Ammonia (NH3) in a terrestrial planet atmosphere is generally a good biosignature gas, primarily because terrestrial planets have no significant known abiotic NH3 source. The conditions required for ...NH3 to accumulate in the atmosphere are, however, stringent. NH3's high water solubility and high bio-useability likely prevent NH3 from accumulating in the atmosphere to detectable levels unless life is a net source of NH3 and produces enough NH3 to saturate the surface sinks. Only then can NH3 accumulate in the atmosphere with a reasonable surface production flux. For the highly favorable planetary scenario of terrestrial planets with H2-dominated atmospheres orbiting M dwarf stars (M5V), we find a minimum of about 5 ppm column-averaged mixing ratio is needed for NH3 to be detectable with JWST, considering a 10 ppm JWST systematic noise floor. When the surface is saturated with NH3 (i.e., there are no NH3-removal reactions on the surface), the required biological surface flux to reach 5 ppm is on the order of 10^10 molecules cm-2 s-1, comparable to the terrestrial biological production of CH4. However, when the surface is unsaturated with NH3, due to additional sinks present on the surface, life would have to produce NH3 at surface flux levels on the order of 10^15 molecules cm-2 s-1 (approx. 4.5x10^6 Tg year-1). This value is roughly 20,000 times greater than the biological production of NH3 on Earth and about 10,000 times greater than Earth's CH4 biological production. Volatile amines have similar solubilities and reactivities to NH3 and hence share NH3's weaknesses and strengths as a biosignature. Finally, to establish NH3 as a biosignature gas, we must rule out mini-Neptunes with deep atmospheres, where temperatures and pressures are high enough for NH3's atmospheric production.
Low-mass stars show evidence of vigorous magnetic activity in the form of large flares and coronal mass ejections. Such space weather events may have important ramifications for the habitability and ...observational fingerprints of exoplanetary atmospheres. Here, using a suite of three-dimensional coupled chemistry-climate model (CCM) simulations, we explore effects of time-dependent stellar activity on rocky planet atmospheres orbiting G-, K-, and M-dwarf stars. We employ observed data from the MUSCLES campaign and Transiting Exoplanet Satellite Survey and test a range of rotation period, magnetic field strength, and flare frequency assumptions. We find that recurring flares drive K- and M-dwarf planet atmospheres into chemical equilibria that substantially deviate from their pre-flare regimes, whereas G-dwarf planet atmospheres quickly return to their baseline states. Interestingly, simulated O\(_2\)-poor and O\(_2\)-rich atmospheres experiencing flares produce similar mesospheric nitric oxide abundances, suggesting that stellar flares can highlight otherwise undetectable chemical species. Applying a radiative transfer model to our CCM results, we find that flare-driven transmission features of bio-indicating species, such as nitrogen dioxide, nitrous oxide, and nitric acid, show particular promise for detection by future instruments.
About 2.5 billion years ago, microbes learned to harness plentiful solar energy to reduce CO\(_2\) with H\(_2\)O, extracting energy and producing O\(_2\) as waste. O\(_2\) production from this ...metabolic process was so vigorous that it saturated its photochemical sinks, permitting it to reach "runaway" conditions and rapidly accumulate in the atmosphere despite its reactivity. Here we argue that O\(_2\) may not be unique: diverse gases produced by life may experience a "runaway" effect similar to O\(_2\). This runaway occurs because the ability of an atmosphere to photochemically cleanse itself of trace gases is generally finite. If produced at rates exceeding this finite limit, even reactive gases can rapidly accumulate to high concentrations and become potentially detectable. Planets orbiting smaller, cooler stars, such as the M dwarfs that are the prime targets for the James Webb Space Telescope (JWST), are especially favorable for runaway due to their lower UV emission compared to higher-mass stars. As an illustrative case study, we show that on a habitable exoplanet with an H\(_2\)-N\(_2\) atmosphere and net surface production of NH\(_3\) orbiting an M dwarf (the "Cold Haber World" scenario), the reactive biogenic gas NH\(_3\) can enter runaway, whereupon an increase in the surface production flux of one order of magnitude can increase NH\(_3\) concentrations by three orders of magnitude and render it detectable by JWST in just two transits. Our work on this and other gases suggests that diverse signs of life on exoplanets may be readily detectable at biochemically plausible production rates.
The potential detection of ppb levels phosphine (PH3) in the clouds of Venus through millimeter-wavelength astronomical observations is extremely surprising as PH3 is an unexpected component of an ...oxidized environment of Venus. A thorough analysis of potential sources suggests that no known process in the consensus model of Venus' atmosphere or geology could produce PH3 at anywhere near the observed abundance. Therefore, if the presence of PH3 in Venus' atmosphere is confirmed, it is highly likely to be the result of a process not previously considered plausible for Venusian conditions. The source of atmospheric PH3 could be unknown geo- or photochemistry, which would imply that the consensus on Venus' chemistry is significantly incomplete. An even more extreme possibility is that strictly aerial microbial biosphere produces PH3. This paper summarizes the Venusian PH3 discovery and the scientific debate that arose since the original candidate detection one year ago.
A long-term goal of exoplanet studies is the identification and detection of biosignature gases. Beyond the most discussed biosignature gas O\(_2\), only a handful of gases have been considered in ...detail. Here we evaluate phosphine (PH\(_3\)). On Earth, PH\(_3\) is associated with anaerobic ecosystems, and as such is a potential biosignature gas on anoxic exoplanets. We simulate CO\(_2-\) and H\(_2-\)dominated habitable terrestrial planet atmospheres. We find that PH\(_3\) can accumulate to detectable concentrations on planets with surface production fluxes of 10\(^{10}\)-10\(^{14}\) cm\(^{-2}\) s\(^{-1}\) (corresponding to surface concentrations of 10s of ppb to 100s of ppm), depending on atmospheric composition and UV flux. While high, such surface fluxes are comparable to the global terrestrial production rate of CH\(_4\) (10\(^{11}\) cm\(^{-2}\) s\(^{-1}\)) and below the maximum local terrestrial PH\(_3\) production rate (10\(^{14}\) cm\(^{-2}\) s\(^{-1}\)). As with other gases, PH\(_3\) can more readily accumulate on low-UV planets, e.g. planets orbiting quiet M-dwarfs or with a photochemical UV shield. If detected, PH\(_3\) is a promising biosignature gas, as it has no known abiotic false positives on terrestrial planets that could generate the high fluxes required for detection. PH\(_3\) also has 3 strong spectral features such that in any atmosphere scenario 1 of the 3 will be unique compared to other dominant spectroscopic molecules. PH\(_3\)'s weakness as a biosignature gas is its high reactivity, requiring high outgassing for detectability. We calculate that 10s of hours of JWST time are required for a potential detection of PH\(_3\). Yet because PH\(_3\) is spectrally active in the same wavelength regions as other atmospherically important molecules (e.g., H\(_2\)O and CH\(_4\)), searches for PH\(_3\) can be carried out at no additional observational cost to searches for other molecules relevant to exoplanet habitability.
The recent candidate detection of ~1 ppb of phosphine in the middle atmosphere of Venus is so unexpected that it requires an exhaustive search for explanations of its origin. Phosphorus-containing ...species have not been modelled for Venus' atmosphere before and our work represents the first attempt to model phosphorus species in the Venusian atmosphere. We thoroughly explore the potential pathways of formation of phosphine in a Venusian environment, including in the planet's atmosphere, cloud and haze layers, surface, and subsurface. We investigate gas reactions, geochemical reactions, photochemistry, and other non-equilibrium processes. None of these potential phosphine production pathways are sufficient to explain the presence of ppb phosphine levels on Venus. If PH3's presence in Venus' atmosphere is confirmed, it therefore is highly likely to be the result of a process not previously considered plausible for Venusian conditions. The process could be unknown geochemistry, photochemistry, or even aerial microbial life, given that on Earth phosphine is exclusively associated with anthropogenic and biological sources. The detection of phosphine adds to the complexity of chemical processes in the Venusian environment and motivates in situ follow up sampling missions to Venus. Our analysis provides a template for investigation of phosphine as a biosignature on other worlds.
We present a study of the eclipses in the accreting white dwarf EX Dra during TESS Cycles 14 and 15. During both of the two outbursts present in this dataset, the eclipses undergo a hysteretic loop ...in eclipse-depth/out-of-eclipse-flux space. In each case, the direction in which the loops are executed strongly suggests an outburst which is triggered near the inner edge of the accretion disk and propagates outwards. This in turn suggests that the outbursts in EX Dra are 'Inside Out' outbursts; events predicted by previous hydrodynamic studies of dwarf nova accretion disks and confirmed spectroscopically in a number of other accreting white dwarf systems. We therefore propose that the direction of the loop executed in eclipse-depth/out-of-eclipse flux space be used as a test to phenomenologically distinguish between 'inside out' and 'outside in' outbursts in other eclipsing dwarf novae; a reliable and purely photometric test to differentiate between these phenomena.
Measurements of trace-gases in planetary atmospheres help us explore chemical conditions different to those on Earth. Our nearest neighbor, Venus, has cloud decks that are temperate but hyper-acidic. ...We report the apparent presence of phosphine (PH3) gas in Venusian atmosphere, where any phosphorus should be in oxidized forms. Single-line millimeter-waveband spectral detections (quality up to ~15 sigma) from the JCMT and ALMA telescopes have no other plausible identification. Atmospheric PH3 at ~20 parts-per-billion abundance is inferred. The presence of phosphine is unexplained after exhaustive study of steady-state chemistry and photochemical pathways, with no currently-known abiotic production routes in Venusian atmosphere, clouds, surface and subsurface, or from lightning, volcanic or meteoritic delivery. Phosphine could originate from unknown photochemistry or geochemistry, or, by analogy with biological production of phosphine on Earth, from the presence of life. Other PH3 spectral features should be sought, while in-situ cloud and surface sampling could examine sources of this gas.
New sets of young M dwarfs with complex, sharp-peaked, and strictly periodic photometric modulations have recently been discovered with Kepler/K2 (scallop shells) and TESS (complex rotators). All are ...part of star-forming associations, are distinct from other variable stars, and likely belong to a unified class. Suggested hypotheses include star spots, accreting dust disks, co-rotating clouds of material, magnetically constrained material, spots and misaligned disks, and pulsations. Here, we provide a comprehensive overview and add new observational constraints with TESS and SPECULOOS Southern Observatory (SSO) photometry. We scrutinize all hypotheses from three new angles: (1) we investigate each scenario's occurrence rates via young star catalogs; (2) we study the features' longevity using over one year of combined data; and (3) we probe the expected color dependency with multi-color photometry. In this process, we also revisit the stellar parameters accounting for activity effects, study stellar flares as activity indicators over year-long time scales, and develop toy models to simulate typical morphologies. We rule out most hypotheses, and only (i) co-rotating material clouds and (ii) spots and misaligned disks remain feasible - with caveats. For (i), co-rotating dust might not be stable enough, while co-rotating gas alone likely cannot cause percentage-scale features; and (ii) would require misaligned disks around most young M dwarfs. We thus suggest a unified hypothesis, a superposition of large-amplitude spot modulations and sharp transits of co-rotating gas clouds. While the complex rotators' mystery remains, these new observations add valuable pieces to the puzzle going forward.
