arXiv subjects

Tereza Constantinou

Publications and source records attributed to Tereza Constantinou.

10 recordsLinked to original sources

arXiv2026

Detecting habitable exoplanet atmospheres with LIFE, the Large Interferometer for Exoplanets

A key goal of astronomers with the next generation telescopes is to detect signs of life in exoplanet atmospheres. NASA's next flagship is the Habitable Worlds Observatory (HWO). In the context of ESA's Voyage 2050 program, the Senior Committee report prioritises detecting habitable exoplanet atmospheres in the mid-IR. The most suited mission for this is the Large Interferometer for Exoplanets (LIFE) which can detect an even wider range of biosignatures than HWO and at lower concentrations. LIFE is a global science collaboration based out of ETH Zürich. With the UK's expertise in building infrared instruments we could play a leading role in realising an ambitious European-led mission. Notably, LIFE is able to detect necessary planetary context like surface temperature and pressure, along with a key discriminator molecule for biosignature false positives, methane, which will be much harder or impossible with HWO. Also, LIFE will be able to investigate many of the nearby rocky exoplanets known from radial velocity searches that are inaccessible to HWO due to its limited spatial resolution.

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arXiv2026

Equifinality of Venus-like CO$_2$ Atmospheres

While Earth locks much of its carbon in its crust as carbonates, Venus retains a comparable carbon inventory almost entirely in its atmosphere as CO$_2$. On Earth, the geological carbon cycle that has produced this vast crustal carbonate inventory is regulated by biology, liquid water, and plate tectonics, which together have stabilised climate over geological timescales. Venus presently lacks all these processes. We test whether Venus's massive CO$_2$ atmosphere is diagnostic of a specific evolutionary pathway by quantifying three routes: primary magma-ocean outgassing, secondary volcanic degassing in a stagnant-lid regime, and remobilisation of crustal carbonates after climate destabilisation. Using a coupled climate--weathering framework, we find that a past habitable Venus could have stored $\sim$20 bar of CO$_2$ as crustal carbonates. Following transition to runaway conditions, crustal heating releases this reservoir over tens of Myr. In stagnant-lid secondary-degassing models with a MORB-like mantle, outgassing reaches only $\sim$25 bar CO$_2$, limited by progressive mantle volatile depletion. However, Venus-like inventories can be achieved through: (i) magmatic carbon enrichment, (ii) increased magmatic delivery to the surface (high extrusion or melt production), and (iii) the recycling of undegassed carbon back into the planet's interior. Primary magma-ocean outgassing can generate $>10^2$ bar CO$_2$, but the retained fraction after early escape remains uncertain. Ultimately, a Venus-like massive CO$_2$ atmosphere is an equifinal outcome and does not uniquely diagnose a temperate past.

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arXiv2026

Comparative biosignatures with systemic retrievals

The discovery of inhabited exoplanets hinges on identifying biosignature gases. JWST can reveal biosignature gases, though current discoveries have yet to evidence life. The central challenge is attribution: how can we confidently identify biogenic sources while ruling out, or deeming unlikely, abiotic explanations? Attribution is particularly difficult for individual planets, especially given the stochastic abiotic processes that can set atmospheric conditions. To address this, we propose a comparative multi-planet approach centred on systemic retrievals: the analysis of multiple planets within a system to empirically define the `abiotic baseline'. This baseline, constructed from obligate uninhabited planets, serves as a local reference point. Systemic retrievals enable marginalisation over inaccessible, latent, shared abiotic parameters within planet evolution models. This is possible because planets within a system are linked by their birth in the same natal disk, have been irradiated by the same evolving star, and have a linked dynamical history. Observations aligning with the abiotic baseline, where the locally-informed abiotic planet evolution models demonstrate high out-of-sample predictive accuracy, are likely non-biological. Potentially biological anomalies are identified as statistical outliers from the abiotic baseline using Bayesian leave-one-out cross-validation. A comparative biosignature is thus defined: an anomaly where a biotic planetary evolution model provides a superior fit than its abiotic counterpart. Where both abiotic and biotic models yield poor predictive accuracy, the anomaly is flagged as an ``unknown unknown"; a signature of either unconstrained abiotic processes, or life as we don't yet know it.

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arXiv2025

Powerful lightning on Venus constrained by atmospheric NO

Signs of lightning on Venus have long been sought, including by space missions and ground-based telescopes searching for optical flashes, plasma waves, or radio signatures. These efforts have yielded conflicting findings regarding the presence or absence of lightning in Venus's atmosphere. In this study we adopt an indirect approach to constrain the prevalence of lightning on Venus, using the chemical by-products it produces in Venus's atmosphere. Nitric oxide (NO) is a key tracer species of lightning, being exclusively generated by lightning in Venus's lower atmosphere. By calculating the present rate of atmospheric destruction of NO in Venus's atmosphere through photochemical-kinetic modelling, we constrain the lightning power required to sustain the estimated NO abundances on modern Venus. The reported NO constraints require lightning to generate at-least three times the power released on Earth; consistent with either a higher rate of strikes, or greater energy per strike, or a combination of both. Limited detections of optical flashes within the clouds could point to lightning striking deeper in the atmosphere and nearer the surface -- with the result that its optical flashes are obscured by the clouds -- driven by triboelectric charging during volcanic eruptions or wind interactions with surface sediments. Our findings underscore the importance for future missions of confirming lightning on Venus, either by verifying the below-cloud NO abundance, or by detecting another unambiguous lightning signature, to provide the first definitive evidence of lightning on a rocky planet other than Earth.

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arXiv2025

Prediction of sulphate hazes in the lower Venus atmosphere

We study the amount, size distribution and material composition of (sub-)mic aerosol particles in the lower Venus atmosphere < 50 km. Our GGchem phase-equilibrium model predicts metal-chloride and metal-fluoride molecules to be present in the gas over the Venus surface in trace concentrations < 2.E-12, in particular FeCl2, NaCl, KCl and SiF4. Using an improved version of the DiffuDrift model developed by Woitke et al.2020, we find that these molecules deposit to form solid potassium sulphate K2SO4, sodium sulphate Na2SO4, and pyrite FeS2 above about 15.5 km, 9.5 km and 2.4 km, respectively. These heights coincide well with the three potential haze layers found in the Pioneer Venus Large Probe neutral mass spectrometer data by Mogul et al.2023. The particles with radius < 0.3 mic can be dredged up from the ground to reach the sulphuric acid cloud base from below by diffusion. The particle density decreases from ~ 5000/cm3 at ground level to ~100/cm3 at a height of 45 km. Particles larger than about 1 mic are found to stay confined to the ground < 10 km, indicating that the larger, so-called mode 3 particles, if they exist, cannot originate from the surface. All particles are expected to be coated by a thin layer of FeS2, Na2SO4 and K2SO4. We have included the repelling effect of particle charges on the coagulation, without which the model would predict much too steep gradients close to the surface, which is inconsistent with measured opacity data. Our models suggest that the particles must have at least 100 negative charges per micron of particle radius at ground level, and > 50/mic at a height of 45 km.

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arXiv2025

Abiotic Ozone in the Observable Atmospheres of Venus and Venus-like Exoplanets

Ozone is a potential biosignature and disambuguator between Earth-like and Venus-like exoplanets due to its association on Earth with photosynthetically produced oxygen (O$_2$). However, the existence of ozone in Venus's observable atmosphere, a planet with no known life, raises the possibility of ozone biosignature false-positives on Venus-like exoplanets. We use a photochemical model of Venus's atmosphere to investigate the origin of its mesospheric ozone layer, and to predict how similar ozone layers would manifest for Venus-like exoplanets. For Venus, our model shows that the previously proposed fluxes of O atoms produced on the dayside and transported to the nightside cannot generate enough ozone to match the observed nightside ozone concentrations without also producing O$_2$ in excess of the observed upper limit. Nor can sufficient ozone be produced by varying the lower-atmosphere chemistry, atmospheric thermal structure, or received stellar flux in our model of Venus's atmosphere. These results imply that a presently unknown chemical pathway is responsible for the ozone production in Venus's nightside mesosphere. Ozone production rates from this pathway of 10$^5$--10$^7$ cm$^{-3}$s$^{-1}$ above the cloud layer on the nightside can re-produce the observed O$_3$ concentrations. Generalised to Venus-like exoplanets, known chemistry similarly fails to produce ozone in the abundance seen in the Venusian mesosphere. However, until the origin of Venus's ozone is understood, we cannot rule out that ozone production at concentrations observable with JWST will be common on abiotic Venus-like worlds, a possibility that limits the usefulness of ozone as a habsignature and as a biosignature.

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arXiv2024

A dry Venusian interior constrained by atmospheric chemistry

Venus's climatic history provides powerful constraint on the location of the inner-edge of the liquid-water habitable zone. However, two very different histories of water on Venus have been proposed: one where Venus had a temperate climate for billions of years, with surface liquid water, and the other where a hot early Venus was never able to condense surface liquid water. Here we offer a novel constraint on Venus's climate history by inferring the water content of its interior. By calculating the present rate of atmospheric destruction of H$_2$O, CO$_2$ and OCS, which must be restored by volcanism to maintain atmospheric stability, we show Venus's interior is dry. Venusian volcanic gases have at most a 6% water mole fraction, substantially drier than terrestrial magmas degassed at similar conditions. The dry interior is consistent with Venus ending its magma ocean epoch desiccated and thereafter having had a long-lived dry surface. Volcanic resupply to Venus's atmosphere therefore indicates that the planet has never been `liquid-water' habitable.

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arXiv2024

Large Interferometer For Exoplanets (LIFE). XIV. Finding terrestrial protoplanets in the galactic neighborhood

The increased brightness temperature of young rocky protoplanets during their magma ocean epoch makes them potentially amenable to atmospheric characterization to distances from the solar system far greater than thermally equilibrated terrestrial exoplanets, offering observational opportunities for unique insights into the origin of secondary atmospheres and the near surface conditions of prebiotic environments. The Large Interferometer For Exoplanets (LIFE) mission will employ a space-based mid-infrared nulling interferometer to directly measure the thermal emission of terrestrial exoplanets. Here, we seek to assess the capabilities of various instrumental design choices of the LIFE mission concept for the detection of cooling protoplanets with transient high-temperature magma ocean atmospheres, in young stellar associations in particular. Using the LIFE mission instrument simulator (LIFEsim) we assess how specific instrumental parameters and design choices, such as wavelength coverage, aperture diameter, and photon throughput, facilitate or disadvantage the detection of protoplanets. We focus on the observational sensitivities of distance to the observed planetary system, protoplanet brightness temperature using a blackbody assumption, and orbital distance of the potential protoplanets around both G- and M-dwarf stars. Our simulations suggest that LIFE will be able to detect (S/N $\geq$ 7) hot protoplanets in young stellar associations up to distances of $\approx$100 pc from the solar system for reasonable integration times (up to $\sim$hours). Detection of an Earth-sized protoplanet orbiting a solar-sized host star at 1 AU requires less than 30 minutes of integration time. M-dwarfs generally need shorter integration times. The contribution from wavelength regions $<$6 $μ$m is important for decreasing the detection threshold and discriminating emission temperatures.

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arXiv2021

Photochemistry of Venus-Like Planets Orbiting K- and M-Dwarf Stars

Compared to the diversity seen in exoplanets, Venus is a veritable astrophysical twin of the Earth, however its global cloud layer truncates features in transmission spectroscopy, masking its non-Earth-like nature. Observational indicators that can distinguish an exo-Venus from an exo-Earth must therefore survive above the cloud layer. The above-cloud atmosphere is dominated by photochemistry, which depends on the spectrum of the host star and therefore changes between stellar systems. We explore the systematic changes in photochemistry above the clouds of Venus-like exoplanets orbiting K-Dwarf or M-Dwarf host stars, using a recently validated model of the full Venus atmosphere (0-115 km) and stellar spectra from the MUSCLES Treasury survey. SO2, OCS and H2S are key gas species in Venus-like planets that are not present in Earth-like planets, and could therefore act as observational discriminants if their atmospheric abundances are high enough to be detected. We find that SO2, OCS and H2S all survive above the cloud layer when irradiated by the coolest K-Dwarf and all seven M-Dwarfs, whereas these species are heavily photochemically depleted above the clouds of Venus. The production of sulfuric acid molecules that form the cloud layer decreases for decreasing stellar effective temperature. Less steady-state photochemical oxygen and ozone forms with decreasing stellar effective temperature, and the effect of chlorine-catalysed reaction cycles diminish in favour of HOx and SOx catalysed cycles. We conclude that trace sulfur gases will be prime observational indicators of Venus-like exoplanets around M-Dwarf host stars, potentially capable of distinguishing an exo-Venus from an exo-Earth.

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arXiv2021

Hydroxide salts in the clouds of Venus: their effect on the sulfur cycle and cloud droplet pH

The depletion of SO$_2$ and H$_2$O in and above the clouds of Venus (45 -- 65 km) cannot be explained by known gas-phase chemistry and the observed composition of the atmosphere. We apply a full-atmosphere model of Venus to investigate three potential explanations for the SO$_2$ and H$_2$O depletion: (1) varying the below-cloud water vapor (H$_2$O), (2) varying the below-cloud sulfur dioxide (SO$_2$), and (3) the incorporation of chemical reactions inside the sulfuric acid cloud droplets. We find that increasing the below-cloud H$_2$O to explain the SO$_2$ depletion results in a cloud top that is 20 km too high, above-cloud O$_2$ three orders of magnitude greater than observational upper limits and no SO above 80 km. The SO$_2$ depletion can be explained by decreasing the below-cloud SO$_2$ to $20\,{\rm ppm}$. The depletion of SO$_2$ in the clouds can also be explained by the SO$_2$ dissolving into the clouds, if the droplets contain hydroxide salts. These salts buffer the cloud pH. The amount of salts sufficient to explain the SO$_2$ depletion entail a droplet pH of $\sim 1$ at 50 km. Since sulfuric acid is constantly condensing out into the cloud droplets, there must be a continuous and pervasive flux of salts of $\approx 10^{-13} \, {\rm mol \, cm^{-2} \, s^{-1}}$ driving the cloud droplet chemistry. An atmospheric probe can test both of these explanations by measuring the pH of the cloud droplets and the concentrations of gas-phase SO$_2$ below the clouds.

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