Seven-year climatology of dust opacity on Mars

This paper describes the procedure we have used to produce multi-annual dust scenarios for Martian years 24 to 30 from a multi-instrument dataset of total dust opacity observations. This procedure includes gridding the observations on a pre-defined longitude-latitude grid with 1 sol resolution in time, and spatially interpolating the results to obtain complete daily maps of total dust opacity. We used weighted binning as gridding technique, and spatial kriging as method of interpolation. The new dust scenarios are available as NetCDF files, easy to interface to any model including global circulation and mesoscale models for the Martian atmosphere

Is Gliese 581d habitable? Some constraints from radiative-convective climate modeling

The recently discovered exoplanet Gl581d is extremely close to the outer edge of its system's habitable zone, which has led to much speculation on its possible climate. We have performed a range of simulations to assess whether, given simple combinations of chemically stable greenhouse gases, the planet could sustain liquid water on its surface. For best estimates of the surface gravity, surface albedo and cloud coverage, we find that less than 10 bars of CO2 is sufficient to maintain a global mean temperature above the melting point of water. Furthermore, even with the most conservative choices of these parameters, we calculate temperatures above the water melting point for CO2 partial pressures greater than about 40 bar. However, we note that as Gl581d is probably in a tidally resonant orbit, further simulations in 3D are required to test whether such atmospheric conditions are stable against the collapse of CO2 on the surface.Comment: 9 pages, 11 figures. Accepted for publication in Astronomy & Astrophysic

BaCu3O4: High Temperature Magnetic Order in One-Dimensional S=1/2 Diamond-Chains

The magnetic properties of the alkaline earth oxocuprate BaCu3O4 are investigated. We show that the characteristic Cu3O4 layers of this material can be described with diamond chains of antiferromagnetically coupled Cu 1/2 spins with only a weak coupling between two adjacent chains. These Cu3O4 layers seem to represent a so far unique system of weakly coupled one-dimensional magnetic objects where the local AF ordering of the Cu2+ ions leads to an actual net magnetic moment of an isolated diamond chain. We demonstrate a magnetic transition at a high N\'eel temperature T_{N}=336 K

3D climate modeling of close-in land planets: Circulation patterns, climate moist bistability and habitability

The inner edge of the classical habitable zone is often defined by the critical flux needed to trigger the runaway greenhouse instability. This 1D notion of a critical flux, however, may not be so relevant for inhomogeneously irradiated planets, or when the water content is limited (land planets). Here, based on results from our 3D global climate model, we find that the circulation pattern can shift from super-rotation to stellar/anti stellar circulation when the equatorial Rossby deformation radius significantly exceeds the planetary radius. Using analytical and numerical arguments, we also demonstrate the presence of systematic biases between mean surface temperatures or temperature profiles predicted from either 1D or 3D simulations. Including a complete modeling of the water cycle, we further demonstrate that for land planets closer than the inner edge of the classical habitable zone, two stable climate regimes can exist. One is the classical runaway state, and the other is a collapsed state where water is captured in permanent cold traps. We identify this "moist" bistability as the result of a competition between the greenhouse effect of water vapor and its condensation. We also present synthetic spectra showing the observable signature of these two states. Taking the example of two prototype planets in this regime, namely Gl581c and HD85512b, we argue that they could accumulate a significant amount of water ice at their surface. If such a thick ice cap is present, gravity driven ice flows and geothermal flux should come into play to produce long-lived liquid water at the edge and/or bottom of the ice cap. Consequently, the habitability of planets at smaller orbital distance than the inner edge of the classical habitable zone cannot be ruled out. Transiting planets in this regime represent promising targets for upcoming observatories like EChO and JWST.Comment: Accepted for publication in Astronomy and Astrophysics, complete abstract in the pdf, 18 pages, 18 figure

Exploring the spatial, temporal, and vertical distribution of methane in Pluto's atmosphere

High-resolution spectra of Pluto in the 1.66 um region, recorded with the VLT/CRIRES instrument in 2008 (2 spectra) and 2012 (5 spectra), are analyzed to constrain the spatial and vertical distribution of methane in Pluto's atmosphere and to search for mid-term (4 year) variability. A sensitivity study to model assumptions (temperature structure, surface pressure, Pluto's radius) is performed. Results indicate that (i) no variation of the CH4 atmospheric content (column-density or mixing ratio) with Pluto rotational phase is present in excess of 20 % (ii) CH4 column densities show at most marginal variations between 2008 and 2012, with a best guess estimate of a ~20 % decrease over this time frame. As stellar occultations indicate that Pluto's surface pressure has continued to increase over this period, this implies a concomitant decrease of the methane mixing ratio (iii) the data do not show evidence for an altitude-varying methane distribution; in particular, they imply a roughly uniform mixing ratio in at least the first 22-27 km of the atmosphere, and high concentrations of low-temperature methane near the surface can be ruled out. Our results are also best consistent with a relatively large (> 1180 km) Pluto radius. Comparison with predictions from a recently developed global climate model GCM indicates that these features are best explained if the source of methane occurs in regional-scale CH4 ice deposits, including both low latitudes and high Northern latitudes, evidence for which is present from the rotational and secular evolution of the near-IR features due to CH4 ice. Our "best guess" predictions for the New Horizons encounter in 2015 are: a 1184 km radius, a 17 ubar surface pressure, and a 0.44 % CH4 mixing ratio with negligible longitudinal variations.Comment: 21 pages, 6 figure

Martian meso/micro-scale winds and surface energy budget

Regional, diurnal and seasonal variations of surface temperature are particularly large on Mars. This is mostly due to the Martian surface remaining close to radiative equilibrium. Contrary to most terrestrial locations, contributions of sensible heat flux (i.e. conduction/convection exchanges between atmosphere and surface) to the surface energy budget [hereinafter SEB] are negligible on Mars owing to lowatmospheric density and heat capacity (e.g. Figure 2 in Savijärvi and Kauhanen, 2008). This radiative control of surface temperature is a key characteristic of the Martian environment and has crucial consequences on the the Martian geology, meteorology, exobiology, etc. In order to identify the impact of this Martian peculiarity to near-surface regional-to-local atmospheric circulations, we employ our recently-built Martian limited-area meteorological model (Spiga and Forget, 2009). We use horizontal resolutions adapted to the dynamical phenomena we aim to resolve: from several tens of kilometers to compute regional winds (mesoscale simulations) to several tens of meters to compute atmospheric boundary-layer winds (microscale or turbulent-resolving simulations, also called Large-Eddy Simulations, LES)

A global glacial ocean state estimate constrained by upper-ocean temperature proxies

Author Posting. © American Meteorological Society, 2018. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 31 (2018): 8059-8079, doi:10.1175/JCLI-D-17-0769.1.We use the method of least squares with Lagrange multipliers to fit an ocean general circulation model to the Multiproxy Approach for the Reconstruction of the Glacial Ocean Surface (MARGO) estimate of near sea surface temperature (NSST) at the Last Glacial Maximum (LGM; circa 23–19 thousand years ago). Compared to a modern simulation, the resulting global, last-glacial ocean state estimate, which fits the MARGO data within uncertainties in a free-running coupled ocean–sea ice simulation, has global-mean NSSTs that are 2°C lower and greater sea ice extent in all seasons in both the Northern and Southern Hemispheres. Increased brine rejection by sea ice formation in the Southern Ocean contributes to a stronger abyssal stratification set principally by salinity, qualitatively consistent with pore fluid measurements. The upper cell of the glacial Atlantic overturning circulation is deeper and stronger. Dye release experiments show similar distributions of Southern Ocean source waters in the glacial and modern western Atlantic, suggesting that LGM NSST data do not require a major reorganization of abyssal water masses. Outstanding challenges in reconstructing LGM ocean conditions include reducing effects from model biases and finding computationally efficient ways to incorporate abyssal tracers in global circulation inversions. Progress will be aided by the development of coupled ocean–atmosphere–ice inverse models, by improving high-latitude model processes that connect the upper and abyssal oceans, and by the collection of additional paleoclimate observations.DEA was supported by a NSF Graduate Research Fellowship and NSF Grant OCE-1060735. OM acknowledges support from the NSF. GF was supported by NASA Award 1553749 and Simons Foundation Award 549931

Spin Dynamics in Cuprates: Optical Conductivity of HgBa2CuO4

The electron-boson spectral density function I^2ChiOmega responsible for carrier scattering of the high temperature superconductor HgBa2CuO4 (Tc = 90 K) is calculated from new data on the optical scattering rate. A maximum entropy technique is used. Published data on HgBa2Ca2Cu3O8 (Tc = 130 K) are also inverted and these new results are put in the context of other known cases. All spectra (with two notable exceptions) show a peak at an energy (Omega_r) proportional to the superconducting transition temperature Omega_r ~= 6.3 kB.Tc. This charge channel relationship follows closely the magnetic resonance seen by polarized neutron scattering, Omega_r^{neutron} ~= 5.4 kB.Tc. The amplitudes of both peaks decrease strongly with increasing temperature. In some cases, the peak at Omega_r is weak and the spectrum can have additional maxima and a background extending up to several hundred meV

Angle-resolved photoemission study of the role of nesting and orbital orderings in the antiferromagnetic phase of BaFe2As2

We present a detailed comparison of the electronic structure of BaFe2As2 in its paramagnetic and antiferromagnetic (AFM) phases, through angle-resolved photoemission studies. Using different experimental geometries, we resolve the full elliptic shape of the electron pockets, including parts of dxy symmetry along its major axis that are usually missing. This allows us to define precisely how the hole and electron pockets are nested and how the different orbitals evolve at the transition. We conclude that the imperfect nesting between hole and electron pockets explains rather well the formation of gaps and residual metallic droplets in the AFM phase, provided the relative parity of the different bands is taken into account. Beyond this nesting picture, we observe shifts and splittings of numerous bands at the transition. We show that the splittings are surface sensitive and probably not a reliable signature of the magnetic order. On the other hand, the shifts indicate a significant redistribution of the orbital occupations at the transition, especially within the dxz/dyz system, which we discuss