Abstract
Direct-imaging observations of terrestrial exoplanets will enable their atmospheric characterization and habitability assessment. Considering Earth, the key atmospheric signatures for the ...biosphere are O
2
and the photochemical product O
3
. However, this O
2
–O
3
biosignature is not detectable in the visible wavelengths for most of the time after the emergence of oxygenic photosynthesis life (i.e., Proterozoic Earth). Here we demonstrate spectroscopic observations in the ultraviolet wavelengths for detecting and characterizing O
2
and O
3
in Proterozoic-Earth-like planets, using
ExoReL
R
. For an O
2
mixing ratio 2–3 orders of magnitude less than the present-day Earth and an O
3
mixing ratio of 10
−7
to 10
−6
, we find that O
3
can be detected and its mixing ratio can be measured precisely (within 1 order of magnitude) in the ultraviolet (0.25–0.4
μ
m), in addition to visible-wavelength spectroscopy. With modest spectral resolution (
R
= 7) and signal-to-noise ratio (∼10) in the ultraviolet, the O
3
detection is robust against other potential gases absorbing in the ultraviolet (e.g., H
2
S and SO
2
), as well as the short-wavelength cutoff between 0.2 and 0.25
μ
m. While the O
3
detection does not rely on the near-infrared spectra, extending the wavelength coverage to the near-infrared (1–1.8
μ
m) would provide essential information to interpret the O
3
biosignature, including the mixing ratio of H
2
O, the cloud pressure, and the determination of the dominant gas of the atmosphere. The ultraviolet and near-infrared capabilities should thus be evaluated as critical components for future missions aiming at imaging and characterizing terrestrial exoplanets, such as the Habitable Worlds Observatory.
We present a study of the photochemistry of abiotic habitable planets with anoxic CO2-N2 atmospheres. Such worlds are representative of early Earth, Mars, and Venus and analogous exoplanets. ...Photodissociation of H2O controls the atmospheric photochemistry of these worlds through production of reactive OH, which dominates the removal of atmospheric trace gases. The near-UV (NUV; >200 nm) absorption cross sections of H2O play an outsized role in OH production; these cross sections were heretofore unmeasured at habitable temperatures (<373 K). We present the first measurements of NUV H2O absorption at 292 K and show it to absorb orders of magnitude more than previously assumed. To explore the implications of these new cross sections, we employ a photochemical model; we first intercompare it with two others and resolve past literature disagreement. The enhanced OH production due to these higher cross sections leads to efficient recombination of CO and O2, suppressing both by orders of magnitude relative to past predictions and eliminating the low-outgassing "false-positive" scenario for O2 as a biosignature around solar-type stars. Enhanced OH increases rainout of reductants to the surface, relevant to prebiotic chemistry, and may also suppress CH4 and H2; the latter depends on whether burial of reductants is inhibited on the underlying planet, as is argued for abiotic worlds. While we focus on CO2-rich worlds, our results are relevant to anoxic planets in general. Overall, our work advances the state of the art of photochemical models by providing crucial new H2O cross sections and resolving past disagreement in the literature and suggests that detection of spectrally active trace gases like CO in rocky exoplanet atmospheres may be more challenging than previously considered.
ABSTRACT Warm Neptune- and sub-Neptune-sized exoplanets in orbits smaller than Mercury's are thought to have experienced extensive atmospheric evolution. Here we propose that a potential outcome of ...this atmospheric evolution is the formation of helium-dominated atmospheres. The hydrodynamic escape rates of Neptune- and sub-Neptune-sized exoplanets are comparable to the diffusion-limited escape rate of hydrogen, and therefore the escape is heavily affected by diffusive separation between hydrogen and helium. A helium atmosphere can thus be formed-from a primordial hydrogen-helium atmosphere-via atmospheric hydrodynamic escape from the planet. The helium atmosphere has very different abundances of major carbon and oxygen species from those of a hydrogen atmosphere, leading to distinctive transmission and thermal emission spectral features. In particular, the hypothesis of a helium-dominated atmosphere can explain the thermal emission spectrum of GJ 436b, a warm Neptune-sized exoplanet, while also being consistent with the transmission spectrum. This model atmosphere contains trace amounts of hydrogen, carbon, and oxygen, with the predominance of CO over CH4 as the main form of carbon. With our atmospheric evolution model, we find that if the mass of the initial atmosphere envelope is 10−3 planetary mass, hydrodynamic escape can reduce the hydrogen abundance in the atmosphere by several orders of magnitude in ∼10 billion years. Observations of exoplanet transits may thus detect signatures of helium atmospheres and probe the evolutionary history of small exoplanets.
Most known terrestrial planets orbit small stars with radii less than 60 per cent of that of the Sun
. Theoretical models predict that these planets are more vulnerable to atmospheric loss than their ...counterparts orbiting Sun-like stars
. To determine whether a thick atmosphere has survived on a small planet, one approach is to search for signatures of atmospheric heat redistribution in its thermal phase curve
. Previous phase curve observations of the super-Earth 55 Cancri e (1.9 Earth radii) showed that its peak brightness is offset from the substellar point (latitude and longitude of 0 degrees)-possibly indicative of atmospheric circulation
. Here we report a phase curve measurement for the smaller, cooler exoplanet LHS 3844b, a 1.3-Earth-radii world in an 11-hour orbit around the small nearby star LHS 3844. The observed phase variation is symmetric and has a large amplitude, implying a dayside brightness temperature of 1,040 ± 40 kelvin and a nightside temperature consistent with zero kelvin (at one standard deviation). Thick atmospheres with surface pressures above 10 bar are ruled out by the data (at three standard deviations), and less-massive atmospheres are susceptible to erosion by stellar wind. The data are well fitted by a bare-rock model with a low Bond albedo (lower than 0.2 at two standard deviations). These results support theoretical predictions that hot terrestrial planets orbiting small stars may not retain substantial atmospheres.
Abstract
M dwarf stars are known for their vigorous flaring. This flaring could impact the climate of orbiting planets, making it important to characterize M dwarf flares at the short wavelengths ...that drive atmospheric chemistry and escape. We conducted a far-ultraviolet flare survey of six M dwarfs from the recent MUSCLES (Measurements of the Ultraviolet Spectral Characteristics of Low-mass Exoplanetary Systems) observations, as well as four highly active M dwarfs with archival data. When comparing absolute flare energies, we found the active-M-star flares to be about 10× more energetic than inactive-M-star flares. However, when flare energies were normalized by the star’s quiescent flux, the active and inactive samples exhibited identical flare distributions, with a power-law index of
(cumulative distribution). The rate and distribution of flares are such that they could dominate the FUV energy budget of M dwarfs, assuming the same distribution holds to flares as energetic as those cataloged by
Kepler
and ground-based surveys. We used the observed events to create an idealized model flare with realistic spectral and temporal energy budgets to be used in photochemical simulations of exoplanet atmospheres. Applied to our own simulation of direct photolysis by photons alone (no particles), we find that the most energetic observed flares have little effect on an Earth-like atmosphere, photolyzing ∼0.01% of the total O
3
column. The observations were too limited temporally (73 hr cumulative exposure) to catch rare, highly energetic flares. Those that the power-law fit predicts occur monthly would photolyze ∼1% of the O
3
column and those it predicts occur yearly would photolyze the full O
3
column. Whether such energetic flares occur at the rate predicted is an open question.
Over the past decade, observations of giant exoplanets (Jupiter-size) have provided key insights into their atmospheres, but the properties of lower-mass exoplanets (sub-Neptune) remain largely ...unconstrained because of the challenges of observing small planets. Numerous efforts to observe the spectra of super-Earths--exoplanets with masses of one to ten times that of Earth--have so far revealed only featureless spectra. Here we report a longitudinal thermal brightness map of the nearby transiting super-Earth 55 Cancri e (refs 4, 5) revealing highly asymmetric dayside thermal emission and a strong day-night temperature contrast. Dedicated space-based monitoring of the planet in the infrared revealed a modulation of the thermal flux as 55 Cancri e revolves around its star in a tidally locked configuration. These observations reveal a hot spot that is located 41 ± 12 degrees east of the substellar point (the point at which incident light from the star is perpendicular to the surface of the planet). From the orbital phase curve, we also constrain the nightside brightness temperature of the planet to 1,380 ± 400 kelvin and the temperature of the warmest hemisphere (centred on the hot spot) to be about 1,300 kelvin hotter (2,700 ± 270 kelvin) at a wavelength of 4.5 micrometres, which indicates inefficient heat redistribution from the dayside to the nightside. Our observations are consistent with either an optically thick atmosphere with heat recirculation confined to the planetary dayside, or a planet devoid of atmosphere with low-viscosity magma flows at the surface.
Geological evidence shows that ancient Mars had large volumes of liquid water. Models of past hydrogen escape to space, calibrated with observations of the current escape rate, cannot explain the ...present-day deuterium-to-hydrogen isotope ratio (D/H). We simulated volcanic degassing, atmospheric escape, and crustal hydration on Mars, incorporating observational constraints from spacecraft, rovers, and meteorites. We found that ancient water volumes equivalent to a 100 to 1500 meter global layer are simultaneously compatible with the geological evidence, loss rate estimates, and D/H measurements. In our model, the volume of water participating in the hydrological cycle decreased by 40 to 95% over the Noachian period (~3.7 billion to 4.1 billion years ago), reaching present-day values by ~3.0 billion years ago. Between 30 and 99% of martian water was sequestered through crustal hydration, demonstrating that irreversible chemical weathering can increase the aridity of terrestrial planets.
ABSTRACT We present a catalog of panchromatic spectral energy distributions (SEDs) for 7 M and 4 K dwarf stars that span X-ray to infrared wavelengths (5 -5.5 m). These SEDs are composites of Chandra ...or XMM-Newton data from 5-∼50 , a plasma emission model from ∼50-100 , broadband empirical estimates from 100-1170 , Hubble Space Telescope data from 1170-5700 , including a reconstruction of stellar Ly emission at 1215.67 , and a PHOENIX model spectrum from 5700-55000 . Using these SEDs, we computed the photodissociation rates of several molecules prevalent in planetary atmospheres when exposed to each star's unattenuated flux ("unshielded" photodissociation rates) and found that rates differ among stars by over an order of magnitude for most molecules. In general, the same spectral regions drive unshielded photodissociations both for the minimally and maximally FUV active stars. However, for O3 visible flux drives dissociation for the M stars whereas near-UV flux drives dissociation for the K stars. We also searched for an far-UV continuum in the assembled SEDs and detected it in 5/11 stars, where it contributes around 10% of the flux in the range spanned by the continuum bands. An ultraviolet continuum shape is resolved for the star ϵ Eri that shows an edge likely attributable to Si ii recombination. The 11 SEDs presented in this paper, available online through the Mikulski Archive for Space Telescopes, will be valuable for vetting stellar upper-atmosphere emission models and simulating photochemistry in exoplanet atmospheres.
Habitable rocky planets around M dwarfs that have H2-dominated atmospheres, if they exist, would permit characterizing habitable exoplanets with detailed spectroscopy using JWST, owing to their ...extended atmospheres and small stars. However, the H2-dominated atmospheres that are consistent with habitable conditions cannot be too massive, and a moderate-sized H2-dominated atmosphere will lose mass to irradiation-driven atmospheric escape on rocky planets around M dwarfs. We evaluate volcanic outgassing and serpentinization as two potential ways to supply H2 and form a steady-state H2-dominated atmosphere. For rocky planets of 1–7 M⊕ and early-, mid-, and late M-type dwarfs, the expected volcanic outgassing rates from a reduced mantle fall short of the escape rates by > ∼ 1 order of magnitude, and a generous upper limit of the serpentinization rate is still less than the escape rate by a factor of a few. Special mechanisms that may sustain the steady-state H2-dominated atmosphere include direct interaction between liquid water and mantle, heat-pipe volcanism from a reduced mantle, and hydrodynamic escape slowed down by efficient upper-atmospheric cooling. It is thus unlikely to find moderate-size, H2-dominated atmospheres on rocky planets of M dwarfs that would support habitable environments.
Abstract
Exoplanets that receive stellar irradiance approximately equal to Earth’s or less have been discovered and many are suitable for spectral characterization. Here, we focus on the temperate ...planets that have massive H
2
-dominated atmospheres, and trace the chemical reactions and transport following the photodissociation of H
2
O, CH
4
, NH
3
, and H
2
S, with K2-18 b, PH2 b, and Kepler-167 e representing temperate/cold planets around M and G/K stars. We find that NH
3
is likely depleted by photodissociation to the cloud deck on planets around G/K stars but remains intact in the middle atmosphere of planets around M stars. A common phenomenon on temperate planets is that the photodissociation of NH
3
in the presence of CH
4
results in HCN as the main photochemical product. The photodissociation of CH
4
together with H
2
O leads to CO and CO
2
, and the synthesis of hydrocarbon is suppressed. Temperate planets with a supersolar atmospheric metallicity and appreciable internal heat may have additional CO and CO
2
from the interior and less NH
3
, and thus less HCN. Our models of K2-18 b can explain the transmission spectrum measured by the Hubble Space Telescope, and indicate that future observations in 0.5–5.0
μ
m wavelength range would provide the sensitivity to detect the equilibrium gases CH
4
, H
2
O, and NH
3
, the photochemical gas HCN, as well as CO
2
in some cases. Temperate and H
2
-rich exoplanets are thus laboratories of atmospheric chemistry that operate in regimes not found in the solar system, and spectral characterization of these planets in transit or reflected starlight promises to greatly expand the types of molecules detected in exoplanet atmospheres.
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