540 research outputs found
Atmospheric studies of habitability in the Gliese 581 system
The M-type star Gliese 581 is orbited by at least one terrestrial planet
candidate in the habitable zone, i.e. GL 581 d. Orbital simulations have shown
that additional planets inside the habitable zone of GL 581 would be
dynamically stable. Recently, two further planet candidates have been claimed,
one of them in the habitable zone.
In view of the ongoing search for planets around M stars which is expected to
result in numerous detections of potentially habitable Super-Earths, we take
the GL 581 system as an example to investigate such planets. In contrast to
previous studies of habitability in the GL 581 system, we use a consistent
atmospheric model to assess surface conditions and habitability. Furthermore,
we perform detailed atmospheric simulations for a much larger subset of
potential planetary and atmospheric scenarios than previously considered.
A 1D radiative-convective atmosphere model is used to calculate temperature
and pressure profiles of model atmospheres, which we assumed to be composed of
molecular nitrogen, water, and carbon dioxide. In these calculations, key
parameters such as surface pressure and CO2 concentration as well as orbital
distance and planetary mass are varied.
Results imply that surface temperatures above freezing could be obtained,
independent of the here considered atmospheric scenarios, at an orbital
distance of 0.117 AU. For an orbital distance of 0.146 AU, CO2 concentrations
as low as 10 times the present Earth's value are sufficient to warm the surface
above the freezing point of water. At 0.175 AU, only scenarios with CO2
concentrations of 5% and 95% were found to be habitable. Hence, an additional
Super-Earth planet in the GL 581 system in the previously determined dynamical
stability range would be considered a potentially habitable planet.Comment: 5 pages, 4 figures, accepted in Astronomy&Astrophysic
Clouds in the atmospheres of extrasolar planets. II. Thermal emission spectra of Earth-like planets influenced by low and high-level clouds
We study the impact of multi-layered clouds (low-level water and high-level
ice clouds) on the thermal emission spectra of Earth-like planets orbiting
different types of stars. Clouds have an important influence on such planetary
emission spectra due to their wavelength dependent absorption and scattering
properties. We also investigate the influence of clouds on the ability to
derive information about planetary surface temperatures from low-resolution
spectra.Comment: accepted for publication in A&
The extrasolar planet Gliese 581 d: a potentially habitable planet? (Corrigendum to arXiv:1009.5814)
We report here that the equation for H2O Rayleigh scattering was incorrectly
stated in the original paper [arXiv:1009.5814]. Instead of a quadratic
dependence on refractivity r, we accidentally quoted an r^4 dependence. Since
the correct form of the equation was implemented into the model, scientific
results are not affected.Comment: accepted to Astronomy&Astrophysic
Estimating precipitation on early Mars using a radiative-convective model of the atmosphere and comparison with inferred runoff from geomorphology
We compare estimates of atmospheric precipitation during the Martian
Noachian-Hesperian boundary 3.8 Gyr ago as calculated in a radiative-convective
column model of the atmosphere with runoff values estimated from a
geomorphological analysis of dendritic valley network discharge rates. In the
atmospheric model, we assume CO2-H2O-N2 atmospheres with surface pressures
varying from 20 mb to 3 bar with input solar luminosity reduced to 75% the
modern value.
Results from the valley network analysis are of the order of a few mm d-1
liquid water precipitation (1.5-10.6 mm d-1, with a median of 3.1 mm d-1).
Atmospheric model results are much lower, from about 0.001-1 mm d-1 of snowfall
(depending on CO2 partial pressure). Hence, the atmospheric model predicts a
significantly lower amount of precipitated water than estimated from the
geomorphological analysis. Furthermore, global mean surface temperatures are
below freezing, i.e. runoff is most likely not directly linked to
precipitation. Therefore, our results strongly favor a cold early Mars with
episodic snowmelt as a source for runoff.
Our approach is challenged by mostly unconstrained parameters, e.g.
greenhouse gas abundance, global meteorology (for example, clouds) and
planetary parameters such as obliquity- which affect the atmospheric result -
as as well as by inherent problems in estimating discharge and runoff on
ancient Mars, such as a lack of knowledge on infiltration and evaporation rates
and on flooding timescales, which affect the geomorphological data.
Nevertheless, our work represents a first step in combining and interpreting
quantitative tools applied in early Mars atmospheric and geomorphological
studies.Comment: accepted in Planetary and Space Science, 37 pages, 14 figures, 2
table
N2-associated surface warming on early Mars
Early Mars may have had a warmer and denser atmosphere allowing for the
presence of liquid water on the surface. However, climate model studies have
not been able to reproduce these conditions even with a CO2 atmosphere of
several bars. Recent 3D simulations of the early Mars climate show that mean
surface temperatures only slightly below 273K could be reached locally.
We want to investigate the effect of increased partial pressures of N2 on
early Mars' surface temperature by including pressure broadening of absorption
lines and collision-induced N2-N2 absorption.
A 1D radiative-convective cloud-free atmospheric model was used to calculate
temperature profiles and surface conditions. We performed a parameter study
varying the N2 partial pressures from 0 to 0.5bar at CO2 partial pressures
between 0.02bar and 3bar. These values are consistent with existing estimates
of the initial, pre-Noachian reservoir. Solar insolation was set to be
consistent with the late Noachian.
Our 1D global mean simulations clearly show that enhanced N2 content in the
Martian atmosphere could have increased surface temperatures. An additional
greenhouse warming of up to 13K was found at a high N2 partial pressure of
0.5bar. Still, even at this N2 partial pressure, global mean surface
temperatures remained below 273K, i.e. the freezing point of water. However,
given the magnitude of the N2-induced surface warming and the results of recent
3D studies which show that local mean surface temperatures are not much lower
than 273K, our results imply that the presence of atmospheric N2 could have led
to almost continously habitable mean surface conditions in some regions. In
addition, atmospheric water column amounts increased by up to a factor of 6 in
response to the surface warming, indicating that precipitation might also
increase upon increasing N2 partial pressure.Comment: 6 pages, 3 figures, accepted for publication in Planetary and Space
Scienc
Clouds in the atmospheres of extrasolar planets. I. Climatic effects of multi-layered clouds for Earth-like planets and implications for habitable zones
The effects of multi-layered clouds in the atmospheres of Earth-like planets
orbiting different types of stars are studied. The radiative effects of cloud
particles are directly correlated with their wavelength-dependent optical
properties. Therefore the incident stellar spectra may play an important role
for the climatic effect of clouds. We discuss the influence of clouds with mean
properties measured in the Earth's atmosphere on the surface temperatures and
Bond albedos of Earth-like planets orbiting different types of main sequence
dwarf stars.Comment: accepted for publication in A&
A new analysis of the GJ581 extrasolar planetary system
We have done a new analysis of the available observations for the GJ581
exoplanetary system. Today this system is controversial due to choices that can
be done in the orbital determination. The main ones are the ocurrence of
aliases and the additional bodies - the planets f and g - announced in Vogt et
al. 2010. Any dynamical study of exoplanets requires the good knowledge of the
orbital elements and the investigations involving the planet g are particularly
interesting, since this body would lie in the Habitable Zone (HZ) of the star
GJ581. This region,for this system, is very attractive of the dynamical point
of view due to several resonances of two and three bodies present there. In
this work, we investigate the conditions under which the planet g may exist. We
stress the fact that the planet g is intimately related with the orbital
elements of the planet d; more precisely, we conclude that it is not possible
to disconnect its existence from the determination of the eccentricity of the
planet d. Concerning the planet f, we have found one solution with period
days, but we are judicious about any affirmation concernig this
body because its signal is in the threshold of detection and the high period is
in a spectral region where the ocorruence of aliases is very common. Besides,
we outline some dynamical features of the habitable zone with the dynamical map
and point out the role played by some resonances laying there.Comment: 12 pages, 9 figure
Characterization of CoRoT Target Fields with the Berlin Exoplanet Search Telescope: Identification of Periodic Variable Stars in the LRa1 Field
In this paper, we report on observations of the CoRoT LRa1 field with the
Berlin Exoplanet Search Telescope (BEST). The current paper is part of a series
of papers describing the results of our stellar variability survey. The BEST is
a small aperture telescope with a wide field of view (FOV). It is dedicated to
searching for stellar variability within the target fields of the CoRoT space
mission to aid in minimizing false-alarm rates and identify potential targets
for additional science. The LRa1 field is CoRoT's second long run field located
in the galactic anticenter direction. We observed the LRa1 stellar field on 23
nights between November and March 2005/2006. From 6099 stars marked as
variable, 39 were classified as periodic variable stars and 27 of them are
within the CoRoT FOV. We also confirmed the variability for four stars listed
in the General Catalogue of Variable Stars (GCVS)
The unstable CO2 feedback cycle on ocean planets
Ocean planets are volatile-rich planets, not present in our Solar system, which are thought to be dominated by deep, global oceans. This results in the formation of high-pressure water ice, separating the planetary crust from the liquid ocean and, thus, also from the atmosphere. Therefore, instead of a carbonate-silicate cycle like on the Earth, the atmospheric carbon dioxide concentration is governed by the capability of the ocean to dissolve carbon dioxide (CO2). In our study, we focus on the CO2 cycle between the atmosphere and the ocean which determines the atmospheric CO2 content. The atmospheric amount of CO2 is a fundamental quantity for assessing the potential habitability of the planet's surface because of its strong greenhouse effect, which determines the planetary surface temperature to a large degree. In contrast to the stabilizing carbonate-silicate cycle regulating the long-term CO2 inventory of the Earth atmosphere, we find that the CO2 cycle feedback on ocean planets is negative and has strong destabilizing effects on the planetary climate. By using a chemistry model for oceanic CO2 dissolution and an atmospheric model for exoplanets, we show that the CO2 feedback cycle can severely limit the extension of the habitable zone for ocean planet
Effect of particle size and Debye length on order parameters of colloidal silica suspensions under confinement
Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.Using atomic force microscopy (AFM) and small angle X-ray scattering (SAXS), we show a full comparison between structuring of nanoparticles in confinement and in bulk in order to explain the effect of confinement on characteristic lengths and the scaling law of the characteristic lengths. Three different-sized particle suspensions are used to check the generalization and the correlation between the characteristic lengths and the system parameters, like particle diameter and Debye length. The two characteristic lengths obtained from AFM force curves, the oscillatory wavelength λ, which is related to the average particle distance, and the decay length ξ, which measures how far particle correlates to obtain periodic oscillations, are in good agreement with the mean particle distance 2π/qmax and the correlation length 2/Δq in bulk, respectively, obtained from the structure peaks of SAXS diagrams. Although confinement causes layering of nanoparticles parallel to the confining surfaces, the characteristic lengths in the direction perpendicular to the confining surfaces follow the bulk behavior. The wavelength scales as ρ−1/3 with the particle number density ρ irrespective of the particle size and the ionic strength and shows a pure volume effect. Upon comparing with literature results, the λ = ρ−1/3 scaling law can be applied more generally for charged particles, as long as the repulsive interaction is sufficiently long-ranged, than the previous expression of λ = 2(R + κ−1), which only approaches the value of average particle distance under specific conditions. The decay length ξ is controlled both by the particle size and the ionic strength of the suspensions, and ξ = R + κ−1 is proposed in the paper. In addition, the interaction strength, the force amplitude and maximum scattering intensity, increases linearly with particle concentration. On the other hand, the Monte Carlo (MC) simulations and approximate hypernetted chain (HNC) closure calculation based on Derjaguin-Landau-Verwey-Overbeek (DLVO) potential are employed to study the characteristic lengths from the theoretical point of view. The experimental wavelengths are in good agreement with the theoretical counterparts and the experimental decay lengths show the same qualitative behavior as theoretical ones on the particle size and ionic strength.DFG, SPP 1273, KolloidverfahrenstechnikDFG, GRK 1524, Self-Assembled Soft-Matter Nanostructures at Interface
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