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Sukrit Ranjan

CIERA Postdoctoral Fellow @ NU

1800 Sherman Avenue #8041
Evanston, IL 60201

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Sukrit Ranjan
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In Press
Zhan, Zhuchang, Sara Seager, Janusz Petkowski, Clara Sousa-Silva, Sukrit Ranjan, Jingcheng Huang, and William Bains. In Press. “Assessment of Isoprene as a Possible Biosignature Gas in Exoplanets with Anoxic Atmospheres.” Astrobiology.
Submitted
An, S., S. Ranjan, K. Yuan, X. Yang, and R. Skodje. Submitted. “The Role of the Three Body Photodissociation Channel of Water in the Evolution of Dioxygen in Astrophysical Applications.” Physical Chemistry, Chemical Physics.
Rimmer, Paul, Sukrit Ranjan, and Sarah Rugheimer. Submitted. “Starting and Searching for Life on Rocky Planets.” Life.
Ranjan, Sukrit, Corinna Kufner, Gabriella Lozano, Zoe Todd, Azra Haseki, and Dimitar Sasselov. Submitted. “UV Transmission in Natural Waters on Prebiotic Earth: Halide and Ferrous Species.” Astrobiology.
Greaves, Jane, William Bains, Janus Petkowski, Sara Seager, Clara Sousa-Silva, Sukrit Ranjan, David Clements, et al. Submitted. “On the Robustness of Phosphine Signatures in Venus’ Clouds.” Nature Astronomy Matters Arising.
Bains, William, Janusz Petkowski, Sara Seager, Sukrit Ranjan, Clara Sousa-Silva, Paul Rimmer, Zhuchang Zhan, Jane Greaves, and Anita Richards. Submitted. “Phosphine on Venus Cannot be Explained by Conventional Processes.” Astrobiology. Publisher's Version Abstract
The recent candidate detection of 20 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 Venusian atmosphere before and our work represents the first attempt to model phosphorus species in 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. The presence of PH3, therefore, must 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.
Huang, Jingcheng, Janusz Petkowski, Sara Seager, Sukrit Ranjan, and Zhuchang Zhan. Submitted. “Assessment of Ammonia as a Biosignature Gas in Exoplanet Atmospheres.” Astrobiology.
2020
Greaves, Jane, Anita Richards, William Bains, Paul Rimmer, Hideo Sagawa, David Clements, Sara Seager, et al. 2020. “Phosphine gas in the cloud decks of Venus.” Nature Astronomy. Publisher's Version Abstract

Measurements of trace gases in planetary atmospheres help us explore chemical conditions different to those on Earth. Our nearest neighbour, Venus, has cloud decks that are temperate but hyperacidic. Here we report the apparent presence of phosphine (PH3) gas in Venus’s atmosphere, where any phosphorus should be in oxidized forms. Single-line millimetre-waveband spectral detections (quality up to ~15σ) from the JCMT and ALMA telescopes have no other plausible identification. Atmospheric PH3 at ~20 ppb abundance is inferred. The presence of PH3 is unexplained after exhaustive study of steady-state chemistry and photochemical pathways, with no currently known abiotic production routes in Venus’s atmosphere, clouds, surface and subsurface, or from lightning, volcanic or meteoritic delivery. PH3 could originate from unknown photochemistry or geochemistry, or, by analogy with biological production of PH3 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.

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Seager, Sara, Janusz Petkowski, Peter Gao, William Bains, Noelle Bryan, Sukrit Ranjan, and Jane Greaves. 2020. “The Venusian Lower Atmosphere Haze as a Depot for Desiccated Microbial Life: A Proposed Life Cycle for Persistence of the Venusian Aerial Biosphere.” Astrobiology. Publisher's Version Abstract

We revisit the hypothesis that there is life in the Venusian clouds to propose a life cycle that resolves the conundrum of how life can persist aloft for hundreds of millions to billions of years. Most discussions of an aerial biosphere in the Venus atmosphere temperate layers never address whether the life—small microbial-type particles—is free floating or confined to the liquid environment inside cloud droplets. We argue that life must reside inside liquid droplets such that it will be protected from a fatal net loss of liquid to the atmosphere, an unavoidable problem for any free-floating microbial life forms. However, the droplet habitat poses a lifetime limitation: Droplets inexorably grow (over a few months) to large enough sizes that are forced by gravity to settle downward to hotter, uninhabitable layers of the Venusian atmosphere. (Droplet fragmentation—which would reduce particle size—does not occur in Venusian atmosphere conditions.) We propose for the first time that the only way life can survive indefinitely is with a life cycle that involves microbial life drying out as liquid droplets evaporate during settling, with the small desiccated “spores” halting at, and partially populating, the Venus atmosphere stagnant lower haze layer (33–48 km altitude). We, thus, call the Venusian lower haze layer a “depot” for desiccated microbial life. The spores eventually return to the cloud layer by upward diffusion caused by mixing induced by gravity waves, act as cloud condensation nuclei, and rehydrate for a continued life cycle. We also review the challenges for life in the extremely harsh conditions of the Venusian atmosphere, refuting the notion that the “habitable” cloud layer has an analogy in any terrestrial environment.

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Ranjan, Sukrit, Edward Schwieterman, Chester Harman, Alexander Fateev, Clara Sousa-Silva, Sara Seager, and Renyu Hu. 2020. “Photochemistry of Anoxic Abiotic Habitable Planet Atmospheres: Impact of New H2O Cross Sections.” The Astrophysical Journal 896: 148. Publisher's Version Abstract

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.

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Todd, Zoe, Albert Fahrenbach, Sukrit Ranjan, Christopher Magnani, Jack Szostak, and Dimitar Sasselov. 2020. “Ultraviolet-Driven Deamination of Cytidine Ribonucleotides Under Planetary Conditions.” Astrobiology 20 (7): 878-888. Publisher's Version Abstract

A previously proposed synthesis of pyrimidine ribonucleotides makes use of ultraviolet (UV) light to convert β-d-ribocytidine-2′,3′-cyclic phosphate to β-d-ribouridine-2′,3′-cyclic phosphate, while simultaneously selectively degrading synthetic byproducts. Past studies of the photochemical reactions of pyrimidines have employed mercury arc lamps, characterized by narrowband emission centered at 254 nm, which is not representative of the UV environment of the early Earth. To further assess this process under more realistic circumstances, we investigated the wavelength dependence of the UV-driven conversion of β-d-ribocytidine-2′,3′-cyclic phosphate to β-d-ribouridine-2′,3′-cyclic phosphate. We used constraints provided by planetary environments to assess the implications for pyrimidine nucleotides on the early Earth. We found that the wavelengths of light (255–285 nm) that most efficiently drive the deamination of β-d-ribocytidine-2′,3′-cyclic phosphate to β-d-ribouridine-2′,3′-cyclic phosphate are accessible on planetary surfaces such as those of the Hadean-Archaean Earth for CO2-N2-dominated atmospheres. However, continued irradiation could eventually lead to low levels of ribocytidine in a low-temperature, highly irradiated environment, if production rates are slow.

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Gunther, Maximilian N., Zhuchang Zhan, Sara Seager, Paul B. Rimmer, Sukrit Ranjan, Keivan G. Stassun, Ryan J. Oelkers, et al. 2020. “Stellar Flares from the First TESS Data Release: Exploring a New Sample of M Dwarfs.” The Astronomical Journal 159 (2): 60. Publisher's Version Abstract

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.

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Sousa-Silva, Clara, Sara Seager, Sukrit Ranjan, Janusz J. Petkowski, Zhuchang Zhan, Renyu Hu, and Williams Bains. 2020. “Phosphine as a Biosignature Gas in Exoplanet Atmospheres.” Astrobiology 20 (2). University Press: 235-268. Publisher's Version Abstract

A long-term goal of exoplanet studies is the identification and detection of biosignature gases. Beyond the most discussed biosignature gas O2, only a handful of gases have been considered in detail. In this study, we evaluate phosphine (PH3). On Earth, PH3 is associated with anaerobic ecosystems, and as such, it is a potential biosignature gas in anoxic exoplanets. We simulate the atmospheres of habitable terrestrial planets with CO2- and H2-dominated atmospheres and find that PH3 can accumulate to detectable concentrations on planets with surface production fluxes of 1010 to 1014 cm−2 s−1 (corresponding to surface concentrations of 10s of ppb to 100s of ppm), depending on atmospheric composition and ultraviolet (UV) irradiation. While high, the surface flux values are comparable to the global terrestrial production rate of methane or CH4 (1011 cm−2 s−1) and below the maximum local terrestrial PH3 production rate (1014 cm−2 s−1). As with other gases, PH3 can more readily accumulate on low-UV planets, for example, planets orbiting quiet M dwarfs or with a photochemically generated UV shield. PH3 has three strong spectral features such that in any atmosphere scenario one of the three will be unique compared with other dominant spectroscopic molecules. Phosphine's weakness as a biosignature gas is its high reactivity, requiring high outgassing rates for detectability. We calculate that tens of hours of JWST (James Webb Space Telescope) time are required for a potential detection of PH3. Yet, because PH3 is spectrally active in the same wavelength regions as other atmospherically important molecules (such as H2O and CH4), searches for PH3 can be carried out at no additional observational cost to searches for other molecular species relevant to characterizing exoplanet habitability. Phosphine is a promising biosignature gas, as it has no known abiotic false positives on terrestrial planets from any source that could generate the high fluxes required for detection.

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2019
Ranjan, Sukrit, Zoe R. Todd, Paul B. Rimmer, Dimitar. D. Sasselov, and Andrew R. Babbin. 2019. “Nitrogen Oxide Concentrations in Natural Waters on Early Earth.” Geochemistry, Geophysics, Geosystems 20. University Press: 2021-2039. Publisher's Version Abstract
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 ( ggge21866-math-0001), in particular nitrate ( ggge21866-math-0002) and nitrite ( ggge21866-math-0003), 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 ofggge21866-math-0004 from the prebiotic atmosphere to the ocean and reported steady state [ ggge21866-math-0005] to be high across all plausible parameter space. These findings rest on the assumption that ggge21866-math-0006 is stable in natural waters unless processed at a hydrothermal vent. Here, we show that ggge21866-math-0007 is unstable in the reducing environment of early Earth. Sinks due to ultraviolet photolysis and reactions with reduced iron (Fe2+) suppress [ ggge21866-math-0008] 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 [ ggge21866-math-0009]  <1μM in the prebiotic ocean. On the other hand, prebiotic ponds with favorable drainage characteristics may have sustained [ ggge21866-math-0010]  ≥1μM. As on modern Earth, most ggge21866-math-0011 on prebiotic Earth should have been present as ggge21866-math-0012, 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 ggge21866-math-0013 kinetics to reduce the considerable uncertainties in predicting [ ggge21866-math-0014] on early Earth.
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Press: MIT,Boston Globe, Times of India, The Hindu. 
2018
Todd, Zoe R., Albert C Fahrenbach, Christopher J. Magnani, Sukrit Ranjan, Anders Bjorkbom, Jack W. Szostak, and Dimitar D. Sasselov. 2018. “Solvated-electron production using cyanocuprates is compatible with the UV-environment on a Hadean-Archaean Earth.” Chemical Communications 54: 1121-1124. Publisher's Version Abstract
UV-driven photoredox processing of cyanocuprates can generate simple sugars necessary for prebiotic synthesis. We investigate the wavelength dependence of this process from 215 to 295 nm and generally observe faster rates at shorter wavelengths. The most efficient wavelengths are accessible to a range of potential prebiotic atmospheres, supporting the potential role of cyanocuprate photochemistry in prebiotic synthesis on the early Earth.
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Press: Harvard Gazette.
Xu, J., Dougal J. Ritson, Sukrit Ranjan, Zoe R. Todd, Dimitar D. Sasselov, and John D. Sutherland. 2018. “Photochemical reductive homologation of hydrogen cyanide using sulfite and ferrocyanide.” Chemical Communications 54: 5566-5569. Publisher's Version Abstract
Photoredox cycling during UV irradiation of ferrocyanide ([FeII(CN)6]4−) in the presence of stoichiometric sulfite (SO32−) is shown to be an extremely effective way to drive the reductive homologation of hydrogen cyanide (HCN) to simple sugars and precursors of hydroxy acids and amino acids.
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Ranjan, Sukrit, Zoe R. Todd, John D. Sutherland, and Dimitar D. Sasselov. 2018. “Sulfidic Anion Concentrations on Early Earth for Surficial Origins-of-Life Chemistry.” Astrobiology 18 (9). University Press: 1023-1040. Publisher's Version Abstract

A key challenge in origin-of-life studies is understanding the environmental conditions on early Earth under which abiogenesis occurred. While some constraints do exist (e.g., zircon evidence for surface liquid water), relatively few constraints exist on the abundances of trace chemical species, which are relevant to assessing the plausibility and guiding the development of postulated prebiotic chemical pathways which depend on these species. In this work, we combine literature photochemistry models with simple equilibrium chemistry calculations to place constraints on the plausible range of concentrations of sulfidic anions (HS−, HSO3−, SO32−) available in surficial aquatic reservoirs on early Earth due to outgassing of SO2 and H2S and their dissolution into small shallow surface water reservoirs like lakes. We find that this mechanism could have supplied prebiotically relevant levels of SO2-derived anions, but not H2S-derived anions. Radiative transfer modeling suggests UV light would have remained abundant on the planet surface for all but the largest volcanic explosions. We apply our results to the case study of the proposed prebiotic reaction network of Patel et al. (2015) and discuss the implications for improving its prebiotic plausibility. In general, epochs of moderately high volcanism could have been especially conducive to cyanosulfidic prebiotic chemistry. Our work can be similarly applied to assess and improve the prebiotic plausibility of other postulated surficial prebiotic chemistries that are sensitive to sulfidic anions, and our methods adapted to study other atmospherically derived trace species.

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Press: MIT,  Boston Globe, Popular Mechanics, The Crimson, The Register.
2017
Ranjan, Sukrit, Robin D. Wordsworth, and Dimitar D. Sasselov. 2017. “The Surface UV Environment on Planets Orbiting M Dwarfs: Implications for Prebiotic Chemistry and the Need for Experimental Follow-up.” The Astrophysical Journal 843: 110. Publisher's Version Abstract
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 steadystate 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.
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Press: CfA, Smithsonian Insider,  Scientific American, The Hindu, Deccan Herald, Centauri Dreams, IFLS. See also TV interview by Science for the Public.
Ranjan, Sukrit, Robin D. Wordsworth, and Dimitar D. Sasselov. 2017. “Atmospheric Constraints on the Surface UV Environment of Mars at 3.9 Ga Relevant to Prebiotic Chemistry.” Astrobiology 17 (8): 687-708. Publisher's Version Abstract

Recent findings suggest that Mars may have been a clement environment for the emergence of life and may even have compared favorably to Earth in this regard. These findings have revived interest in the hypothesis that prebiotically important molecules or even nascent life may have formed on Mars and been transferred to Earth. UV light plays a key role in prebiotic chemistry. Characterizing the early martian surface UV environment is key to understanding how Mars compares to Earth as a venue for prebiotic chemistry.

Here, we present two-stream, multilayer calculations of the UV surface radiance on Mars at 3.9 Ga to constrain the surface UV environment as a function of atmospheric state. We explore a wide range of atmospheric pressures, temperatures, and compositions that correspond to the diversity of martian atmospheric states consistent with available constraints. We include the effects of clouds and dust. We calculate dose rates to quantify the effect of different atmospheric states on UV-sensitive prebiotic chemistry.

We find that, for normative clear-sky CO2-H2O atmospheres, the UV environment on young Mars is comparable to young Earth. This similarity is robust to moderate cloud cover; thick clouds (τcloud ≥ 100) are required to significantly affect the martian UV environment, because cloud absorption is degenerate with atmospheric CO2. On the other hand, absorption from SO2, H2S, and dust is nondegenerate with CO2, meaning that, if these constituents build up to significant levels, surface UV fluence can be suppressed. These absorbers have spectrally variable absorption, meaning that their presence affects prebiotic pathways in different ways. In particular, high SO2 environments may admit UV fluence that favors pathways conducive to abiogenesis over pathways unfavorable to it. However, better measurements of the spectral quantum yields of these pathways are required to evaluate this hypothesis definitively.

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Press: SciNews. 
Ranjan, Sukrit, and Dimitar D. Sasselov. 2017. “Constraints on the Early Terrestrial Surface UV Environment Relevant to Prebiotic Chemistry.” Astrobiology 17 (3): 169-204. Publisher's Version Abstract

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 CO2, 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 CO2 also means that the UV surface fluence is insensitive to plausible levels of CH4, O2, and O3. At scattering wavelengths, UV fluence drops off comparatively slowly with increasing CO2 levels. However, if SO2 and/or H2S 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.

H2O 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 H2O and is available across a broad range of , meaning that mercury lamps are suitable for initial studies regardless of the uncertainty in primordial H2O and CO2 levels.

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