Assessment of Ammonia as a Biosignature Gas in Exoplanet Atmospheres

Abstract: 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.

• The paper demonstrates that biologically produced ammonia can accumulate in exoplanet atmospheres under specific conditions.
• The study employs extensive photochemical models with over 800 reactions to simulate atmospheric compositions and deposition processes.
• The research establishes that a minimum ammonia mixing ratio of 5 ppm is needed for detection, highlighting challenges in matching Earth-like production rates.

Assessment of Ammonia as a Biosignature Gas in Exoplanet Atmospheres

The paper's primary aim is to evaluate the validity of ammonia (NH₃) as a potential biosignature gas in the atmospheres of terrestrial exoplanets. The authors provide a comprehensive analysis of the conditions under which ammonia might be both produced and detectable in sufficient quantities to serve as an indicator of biological activity. Notably, terrestrial planets lack significant abiotic sources of ammonia, which enhances its potential as a biosignature.

Key Insights and Modeling Approach

The researchers emphasize the challenges associated with the accumulation of ammonia in the atmosphere due to its high water solubility and usefulness to biology, which tends to prevent its buildup unless life actively produces it in amounts sufficient to saturate surface sinks. The paper identifies and discusses planetary scenarios favorable to the accumulation of ammonia, especially focusing on planets with hydrogen-dominated atmospheres around M dwarf stars.

Required Conditions for Detectability

1. Minimum Mixing Ratio: The paper asserts that for ammonia to be detectable with instruments like the James Webb Space Telescope (JWST), a minimum column-averaged mixing ratio of about 5 ppm is necessary. This estimation considers JWST's systematic noise floor of approximately 10 ppm.
2. Surface Production Flux: If the planet's surface is unsaturated with ammonia, the required biological surface flux is orders of magnitude greater than Earth's current biological production rates, rendering detectability challenging without considerably high anabolic activities.

Biosignature Potential and Comparative Analysis

The identification of ammonia as a viable biosignature arises from its distinct chemical properties and biological relevance. Comparatively, many other proposed biosignature gases have much lower solubility in water, which affects their atmospheric accumulation under different planetary conditions. Furthermore, the paper extends its findings to volatile amines, which share chemical characteristics with ammonia, thereby suggesting similar implications for their potential as biosignatures.

Atmospheric Modeling

The authors employ detailed photochemical and spectroscopy models to simulate various atmospheric compositions and surface conditions. These models incorporate over 800 chemical reactions to predict ammonia behavior in exoplanetary environments, focusing particularly on:

• Hydrogen, nitrogen, and carbon dioxide-dominated atmospheres.
• Dry and wet deposition processes that influence ammonia concentration.

Practical Implications and Future Prospects

The practical implications of this research bridge observational astronomy and biochemistry, potentially guiding the design of future telescopes and observational strategies aimed at identifying signs of life on exoplanets. Furthermore, by illustrating the atmospheric scenarios where ammonia can accumulate to detectable levels, the paper aids in narrowing down the target planets for follow-up observations.

Speculation on Advancements and Methodologies

The authors acknowledge that while JWST provides significant observational capabilities, the detection of ammonia may benefit from future space-based telescopes with enhanced functionalities or advanced ground-based instruments. Such upgrades could potentially enable the observation of ammonia in non-hydrogen dominated atmospheres or around larger host stars.

Conclusion

In summary, the paper presents a detailed investigation into ammonia's viability as a biosignature gas. It offers essential insights into biogeochemical cycles in extraterrestrial environments and sets the stage for further research and development in exoplanetary science. However, asserting ammonia as a definitive biosignature will necessitate careful interpretation to discern between biological and non-biological origins, especially in light of potential abiotic production in mini-Neptune-like bodies.