ABSTRACT Several small dense exoplanets are now known, inviting comparisons to Earth and Venus. Such comparisons require translating their masses and sizes to composition models of evolved ...multi-layer interior planets. Such theoretical models rely on our understanding of the Earth's interior, as well as independently derived equations of state, but so far have not involved direct extrapolations from Earth's seismic model: the Preliminary Reference Earth Model (PREM). To facilitate more detailed compositional comparisons between small exoplanets and the Earth, we derive here a semi-empirical mass-radius relation for two-layer rocky planets based on PREM, , where CMF stands for core mass fraction. It is applicable to 1 ∼ 8 M⊕ and a CMF of 0.0 ∼ 0.4. Applying this formula to Earth and Venus and several known small exoplanets with radii and masses measured to better than ∼30% precision gives a CMF fit of 0.26 0.07.
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.
The radii and orbital periods of 4,000+ confirmed/candidate exoplanets have been precisely measured by the Kepler mission. The radii show a bimodal distribution, with two peaks corresponding to ...smaller planets (likely rocky) and larger intermediate-size planets, respectively. While only the masses of the planets orbiting the brightest stars can be determined by ground-based spectroscopic observations, these observations allow calculation of their average densities placing constraints on the bulk compositions and internal structures. However, an important question about the composition of planets ranging from 2 to 4 Earth radii (R⊕) still remains. They may either have a rocky core enveloped in a H₂–He gaseous envelope (gas dwarfs) or contain a significant amount of multicomponent, H₂O-dominated ices/fluids (water worlds). Planets in the mass range of 10–15 M⊕, if half-ice and half-rock by mass, have radii of 2.5 R⊕, which exactly match the second peak of the exoplanet radius bimodal distribution. Any planet in the 2- to 4-R⊕ range requires a gas envelope of at most a few mass percentage points, regardless of the core composition. To resolve the ambiguity of internal compositions, we use a growth model and conduct Monte Carlo simulations to demonstrate that many intermediate-size planets are “water worlds.”
Life as we know it is homochiral, but the origins of biological homochirality on early Earth remain elusive. Shallow closed-basin lakes are a plausible prebiotic environment on early Earth, and most ...are expected to have significant sedimentary magnetite deposits. We hypothesize that ultraviolet (200- to 300-nm) irradiation of magnetite deposits could generate hydrated spin-polarized electrons sufficient to induce enantioselective prebiotic chemistry. Such electrons are potent reducing agents that drive reduction reactions where the spin polarization direction can enantioselectively alter the reaction kinetics. Our estimate of this chiral bias is based on the strong effective spin-orbit coupling observed in the chiral-induced spin selectivity (CISS) effect, as applied to energy differences in reduction reactions for different isomers. In the original CISS experiments, spin-selective electron transmission through a monolayer of double-strand DNA molecules is observed at room temperature—indicating a strong coupling between molecular chirality and electron spin. We propose that the chiral symmetry breaking due to the CISS effect, when applied to reduction chemistry, can induce enantioselective synthesis on the prebiotic Earth and thus facilitate the homochiral assembly of life’s building blocks.
We advocate an integrative approach between laboratory experiments in prebiotic chemistry and geologic, geochemical, and astrophysical observations to help assemble a robust chemical pathway to life ...that can be reproduced in the laboratory. The cyanosulfidic chemistry scenario described here was developed by such an integrative iterative process. We discuss how it maps onto evolving planetary surface environments on early Earth and Mars and the value of comparative planetary evolution. The results indicate that Mars can offer direct evidence for geochemical conditions similar to prebiotic Earth, whose early record has been erased. The Jezero crater is now the chosen landing site for NASA's Mars 2020 rover, making this an extraordinary opportunity for a breakthrough in understanding life's origins.
The UV environment is a key boundary condition to abiogenesis. However, considerable uncertainty exists as to planetary conditions and hence surface UV at abiogenesis. Here, we present two-stream ...multilayer clear-sky calculations of the UV surface radiance on Earth at 3.9 Ga to constrain the UV surface fluence as a function of albedo, solar zenith angle (SZA), and atmospheric composition. Variation in albedo and latitude (through SZA) can affect maximum photoreaction rates by a factor of >10.4; for the same atmosphere, photoreactions can proceed an order of magnitude faster at the equator of a snowball Earth than at the poles of a warmer world. Hence, surface conditions are important considerations when computing prebiotic UV fluences. For climatically reasonable levels of CO
, fluence shortward of 189 nm is screened out, meaning that prebiotic chemistry is robustly shielded from variations in UV fluence due to solar flares or variability. Strong shielding from CO
also means that the UV surface fluence is insensitive to plausible levels of CH
, O
, and O
. At scattering wavelengths, UV fluence drops off comparatively slowly with increasing CO
levels. However, if SO
and/or H
S can build up to the ≥1-100 ppm level as hypothesized by some workers, then they can dramatically suppress surface fluence and hence prebiotic photoprocesses. H
O is a robust UV shield for λ < 198 nm. This means that regardless of the levels of other atmospheric gases, fluence ≲198 nm is only available for cold, dry atmospheres, meaning sources with emission ≲198 (e.g., ArF excimer lasers) can only be used in simulations of cold environments with low abundance of volcanogenic gases. On the other hand, fluence at 254 nm is unshielded by H
O and is available across a broad range of Formula: see text, meaning that mercury lamps are suitable for initial studies regardless of the uncertainty in primordial H
O and CO
levels. Key Words: Radiative transfer-Origin of life-Planetary environments-UV radiation-Prebiotic chemistry. Astrobiology 17, 169-204.
ABSTRACT GJ 1132b is a nearby Earth-sized exoplanet transiting an M dwarf, and is among the most highly characterizable small exoplanets currently known. In this paper, we study the interaction of a ...magma ocean with a water-rich atmosphere on GJ 1132b and determine that it must have begun with more than 5 wt% initial water in order to still retain a water-based atmosphere. We also determine the amount of O2 that can build up in the atmosphere as a result of hydrogen dissociation and loss. We find that the magma ocean absorbs at most ∼10% of the O2 produced, whereas more than 90% is lost to space through hydrodynamic drag. The most common outcome for GJ 1132b from our simulations is a tenuous atmosphere dominated by O2, though, for very large initial water abundances, atmospheres with several thousands of bars of O2 are possible. A substantial steam envelope would indicate either the existence of an earlier H2 envelope or low XUV flux over the system's lifetime. A steam atmosphere would also imply the continued existence of a magma ocean on GJ 1132b. Further modeling is needed to study the evolution of CO2 or N2-rich atmospheres on GJ 1132b.
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
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