167 research outputs found
Organic Haze as a Biosignature in Anoxic Earth-like Atmospheres
Early Earth may have hosted a biologically-mediated global organic haze
during the Archean eon (3.8-2.5 billion years ago). This haze would have
significantly impacted multiple aspects of our planet, including its potential
for habitability and its spectral appearance. Here, we model worlds with
Archean-like levels of carbon dioxide orbiting the ancient sun and an M4V dwarf
(GJ 876) and show that organic haze formation requires methane fluxes
consistent with estimated Earth-like biological production rates. On planets
with high fluxes of biogenic organic sulfur gases (CS2, OCS, CH3SH, and
CH3SCH3), photochemistry involving these gases can drive haze formation at
lower CH4/CO2 ratios than methane photochemistry alone. For a planet orbiting
the sun, at 30x the modern organic sulfur gas flux, haze forms at a CH4/CO2
ratio 20% lower than at 1x the modern organic sulfur flux. For a planet
orbiting the M4V star, the impact of organic sulfur gases is more pronounced:
at 1x the modern Earth organic sulfur flux, a substantial haze forms at CH4/CO2
~ 0.2, but at 30x the organic sulfur flux, the CH4/CO2 ratio needed to form
haze decreases by a full order of magnitude. Detection of haze at an
anomalously low CH4/CO2 ratio could suggest the influence of these biogenic
sulfur gases, and therefore imply biological activity on an exoplanet. When
these organic sulfur gases are not readily detectable in the spectrum of an
Earth-like exoplanet, the thick organic haze they can help produce creates a
very strong absorption feature at UV-blue wavelengths detectable in reflected
light at a spectral resolution as low as 10. In direct imaging, constraining
CH4 and CO2 concentrations will require higher spectral resolution, and R > 170
is needed to accurately resolve the structure of the CO2 feature at 1.57
{\mu}m, likely, the most accessible CO2 feature on an Archean-like exoplanet.Comment: accepted for publication in Astrobiolog
Nonphotosynthetic Pigments as Potential Biosignatures
Previous work on possible surface reflectance biosignatures for Earth-like
planets has typically focused on analogues to spectral features produced by
photosynthetic organisms on Earth, such as the vegetation red edge. Although
oxygenic photosynthesis, facilitated by pigments evolved to capture photons, is
the dominant metabolism on our planet, pigmentation has evolved for multiple
purposes to adapt organisms to their environment. We present an
interdisciplinary study of the diversity and detectability of nonphotosynthetic
pigments as biosignatures, which includes a description of environments that
host nonphotosynthetic biologically pigmented surfaces, and a lab-based
experimental analysis of the spectral and broadband color diversity of
pigmented organisms on Earth. We test the utility of broadband color to
distinguish between Earth-like planets with significant coverage of
nonphotosynthetic pigments and those with photosynthetic or nonbiological
surfaces, using both 1-D and 3-D spectral models. We demonstrate that, given
sufficient surface coverage, nonphotosynthetic pigments could significantly
impact the disk-averaged spectrum of a planet. However, we find that due to the
possible diversity of organisms and environments, and the confounding effects
of the atmosphere and clouds, determination of substantial coverage by
biologically produced pigments would be difficult with broadband colors alone
and would likely require spectrally resolved data.Comment: 21 pages, 12 figures, 5 tables. Full, published articl
The polarization of the planet-hosting WASP-18 system
We report observations of the linear polarization of the WASP-18 system,
which harbors a very massive ( approx 10 M_J) planet orbiting very close to its
star with an orbital period of 0.94 days. We find the WASP-18 system is
polarized at about 200 parts-per-million (ppm), likely from the interstellar
medium predominantly, with no strong evidence for phase dependent modulation
from reflected light from the planet. We set an upper limit of 40 ppm (99%
confidence level) on the amplitude of a reflected polarized light planetary
signal. We compare the results with models for a number of processes that may
produce polarized light in a planetary system to determine if we can rule out
any phenomena with this limit. Models of reflected light from thick clouds can
approach or exceed this limit, but such clouds are unlikely at the high
temperature of the WASP-18b atmosphere. Additionally, we model the expected
polarization resulting from the transit of the planet across the star and find
this has an amplitude of about 1.6 ppm, which is well below our detection
limits. We also model the polarization due to the tidal distortion of the star
by the massive planet and find this is also too small to be measured currently.Comment: 23 pages, 10 Figures, 6 Tables, Accepted to A
There's more to life than O: Simulating the detectability of a range of molecules for ground-based high-resolution spectroscopy of transiting terrestrial exoplanets
Within the next decade, atmospheric O on Earth-like M dwarf planets may
be accessible with visible--near-infrared, high spectral resolution extremely
large ground-based telescope (ELT) instruments. However, the prospects for
using ELTs to detect environmental properties that provide context for O
have not been thoroughly explored. Additional molecules may help indicate
planetary habitability, rule out abiotically generated O, or reveal
alternative biosignatures. To understand the accessibility of environmental
context using ELT spectra, we simulate high-resolution transit transmission
spectra of previously-generated evolved terrestrial atmospheres. We consider
inhabited pre-industrial and Archean Earth-like atmospheres, and lifeless
worlds with abiotic O buildup from CO and HO photolysis. All
atmospheres are self-consistent with M2V--M8V dwarf host stars. Our simulations
include explicit treatment of systematic and telluric effects to model
high-resolution spectra for GMT, TMT, and E-ELT configurations for systems 5
and 12 pc from Earth. Using the cross-correlation technique, we determine the
detectability of major species in these atmospheres: O, O, CH,
CO, CO, HO, and CH. Our results suggest that CH and CO
are the most accessible molecules for terrestrial planets transiting a range of
M dwarf hosts using an E-ELT, TMT, or GMT sized telescope, and that the O
NIR and HO 0.9 m bands may also be accessible with more observation
time. Although this technique still faces considerable challenges, the ELTs
will provide access to the atmospheres of terrestrial planets transiting
earlier-type M-dwarf hosts that may not be possible using JWST.Comment: Accepted for publication in The Planetary Science Journa
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