185 research outputs found
Hydrodynamic simulations of captured protoatmospheres around Earth-like planets
Young terrestrial planets, when they are still embedded in a circumstellar
disk, accumulate an atmosphere of nebula gas. The evolution and eventual
evaporation of the protoplanetary disk affect the structure and dynamics of the
planetary atmosphere. These processes, combined with other mass loss
mechanisms, such as thermal escape driven by extreme ultraviolet and soft X-ray
radiation (XUV) from the young host star, determine how much of the primary
atmosphere, if anything at all, survives into later stages of planetary
evolution. Our aim is to explore the structure and the dynamic outflow
processes of nebula-accreted atmospheres in dependency on changes in the
planetary environment. We integrate stationary hydrostatic models and perform
time-dependent dynamical simulations to investigate the effect of a changing
nebula environment on the atmospheric structure and the timescales on which the
protoatmosphere reacts to these changes. We find that the behavior of the
atmospheres strongly depends on the mass of the planetary core. For planets of
about Mars-mass the atmospheric structure, and in particular the atmospheric
mass, changes drastically and on very short timescales whereas atmospheres
around higher mass planets are much more robust and inert
Heating efficiency in hydrogen-dominated upper atmospheres
Context. The heating efficiency is defined as the ratio of the net local
gas-heating rate to the rate of stellar radiative energy absorption. It plays
an important role in thermal-escape processes from the upper atmospheres of
planets that are exposed to stellar soft X-rays and extreme ultraviolet
radiation (XUV). Aims. We model the thermal-escape-related heating efficiency
of the stellar XUV radiation in the hydrogen-dominated upper atmosphere of the
extrasolar gas giant HD 209458b. The model result is then compared with
previous thermal-hydrogen-escape studies which assumed heating efficiency
values between 10-100%. Methods. The photolytic and electron impact processes
in the thermosphere were studied by solving the kinetic Boltzmann equation and
applying a Direct Simulation Monte Carlo model. We calculated the energy
deposition rates of the stellar XUV flux and that of the accompanying primary
photoelectrons that are caused by electron impact processes in the H2 to H
transition region in the upper atmosphere. Results. The heating by XUV
radiation of hydrogen-dominated upper atmospheres does not reach higher than
20% above the main thermosphere altitude, if the participation of photoelectron
impact processes is included. Conclusions. Hydrogen-escape studies from
exoplanets that assume heating efficiency values that are >= 20 % probably
overestimate the thermal escape or mass-loss rates, while those who assumed
values that are < 20% probably produce more realistic atmospheric-escape rates.Comment: 7 pages, 4 figures, accepted to A&
The Extreme Ultraviolet and X-Ray Sun in Time: High-Energy Evolutionary Tracks of a Solar-Like Star
Aims. We aim to describe the pre-main sequence and main-sequence evolution of
X-ray and extreme-ultaviolet radiation of a solar mass star based on its
rotational evolution starting with a realistic range of initial rotation rates.
Methods. We derive evolutionary tracks of X-ray radiation based on a
rotational evolution model for solar mass stars and the rotation-activity
relation. We compare these tracks to X-ray luminosity distributions of stars in
clusters with different ages.
Results. We find agreement between the evolutionary tracks derived from
rotation and the X-ray luminosity distributions from observations. Depending on
the initial rotation rate, a star might remain at the X-ray saturation level
for very different time periods, approximately from 10 Myr to 300 Myr for slow
and fast rotators, respectively.
Conclusions. Rotational evolution with a spread of initial conditions leads
to a particularly wide distribution of possible X-ray luminosities in the age
range of 20 to 500 Myrs, before rotational convergence and therefore X-ray
luminosity convergence sets in. This age range is crucial for the evolution of
young planetary atmospheres and may thus lead to very different planetary
evolution histories.Comment: 4 pages, 4 figures, accepted for publication in A&
The Role of N2 as a Geo-Biosignature for the Detection and Characterization of Earth-like Habitats
Since the Archean, N2 has been a major atmospheric constituent in Earth's
atmosphere. Nitrogen is an essential element in the building blocks of life,
therefore the geobiological nitrogen cycle is a fundamental factor in the long
term evolution of both Earth and Earth-like exoplanets. We discuss the
development of the Earth's N2 atmosphere since the planet's formation and its
relation with the geobiological cycle. Then we suggest atmospheric evolution
scenarios and their possible interaction with life forms: firstly, for a
stagnant-lid anoxic world, secondly for a tectonically active anoxic world, and
thirdly for an oxidized tectonically active world. Furthermore, we discuss a
possible demise of present Earth's biosphere and its effects on the atmosphere.
Since life forms are the most efficient means for recycling deposited nitrogen
back into the atmosphere nowadays, they sustain its surface partial pressure at
high levels. Also, the simultaneous presence of significant N2 and O2 is
chemically incompatible in an atmosphere over geological timescales. Thus, we
argue that an N2-dominated atmosphere in combination with O2 on Earth-like
planets within circumstellar habitable zones can be considered as a
geo-biosignature. Terrestrial planets with such atmospheres will have an
operating tectonic regime connected with an aerobe biosphere, whereas other
scenarios in most cases end up with a CO2-dominated atmosphere. We conclude
with implications for the search for life on Earth-like exoplanets inside the
habitable zones of M to K-stars
The Upper Atmospheres of Terrestrial Planets: Carbon Dioxide Cooling and the Earth's Thermospheric Evolution
Context: The thermal and chemical structures of the upper atmospheres of
planets crucially influence losses to space and must be understood to constrain
the effects of losses on atmospheric evolution.
Aims: We develop a 1D first-principles hydrodynamic atmosphere model that
calculates atmospheric thermal and chemical structures for arbitrary planetary
parameters, chemical compositions, and stellar inputs. We apply the model to
study the reaction of the Earth's upper atmosphere to large changes in the
CO abundance and to changes in the input solar XUV field due to the Sun's
activity evolution from 3~Gyr in the past to 2.5~Gyr in the future.
Methods: For the thermal atmosphere structure, we consider heating from the
absorption of stellar X-ray, UV, and IR radiation, heating from exothermic
chemical reactions, electron heating from collisions with non-thermal
photoelectrons, Joule heating, cooling from IR emission by several species,
thermal conduction, and energy exchanges between the neutral, ion, and electron
gases. For the chemical structure, we consider 500 chemical reactions,
including 56 photoreactions, eddy and molecular diffusion, and advection. In
addition, we calculate the atmospheric structure by solving the hydrodynamic
equations. To solve the equations in our model, we develop the Kompot code and
provide detailed descriptions of the numerical methods used in the appendices.
Results: We verify our model by calculating the structures of the upper
atmospheres of the modern Earth and Venus. By varying the CO abundances at
the lower boundary (65~km) of our Earth model, we show that the atmospheric
thermal structure is significantly altered. [Abstract Truncated]Comment: 37 pages, 14 figures, to be published in A&
Remote sensing of the Io torus plasma ribbon using natural radio occultation of the Jovian radio emissions
International audienceWe study the Jovian hectometric (HOM) emissions recorded by the RPWS (Radio and Plasma Wave Science) experiment onboard the Cassini spacecraft during its Jupiter flyby. We analyze the attenuation band associated with the intensity extinction of HOM radiation. This phenomenon is interpreted as a refraction effect of the Jovian hectometric emission inside the Io plasma torus. This attenuation band was regularly observed during periods of more than 5 months, from the beginning of October 2000 to the end of March 2001. We estimate for this period the variation of the electron density versus the central meridian longitude (CML). We find a clear local time dependence. Hence the electron density was not higher than 5.0 × 104 cm−3 during 2 months, when the spacecraft approached the planet on the dayside. In the late afternoon and evening sectors, the electron density increases to 1.5 × 105 cm−3 and reach a higher value at some specific occasions. Additionally, we show that ultraviolet and hectometric wavelength observations have common features related to the morphology of the Io plasma torus. The maxima of enhancements/attenuations of UV/HOM observations occur close to the longitudes of the tip of the magnetic dipole in the southern hemisphere (20° CML) and in the northern hemisphere (200° CML), respectively. This is a significant indication about the importance of the Jovian magnetic field as a physical parameter in the coupling process between Jupiter and the Io satellite
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