Atmospheric pressure and CO2 concentration of potential habitable planet HD40307g

HD40307g is the closest potentially habitable planet discovered today orbiting a K2V star and will be a prime target for future TPF-like missions if its existence is confirmed.Although the atmosphere of HD40307g should be denser and contain more CO2 judging from the amount of radiation it receives from its star,it is unknown how dense and how much CO2 the planetary atmosphere should have.Thus more knowledge on its atmosphere is useful.For HD40307g to have Earth-like climate(288 K global mean surface temperature),we obtain the following combination of atmospheric pressure and CO2 mixing ratio:(1)10-bar+3%CO2;(2)5-bar+10%CO2;(3)3-bar+30%CO2.     

The nutrient cycle through snow and ice, a review

This paper reviews the merging of the nutrient cycle with the water cycle in the seasonal alpine snow cover, emphasizing physical processes at the snowpack and snow grain scale. Nutrients are incorporated into snowflakes growing in the atmosphere, they are part of the dry deposition from the atmosphere to the snowpack and they reach the snow as plant litter. The physical processes of the accumulation of nutrients and their redistribution in and on the snow grains and in the pore space of the snow matrix are described.¶The first flush of meltwater that reaches the soil carries a solution of nutrients and acids in a concentration several times higher than bulk values, an effect that increases with the age of the snow and the number of melt/freeze cycles and is more pronounced for sulfate than for chloride. Species that are attached to insoluble particles will be concentrated near the snow surface and will display peak concentrations in the final fraction of meltwater.     

First interactions with the hydrologic cycle determine pyrogenic carbon's fate in the Earth system

Fires produce an aromatic particulate residue commonly referred to as pyrogenic carbon (PyC). Particulate PyC is low density, high porosity, and is predominantly deposited on the soil surface in post-fire landscapes. These characteristics create a material that is prone to mobility, both vertically down the soil profile and laterally across the landscape even in low-relief landforms. Because of its tendency for lateral mobilization, we argue here that PyC's first interaction with water determines its environmental fate and persistence, not its interactions with soil minerals or microbes. PyC's first interactions with water determine: the amount of PyC that will enter the soil profile and experience microbial and geochemical alterations, whether it will be buried in depositional environments and stored on the landscape, or if it will be transported to streams and eventually to the ocean. Here we posit that this crucial first interaction with the hydrologic cycle occurs on the timescale of days to weeks, and therefore supersedes microbial decomposition as the primary control on PyC's environmental persistence. © 2020 John Wiley & Sons, Ltd.     

The origin of the Earth

It is not possible to consider the formation of the Earth in isolation without reference to the formation of the rest of the solar system. A brief account is given of the current scientific consensus on that topic, explaining the origin of an inner solar system rocky planet depleted in most of the gaseous and icy components of the original solar nebula. Volatile element depletion occurred at a very early stage in the nebula, and was probably responsible for the formation of Jupiter before that of the inner planets. The Earth formed subsequently from accumulation of a hierarchy of planetesimals. Evidence of these remains in the ancient cratered surfaces and the obliquities (tilts) of most planets. Earth melting occurred during this process, as well as from the giant Moon-forming impact. The strange density and chemistry of the Moon are consistent with an origin from the mantle of the impactor. Core-mantle separation on the Earth was coeval with accretion. Some speculations are given on the origin of the hydrosphere.     

Constant Q and a Fractal, Stratified Earth

— Frequency-dependent measurements of the quality factor Q typically show a constant behaviour for low frequencies and a positive power-law dependence for higher frequencies. In particular, the constant Q pattern is usually explained using intrinsic attenuation models due to anelasticity with either a single or multiple superposed relaxation mechanisms — each with a particular resonance peak.¶However, in this study, I show using wave localisation theory that a constant Q may also be due to apparent attenuation due to scattering losses. Namely, this phenomenon occurs if the earth displays fractal characteristics. Moreover, if fractal characteristics exist over a limited range of scales only, even an absorption band can be created—in accordance with observations. This indicates that it may be very difficult to distinguish between intrinsic and scattering attenuation on the basis of frequency-dependent measurements of the quality factor only.     

一个令人振奋的新创举--地球镜

美国国家科学基金会地球科学部与科学团体正在谱写一曲新的和令人振奋的、称为地球镜的乐章,它由一些现代化的地震和大地测量仪台阵组成,多数台阵集中在太平洋活动的大地构造带上和美国西部的北美板块边界上.沿圣安德烈斯断层帕克菲尔德地段布设的地球深部观测系统也将是这个观测镜的一部分.     

The cooling Earth: A reappraisal

By treating the lithosphere as a diffusive boundary layer to mantle convection, the convective speed or mantle creep rate, $\text{?}\text{?}$, can be related to the mantle-derived heat flux, $\text{Q}\text{?}$. If cell size is independent of $\text{Q}\text{?}{}^2$ then $\text{?}\text{?}\text{∝}\text{Q}\text{?}$. (If cell size varies with $\text{Q}\text{?}$, then a different power law prevails, but the essential conclusions are unaffected.) Then the factthat for constant thermodynamic efficiency of mantle convection, the mechanical power dissipation is proportionalto $\text{Q}\text{?}$, gives convective stress $\text{σ ∝}\text{Q}\text{?}{}^{\mathrm{?1}}$, i.e. the stress increases as the convection slows. This means an increasing viscosityor stiffness of the mantle which can be identified with a cooling rate in terms of a temperature-dependent creep law. If we suppose that the mantle was at or close to its melting point within 1 or 2 × 10⁸ years of accretionof the Earth, the whole scale of cooling is fixed. The present rate of cooling is estimated to be about 4.6 × 10^?8 deg y^?1 for the average mantle temperature, assumed to be 2500 K, but this very slow cooling rate represents a loss ofresidual mantle heat of 7 × 10¹² W, about 30% of the total mantle-derived heat flux. This conclusion requires theEarth to be deficient in radioactive heat, relative to carbonaceous chondrites. A consideration of mantle outgassing and atmospheric argon leads to the conclusion that the deficiency is due to depletion of potassium, and that the K/U ratio of the mantle is only about 2500, much less than either the crustal or carbonaceous chondritic values. Thetotal terrestrial potassium is estimated to be about 6 × 10²⁰ kg. Acceptance of the cooling of the Earth removes the necessity for potassium in the core.     

Cenozoic evolution of the sulfur cycle: Insight from oxygen isotopes in marine sulfate

We report new data on oxygen isotopes in marine sulfate (δ¹⁸O_SO4), measured in marine barite (BaSO₄), over the Cenozoic. The δ¹⁸O_SO4 varies by 6‰ over the Cenozoic, with major peaks 3, 15, 30 and 55 Ma. The δ¹⁸O_SO4 does not co-vary with the δ³⁴S_SO4, emphasizing that different processes control the oxygen and sulfur isotopic composition of sulfate. This indicates that temporal changes in the δ¹⁸O_SO4 over the Cenozoic must reflect changes in the isotopic fractionation associated with the sulfide reoxidation pathway. This suggests that variations in the aerial extent of different types of organic-rich sediments may have a significant impact on the biogeochemical sulfur cycle and emphasizes that the sulfur cycle is less sensitive to net organic carbon burial than to changes in the conditions of that organic carbon burial. The δ¹⁸O_SO4 also does not co-vary with the δ¹⁸O measured in benthic foraminifera, emphasizing that oxygen isotopes in water and sulfate remain out of equilibrium over the lifetime of sulfate in the ocean. A simple box model was used to explore dynamics of the marine sulfur cycle with respect to both oxygen and sulfur isotopes over the Cenozoic. We interpret variability in the δ¹⁸O_SO4 to reflect changes in the aerial distribution of conditions within organic-rich sediments, from periods with more localized, organic-rich sediments, to periods with more diffuse organic carbon burial. While these changes may not impact the net organic carbon burial, they will greatly affect the way that sulfur is processed within organic-rich sediments, impacting the sulfide reoxidation pathway and thus the δ¹⁸O_SO4. Our qualitative interpretation of the record suggests that sulfate concentrations were probably lower earlier in the Cenozoic.     

The mass and moment of inertia of the Earth

Recent revisions of geodetic and astronomical constants by the International Association of Geodesy and the International Astronomical Union lead to improved values for the earth's mass and moment of inertia. Corrections to be applied to these values before they are used as constraints in the inversion of seismic data are discussed.     

The solar wind and the Sun–Earth link

Plasma streams out from the Sun in the form of the solar wind. Shadia Rifai Habbal and Richard Woo examine the workings of this enigmatic link between the Earth and the Sun.     

The Earth’s plasmasphere: State of studies (a Review)

The main results of the experimental and theoretical studies of the Earth’s plasmasphere physics obtained until the present are reviewed. The review is aimed at attracting attention of scientists to studying this region of the Earth’s magnetosphere since many problems of the plasmasphere physics, first of all, the problems of plasmapause formation and plasmasphere filling and erosion, which are of importance in understanding the relation of the processes proceeding in the Sun and solar wind to the processes observed in the Earth’s ionosphere and atmosphere, remain unclear.     

The Skin Effect of the Anelastic Earth

--Assuming that the rheology of the earth's crust is defined by stress-strain relations containing a memory mechanism, it is seen that the stress field and the maximum shear stress (mss), caused by elongated mountain ranges, are limited mostly to the crust. In a geological, relatively short time, where relaxation is reached, the mss assumes the depth distribution (2~x)exp(-2~x) where x is the depth h measured in units of wavelength w of the mountain range; the mss reaches its maximum value, of about 1/3 the surface load, at the depth x = 1/2~ (x = h/w). Contrary to the case of the perfectly elastic earth model, where the mss at the surface is about 0.25 the maximum surface load, in the case of the anelastic earth model the mss is asymptotically nil at the surface and for depths larger than w, the mss is less than 1.2% the maximum surface load. It is seen that the relaxation of the medium causes a rotation of the planes of the mss in a layer near the surface and that, in the case when the surface load is applied periodically, there is a phase shift between the surface load and the stress at depth which is less than four days when the period is one year; it is smaller for shorter periods. It is also seen that the creation of reservoirs lasting a finite time causes a small variation of the mss in the crust which, when the reservoir is removed, leaves a residual mss with maximum at the surface and decreasing exponentially with depth; for a load lasting one year it takes 250 days to reduce to 40% the stress at depth. As a consequence of the residual mss, caused by the load of the topography, after earthquakes the local mss will not drop to zero; this decreases the time required by tectonic forces to release the next earthquake and increases the rate of seismicity.     

The early Paleozoic carbon cycle

A review of O, C, Sr and S isotope trends for the entire Phanerozoic shows that the present-day values of isotope signals are similar to those at the Proterozoic termination. The sharp rise in ⁸⁷Sr/⁸⁶Sr since 65 Ma has been attributed to an uplift and subsequent metamorphism and erosion associated with the Himalayas and Tibet. This orogenic evolution has been postulated to have influenced the global organic and inorganic carbon cycles and climate as well. A similar large-scale orogeny, the Pan-African event, also dominated the Neoproterozoic (Vendian) times, and the similarity of modern and Neoproterozoic isotope values for seawater may therefore have had a comparable tectonic cause. In this contribution, we present the results of a numerical model of the coupled C–alkalinity–S–Sr cycles suggesting that the early Paleozoic (from early Cambrian to late Devonian) evolution of Sr, O, C and S seawater isotope signals could have been the consequence of progressive oxidation of a large reduced carbon reservoir exhumed during the Pan-African orogeny. The δ¹⁸O measured in brachiopod shells is used as a forcing of the model, postulating that any change in the oxygen isotopic composition of seawater is the result of a disequilibrium in the organic carbon subcycle through the coupling of the oxygen isotopic and carbon cycles. The calculated δ¹³C, ⁸⁷Sr/⁸⁶Sr and δ³⁴S are in good agreement with the data, as is the reasonable calculated history for atmospheric pCO₂ and its relation to global climate.     

The Deformational Response of a Viscoelastic Solid Earth Model Coupled to a Thermomechanical Ice Sheet Model

We apply a coupled thermomechanical ice sheet—self-gravitating viscoelastic solid Earth model (SGVEM), allowing for the dynamic exchange of ice thickness and bedrock deformation, in order to investigate the effect of viscoelastic deformation on ice dynamics and vice versa. In a synthetic glaciation scenario, we investigate the interaction between the ice sheet and the solid Earth deformation, the glacial-isostatic adjustment (GIA), accounting for an atmospheric forcing depending on the ice sheet surface altitude. We compare the results from the coupled model to runs with the common elastic lithosphere/relaxing asthenosphere (ELRA) model, where the lithosphere is represented by a thin plate and the mantle relaxes with one characteristic relaxation time, as well as to a rigid Earth without any deformation. We find that the deformational behaviour of the SGVEM on ice dynamics (i.e. stored ice volume, ice thickness and velocity field) is comparable to the ELRA for an optimal choice of the parameters in steady state, but exhibits differences in the transient behaviour. Beyond the ice sheet, in the region of peripheral forebulge, the differences in the transient surface deformation between ELRA and SGVEM are substantial, demonstrating the inadequacy of the ELRA model for interpreting constraints on GIA in the periphery of the ice sheet, such as sea-level indicators and GPS uplift rates.     

The axial momentum balance of Earth and its fluid envelope

The emergence of greatly improved data sets over the past decade has heightened awareness of the close relationship between changes in the axial component of the angular momentum of the atmosphere and that of the solid Earth, the latter being reflected in small, though detectable, changes in the planet's rate of rotation. Changes in the large-scale wind field, and hence in atmospheric angular momentum, on intraseasonal through interannual time scales can be associated with a number of identifiable meteorological phenomena, whose further study has been given new impetus by the discovery of their signals in Earth's rotation. Future advances in the subject are apt to occur in connection with new data sets that will help address questions remaining about rapid changes in Earth rotation and the torques responsible for the momentum changes. Also in the coming decade, both new data and modeling approaches should help clarify the role of the oceanic portion of Earth's fluid envelope in the planetary momentum balance.