60 research outputs found
Nitrogen Oxide Concentrations in Natural Waters on Early Earth
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
(NO), in particular nitrate (NO) and nitrite
(NO), 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 NO from the prebiotic atmosphere to the
ocean, and reported steady-state [NO] to be high across all plausible
parameter space. These findings rest on the assumption that NO is
stable in natural waters unless processed at a hydrothermal vent. Here, we show
that NO is unstable in the reducing environment of early Earth. Sinks
due to UV photolysis and reactions with reduced iron (Fe) suppress
[NO] by several orders of magnitude relative to past predictions. For
pH and C, we find that it is most probable that
NO]M in the prebiotic ocean. On the other hand, prebiotic
ponds with favorable drainage characteristics may have sustained
[NO]M. As on modern Earth, most NO on prebiotic
Earth should have been present as NO, 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
NO kinetics to reduce the considerable uncertainties in predicting
[NO] on early Earth.Comment: In review for publication at Geochemistry, Geophysics, and Geosystems
(G-cubed). Comments, questions, and criticism solicited; please contact
corresponding author at [email protected]. SI at:
https://web-cert.mit.edu/sukrit/Public/nox_si.pdf. GitHub at:
https://github.com/sukritranjan/no
UV Spectral Characterization of Low-Mass Stars With AstroSat UVIT for Exoplanet Applications: The Case Study of HIP 23309
Characterizing rocky exoplanet atmospheres is a key goal of exoplanet
science, but interpreting such observations will require understanding the
stellar UV irradiation incident on the planet from its host star. Stellar UV
mediates atmospheric escape, photochemistry, and planetary habitability, and
observations of rocky exoplanets can only be understood in the context of the
UV SED of their host stars. Particularly important are SEDs from
observationally favorable but poorly understood low-mass M-dwarf stars, which
are the only plausible targets for rocky planet atmospheric characterization
for the next 1-2 decades. In this work, we explore the utility of AstroSat UVIT
for the characterization of the UV SEDs of low-mass stars. We present
observations of the nearby M0 star HIP 23309 in the FUV and NUV gratings of
UVIT. Our FUV spectra are consistent with contemporaneous HST data and our NUV
spectra are stable between orbits, suggesting UVIT is a viable tool for the
characterization of the SEDs of low-mass stars. We apply our measured spectra
to simulations of photochemistry and habitability for a hypothetical rocky
planet orbiting HIP 23309 and elucidate the utility and limitations of UVIT in
deriving UV SEDs of M-dwarf exoplanet hosts. Our work validates UVIT as a tool
to complement HST in the characterization of exoplanet host stars and carries
implications for its successor missions like INSIST.Comment: Accepted to A
Photochemistry of Anoxic Abiotic Habitable Planet Atmospheres: Impact of New HO Cross-Sections
We present a study of the photochemistry of abiotic habitable planets with
anoxic CO-N atmospheres. Such worlds are representative of early Earth,
Mars and Venus, and analogous exoplanets. HO photodissociation controls the
atmospheric photochemistry of these worlds through production of reactive OH,
which dominates the removal of atmospheric trace gases. The near-UV (NUV;
nm) absorption cross-sections of HO play an outsized role in OH
production; these cross-sections were heretofore unmeasured at habitable
temperatures ( K). We present the first measurements of NUV HO
absorption at 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 O,
suppressing both by orders of magnitude relative to past predictions and
eliminating the low-outgassing "false positive" scenario for O as a
biosignature around solar-type stars. Enhanced [OH] increases rainout of
reductants to the surface, relevant to prebiotic chemistry, and may also
suppress CH and H; the latter depends on whether burial of reductants
is inhibited on the underlying planet, as is argued for abiotic worlds. While
we focus on CO-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 HO 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.Comment: Manuscript (this version) accepted to ApJ. Cross-section data
available at https://github.com/sukritranjan/ranjanschwietermanharman2020.
Feedback continues to be solicite
Recommended from our members
Atmospheric Constraints on the Surface UV Environment of Mars at 3.9 Ga Relevant to Prebiotic Chemistry
Recent findings suggest 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 multi-layer 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, corresponding 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 (Ï„>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 if they can build up to high levels, surface UV fluence will 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.Earth and Planetary Science
A Re-Appraisal of CO/O Runaway on Habitable Planets Orbiting Low-Mass Stars
Efforts to spectrally characterize the atmospheric compositions of temperate
terrestrial exoplanets orbiting M-dwarf stars with the James Webb Space
Telescope (JWST) are now underway. Key molecular targets of such searches
include O and CO, which are potential indicators of life. Recently, it was
proposed that CO photolysis generates abundant ( bar) abiotic
O and CO in the atmospheres of habitable M-dwarf planets with CO-rich
atmospheres, constituting a strong false positive for O as a biosignature
and further complicating efforts to use CO as a diagnostic of surface biology.
Significantly, this implied that TRAPPIST-1e and TRAPPIST-1f, now under
observation with JWST, would abiotically accumulate abundant O and CO, if
habitable. Here, we use a multi-model approach to re-examine photochemical
O and CO accumulation on planets orbiting M-dwarf stars. We show that
photochemical O remains a trace gas on habitable CO-rich M-dwarf
planets, with earlier predictions of abundant O and CO due to an
atmospheric model top that was too low to accurately resolve the unusually-high
CO photolysis peak on such worlds. Our work strengthens the case for O
as a biosignature gas, and affirms the importance of CO as a diagnostic of
photochemical O production. However, observationally relevant false
positive potential remains, especially for O's photochemical product O,
and further work is required to confidently understand O and O as
biosignature gases on M-dwarf planets.Comment: Submitted to AAS Journals; comments and criticism solicited at
[email protected]. 3 Figures, 1 Table in main text; 3Figures, 5 Tables in S
The Emergent 1.1-1.7 μm Spectrum of the Exoplanet CoRoT-2b as Measured Using the Hubble Space Telescope
We have used Hubble/WFC3 and the G141 grism to measure the secondary eclipse of the transiting, very hot Jupiter CoRoT-2b in the 1.1-1.7 μm spectral region. We find an eclipse depth averaged over this band equal to 395^(+69)_(-45) parts per million, equivalent to a blackbody temperature of 1788 ± 18 K. We study and characterize several WFC3 instrumental effects, especially the "hook" phenomenon described by Deming et al. We use data from several transiting exoplanet systems to find a quantitative relation between the amplitude of the hook and the exposure level of a given pixel. Although the uncertainties in this relation are too large to allow us to develop an empirical correction for our data, our study provides a useful guide for optimizing exposure levels in future WFC3 observations. We derive the planet's spectrum using a differential method. The planet-to-star contrast increases to longer wavelength within the WFC3 bandpass, but without water absorption or emission to a 3σ limit of 85 ppm. The slope of the WFC3 spectrum is significantly less than the slope of the best-fit blackbody. We compare all existing eclipse data for this planet to a blackbody spectrum, and to spectra from both solar abundance and carbon-rich (C/O = 1) models. A blackbody spectrum is an acceptable fit to the full data set. Extra continuous opacity due to clouds or haze, and flattened temperature profiles, are strong candidates to produce quasi-blackbody spectra, and to account for the amplitude of the optical eclipses. Our results show ambiguous evidence for a temperature inversion in this planet
- …