1,034 research outputs found
Galactic cosmic rays on extrasolar Earth-like planets I. Cosmic ray flux
(abridged abstract) Theoretical arguments indicate that close-in terrestial
exoplanets may have weak magnetic fields, especially in the case of planets
more massive than Earth (super-Earths). Planetary magnetic fields, however,
constitute one of the shielding layers that protect the planet against
cosmic-ray particles. In particular, a weak magnetic field results in a high
flux of Galactic cosmic rays that extends to the top of the planetary
atmosphere. We wish to quantify the flux of Galactic cosmic rays to an
exoplanetary atmosphere as a function of the particle energy and of the
planetary magnetic moment. We numerically analyzed the propagation of Galactic
cosmic-ray particles through planetary magnetospheres. We evaluated the
efficiency of magnetospheric shielding as a function of the particle energy (in
the range 16 MeV E 524 GeV) and as a function of the planetary
magnetic field strength (in the range 0 {M} 10
). Combined with the flux outside the planetary magnetosphere, this
gives the cosmic-ray energy spectrum at the top of the planetary atmosphere as
a function of the planetary magnetic moment. We find that the particle flux to
the planetary atmosphere can be increased by more than three orders of
magnitude in the absence of a protecting magnetic field. For a weakly
magnetized planet (), only particles with energies
below 512 MeV are at least partially shielded. For a planet with a magnetic
moment similar to Earth, this limit increases to 32 GeV, whereas for a strongly
magnetized planet (), partial shielding extends up to 200
GeV. We find that magnetic shielding strongly controls the number of cosmic-ray
particles reaching the planetary atmosphere. The implications of this increased
particle flux are discussed in a companion article.Comment: 10 pages, 9 figures; accepted in A&
Galactic cosmic rays on extrasolar Earth-like planets: II. Atmospheric implications
(abridged abstract) Theoretical arguments indicate that close-in terrestial
exoplanets may have weak magnetic fields. As described in the companion article
(Paper I), a weak magnetic field results in a high flux of galactic cosmic rays
to the top of the planetary atmosphere. We investigate effects that may result
from a high flux of galactic cosmic rays both throughout the atmosphere and at
the planetary surface. Using an air shower approach, we calculate how the
atmospheric chemistry and temperature change under the influence of galactic
cosmic rays for Earth-like (N_2-O_2 dominated) atmospheres. We evaluate the
production and destruction rate of atmospheric biosignature molecules. We
derive planetary emission and transmission spectra to study the influence of
galactic cosmic rays on biosignature detectability. We then calculate the
resulting surface UV flux, the surface particle flux, and the associated
equivalent biological dose rates. We find that up to 20% of stratospheric ozone
is destroyed by cosmic-ray protons. The reduction of the planetary ozone layer
leads to an increase in the weighted surface UV flux by two orders of magnitude
under stellar UV flare conditions. The resulting biological effective dose rate
is, however, too low to strongly affect surface life. We also examine the
surface particle flux: For a planet with a terrestrial atmosphere, a reduction
of the magnetic shielding efficiency can increase the biological radiation dose
rate by a factor of two. For a planet with a weaker atmosphere (with a surface
pressure of 97.8 hPa), the planetary magnetic field has a much stronger
influence on the biological radiation dose, changing it by up to two orders of
magnitude.Comment: 14 pages, 9 figures, published in A&
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
The habitability of a stagnant-lid Earth
Plate tectonics is a fundamental component for the habitability of the Earth.
Yet whether it is a recurrent feature of terrestrial bodies orbiting other
stars or unique to the Earth is unknown. The stagnant lid may rather be the
most common tectonic expression on such bodies. To understand whether a
stagnant-lid planet can be habitable, i.e. host liquid water at its surface, we
model the thermal evolution of the mantle, volcanic outgassing of HO and
CO, and resulting climate of an Earth-like planet lacking plate tectonics.
We used a 1D model of parameterized convection to simulate the evolution of
melt generation and the build-up of an atmosphere of HO and CO over 4.5
Gyr. We then employed a 1D radiative-convective atmosphere model to calculate
the global mean atmospheric temperature and the boundaries of the habitable
zone (HZ). The evolution of the interior is characterized by the initial
production of a large amount of partial melt accompanied by a rapid outgassing
of HO and CO. At 1 au, the obtained temperatures generally allow for
liquid water on the surface nearly over the entire evolution. While the outer
edge of the HZ is mostly influenced by the amount of outgassed CO, the
inner edge presents a more complex behaviour that is dependent on the partial
pressures of both gases. At 1 au, the stagnant-lid planet considered would be
regarded as habitable. The width of the HZ at the end of the evolution, albeit
influenced by the amount of outgassed CO, can vary in a non-monotonic way
depending on the extent of the outgassed HO reservoir. Our results suggest
that stagnant-lid planets can be habitable over geological timescales and that
joint modelling of interior evolution, volcanic outgassing, and accompanying
climate is necessary to robustly characterize planetary habitability
Consistently Simulating a Wide Range of Atmospheric Scenarios for K2-18b with a Flexible Radiative Transfer Module
The atmospheres of small, potentially rocky exoplanets are expected to cover
a diverse range in composition and mass. Studying such objects therefore
requires flexible and wide-ranging modeling capabilities. We present in this
work the essential development steps that lead to our flexible radiative
transfer module, REDFOX, and validate REDFOX for the Solar system planets
Earth, Venus and Mars, as well as for steam atmospheres. REDFOX is a
k-distribution model using the correlated-k approach with random overlap method
for the calculation of opacities used in the -two-stream approximation
for radiative transfer. Opacity contributions from Rayleigh scattering, UV /
visible cross sections and continua can be added selectively. With the improved
capabilities of our new model, we calculate various atmospheric scenarios for
K2-18b, a super-Earth / sub-Neptune with 8 M orbiting in the
temperate zone around an M-star, with recently observed HO spectral
features in the infrared. We model Earth-like, Venus-like, as well as H-He
primary atmospheres of different Solar metallicity and show resulting climates
and spectral characteristics, compared to observed data. Our results suggest
that K2-18b has an H-He atmosphere with limited amounts of HO and
CH. Results do not support the possibility of K2-18b having a water
reservoir directly exposed to the atmosphere, which would reduce atmospheric
scale heights, hence too the amplitudes of spectral features inconsistent with
the observations. We also performed tests for H-He atmospheres up to 50
times Solar metallicity, all compatible with the observations.Comment: 28 pages, 13 figures, accepted for publication in Ap
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&
Blue Dots Team Transits Working Group Review
Transiting planet systems offer an unique opportunity to observationally
constrain proposed models of the interiors (radius, composition) and
atmospheres (chemistry, dynamics) of extrasolar planets. The spectacular
successes of ground-based transit surveys (more than 60 transiting systems
known to-date) and the host of multi-wavelength, spectro-photometric follow-up
studies, carried out in particular by HST and Spitzer, have paved the way to
the next generation of transit search projects, which are currently ongoing
(CoRoT, Kepler), or planned. The possibility of detecting and characterizing
transiting Earth-sized planets in the habitable zone of their parent stars
appears tantalizingly close. In this contribution we briefly review the power
of the transit technique for characterization of extrasolar planets, summarize
the state of the art of both ground-based and space-borne transit search
programs, and illustrate how the science of planetary transits fits within the
Blue Dots perspective.Comment: 9 pages, 3 figures, to be published in the proceedings (ASP Conf.
Ser.) of the "Pathways Towards Habitable Planets" conference, held in
Barcelona (14-18 Sep 2009
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
Tobacco smoking and all-cause mortality in a large Australian cohort study: findings from a mature epidemic with current low smoking prevalence
This study finds that up to two-thirds of deaths in current smokers in Australia can be attributed to smoking.
Abstract
Background
The smoking epidemic in Australia is characterised by historic levels of prolonged smoking, heavy smoking, very high levels of long-term cessation, and low current smoking prevalence, with 13% of adults reporting that they smoked daily in 2013. Large-scale quantitative evidence on the relationship of tobacco smoking to mortality in Australia is not available despite the potential to provide independent international evidence about the contemporary risks of smoking.
Methods
This is a prospective study of 204,953 individuals aged ≥45 years sampled from the general population of New South Wales, Australia, who joined the 45 and Up Study from 2006–2009, with linked questionnaire, hospitalisation, and mortality data to mid-2012 and with no history of cancer (other than melanoma and non-melanoma skin cancer), heart disease, stroke, or thrombosis. Hazard ratios (described here as relative risks, RRs) for all-cause mortality among current and past smokers compared to never-smokers were estimated, adjusting for age, education, income, region of residence, alcohol, and body mass index.
Results
Overall, 5,593 deaths accrued during follow-up (874,120 person-years; mean: 4.26 years); 7.7% of participants were current smokers and 34.1% past smokers at baseline. Compared to never-smokers, the adjusted RR (95% CI) of mortality was 2.96 (2.69–3.25) in current smokers and was similar in men (2.82 (2.49–3.19)) and women (3.08 (2.63–3.60)) and according to birth cohort. Mortality RRs increased with increasing smoking intensity, with around two- and four-fold increases in mortality in current smokers of ≤14 (mean 10/day) and ≥25 cigarettes/day, respectively, compared to never-smokers. Among past smokers, mortality diminished gradually with increasing time since cessation and did not differ significantly from never-smokers in those quitting prior to age 45. Current smokers are estimated to die an average of 10 years earlier than non-smokers.
Conclusions
In Australia, up to two-thirds of deaths in current smokers can be attributed to smoking. Cessation reduces mortality compared with continuing to smoke, with cessation earlier in life resulting in greater reductions
Detailed Spectroscopic and Photometric Analysis of DQ White Dwarfs
We present an analysis of spectroscopic and photometric data for cool DQ
white dwarfs based on improved model atmosphere calculations. In particular, we
revise the atmospheric parameters of the trigonometric parallax sample of
Bergeron et al.(2001), and discuss the astrophysical implications on the
temperature scale and mean mass, as well as the chemical evolution of these
stars. We also analyze 40 new DQ stars discovered in the first data release of
the Sloan Digital Sky Survey.Comment: 6 pages,3 figures, 14th European Workshop on White Dwarfs, ASP
Conference Series, in pres
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