In order to understand the early history of telluric interiors and atmospheres during the ocean magma stage, a coupled interior‐atmosphere‐escape model is being developed. This paper describes the ...atmospheric part and its first preliminary results. A unidimensional, radiative‐convective, H2O‐CO2 atmosphere is modeled following a vertical T(z) profile similar to Kasting (1988) and Abe and Matsui (1988). Opacities in the thermal IR are then computed using a k‐correlated code (KSPECTRUM), tabulated continuum opacities for H2O‐H2O and CO2‐CO2 absorption, and water or sulphuric acid clouds in the moist convective zone (whenever present). The first results show the existence of two regimes depending on the relative value of the surface temperature Ts compared to a threshold temperature Tc depending on the total gaseous inventory. For Ts < Tc, efficient blanketing results in a cool upper atmosphere, a cloud cover, and a long lifetime for the underneath magma ocean with a net thermal IR flux between 200 and 280 Wm−2. For Ts > Tc, the blanketing is not efficient enough to prevent large radiative heat loss to space through a hot, cloudless atmosphere. Our current calculations may underestimate the thermal flux in the case of hot surfaces with little gaseous content in the atmosphere.
Key Points
Atmospheric blanketing above magma oceans rules their cooling time
Blanketing efficiency depends on surface temperature and atmospheric content
This simple model is perfectly suited for integration into coupled models
Context.
The TRAPPIST-1 planetary system is favourable for transmission spectroscopy and offers the unique opportunity to study rocky planets with possibly non-primary envelopes. We present here the ...transmission spectrum of the seventh planet of the TRAPPIST-1 system, TRAPPIST-1 h (
R
P
= 0.752
R
⊕
,
T
eq
= 173 K) using
Hubble
Space Telescope (HST), Wide Field Camera 3 Grism 141 (WFC3/G141) data.
Aims.
Our purpose is to reduce the HST observations of the seventh planet of the TRAPPIST-1 system and, by testing a simple atmospheric hypothesis, to put a new constraint on the composition and the nature of the planet.
Methods.
First we extracted and corrected the raw data to obtain a transmission spectrum in the near-infrared (NIR) band (1.1–1.7 μm). TRAPPIST-1 is a cold M-dwarf and its activity could affect the transmission spectrum. We corrected for stellar modulations using three different stellar contamination models; while some fit the data better, they are statistically not significant and the conclusion remains unchanged concerning the presence or lack thereof of an atmosphere. Finally, using a Bayesian atmospheric retrieval code, we put new constraints on the atmosphere composition of TRAPPIST-1h.
Results.
According to the retrieval analysis, there is no evidence of molecular absorption in the NIR spectrum. This suggests the presence of a high cloud deck or a layer of photochemical hazes in either a primary atmosphere or a secondary atmosphere dominated by heavy species such as nitrogen. This result could even be the consequence of the lack of an atmosphere as the spectrum is better fitted using a flat line. Variations in the transit depth around 1.3 μm are likely due to remaining scattering noise and the results do not improve while changing the spectral resolution. TRAPPIST-1 h has probably lost its atmosphere or possesses a layer of clouds and hazes blocking the NIR signal. We cannot yet distinguish between a primary cloudy or a secondary clear envelope using HST/WFC3 data; however, in most cases with more than 3
σ
confidence, we can reject the hypothesis of a clear atmosphere dominated by hydrogen and helium. By testing the forced secondary atmospheric scenario, we find that a CO-rich atmosphere (i.e. with a volume mixing ratio of 0.2) is one of the best fits to the spectrum with a Bayes factor of 1.01, corresponding to a 2.1
σ
detection.
Context.
LHS 1140 is an M dwarf known to host two transiting planets at orbital periods of 3.77 and 24.7 days. They were detected with HARPS and
Spitzer
. The external planet (LHS 1140 b) is a rocky ...super-Earth that is located in the middle of the habitable zone of this low-mass star. All these properties place this system at the forefront of the habitable exoplanet exploration, and it therefore constitutes a relevant case for further astrobiological studies, including atmospheric observations.
Aims.
We further characterize this system by improving the physical and orbital properties of the known planets, search for additional planetary-mass components in the system, and explore the possibility of co-orbitals.
Methods.
We collected 113 new high-precision radial velocity observations with ESPRESSO over a 1.5-yr time span with an average photon-noise precision of 1.07 m s
−1
. We performed an extensive analysis of the HARPS and ESPRESSO datasets and also analyzed them together with the new TESS photometry. We analyzed the Bayesian evidence of several models with different numbers of planets and orbital configurations.
Results.
We significantly improve our knowledge of the properties of the known planets LHS 1140 b (
P
b
~ 24.7 days) and LHS 1140 c (
P
c
~ 3.77 days). We determine new masses with a precision of 6% for LHS 1140 b (6.48 ± 0.46
M
⊕
) and 9% for LHS 1140 c (
m
c
= 1.78 ± 0.17
M
⊕
). This reduces the uncertainties relative to previously published values by half. Although both planets have Earth-like bulk compositions, the internal structure analysis suggests that LHS 1140 b might be iron-enriched and LHS 1140 c might be a true Earth twin. In both cases, the water content is compatible to a maximum fraction of 10–12% in mass, which is equivalent to a deep ocean layer of 779 ± 650 km for the habitable-zone planet LHS 1140 b. Our results also provide evidence for a new planet candidate in the system (
m
d
= 4.8 ± 1.1
M
⊕
) on a 78.9-day orbital period, which is detected through three independent methods. The analysis also allows us to discard other planets above 0.5
M
⊕
for periods shorter than 10 days and above 2
M
⊕
for periods up to one year. Finally, our co-orbital analysis discards co-orbital planets in the tadpole and horseshoe configurations of LHS 1140 b down to 1
M
⊕
with a 95% confidence level (twice better than with the previous HARPS dataset). Indications for a possible co-orbital signal in LHS 1140 c are detected in both radial velocity (alternatively explained by a high eccentricity) and photometric data (alternatively explained by systematics), however.
Conclusions.
The new precise measurements of the planet properties of the two transiting planets in LHS 1140 as well as the detection of the planet candidate LHS 1140 d make this system a key target for atmospheric studies of rocky worlds at different stellar irradiations.
The thermal evolution of magma oceans produced by collision with giant impactors late in accretion is expected to depend on the composition and structure of the atmosphere through the greenhouse ...effect of CO2 and H2O released from the magma during its crystallization. In order to constrain the various cooling timescales of the system, we developed a 1‐D parameterized convection model of a magma ocean coupled with a 1‐D radiative‐convective model of the atmosphere. We conducted a parametric study and described the influences of the initial volatile inventories, the initial depth of the magma ocean, and the Sun‐planet distance. Our results suggest that a steam atmosphere delays the end of the magma ocean phase by typically 1 Myr. Water vapor condenses to an ocean after 0.1, 1.5, and 10 Myr for, respectively, Mars, Earth, and Venus. This time would be virtually infinite for an Earth‐sized planet located at less than 0.66 AU from the Sun. Using a more accurate calculation of opacities, we show that Venus is much closer to this threshold distance than in previous models. So there are conditions such as no water ocean is formed on Venus. Moreover, for Mars and Earth, water ocean formation timescales are shorter than typical time gaps between major impacts. This implies that successive water oceans may have developed during accretion, making easier the loss of their atmospheres by impact erosion. On the other hand, Venus could have remained in the magma ocean stage for most of its accretion.
Key PointsWe developed a parameterized magma ocean/atmospheric evolution modelWe show the critical influence of an atmosphere on primitive plates formationWe investigate the conditions for appearance of a liquid water ocean
Aims. Following the announcement of the detection of phosphine (PH3) in the cloud deck of Venus at millimeter wavelengths, we have searched for other possible signatures of this molecule in the ...infrared range.Methods. Since 2012, we have been observing Venus in the thermal infrared at various wavelengths to monitor the behavior of SO2 and H2O at the cloud top. We have identified a spectral interval recorded in March 2015 around 950 cm−1 where a PH3 transition is present.Results. From the absence of any feature at this frequency, we derive, on the disk-integrated spectrum, a 3-σ upper limit of 5 ppbv for the PH3 mixing ratio, assumed to be constant throughout the atmosphere. This limit is 4 times lower than the disk-integrated mixing ratio derived at millimeter wavelengths.Conclusions. Our result brings a strong constraint on the maximum PH3 abundance at the cloud top and in the lower mesosphere of Venus.
•H2O mixing ratio and the cloud top altitudes were measured by SPICAV IR on VEx over 8.5 years.•The average H2O mixing ratio equals 5–7ppm at effective altitudes of 60–62km.•Two maxima in the ...latitudinal distribution of H2O were observed: near equator and near the pole.•A asymmetry of H2O longitudinal distribution has been observed in low latitudes.•No prominent long-term nor local time variations of H2O and the cloud tops were detected.
SPICAV VIS-IR spectrometer on-board the Venus Express mission measured the H2O abundance above Venus’ clouds in the 1.38µm band, and provided an estimation of the cloud top altitude based on CO2 bands in the range of 1.4–1.6µm. The H2O content and the cloud top altitude have been retrieved for the complete Venus Express dataset from 2006 to 2014 taking into account multiple scattering in the cloudy atmosphere. The cloud top altitude, corresponding to unit nadir aerosol optical depth at 1.48µm, varies from 68 to 73km at latitudes from 40ºS to 40ºN with an average of 70.2±0.8km assuming the aerosol scale height of 4km. In high northern latitudes, the cloud top decreases to 62–68km. The altitude of formation of water lines ranges from 59 to 66km. The H2O mixing ratio at low latitudes (20ºS-20ºN) is equal to 6.1±1.2ppm with variations from 4 to 11ppm and the effective altitude of 61.9±0.5km. Between 30º and 50º of latitude in both hemispheres, a local minimum was observed with a value of 5.4±1ppm corresponding to the effective altitude of 62.1±0.6km and variations from 3 to 8ppm. At high latitudes in both hemispheres, the water content varies from 4 to 12ppm with an average of 7.2±1.4ppm which corresponds to 60.6±0.5km. Observed variations of water vapor within a factor of 2-3 on the short timescale appreciably exceed individual measurement errors and could be explained as a real variation of the mixing ratio or/and possible variations of the cloud opacity within the clouds. The maximum of water at lower latitudes supports a possible convection and injection of water from lower atmospheric layers. The vertical gradient of water vapor inside the clouds explains well the increase of water near the poles correlating with the decrease of the cloud top altitude and the H2O effective altitude. On the contrary, the depletion of water in middle latitudes does not correlate with the H2O effective altitude and cannot be completely explained by the vertical gradient of water vapor within the clouds. Retrieved H2O mixing ratio is higher than those obtained in 2.56µm from VIRTIS-H data (Cottini et al., 2015 Planet. Space Sci., 113, 219–225 ) at altitudes of 68–70km which is well consistent with the lower altitudes of water mixing ratio from the 1.38µm band. Observations for different solar and emission angles allowed to constrain also the average vertical distribution of H2O mixing ratio in the clouds with 2ppm at 66km and 7–7.5ppm at 59–61km. The water vapor latitudinal-longitudinal distribution does not show any direct correlation with the cloud tops. Yet a strong asymmetry of H2O longitudinal distribution has been observed with a maximum of 7–7.5ppm from −120º to 30º of longitude and shifted to the southern hemisphere (20ºS-10ºN). To the east, the minimum is observed with values not in excess of 6ppm and over a wide range of longitudes from 30º to 160º. Bertaux et al. (2015) announced a correlation between the zonal wind pattern in the equatorial region and underlying topography of Aphrodite Terra as the result of stationary gravity waves produced at the ground level near the mountains. The water minimum corresponds to the Aphrodite Terra highlands and can be also associated with the influence of Venus topography. No prominent long-term on the time scale of 8.5 years nor local time variations of water vapor and the cloud top altitude were detected.
How the volatile content influences the primordial surface conditions of terrestrial planets and, thus, their future geodynamic evolution is an important question to answer. We simulate the secular ...convective cooling of a 1‐D magma ocean (MO) in interaction with its outgassed atmosphere. The heat transfer in the atmosphere is computed either using the grey approximation or using a k‐correlated method. We vary the initial CO2 and H2O contents (respectively from 0.1 × 10−2 to 14 × 10−2 wt % and from 0.03 to 1.4 times the Earth Ocean current mass) and the solar distance—from 0.63 to 1.30 AU. A first rapid cooling stage, where efficient MO cooling and degassing take place, producing the atmosphere, is followed by a second quasi steady state where the heat flux balance is dominated by the solar flux. The end of the rapid cooling stage (ERCS) is reached when the mantle heat flux becomes negligible compared to the absorbed solar flux. The resulting surface conditions at ERCS, including water ocean's formation, strongly depend both on the initial volatile content and solar distance D. For D > DC, the “critical distance,” the volatile content controls water condensation and a new scaling law is derived for the water condensation limit. Although today's Venus is located beyond DC due to its high albedo, its high CO2/H2O ratio prevents any water ocean formation. Depending on the formation time of its cloud cover and resulting albedo, only 0.3 Earth ocean mass might be sufficient to form a water ocean on early Venus.
Plain Language Summary
Early in their history, Earth‐like planets are impacted by small rocky bodies, and the energy brought by the impactors heats the planet. Giant impactors can even remove the atmosphere and melt a large and deep fraction of the planet, leading to the formation of an “ocean” of molten rocks. From this initial stage, cooling and solidification proceed, expelling volatiles to rebuild an atmosphere. Varying the initial CO2 and H2O contents for planets located at different distances from the star, we study their influence on the planet evolution and on the surface temperature and pressure. These will condition the formation of a water ocean and the tectonic regime of the solid‐state planet. From our calculations, we derived simple relations to forecast water ocean formation. They suggest that a water ocean might have formed on Venus early in its history.
Key Points
Magma ocean and atmospheric coupled modeling during the first million years
Critical distance for water ocean formation obeys simple scaling laws
Venus might have condensed a water ocean during its history
Abstract
Short-period, low-mass water-rich planets are subject to strong irradiation from their host star, resulting in hydrospheres in a supercritical state. In this context, we explore the role of ...irradiation on small terrestrial planets that are moderately wet in the low-mass regime (0.2–1
M
⊕
). We investigate their bulk properties for water content in the 0.01–5% range by making use of an internal structure model that is coupled to an atmosphere model. This coupling allows us to take into account both the compression of the interior due to the weight of the hydrosphere and the possibility of atmospheric instability in the low-mass regime. We show that, even for low masses and low water content, these planets display inflated atmospheres. For extremely low planetary masses and high irradiation temperatures, we find that steam atmospheres become gravitationally unstable when the ratio
η
of their scale height to planetary radius exceeds a critical value of ∼0.1. This result is supported by observational data, as all currently detected exoplanets exhibit values of
η
smaller than 0.013. Depending on their water content, our results show that highly irradiated, low-mass planets up to 0.9
M
⊕
with significative hydrospheres are not in a stable form and should lose their volatile envelope.
This paper presents an updated version of the simple 1‐D radiative‐convective H2O‐CO2 atmospheric model from Marcq (2012) and used by Lebrun et al. (2013) in their coupled interior‐atmosphere model. ...This updated version includes a correction of a major miscalculation of the outgoing longwave radiation (OLR) and extends the validity of the model (P coordinate system, possible inclusion of N2, and improved numerical stability). It confirms the qualitative findings of Marcq (2012), namely, (1) the existence of a blanketing effect in any H2O‐dominated atmosphere: the outgoing longwave radiation (OLR) reaches an asymptotic value, also known as Nakajima's limit and first evidenced by Nakajima et al. (1992), around 280 W/m2 neglecting clouds, significantly higher than our former estimate from Marcq (2012). (2) The blanketing effect breaks down for a given threshold temperature Tϵ, with a fast increase of OLR with increasing surface temperature beyond this threshold, making extrasolar planets in such an early stage of their evolution easily detectable near 4 μm provided they orbit a red dwarf. Tϵ increases strongly with H2O surface pressure, but increasing CO2 pressure leads to a slight decrease of Tϵ. (3) Clouds act both by lowering Nakajima's limit by up to 40% and by extending the blanketing effect, raising the threshold temperature Tϵ by about 10%.
Plain Language Summary
Recently formed Earth‐sized planets experience a “magma ocean” stage, where molten rocks extend from the core up to the surface. These planets are able to cool themselves by radiating more heat through their thick atmospheres than they absorb from their parent star. We have investigated the effect of the total atmospheric content (assumed to consist mostly of water vapor and carbon dioxide) and of the surface temperature of the magma ocean upon the rapidity of the cooling. Our main finding is that there are two stages: for very high surface temperatures, cooling is fast, and only thin clouds can form. Such planets would be quite easily detected since they radiate very efficiently in the infrared range. Conversely, relatively cool surface temperatures lead to cooler upper atmospheres, harboring thick water clouds. Such planets would be very difficult to distinguish from more mature planets such as Earth or Venus from the point of view of a remote observer.
Key Points
Thermal blanketing confirmed at almost equal to 280 W/m2, for steam‐dominated atmospheres (Nakajima's limit) around magma ocean planets
Thermal blanketing effective only for surface temperatures lower than a threshold, depending mainly upon the atmospheric water content
Detectability prospects for hot extrasolar magma ocean planets orbiting red dwarfs are optimal (contrast over 10%) near 4 micrometer
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