89 research outputs found
Methane release on Early Mars by atmospheric collapse and atmospheric reinflation
A candidate explanation for Early Mars rivers is atmospheric warming due to
surface release of H or CH gas. However, it remains unknown how much
gas could be released in a single event. We model the CH release by one
mechanism for rapid release of CH from clathrate. By modeling how
CH-clathrate release is affected by changes in Mars' obliquity and
atmospheric composition, we find that a large fraction of total outgassing from
CH clathrate occurs following Mars' first prolonged atmospheric collapse.
This atmosphere-collapse-initiated CH-release mechanism has three stages.
(1) Rapid collapse of Early Mars' carbon dioxide atmosphere initiates a slower
shift of water ice from high ground to the poles. (2) Upon subsequent
CO-atmosphere re-inflation and CO-greenhouse warming, low-latitude
clathrate decomposes and releases methane gas. (3) Methane can then perturb
atmospheric chemistry and surface temperature, until photochemical processes
destroy the methane. Within our model, we find that under some circumstances a
Titan-like haze layer would be expected to form, consistent with transient
deposition of abundant complex abiotic organic matter on the Early Mars
surface. We also find that this CH-release mechanism can warm Early Mars,
but special circumstances are required in order to uncork 10 kg of
CH, the minimum needed for strong warming. Specifically, strong warming
only occurs when the fraction of the hydrate stability zone that is initially
occupied by clathrate exceeds 10%, and when Mars' first prolonged atmospheric
collapse occurs for atmospheric pressure > 1 bar.Comment: Accepted by Planetary and Space Scienc
The CO2 –broadened H2O continuum in the 100–1500 cm -1 region: Measurements, predictions and empirical model
Transmission spectra of H2O + CO2 mixtures have been recorded, at 296, 325 and 366 K, for various pres- sures and mixture compositions using two experimental setups. Their analysis enables to retrieve values of the “continuum”absorption by the CO2 -broadened H2O line wings between 100 and 1500 cm-1 . The results are in good agreement with those, around 1300 cm-1 , of the single previous experimental study available. Comparisons are also made with direct predictions based on line-shape correction factors χ calculated, almost thirty years ago, using a quasistatic approach and an input H2O –CO2 intermolecular potential. They show that this model quite nicely predicts, with slightly overestimated values, the con- tinuum over a spectral range where it varies by more than three orders of magnitude. An empirical cor- rection is proposed, based on the experimental data, which should be useful for radiative transfer and climate studies in CO2 rich planetary atmospheres
3D modelling of the climatic impact of outflow channel formation events on early Mars
Mars was characterized by cataclysmic groundwater-sourced surface flooding that formed large outflow channels and that may have altered the climate for extensive periods during the Hesperian era. In particular, it has been speculated that such events could have induced significant rainfall and caused the formation of late-stage valley networks.
We present the results of 3-D Global Climate Model simulations reproducing the short and long term climatic impact of a wide range of outflow channel formation events under cold ancient Mars conditions. We find that the most intense of these events (volumes of water up to 107km3 and released at temperatures up to 320 Kelvins) cannot trigger long-term greenhouse global warming, regardless of how favorable are the external conditions (e.g. obliquity and seasons). In any case, outflow channel formation events at any atmospheric pressure are unable to produce rainfall or significant snowmelt at latitudes below 40∘N.
On the long term, for an obliquity of ∼45∘ and atmospheric pressures > 80 mbar, we find that the lake ice (formed quickly after the outflow event) is transported progressively southward through the mechanisms of sublimation and adiabatic cooling. At the same time, and as long as the initial water reservoir is not entirely sublimated, ice deposits remain in the West Echus Chasma Plateau region where hints of hydrological activity contemporaneous with outflow channel formation events have been observed. However, because the high albedo of ice drives Mars to even colder temperatures, snowmelt produced by seasonal solar forcing is difficult to attain.Earth and Planetary Science
RISTRETTO: a pathfinder instrument for exoplanet atmosphere characterization
We introduce the RISTRETTO instrument for ESO VLT, an evolution from the
original idea of connecting the SPHERE high-contrast facility to the ESPRESSO
spectrograph (Lovis et al 2017). RISTRETTO is an independent, AO-fed
spectrograph proposed as a visitor instrument, with the goal of detecting
nearby exoplanets in reflected light for the first time. RISTRETTO aims at
characterizing the atmospheres of Proxima b and several other exoplanets using
the technique of high-contrast, high-resolution spectroscopy. The instrument is
composed of two parts: a front-end to be installed on VLT UT4 providing a
two-stage adaptive optics system using the AOF facility with coronagraphic
capability and a 7-fiber IFU, and a diffraction-limited R=135,000 spectrograph
in the 620-840 nm range. We present the requirements and the preliminary design
of the instrument
Sensitive Probing of Exoplanetary Oxygen via Mid Infrared Collisional Absorption
The collision-induced fundamental vibration-rotation band at 6.4 um is the
most significant absorption feature from O2 in the infrared (Timofeyev and
Tonkov, 1978; Rinslandet al., 1982, 1989), yet it has not been previously
incorporated into exoplanet spectral analyses for several reasons. Either CIAs
were not included or incomplete/obsolete CIA databases were used. Also, the
current version of HITRAN does not include CIAs at 6.4 um with other collision
partners (O2-X). We include O2-X CIA features in our transmission spectroscopy
simulations by parameterizing the 6.4 um O2-N2 CIA based on Rinsland et
al.(1989) and the O2-CO2 CIA based on Baranov et al. (2004). Here we report
that the O2-X CIA may be the most detectable O2 feature for transit
observations. For a potentialTRAPPIST-1e analogue system within 5 pc of the
Sun, it could be the only O2 detectable signature with JWST (using MIRI LRS)
for a modern Earth-like cloudy atmosphere with biological quantities of O2.
Also, we show that the 6.4 um O2-X CIA would be prominent for O2-rich
desiccated atmospheres (Luger and Barnes, 2015) and could be detectable with
JWST in just a few transits. For systems beyond 5 pc, this feature could
therefore be a powerful discriminator of uninhabited planets with
non-biological "false positive" O2 in their atmospheres - as they would only be
detectable at those higher O2 pressures.Comment: Published in Nature Astronom
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The CO₂–broadened H₂O continuum in the 100–1500cm-1 region: Measurements, predictions and empirical model
Transmission spectra of H₂O+CO₂ mixtures have been recorded, at 296, 325 and 366 K, for various pressures and mixture compositions using two experimental setups. Their analysis enables to retrieve values of the “continuum” absorption by the CO₂-broadened H2₂O line wings between 100 and 1500 cm-1. The results are in good agreement with those, around 1300 cm-1, of the single previous experimental study available. Comparisons are also made with direct predictions based on line-shape correction factors χ calculated, almost thirty years ago, using a quasistatic approach and an input H₂O—CO₂ intermolecular potential. They show that this model quite nicely predicts, with slightly overestimated values, the continuum over a spectral range where it varies by more than three orders of magnitude. An empirical correction is proposed, based on the experimental data, which should be useful for radiative transfer and climate studies in CO2 rich planetary atmospheres
Water Condensation Zones around Main Sequence Stars
Understanding the set of conditions that allow rocky planets to have liquid
water on their surface -- in the form of lakes, seas or oceans -- is a major
scientific step to determine the fraction of planets potentially suitable for
the emergence and development of life as we know it on Earth. This effort is
also necessary to define and refine the so-called "Habitable Zone" (HZ) in
order to guide the search for exoplanets likely to harbor remotely detectable
life forms. Until now, most numerical climate studies on this topic have
focused on the conditions necessary to maintain oceans, but not to form them in
the first place. Here we use the three-dimensional Generic Planetary Climate
Model (PCM), historically known as the LMD Generic Global Climate Model (GCM),
to simulate water-dominated planetary atmospheres around different types of
Main-Sequence stars. The simulations are designed to reproduce the conditions
of early ocean formation on rocky planets due to the condensation of the
primordial water reservoir at the end of the magma ocean phase. We show that
the incoming stellar radiation (ISR) required to form oceans by condensation is
always drastically lower than that required to vaporize oceans. We introduce a
Water Condensation Limit, which lies at significantly lower ISR than the inner
edge of the HZ calculated with three-dimensional numerical climate simulations.
This difference is due to a behavior change of water clouds, from low-altitude
dayside convective clouds to high-altitude nightside stratospheric clouds.
Finally, we calculated transit spectra, emission spectra and thermal phase
curves of TRAPPIST-1b, c and d with H2O-rich atmospheres, and compared them to
CO2 atmospheres and bare rock simulations. We show using these observables that
JWST has the capability to probe steam atmospheres on low-mass planets, and
could possibly test the existence of nightside water clouds.Comment: Accepted for publication in Astronomy & Astrophysic
Transmission Spectroscopy of the Habitable Zone Exoplanet LHS 1140 b with JWST/NIRISS
LHS 1140 b is the second-closest temperate transiting planet to Earth with an equilibrium temperature low enough to support surface liquid water. At 1.730 ± 0.025 R ⊕, LHS 1140 b falls within the radius valley separating H2-rich mini-Neptunes from rocky super-Earths. Recent mass and radius revisions indicate a bulk density significantly lower than expected for an Earth-like rocky interior, suggesting that LHS 1140 b could be either a mini-Neptune with a small envelope of hydrogen (∼0.1% by mass) or a water world (9%–19% water by mass). Atmospheric characterization through transmission spectroscopy can readily discern between these two scenarios. Here we present two JWST/NIRISS transit observations of LHS 1140 b, one of which captures a serendipitous transit of LHS 1140 c. The combined transmission spectrum of LHS 1140 b shows a telltale spectral signature of unocculted faculae (5.8σ), covering ∼20% of the visible stellar surface. Besides faculae, our spectral retrieval analysis reveals tentative evidence of residual spectral features, best fit by Rayleigh scattering from a N2-dominated atmosphere (2.3σ), irrespective of the consideration of atmospheric hazes. We also show through Global Climate Models (GCMs) that H2-rich atmospheres of various compositions (100×, 300×, 1000× solar metallicity) are ruled out to >10σ. The GCM calculations predict that water clouds form below the transit photosphere, limiting their impact on transmission data. Our observations suggest that LHS 1140 b is either airless or, more likely, surrounded by an atmosphere with a high mean molecular weight. Our tentative evidence of a N2-rich atmosphere provides strong motivation for future transmission spectroscopy observations of LHS 1140 b
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