860 research outputs found
Atmospheric effects of stellar cosmic rays on Earth-like exoplanets orbiting M-dwarfs
M-dwarf stars are generally considered favourable for rocky planet detection.
However, such planets may be subject to extreme conditions due to possible high
stellar activity. The goal of this work is to determine the potential effect of
stellar cosmic rays on key atmospheric species of Earth-like planets orbiting
in the habitable zone of M-dwarf stars and show corresponding changes in the
planetary spectra. We build upon the cosmic rays model scheme of Grenfell et
al. (2012), who considered cosmic ray induced NOx production, by adding further
cosmic ray induced production mechanisms (e.g. for HOx) and introducing primary
protons of a wider energy range (16 MeV - 0.5 TeV). Previous studies suggested
that planets in the habitable zone that are subject to strong flaring
conditions have high atmospheric methane concentrations, while their ozone
biosignature is completely destroyed. Our current study shows, however, that
adding cosmic ray induced HOx production can cause a decrease in atmospheric
methane abundance of up to 80\%. Furthermore, the cosmic ray induced HOx
molecules react with NOx to produce HNO, which produces strong HNO
signals in the theoretical spectra and reduces NOx-induced catalytic
destruction of ozone so that more than 25\% of the ozone column remains. Hence,
an ozone signal remains visible in the theoretical spectrum (albeit with a
weaker intensity) when incorporating the new cosmic ray induced NOx and HOx
schemes, even for a constantly flaring M-star case. We also find that HNO
levels may be high enough to be potentially detectable. Since ozone
concentrations, which act as the key shield against harmful UV radiation, are
affected by cosmic rays via NOx-induced catalytic destruction of ozone, the
impact of stellar cosmic rays on surface UV fluxes is also studied.Comment: 14 pages, 12 figure
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&
New Insights into Cosmic Ray induced Biosignature Chemistry in Earth-like Atmospheres
With the recent discoveries of terrestrial planets around active M-dwarfs,
destruction processes masking the possible presence of life are receiving
increased attention in the exoplanet community. We investigate potential
biosignatures of planets having Earth-like (N-O) atmospheres orbiting
in the habitable zone of the M-dwarf star AD Leo. These are bombarded by high
energetic particles which can create showers of secondary particles at the
surface. We apply our cloud-free 1D climate-chemistry model to study the
influence of key particle shower parameters and chemical efficiencies of NOx
and HOx production from cosmic rays. We determine the effect of stellar
radiation and cosmic rays upon atmospheric composition, temperature, and
spectral appearance. Despite strong stratospheric O destruction by cosmic
rays, smog O can significantly build up in the lower atmosphere of our
modeled planet around AD Leo related to low stellar UVB. NO abundances
decrease with increasing flaring energies but a sink reaction for NO with
excited oxygen becomes weaker, stabilizing its abundance. CH is removed
mainly by Cl in the upper atmosphere for strong flaring cases and not via
hydroxyl as is otherwise usually the case. Cosmic rays weaken the role of
CH in heating the middle atmosphere so that HO absorption becomes more
important. We additionally underline the importance of HNO as a possible
marker for strong stellar particle showers. In a nutshell, uncertainty in NOx
and HOx production from cosmic rays significantly influences biosignature
abundances and spectral appearance.Comment: Manuscript version after addressing all referee comments. Published
in Ap
INCREASE: An updated model suite to study the INfluence of Cosmic Rays on Exoplanetary AtmoSpherEs
Exoplanets are as diverse as they are fascinating. They vary from ultrahot Jupiter-like low-density planets to presumed gas-ice-rock mixture worlds such as GJ 1214b or worlds as LHS 1140b, which features twice the Earth\u27s bulk density. Regarding the great diversity of exoplanetary atmospheres, much remains to be explored. For a few selected objects such as GJ1214b, Proxima Centauri b, and the TRAPPIST-1 planets, the first observations of their atmospheres have already been achieved or are expected in the near future with the launch of the James Webb Space Telescope envisaged in October 2021. However, in order to interpret these observations, model studies of planetary atmospheres that account for various processes—such as atmospheric escape, outgassing, climate, photochemistry, as well as the physics of air showers and the transport of stellar energetic particles and galactic cosmic rays through the stellar astrospheres and planetary magnetic fields—are necessary. Here, we present our model suite INCREASE, a planned extension of the model suite discussed in Herbst, Grenfell, et al. (2019)
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
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
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