter that measures the angle of the Mole axis to the local gravity vector. The Mole includes temperature sensors and heaters to measure the regolith thermal conductivity to better than 3.5% (1-
σ
) using the Mole as a modified line heat source. The Mole is planned to advance at least 3 m—sufficiently deep to reduce errors from daily surface temperature forcings—and up to 5 m into the martian regolith. After landing, HP
3
will be deployed onto the martian surface by a robotic arm after choosing an instrument placement site that minimizes disturbances from shadows caused by the lander and the seismometer. The Mole will then execute hammering cycles, advancing 50 cm into the subsurface at a time, followed by a cooldown period of at least 48 h to allow heat built up during hammering to dissipate. After an equilibrated thermal state has been reached, a thermal conductivity measurement is executed for 24 h. This cycle is repeated until the final depth of 5 m is reached or further progress becomes impossible. The subsequent monitoring phase consists of hourly temperature measurements and lasts until the end of the mission. Model calculations show that the duration of temperature measurement required to sufficiently reduce the error introduced by annual surface temperature forcings is 0.6 martian years for a final depth of 3 m and 0.1 martian years for the target depth of 5 m.
Volcanic outgassing of CO2 and H2O on Mars Grott, M.; Morschhauser, A.; Breuer, D. ...
Earth and planetary science letters,
08/2011, Letnik:
308, Številka:
3-4
Journal Article
Recenzirano
Volcanic outgassing is one of the main sources of volatiles for the martian atmosphere and degassing of the martian interior potentially influenced the early martian climate. Using a parameterized ...thermo-chemical evolution model and considering two end-member melting models, we self-consistently calculate the amount of CO2 and H2O outgassed during the martian evolution. Outgassing rates are found to depend primarily on a factor describing the outgassing efficiency, the bulk mantle water content, the mantle oxygen fugacity, and the local melt fraction in the magma source regions. We find that significant outgassing ceased around 3.5-2Gyr ago, depending on the adopted melting model. A total of 0.9-1bar CO2 is outgassed during this time period if a mantle oxygen fugacity corresponding to one log10 unit above the iron-wustite buffer is assumed. Additionally, a total of 17-61m of water is delivered to the surface. Outgassing is most efficient in the pre-Noachian (up to 4.1Gyr), but still significant during the Noachian, and 5-15m of water and a arrow right 4250mbar of CO2 are outgassed between 4.1 and 3.7Gyr. Although this amount is probably insufficient for an appreciable greenhouse effect, pressures are found to be sufficient to stabilize transient liquid water on the surface well into the Hesperian period. Therefore, our results support the hypothesis that rather than being warm-and-wet, the martian climate was probably cold-and-wet. Outgassing is found to strongly decline during the Hesperian, and is insignificant during the Amazonian period. A simple parameterization for the outgassing of CO2 and H2O as a function of time is presented.
Regardless of the steady increase of computing power during the last decades, numerical models in a 3D spherical shell are only used in specific setups to investigate the thermochemical convection in ...planetary interiors, while 2D geometries are typically favored in most exploratory studies involving a broad range of parameters. The 2D cylindrical and the more recent 2D spherical annulus geometries are predominantly used in this context, but the extent to how well they reproduce the 3D spherical shell results in comparison to each other and in which setup has not yet been extensively investigated. Here we performed a thorough and systematic study in order to assess which 2D geometry reproduces best the 3D spherical shell. In a first set of models, we investigated the effects of the geometry on thermal convection in steady‐state setups while varying a broad range of parameters. Additional thermal evolution models of three terrestrial bodies, namely Mercury, the Moon, and Mars, which have different interior structures, were used to compare the 2D and 3D geometries. Our investigations show that the 2D spherical annulus geometry provides results closer to models in a 3D spherical shell compared to the 2D cylindrical geometry. Our study indicates where acceptable differences can be expected when using a 2D instead of a 3D geometry and where to be cautious when interpreting the results.
Plain Language Summary
In geodynamic modeling, numerical models are used in order to investigate how the interior of a terrestrial planet evolves from the earliest stage, after the planetary formation, up to present day. Often, the mathematical equations that are used to model the physical processes in the interior of rocky planets are discretized and solved using geometric meshes. The most commonly applied geometries are the 3D spherical shell, the 2D cylinder, and the 2D spherical annulus. While being the most accurate and realistic, the 3D geometry is expensive in terms of computing power and time of execution. On the other hand, 2D geometries provide a reduced accuracy but are computationally faster. Here we perform an extensive comparison between 2D and 3D geometries in scenarios of increasing complexity. The 2D spherical annulus geometry shows much closer results to the 3D spherical shell when compared to the 2D cylinder and should be given preference in 2D modeling studies.
Key Points
Interior dynamics models using the 2D spherical annulus geometry match the results of a 3D spherical shell better than the 2D cylinder
The difference between 2D and 3D geometries decreases when models are heated from below by the core and from within by radioactive elements
The 2D spherical annulus shows negligible differences to 3D for the thermal evolution of Mercury and the Moon, and acceptable values for Mars
Fitting the isotopic composition of lunar rocks to a new thermal and crystallization model shows the Moon formed 4.40 to 4.45 Ga.
A giant impact onto Earth led to the formation of the Moon, resulted ...in a lunar magma ocean (LMO), and initiated the last event of core segregation on Earth. However, the timing and temporal link of these events remain uncertain. Here, we demonstrate that the low thermal conductivity of the lunar crust combined with heat extraction by partial melting of deep cumulates undergoing convection results in an LMO solidification time scale of 150 to 200 million years. Combining this result with a crystallization model of the LMO and with the ages and isotopic compositions of lunar samples indicates that the Moon formed 4.425 ± 0.025 billion years ago. This age is in remarkable agreement with the U-Pb age of Earth, demonstrating that the U-Pb age dates the final segregation of Earth’s core.
The very limited amount of global contraction observed on Mercury's surface poses severe constraints on models of the planet's thermo-chemical evolution and current models rely on a very refractory, ...Thorium rich composition to slow planetary cooling. However, a refractory composition appears to be incompatible with evidence for pyroclastic eruptions, which require a substantial amount of volatiles to be present in the planetary interior. Furthermore, volcanic activity appears to have been ongoing for a considerable part of the planet's history, while current models predict an early cessation of crustal production. To address these inconsistencies we have reinvestigated the thermo-chemical evolution of Mercury using a non-refractory compositional model, taking the presence of a thermally insulating regolith layer into account. We find that models with a stiff mantle rheology satisfy the observational constraints if the regolith layer is at least 2
km thick. In these models, inefficient mantle convection and thermal insulation significantly slow planetary cooling and prolong the phase of crustal production to 2.5
Gyr after core formation, allowing the volume increase associated with mantle differentiation to offset some of the radial contraction caused by planetary cooling. Models furthermore predict substantial core sulfur contents above 6
wt.%, average crustal thicknesses between 10 and 40
km, and secular cooling rates of 30
K/Gyr.
► Mercury's thermal and chemical evolution is modeled. ► An insulating crust and regolith layer slows planetary cooling. ► Volcanism persists up to 2.5 Gyr after core formation. ► Small radial contraction is found to be compatible with a volatile rich composition.
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