A magma ocean origin for Mercury's earliest exosphere

2020 ◽  
Author(s):  
Diana Gamborino ◽  
Noah Jäggi ◽  
Dan J. Bower ◽  
Aaron Wolf ◽  
Paolo Sossi ◽  
...  
Keyword(s):  
Surface Composition ◽  
Building Blocks ◽  
Atmospheric Composition ◽  
Initial Surface ◽  
Magma Ocean ◽  
Loss Mechanism ◽  
Near Surface ◽  
Enstatite Chondrite ◽  
Atmospheric Loss ◽  
Rapid Evacuation

<p>MESSENGER observations used to constrain surface composition suggest a global magma ocean formed on early Mercury [1, 2]. Our study models the coupled evolution of the Hermean magma ocean and atmosphere, and determines the extent of exospheric loss from an atmosphere formed by evaporation of a magma ocean. Using our framework to couple the interior and exterior chemical reservoirs, we evaluate a range of possible atmospheric evolution scenarios for early Mercury. These include the possibility that Mercury was fully-accreted before a mantle stripping event caused by a giant impact [3] led to degassing, or alternatively that the building blocks of Mercury were already relatively volatile-free.</p> <p>Assuming an initial surface temperature of 2500 K and an oxygen fugacity fixed at 1 log unit below the Iron-Wüstite buffer, we find that the Hermean magma ocean cooled to 1500 K (the rheological transition) in around 400 to 9000 yrs, depending on the efficiency of radiative heat transfer in the atmosphere. We investigate the behaviour of two endmember cases: (1) a present-day sized Mercury that is volatile free and thus cools in a manner similar to that of a blackbody, and (2) a larger Mercury that is sufficiently volatile-rich to provide a greenhouse atmosphere that delays cooling. During the magma ocean stage, evaporation and sublimation of oxide components from the molten silicate magma contribute to the growth of the atmosphere, in addition to volatile outgassing. For the endmember case with initial volatile abundances based on enstatite chondrite-like precursors, the atmosphere is dominated by CO, CO<sub>2</sub>, H<sub>2</sub>, and H<sub>2</sub>O, with minor abundances of SiO, Na, K, Mg, and Fe gas species. For the endmember case without major volatiles (C, H), the atmospheric composition is dominated by metal- and metal oxide gas species only.</p> <p>We apply a Monte Carlo (MC) atmospheric loss model [4] to calculate exospheric losses for all pathways and find that photoionization is the dominant loss mechanism for early Mercury. Cases both with and without major volatiles reveal that the atmosphere lasts between 100 and 250 years before the near-surface of Mercury becomes solid. Preliminary MC results show that photo-dissociation considerably alters the atmospheric composition, efficiently breaking SiO<sub>2</sub> into SiO<sup>+</sup> and O. Solar wind and magnetospheric plasma leads to rapid evacuation of the ionized species from the neutral exosphere whereas oxygen is lost efficiently by Jeans escape.</p> <p>Using our coupled framework, future modelling efforts will aim to understand if and how the evaporation of the magma ocean of early Mercury has modified its surface composition, with a view to interpreting BepiColombo observations. Hence our work provides an important step to connect Mercury’s formation, earliest evolution, and upcoming observations.</p> <p>References: [1] Vander Kaaden, K. E. and McCubbin, F. M. (2016). Cosmochim. Ac., 173, 246–263. [2] Berthet, S., Malavergne, V., and Righter, K. (2009). Geochim. Cosmochim. Ac., 73(20), 6402–6420. [3] Benz, W., et al. (2008). Mercury, pp. 7–20. Springer. [4] Wurz, P., and Lammer, H. (2003). <em>Icarus</em>, <em>164</em>(1), 1–13</p> <p> </p> <p> </p>

Implications of magma oceans for astrophysical observations: mass-radius and atmospheric composition

2020 ◽  
Author(s):  
Dan J. Bower ◽  
Daniel Kitzmann ◽  
Aaron Wolf ◽  
Patrick Sanan ◽  
Caroline Dorn ◽  
...  
Keyword(s):  
Building Blocks ◽  
Atmospheric Composition ◽  
Terrestrial Planets ◽  
Iron Core ◽  
Magma Ocean ◽  
Static Structure ◽  
Emission Data ◽  
Modelling Framework ◽  
Magma Oceans ◽  
Structure Calculations

<div> <div> <div> <p>The earliest secondary atmosphere of a rocky planet originates from extensive volatile release during one or more magma ocean epochs that occur during and after the assembly of the planet. Magma oceans set the stage for the long-term evolution of terrestrial planets by establishing the major chemical reservoirs of the iron core and silicate mantle, chemical stratification within the mantle, and outgassed atmosphere. Furthermore, current and future exoplanet observations will favour the detection and characterisation of hot and warm planets, potentially with large outgassed atmospheres. In this study, we highlight the potential to combine models of coupled interior–atmosphere evolution with static structure calculations and modelled atmospheric spectra (transmission and emission). By combining these components in a common modelling framework, we acknowledge planets as dynamic entities and leverage their evolution to bridge planet formation, interior-atmosphere interaction, and observations.</p> <p>An interior–atmosphere model is combined with static structure calculations to track the evolving radius of a hot rocky mantle that is outgassing volatiles. We consider oxidised species CO2 and H2O and generate synthetic emission and transmission spectra for CO2 and H2O dominated atmospheres. Atmospheres dominated by CO2 suppress the outgassing of H2O to a greater extent than previously realised, since previous studies have applied an erroneous relationship between volatile mass and partial pressure. Furthermore, formation of a lid at the surface can tie the outgassing of H2O to the efficiency of heat transport through the lid, rather than the radiative timescale of the atmosphere. We extend this work to explore the speciation of a primary atmosphere that is constrained using meteoritic materials as proxies for the planetary building blocks, and find that a range of reducing and oxidising atmospheres are possible.</p> </div> </div> </div><div> <div> <div> <p>Our results demonstrate that a hot molten planet can have a radius several percent larger (about 5%, assuming Earth-like core size) than its equivalent solid counterpart, which may explain the larger radii of some close-in exoplanets. Outgassing of a low molar mass species (such as H2O, compared to CO2) can combat the continual contraction of a planetary mantle and even marginally increase the planetary radius. We further use our models to generate synthetic transmission and emission data to aid in the detection and characterisation of rocky planets via transits and secondary eclipses. Atmospheres of terrestrial planets around M-stars that are dominated by CO2 versus H2O could be distinguished by future observing facilities that have extended wavelength coverage (e.g., JWST). Incomplete magma ocean crystallisation, as may be the case for close-in terrestrial planets, or full or part retention of an early outgassed atmosphere, should be considered in the interpretation of observational data from current and future observing facilities.</p> </div> </div> </div>

A new theory on Mercury's origin

2020 ◽  
Author(s):  
Manuel Scherf ◽  
Nikolay Erkaev ◽  
Helmut Lammer
Keyword(s):  
Solar Nebula ◽  
Surface Composition ◽  
Building Blocks ◽  
The Body ◽  
Terrestrial Planets ◽  
Magma Ocean ◽  
Volatile Elements ◽  
Continuous Growth ◽  
Giant Impact ◽  
First Time

<p>Of all the terrestrial planets in the Solar System Mercury stands out with a remarkably high core-mantle ratio, with its core occupying about 85% of the planetary radius. Several different theories tried to explain its high Fe/Si-ratio; the giant impact theory (e.g. [1]) for instance argues that one or more giant impacts stripped away most of the Hermean mantle, while the core remained and formed the smallest of the terrestrial planets. Another theory explains the high density of Mercury through a partial volatilization during the time of the solar nebula (e.g. [2]). Here, proto-Mercury is assumed to be substantially more massive than at present-day with a composition close to those of the other terrestrial planets. When the planet was surrounded by the hot solar nebula, however, most of the mantle evaporated, ending up with present-day Mercury. Other theories argue with the particular primordial conditions of its orbital location that might have favored the accretion of dense and volatile poor building-blocks such as enstatite chondrites (e.g. [3,4]). Messenger, however, revealed a surface composition that is surprisingly rich in volatile and moderately volatile elements [4]. This is hardly compatible with the giant impact and vaporization theory but supports hypothesis that connect Mercury’s high core-mantle ratio to the particular conditions of its orbital location.</p> <p>Within this talk, we will for the first time present a new model that connects these conditions with accretion and partial planetary evaporation. We will argue that Mercury (in contrast to old evaporation theories) was released out of the nebula as a small planetary embryo, comparable in size to the moon, that was covered with a global magma ocean. While the embryo proceeds to grow through frequent impactors, (moderately) volatile elements evaporate from the magma ocean and are lost into space due to the high surface temperature, the low gravity of the body and the high XUV flux from the young Sun. Here, lighter and more volatile elements are preferentially lost from the embryo, while the heavier and less volatile elements escape less efficient. Due to the continuous growth of proto-Mercury, however, the gravitational energy will start to dominate over the thermal energy of the evaporated particles, making them harder and harder to escape, which ultimately halts the loss of moderately volatile elements. Mercury subsequently finalizes its accretion with relatively volatile rich material and evolves to the body we can observe at present-day. We simulated the escape of (moderately) volatile elements with an adopted version of a 1D hydrodynamic upper atmosphere model (e.g. [5]) and will present our results here for the first time.</p> <p><strong>References:</strong> [1] Benz, W. et al., Space Sciences Series of ISSI, Volume 26, p. 7., 2008. [2] Cameron, A.G.W., Icarus, Volume 64, Issue 2, p. 285-294., 1985. [3] Charlier, B. and Namur, O., Elements, Volume 15, p. 9-14, 2019. [4] Nittler, Larry R. et al., Science, Volume 333, Issue 6051, pp. 1847, 2011. []5 Erkaev, et al., MNRAS, Volume 460, Issue 2, p.1300-1309, 2016.</p>

scholarly journals Evolution of Mercury’s Earliest Atmosphere

2021 ◽  
Vol 2 (6) ◽  
pp. 230
Author(s):  
Noah Jäggi ◽  
Diana Gamborino ◽  
Dan J. Bower ◽  
Paolo A. Sossi ◽  
Aaron S. Wolf ◽  
...  
Keyword(s):  
Total Mass ◽  
Chemical Exchange ◽  
Plasma Heating ◽  
Building Blocks ◽  
Atmospheric Composition ◽  
Magma Ocean ◽  
Heating Efficiency ◽  
Atmospheric Escape ◽  
Total Mass Loss ◽  
Early Atmosphere

Abstract 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 determines 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 provides 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 ∼106 kg s−1. Loss by photoevaporation depends on the timing of Mercury formation and assumed heating efficiency and ranges from ∼106.6 to ∼109.6 kg s−1. 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 10,000 yr. Therefore, escape processes only account for an equivalent loss of less than 2.3 km of crust (0.3% of Mercury’s mass). Accordingly, ≤0.02% of the total mass of H2O 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.

Investigation of atmospheric hydrogen cyanide: a modelling perspective

2021 ◽  
Author(s):  
Antonio G. Bruno ◽  
Jeremy J. Harrison ◽  
David P. Moore ◽  
Martyn P. Chipperfield ◽  
Richard J. Pope
Keyword(s):  
Biomass Burning ◽  
Atmospheric Chemistry ◽  
Hydrogen Cyanide ◽  
Three Dimensional ◽  
Oxidation Reactions ◽  
Loss Mechanism ◽  
Mixing Ratios ◽  
The Individual ◽  
Atmospheric Loss ◽  
Physical And Chemical

<p>Hydrogen cyanide (HCN) is one of the most abundant cyanides present in the global atmosphere, and is a tracer of biomass burning, especially for peatland fires. The HCN lifetime is 2–5 months in the troposphere but several years in the stratosphere. Understanding the physical and chemical mechanisms of HCN variability is important due to its non-negligible role in the nitrogen cycle. The main source of tropospheric HCN is biomass burning with minor contributions from industry and transport. The main loss mechanism of atmospheric HCN is the reaction with the hydroxyl radical (OH). Ocean uptake is also important, while in the stratosphere oxidation by reaction with O(<sup>1</sup>D) needs to be considered.</p><p>HCN variability can be investigated using chemical model simulations, such as three-dimensional (3-D) chemical transport models (CTMs). Here we use an adapted version of the TOMCAT 3-D CTM at a 1.2°x1.2° spatial resolution from the surface to ~60 km for 12 idealised HCN tracers which quantify the main loss mechanisms of HCN, including ocean uptake, atmospheric oxidation reactions and their combinations. The TOMCAT output of the HCN distribution in the period 2004-2020 has been compared with HCN profiles measured by the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) over an altitude grid from 6 to 42 km. HCN model data have also been compared with ground-based measurements of HCN columns from NDACC FTIR stations and with in-situ volume mixing ratios (VMRs) from NOAA ground-based measurement sites.</p><p>The model outputs for the HCN tracer with full treatment of the loss processes generally agree well with ACE-FTS measurements, as long as we use recent laboratory values for the atmospheric loss reactions. Diagnosis of the individual loss terms shows that decay of the HCN profile in the upper stratosphere is due mainly to the O(<sup>1</sup>D) sink. In order to test the magnitude of the tropospheric OH sink and the magnitude of the ocean sink, we also show the comparisons of the model tracers with surface-based observations. The implications of our results for understanding HCN and its variability are then discussed.</p>

scholarly journals The composition of Europa's near-surface atmosphere

2008 ◽  
Vol 4 (S251) ◽  
pp. 327-328
Author(s):  
Mau C. Wong ◽  
Tim Cassidy ◽  
Robert E. Johnson
Keyword(s):  
Solar System ◽  
Surface Composition ◽  
Near Surface ◽  
Water Ice ◽  
Model Studies ◽  
Surface Atmosphere ◽  
Jovian Magnetosphere ◽  
Ice Surface ◽  
Geophysical Phenomenon ◽  
Inorganic Species

AbstractThe presence of an undersurface ocean renders Europa as one of the few planetary bodies in our Solar System that has been conjectured to have possibly harbored life. Some of the organic and inorganic species present in the ocean underneath are expected to transport upwards through the relatively thin ice crust and manifest themselves as impurities of the water ice surface. For this reason, together with its unique dynamic atmosphere and geological features, Europa has attracted strong scientific interests in past decades.Europa is imbedded inside the Jovian magnetosphere, and, therefore, is constantly subjected to the immerse surrounding radiations, similar to the other three Galilean satellites. The magnetosphere-atmosphere-surface interactions form a complex system that provides a multitude of interesting geophysical phenomenon that is unique in the Solar System. The atmosphere of Europa is thought to have created by, mostly, charged particles sputtering of surface materials. Consequently, the study of Europa's atmosphere can be used as a tool to infer the surface composition. In this paper, we will discuss our recent model studies of Europa's near-surface atmosphere. In particular, the abundances and distributions of the dominant O2 and H2O species, and of other organic and inorganic minor species will be addressed.

scholarly journals Surface composition and near-surface hardness studies on high dose boron-implanted 304 stainless steel

1990 ◽  
Vol 13 (5) ◽  
pp. 333-342 ◽  
Author(s):  
A K Goel ◽  
N D Sharma ◽  
R K Mohindra ◽  
P K Ghosh ◽  
M C Bhatnagar
Keyword(s):  
Stainless Steel ◽  
Surface Composition ◽  
Surface Hardness ◽  
304 Stainless Steel ◽  
High Dose ◽  
Near Surface

 The Climate Response to Emissions Reductions due to COVID-19

2021 ◽  
Author(s):  
Chris Jones ◽  
Keyword(s):  
Atmospheric Composition ◽  
Shortwave Radiation ◽  
Initial Condition ◽  
Emissions Reductions ◽  
Near Surface ◽  
Surface Climate ◽  
Climate Signals ◽  
Near Term ◽  
Regional Analyses ◽  
The Impact

<p>Many nations responded to the COVID-19 pandemic by restricting travel and other activities during 2020, resulting in temporarily reduced emissions of CO2, other greenhouse gases and ozone and aerosol precursors. We perform a coordinated Intercomparison, CovidMIP, of Earth System model simulations to assess the impact on climate of these emissions reductions. Eleven models performed multiple initial-condition ensembles to produce over 280 simulations spanning both initial condition and model structural uncertainty. We find model consensus on reduced aerosol amounts (particularly over East Asia) and associated increases in surface shortwave radiation levels. However, any impact on near-surface temperature or rainfall during 2020-2024 is extremely small and is not detectable in this initial analysis. Regional analyses on a finer scale, and closer attention to extremes (especially linked to changes in atmospheric composition and air quality) are required to test the impact of COVID-19-related emission reductions on near-term climate.</p><p>This first-look at results has focussed on surface climate, but future analysis will include attribution of drivers of climate signals; longer term implications of emissions reductions and options for economic recovery; quantifying changes in extremes; influence on atmospheric circulation and the carbon cycle.</p>

scholarly journals Stellar flares versus luminosity: XUV-induced atmospheric escape and planetary habitability

2020 ◽  
Vol 500 (1) ◽  
pp. L1-L5
Author(s):  
Dimitra Atri ◽  
Shane R Carberry Mogan
Keyword(s):  
Extreme Ultraviolet ◽  
Wide Energy Range ◽  
X Rays ◽  
Loss Mechanism ◽  
Atmospheric Escape ◽  
Stellar Flares ◽  
Wide Energy ◽  
A Minor ◽  
Atmospheric Loss ◽  
Planetary Habitability

ABSTRACT Space weather plays an important role in the evolution of planetary atmospheres. Observations have shown that stellar flares emit energy in a wide energy range (1030–1038 erg), a fraction of which lies in X-rays and extreme ultraviolet (XUV). These flares heat the upper atmosphere of a planet, leading to increased escape rates, and can result in atmospheric erosion over a period of time. Observations also suggest that primordial terrestrial planets can accrete voluminous H/He envelopes. Stellar radiation can erode these protoatmospheres over time, and the extent of this erosion has implications for the planet’s habitability. We use the energy-limited equation to calculate hydrodynamic escape rates from these protoatmospheres irradiated by XUV stellar flares and luminosity. We use the flare frequency distribution of 492 FGKM stars observed with TESS to estimate atmospheric loss in habitable zone planets. We find that for most stars, luminosity-induced escape is the main loss mechanism, with a minor contribution from flares. However, flares dominate the loss mechanism of ∼20 per cent M4–M10 stars. M0–M4 stars are most likely to completely erode both their proto- and secondary atmospheres, and M4–M10 are least likely to erode secondary atmospheres. We discuss the implications of these results on planetary habitability.

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