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Register a new userPhosphine as a Biosignature Gas in Exoplanet Atmospheres
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Added by
Clara Sousa-Silva Dr
Publication date
2019
fields
Physics
and research's language is
English
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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.
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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. NH3s 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 Earths CH4 biological production. Volatile amines have similar solubilities and reactivities to NH3 and hence share NH3s 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 NH3s atmospheric production.
Research for possible biosignature gases on habitable exoplanet atmosphere is accelerating. We add isoprene, C5H8, to the roster of biosignature gases. We found that formation of isoprene geochemical formation is highly thermodynamically disfavored and has no known abiotic false positives. The isoprene production rate on Earth rivals that of methane (~ 500 Tg yr-1). On Earth, isoprene is rapidly destroyed by oxygen-containing radicals, but its production is ubiquitous to a diverse array of evolutionarily distant organisms, from bacteria to plants and animals-few, if any at all, volatile secondary metabolite has a larger evolutionary reach. While non-photochemical sinks of isoprene may exist, the destruction of isoprene in an anoxic atmosphere is mainly driven by photochemistry. Motivated by the concept that isoprene might accumulate in anoxic environments, we model the photochemistry and spectroscopic detection of isoprene in habitable temperature, rocky exoplanet anoxic atmospheres with a variety of atmosphere compositions under different host star UV fluxes. Limited by an assumed 10 ppm instrument noise floor, habitable atmosphere characterization using JWST is only achievable with transit signal similar or larger than that for a super-Earth sized exoplanet transiting an M dwarf star with an H2-dominated atmosphere. Unfortunately, isoprene cannot accumulate to detectable abundance without entering a run-away phase, which occurs at a very high production rate, ~ 100 times Earths production rate. In this run-away scenario isoprene will accumulate to > 100 ppm and its spectral features are detectable with ~ 20 JWST transits. One caveat is that some spectral features are hard to be distinguished from that of methane. Despite these challenges, isoprene is worth adding to the menu of potential biosignature gases.
Early Earth may have hosted a biologically-mediated global organic haze during the Archean eon (3.8-2.5 billion years ago). This haze would have significantly impacted multiple aspects of our planet, including its potential for habitability and its spectral appearance. Here, we model worlds with Archean-like levels of carbon dioxide orbiting the ancient sun and an M4V dwarf (GJ 876) and show that organic haze formation requires methane fluxes consistent with estimated Earth-like biological production rates. On planets with high fluxes of biogenic organic sulfur gases (CS2, OCS, CH3SH, and CH3SCH3), photochemistry involving these gases can drive haze formation at lower CH4/CO2 ratios than methane photochemistry alone. For a planet orbiting the sun, at 30x the modern organic sulfur gas flux, haze forms at a CH4/CO2 ratio 20% lower than at 1x the modern organic sulfur flux. For a planet orbiting the M4V star, the impact of organic sulfur gases is more pronounced: at 1x the modern Earth organic sulfur flux, a substantial haze forms at CH4/CO2 ~ 0.2, but at 30x the organic sulfur flux, the CH4/CO2 ratio needed to form haze decreases by a full order of magnitude. Detection of haze at an anomalously low CH4/CO2 ratio could suggest the influence of these biogenic sulfur gases, and therefore imply biological activity on an exoplanet. When these organic sulfur gases are not readily detectable in the spectrum of an Earth-like exoplanet, the thick organic haze they can help produce creates a very strong absorption feature at UV-blue wavelengths detectable in reflected light at a spectral resolution as low as 10. In direct imaging, constraining CH4 and CO2 concentrations will require higher spectral resolution, and R > 170 is needed to accurately resolve the structure of the CO2 feature at 1.57 {mu}m, likely, the most accessible CO2 feature on an Archean-like exoplanet.
Here we review how environmental context can be used to interpret whether O2 is a biosignature in extrasolar planetary observations. This paper builds on the overview of current biosignature research discussed in Schwieterman et al. (2017), and provides an in-depth, interdisciplinary example of biosignature identification and observation that serves as a basis for the development of the general framework for biosignature assessment described in Catling et al., (2017). O2 is a potentially strong biosignature that was originally thought to be an unambiguous indicator for life at high-abundance. We describe the coevolution of life with the early Earths environment, and how the interplay of sources and sinks in the planetary environment may have resulted in suppression of O2 release into the atmosphere for several billion years, a false negative for biologically generated O2. False positives may also be possible, with recent research showing potential mechanisms in exoplanet environments that may generate relatively high abundances of atmospheric O2 without a biosphere being present. These studies suggest that planetary characteristics that may enhance false negatives should be considered when selecting targets for biosignature searches. Similarly our ability to interpret O2 observed in an exoplanetary atmosphere is also crucially dependent on environmental context to rule out false positive mechanisms. We describe future photometric, spectroscopic and time-dependent observations of O2 and the planetary environment that could increase our confidence that any observed O2 is a biosignature, and help discriminate it from potential false positives. By observing and understanding O2 in its planetary context we can increase our confidence in the remote detection of life, and provide a model for biosignature development for other proposed biosignatures.
High-resolution Doppler spectroscopy is a powerful tool for identifying molecular species in the atmospheres of both transiting and non-transiting exoplanets. Currently, such data is analysed using cross-correlation techniques to detect the Doppler shifting signal from the orbiting planet. In this paper we demonstrate that, compared to cross-correlation methods currently used, the technique of Doppler tomography has improved sensitivity in detecting the subtle signatures expected from exoplanet atmospheres. This is partly due to the use of a regularizing statistic, which acts to suppress noise, coupled to the fact that all the data is fit simultaneously. In addition, we show that the technique can also effectively suppress contanimating spectral features that may arise due to overlapping lines, repeating line patterns, or the use of incorrect linelists. These issues can confuse conventional cross-correlation approaches, primarily due to aliasing issues inherent in such techniques, whereas Doppler tomography is less susceptible to such effects. In particular, Doppler tomography shows exceptional promise for simultaneously detecting multiple line species (e.g. isotopologues), even when there are high contrasts between such species -- and far outperforms current CCF analyses in this respect. Finally, we demonstrate that Doppler tomography is capable of recovering molecular signals from exoplanets using real data, by confirming the strong detection of CO in the atmosphere of Tau Boo b. We recover a signal with a planetary radial velocity semi-amplitude Kp = 109.6 +/- 2.2 km/s, in excellent agreement with the previously reported value of 110.0 +/- 3.2 km/s.
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