arXiv subjects

Paolo A. Sossi

Publications and source records attributed to Paolo A. Sossi.

24 records · Page 2Linked to original sources

arXiv2022

Retention of water in terrestrial magma oceans and carbon-rich early atmospheres

Massive steam and CO$_2$ atmospheres have been proposed for magma ocean outgassing of Earth and terrestrial planets. Yet formation of such atmospheres depends on volatile exchange with the molten interior, governed by volatile solubilities and redox reactions. We determine the evolution of magma ocean--atmosphere systems for a range of oxygen fugacities, C/H ratios and hydrogen budgets that include redox reactions for hydrogen (H$_2$--H$_2$O), carbon (CO--CO$_2$), methane (CH$_4$), and solubility laws for H$_2$O and CO$_2$. We find that small initial budgets of hydrogen, high C/H ratios, and oxidizing conditions, suppress outgassing of hydrogen until the late stage of magma ocean crystallization. Hence early atmospheres in equilibrium with magma oceans are dominantly carbon-rich, and specifically CO-rich except at the most oxidizing conditions. The high solubility of H$_2$O limits its outgassing to melt fractions below $\sim$30\%, the fraction at which the mantle transitions from vigorous to sluggish convection with melt percolation. Sluggish melt percolation could enable a surface lid to form, trapping water in the interior and thereby maintaining a carbon-rich atmosphere (equilibrium crystallization). Alternatively, efficient crystal settling could maintain a molten surface, promoting a transition to a water-rich atmosphere (fractional crystallization). However, additional processes, including melt trapping and H dissolution in crystallizing minerals, further conspire to limit the extent of H outgassing, even for fractional crystallization. Hence, much of the water delivered to planets during their accretion can be safely harbored in their interiors during the magma ocean stage, particularly at oxidizing conditions.

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arXiv2022

A model Earth-sized planet in the habitable zone of $α$ Centauri A/B

The bulk chemical composition and interior structure of rocky exoplanets are of fundamental importance to understanding their long-term evolution and potential habitability. Observations of the chemical compositions of the solar system rocky bodies and of other planetary systems have increasingly shown a concordant picture that the chemical composition of rocky planets reflects that of their host stars for refractory elements, whereas this expression breaks down for volatiles. This behavior is explained by devolatilization during planetary formation and early evolution. Here, we apply a devolatilization model calibrated with solar system bodies to the chemical composition of our nearest Sun-like stars -- $α$ Centauri A and B -- to estimate the bulk composition of any habitable-zone rocky planet in this binary system ("$α$-Cen-Earth"). Through further modeling of likely planetary interiors and early atmospheres, we find that compared to Earth, such a planet is expected to have (i) a reduced (primitive) mantle that is similarly dominated by silicates albeit enriched in carbon-bearing species (graphite/diamond); (ii) a slightly larger iron core, with a core mass fraction of $38.4_{-5.1}^{+4.7}$ wt% (cf. Earth's 32.5 $\pm$ 0.3 wt%); (iii) an equivalent water-storage capacity; and (iv) a CO$_2$-CH$_4$-H$_2$O-dominated early atmosphere that resembles that of Archean Earth. Further taking into account its $\sim$ 25% lower intrinsic radiogenic heating from long-lived radionuclides, an ancient $α$-Cen-Earth ($\sim$ 1.5-2.5 Gyr older than Earth) is expected to have less efficient mantle convection and planetary resurfacing, with a potentially prolonged history of stagnant-lid regimes.

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arXiv2021

Evolution of Mercury's Earliest Atmosphere

MESSENGER observations suggest a magma ocean formed on proto-Mercury, during which evaporation of metals and outgassing of C- and H-bearing volatiles produced an early atmosphere. Atmospheric escape subsequently occurred by plasma heating, photoevaporation, Jeans escape, and photoionization. To quantify atmospheric loss, we combine constraints on the lifetime of surficial melt, melt composition, and atmospheric composition. Consideration of two initial Mercury sizes and four magma ocean compositions determine the atmospheric speciation at a given surface temperature. A coupled interior-atmosphere model determines the cooling rate and therefore the lifetime of surficial melt. Combining the melt lifetime and escape flux calculations provide estimates for the total mass loss from early Mercury. Loss rates by Jeans escape are negligible. Plasma heating and photoionization are limited by homopause diffusion rates of $\sim10^{6}$ kg/s. Loss by photoevaporation depends on the timing of Mercury formation and assumed heating efficiency and ranges from $\sim10^{6.6}$ to $\sim10^{9.6}$ kg/s. The material for photoevaporation is sourced from below the homopause and is therefore energy-limited rather than diffusion-limited. The timescale for efficient interior-atmosphere chemical exchange is less than ten thousand years. Therefore, escape processes only account for an equivalent loss of less than 2.3 km of crust ($0.3\%$ of Mercury's mass). Accordingly, $\leq0.02\%$ of the total mass of H$_2$O and Na is lost. Therefore, cumulative loss cannot significantly modify Mercury's bulk mantle composition during the magma ocean stage. Mercury's high core:mantle ratio and volatile-rich surface may instead reflect chemical variations in its building blocks resulting from its solar-proximal accretion environment.

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arXiv2019

Thermodynamics of Element Volatility and its Application to Planetary Processes

Despite its importance in geological sciences, our understanding of interactions between gas and condensed phases (comprising solids and liquids) remains clouded by the fact that, often, only indirect evidence remains for their occurrence. This arises from the tendency for the vapour phase to escape from the condensed phase with which it interacts, owing to its much lower density and thus greater volume. For a gas that is sufficiently tenuous that interactions do not occur between its constituent molecules, this relationship is quantified in the ideal gas law (Clapeyron 1834): $PV=nRT$ (1) where $P$ is the total pressure exerted by the gas, $V$ its volume, $n$ is the number of moles, $R$ the gas constant 8.3145 $Jmol^{-1}K^{-1}$, Horstmann, 1873) and $T$ the absolute temperature. One mole of an ideal gas at 273.15 $K$ and $10^5$ $Pa$ (standard temperature and pressure for gases) has a molar volume of 22,711 $cm^3/mol$, $10^3$ x greater than typical silicate liquids or minerals. As a result, vaporisation processes in nature are often informed by chemical and textural evidence remaining in the condensed phase.

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arXiv2018

Volatile loss following cooling and accretion of the Moon revealed by chromium isotopes

Terrestrial and lunar rocks share chemical and isotopic similarities in refractory elements, suggestive of a common precursor. By contrast, the marked depletion of volatile elements in lunar rocks together with their enrichment in heavy isotopes compared to Earth s mantle suggests that the Moon underwent evaporative loss of volatiles. However, whether equilibrium prevailed during evaporation, and, if so, at what conditions (temperature, pressure and oxygen fugacity) remain unconstrained. Chromium may shed light on this question, as it has several thermodynamically-stable, oxidised gas species that can distinguish between kinetic and equilibrium regimes. Here, we present high-precision Cr isotope measurements in terrestrial and lunar rocks that reveal an enrichment in the lighter isotopes of Cr in the Moon compared to Earths mantle by 100 +/- 40 ppm per atomic mass unit. This observation is consistent with Cr partitioning into an oxygen-rich vapour phase in equilibrium with the proto-Moon, thereby stabilising the CrO2 species that is isotopically heavy compared to CrO in a lunar melt. Temperatures of 1600 to 1800 K and oxygen fugacities near the Fayalite-Magnetite-Quartz buffer are required to explain the elemental and isotopic difference of Cr between Earth s mantle and the Moon. These temperatures are far lower than modelled in the aftermath of a giant impact, implying that volatile loss did not occur contemporaneously with impact but following cooling and accretion of the Moon.

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arXiv2017

Early Solar System irradiation quantified by linked vanadium and beryllium isotope variations in meteorites

X-ray emission in young stellar objects (YSOs) is orders of magnitude more intense than in main sequence stars1,2, suggestive of cosmic ray irradiation of surrounding accretion disks. Protoplanetary disk irradiation has been detected around YSOs by HERSCHEL3. In our solar system, short-lived 10Be (half-life = 1.39 My4), which cannot be produced by stellar nucleosynthesis, was discovered in the oldest solar system solids, the calcium-aluminium-rich inclusions (CAIs)5. The high 10Be abundance, as well as detection of other irradiation tracers6,7, suggest 10Be likely originates from cosmic ray irradiation caused by solar flares8. Nevertheless, the nature of these flares (gradual or impulsive), the target (gas or dust), and the duration and location of irradiation remain unknown. Here we use the vanadium isotopic composition, together with initial 10Be abundance to quantify irradiation conditions in the early Solar System9. For the initial 10Be abundances recorded in CAIs, 50V excesses of a few per mil relative to chondrites have been predicted10,11. We report 50V excesses in CAIs up to 4.4 per mil that co-vary with 10Be abundance. Their co-variation dictates that excess 50V and 10Be were synthesised through irradiation of refractory dust. Modelling of the production rate of 50V and 10Be demonstrates that the dust was exposed to solar cosmic rays produced by gradual flares for less than 300 years at about 0.1 au from the protoSun.

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