Potentially habitable planets orbiting M dwarfs are of intense astrobiological interest because they are the only rocky worlds accessible to biosignature search over the next 10+ years because of a ...confluence of observational effects. Simultaneously, recent experimental and theoretical work suggests that UV light may have played a key role in the origin of life on Earth, especially the origin of RNA. Characterizing the UV environment on M-dwarf planets is important for understanding whether life as we know it could emerge on such worlds. In this work, we couple radiative transfer models to observed M-dwarf spectra to determine the UV environment on prebiotic Earth-analog planets orbiting M dwarfs. We calculate dose rates to quantify the impact of different host stars on prebiotically important photoprocesses. We find that M-dwarf planets have access to 100-1000 times less bioactive UV fluence than the young Earth. It is unclear whether UV-sensitive prebiotic chemistry that may have been important to abiogenesis, such as the only known prebiotically plausible pathways for pyrimidine ribonucleotide synthesis, could function on M-dwarf planets. This uncertainty affects objects like the recently discovered habitable-zone planets orbiting Proxima Centauri, TRAPPIST-1, and LHS 1140. Laboratory studies of the sensitivity of putative prebiotic pathways to irradiation level are required to resolve this uncertainty. If steady-state M-dwarf UV output is insufficient to power these pathways, transient elevated UV irradiation due to flares may suffice; laboratory studies can constrain this possibility as well.
Ultraviolet radiation is common to most planetary environments and could play a key role in the chemistry of molecules relevant to abiogenesis (prebiotic chemistry). In this work, we explore the ...impact of UV light on prebiotic chemistry that might occur in liquid water on the surface of a planet with an atmosphere. We consider effects including atmospheric absorption, attenuation by water, and stellar variability to constrain the UV input as a function of wavelength. We conclude that the UV environment would be characterized by broadband input, and wavelengths below 204 nm and 168 nm would be shielded out by atmospheric CO2 and water, respectively. We compare this broadband prebiotic UV input to the narrowband UV sources (e.g., mercury lamps) often used in laboratory studies of prebiotic chemistry and explore the implications for the conclusions drawn from these experiments. We consider as case studies the ribonucleotide synthesis pathway of Powner et al. (2009) and the sugar synthesis pathway of Ritson and Sutherland (2012). Irradiation by narrowband UV light from a mercury lamp formed an integral component of these studies; we quantitatively explore the impact of more realistic UV input on the conclusions that can be drawn from these experiments. Finally, we explore the constraints solar UV input places on the buildup of prebiotically important feedstock gasses like CH4 and HCN. Our results demonstrate the importance of characterizing the wavelength dependence (action spectra) of prebiotic synthesis pathways to determine how pathways derived under laboratory irradiation conditions will function under planetary prebiotic conditions.
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
The search for signs of life beyond Earth is a crucial driving motivation of exoplanet science, fueling new work on biosignature gases in habitable exoplanet atmospheres. We study carbonyls, ...a category of molecules containing the C=O double bond, following our proposal to systematically identify plausible biosignature gas candidates from a list of all small volatile molecules. We rule out carbonyls as biosignature gases due to both their high water solubility and their high photolysis rate, despite their ubiquitous production by life on Earth, their critical importance in Earth’s life biochemistry, and their uniquely identifiable molecular spectral features in mid- to low-resolution spectroscopy. Even in scenarios where life is a large net source of carbonyls, we demonstrate that detection of carbonyls is not possible on even the most ideal habitable exoplanet, because only 10 ppb of carbonyls can accumulate under our most optimistic assumptions. Moreover, high biological fluxes of organic carbon gases, while mathematically possible, are likely biologically unattainable due to the resulting huge waste of carbon—a main building block for life. Our simulations show that photochemical processing of carbonyls leads to generation of CO in quantities that can reengineer the atmosphere, affirming the ambiguity of CO as an antibiosignature. Overall, we find that the expression of a carbonyl-producing biosphere by CO, though potentially detectable by the James Webb Space Telescope, is unable to be uniquely traced back to carbonyls. While carbonyls fail as a bioindicator, by investigating them we have made a significant step toward systematically assessing the biosignature gas potential of all small volatile molecules.
Abstract
Biosignature gas research has been growing in recent years thanks to next-generation space- and ground-based telescopes. Methanol (CH
3
OH) has many advantages as a biosignature gas ...candidate. First, CH
3
OH’s hydroxyl group (OH) has a unique spectral feature not present in other anticipated gases in the atmospheres of rocky exoplanets. Second, there are no significant known abiotic CH
3
OH sources on terrestrial planets in the solar system. Third, life on Earth produces CH
3
OH in large quantities. However, despite CH
3
OH’s advantages, we consider it a poor biosignature gas in the atmospheres of terrestrial exoplanets due to the enormous production flux required to reach its detection limit. CH
3
OH’s high water solubility makes it very difficult to accumulate in the atmosphere. For the highly favorable planetary scenario of an exoplanet with an H
2
-dominated atmosphere orbiting an M5V dwarf star, we find that only when the column-averaged mixing ratio of CH
3
OH reaches at least 10 ppm can we detect it with the James Webb Space Telescope (JWST). The CH
3
OH bioproduction flux required to reach the JWST detection threshold of 10 ppm must be of the order of 10
14
molecules cm
−2
s
−1
, which is roughly three times the annual O
2
production on Earth. Considering that such an enormous flux of CH
3
OH is essentially a massive waste of organic carbon—a major building block of life, we think this flux, while mathematically possible, is likely biologically unattainable. Although CH
3
OH can theoretically accumulate on exoplanets with CO
2
- or N
2
-dominated atmospheres, such planets’ small atmospheric scale heights and weak atmospheric signals put them out of reach for near-term observations.
Abstract
The search for signs of life on other worlds has largely focused on terrestrial planets. Recent work, however, argues that life could exist in the atmospheres of temperate sub-Neptunes. Here ...we evaluate the usefulness of carbon dioxide isotopologues as evidence of aerial life. Carbon isotopes are of particular interest, as metabolic processes preferentially use the lighter
12
C over
13
C. In principle, the upcoming James Webb Space Telescope (JWST) will be able to spectrally resolve the
12
C and
13
C isotopologues of CO
2
, but not CO and CH
4
. We simulated observations of CO
2
isotopologues in the H
2
-dominated atmospheres of our nearest (<40 pc), temperate (equilibrium temperature of 250–350 K) sub-Neptunes with M-dwarf host stars. We find
13
CO
2
and
12
CO
2
distinguishable if the atmosphere is H
2
dominated with a few percentage points of CO
2
for the most idealized target with an Earth-like composition of the two most abundant isotopologues,
12
CO
2
and
13
CO
2
. With a Neptune-like metallicity of 100× solar and a C/O of 0.55, we are unable to distinguish between
13
CO
2
and
12
CO
2
in the atmospheres of temperate sub-Neptunes. If atmospheric composition largely follows metallicity scaling, the concentration of CO
2
in a H
2
-dominated atmosphere will be too low to distinguish CO
2
isotopologues with JWST. In contrast, at higher metallicities, there will be more CO
2
, but the smaller atmospheric scale height makes the measurement impossible. Carbon dioxide isotopologues are unlikely to be useful biosignature gases for the JWST era. Instead, isotopologue measurements should be used to evaluate formation mechanisms of planets and exosystems.
We perform a study of stellar flares for the 24,809 stars observed with 2 minute cadence during the first two months of the TESS mission. Flares may erode exoplanets' atmospheres and impact their ...habitability, but might also trigger the genesis of life around small stars. TESS provides a new sample of bright dwarf stars in our galactic neighborhood, collecting data for thousands of M dwarfs that might host habitable exoplanets. Here, we use an automated search for flares accompanied by visual inspection. Then, our public allesfitter code robustly selects the appropriate model for potentially complex flares via Bayesian evidence. We identify 1228 flaring stars, 673 of which are M dwarfs. Among 8695 flares in total, the largest superflare increased the stellar brightness by a factor of 16.1. Bolometric flare energies range from 1031.0 to 1036.9 erg, with a median of 1033.1 erg. Furthermore, we study the flare rate and energy as a function of stellar type and rotation period. We solidify past findings that fast rotating M dwarfs are the most likely to flare and that their flare amplitude is independent of the rotation period. Finally, we link our results to criteria for prebiotic chemistry, atmospheric loss through coronal mass ejections, and ozone sterilization. Four of our flaring M dwarfs host exoplanet candidates alerted on by TESS, for which we discuss how these effects can impact life. With upcoming TESS data releases, our flare analysis can be expanded to almost all bright small stars, aiding in defining criteria for exoplanet habitability.
A key challenge in origins‐of‐life studies is estimating the abundances of species relevant to the chemical pathways proposed to have contributed to the emergence of life on early Earth. Dissolved ...nitrogen oxide anions (
NOX−), in particular nitrate (
NO3−) and nitrite (
NO2−), have been invoked in diverse origins‐of‐life chemistry, from the oligomerization of RNA to the emergence of protometabolism. Recent work has calculated the supply of
NOX− from the prebiotic atmosphere to the ocean and reported steady state
NOX− to be high across all plausible parameter space. These findings rest on the assumption that
NOX− is stable in natural waters unless processed at a hydrothermal vent. Here, we show that
NOX− is unstable in the reducing environment of early Earth. Sinks due to ultraviolet photolysis and reactions with reduced iron (Fe2+) suppress
NOX− by several orders of magnitude relative to past predictions. For pH = 6.5–8 and T = 0–50 °C, we find that it is most probable that
NOX− <1μM in the prebiotic ocean. On the other hand, prebiotic ponds with favorable drainage characteristics may have sustained
NOX− ≥1μM. As on modern Earth, most
NOX− on prebiotic Earth should have been present as
NO3−, due to its much greater stability. These findings inform the kind of prebiotic chemistries that would have been possible on early Earth. We discuss the implications for proposed prebiotic chemistries and highlight the need for further studies of
NOX− kinetics to reduce the considerable uncertainties in predicting
NOX− on early Earth.
Key Points
Nitrate and nitrite (
NOX−) are relevant to prebiotic chemistry; past work has argued these molecules were abundant in the early ocean
Fe2+ and UV suppress
NOX− to much lower concentrations than previously thought in the ocean;
NOX− could have been higher in ponds
Most
NOX− should have been nitrate; prebiotic chemistries that use nitrate are more plausible than those that use nitrite
Abstract
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.
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