126 research outputs found
Early evolution of purple retinal pigments on Earth and implications for exoplanet biosignatures
We propose that retinal-based phototrophy arose early in the evolution of
life on Earth, profoundly impacting the development of photosynthesis and
creating implications for the search for life beyond our planet. While the
early evolutionary history of phototrophy is largely in the realm of the
unknown, the onset of oxygenic photosynthesis in primitive cyanobacteria
significantly altered the Earth's atmosphere by contributing to the rise of
oxygen ~2.3 billion years ago. However, photosynthetic chlorophyll and
bacteriochlorophyll pigments lack appreciable absorption at wavelengths about
500-600 nm, an energy-rich region of the solar spectrum. By contrast, simpler
retinal-based light-harvesting systems such as the haloarchaeal purple membrane
protein bacteriorhodopsin show a strong well-defined peak of absorbance
centered at 568 nm, which is complementary to that of chlorophyll pigments. We
propose a scenario where simple retinal-based light-harvesting systems like
that of the purple chromoprotein bacteriorhodopsin, originally discovered in
halophilic Archaea, may have dominated prior to the development of
photosynthesis. We explore this hypothesis, termed the 'Purple Earth,' and
discuss how retinal photopigments may serve as remote biosignatures for
exoplanet research.Comment: Published Open Access in the International Journal of Astrobiology;
10 pages, 6 figure
A Limited Habitable Zone for Complex Life
The habitable zone (HZ) is commonly defined as the range of distances from a
host star within which liquid water, a key requirement for life, may exist on a
planet's surface. Substantially more CO2 than present in Earth's modern
atmosphere is required to maintain clement temperatures for most of the HZ,
with several bars required at the outer edge. However, most complex aerobic
life on Earth is limited by CO2 concentrations of just fractions of a bar. At
the same time, most exoplanets in the traditional HZ reside in proximity to M
dwarfs, which are more numerous than Sun-like G dwarfs but are predicted to
promote greater abundances of gases that can be toxic in the atmospheres of
orbiting planets, such as carbon monoxide (CO). Here we show that the HZ for
complex aerobic life is likely limited relative to that for microbial life. We
use a 1D radiative-convective climate and photochemical models to circumscribe
a Habitable Zone for Complex Life (HZCL) based on known toxicity limits for a
range of organisms as a proof of concept. We find that for CO2 tolerances of
0.01, 0.1, and 1 bar, the HZCL is only 21%, 32%, and 50% as wide as the
conventional HZ for a Sun-like star, and that CO concentrations may limit some
complex life throughout the entire HZ of the coolest M dwarfs. These results
cast new light on the likely distribution of complex life in the universe and
have important ramifications for the search for exoplanet biosignatures and
technosignatures.Comment: Revised including additional discussion. Published Gold OA in ApJ. 9
pages, 5 figures, 5 table
A Quarter-Century of Observations of Comet 10P/Tempel 2 at Lowell Observatory: Continued Spin-Down, Coma Morphology, Production Rates, and Numerical Modeling
We report on photometry and imaging of Comet 10P/Tempel 2 obtained at Lowell
Observatory from 1983 through 2011. We measured a nucleus rotation period of
8.950 +/- 0.002 hr from 2010 September to 2011 January. This rotation period is
longer than the period we previously measured in 1999, which was itself longer
than the period measured in 1988. A nearly linear jet was observed which varied
little during a rotation cycle in both R and CN images acquired during the 1999
and 2010 apparitions. We measured the projected direction of this jet
throughout the two apparitions and, under the assumption that the source region
of the jet was near the comet's pole, determined a rotational pole direction of
RA/Dec = 151deg/+59deg from CN measurements and RA/Dec = 173deg/+57deg from
dust measurements (we estimate a circular uncertainty of 3deg for CN and 4deg
for dust). Different combinations of effects likely bias both gas and dust
solutions and we elected to average these solutions for a final pole of RA/Dec
= 162 +/- 11deg/+58 +/- 1deg. Photoelectric photometry was acquired in 1983,
1988, 1999/2000, and 2010/2011. The activity exhibited a steep turn-on ~3
months prior to perihelion (the exact timing of which varies) and a relatively
smooth decline after perihelion. The activity during the 1999 and 2010
apparitions was similar; limited data in 1983 and 1988 were systematically
higher and the difference cannot be explained entirely by the smaller
perihelion distance. We measured a "typical" composition, in agreement with
previous investigators. Monte Carlo numerical modeling with our pole solution
best replicated the observed coma morphology for a source region located near a
comet latitude of +80deg and having a radius of ~10deg. Our model reproduced
the seasonal changes in activity, suggesting that the majority of Tempel 2's
activity originates from a small active region located near the pole.Comment: Accepted by AJ; 29 pages of text (preprint style), 8 tables, 7
figure
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
Earthshine as an Illumination Source at the Moon
Earthshine is the dominant source of natural illumination on the surface of
the Moon during lunar night, and at locations within permanently shadowed
regions that never receive direct sunlight. As such, earthshine may enable the
exploration of areas of the Moon that are hidden from solar illumination. The
heat flux from earthshine may also influence the transport and cold trapping of
volatiles present in the very coldest areas. In this study, Earth's spectral
radiance at the Moon is examined using a suite of Earth spectral models created
using the Virtual Planetary Laboratory (VPL) three dimensional modeling
capability. At the Moon, the broadband, hemispherical irradiance from Earth
near 0 phase is approximately 0.15 watts per square meter, with comparable
contributions from solar reflectance and thermal emission. Over the simulation
timeframe, spanning two lunations, Earth's thermal irradiance changes less than
a few mW per square meter as a result of cloud variability and the
south-to-north motion of sub-observer position. In solar band, Earth's
diurnally averaged light curve at phase angles < 60 degrees is well fit using a
Henyey Greenstein integral phase function. At wavelengths > 0.7 microns, near
the well known vegetation "red edge", Earth's reflected solar radiance shows
significant diurnal modulation as a result of the longitudinal asymmetry in
projected landmass, as well as from the distribution of clouds. A simple
formulation with adjustable coefficients is presented for estimating Earth's
hemispherical irradiance at the Moon as a function of wavelength, phase angle
and sub-observer coordinates. It is demonstrated that earthshine is
sufficiently bright to serve as a natural illumination source for optical
measurements from the lunar surface.Comment: 27 pages, 15 figures, 1 tabl
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
Is the Pale Blue Dot unique? Optimized photometric bands for identifying Earth-like exoplanets
The next generation of ground and space-based telescopes will image habitable
planets around nearby stars. A growing literature describes how to characterize
such planets with spectroscopy, but less consideration has been given to the
usefulness of planet colors. Here, we investigate whether potentially
Earth-like exoplanets could be identified using UV-visible-to-NIR wavelength
broadband photometry (350-1000 nm). Specifically, we calculate optimal
photometric bins for identifying an exo-Earth and distinguishing it from
uninhabitable planets including both Solar System objects and model exoplanets.
The color of some hypothetical exoplanets - particularly icy terrestrial worlds
with thick atmospheres - is similar to Earth's because of Rayleigh scattering
in the blue region of the spectrum. Nevertheless, subtle features in Earth's
reflectance spectrum appear to be unique. In particular, Earth's reflectance
spectrum has a 'U-shape' unlike all our hypothetical, uninhabitable planets.
This shape is partly biogenic because O2-rich, oxidizing air is transparent to
sunlight, allowing prominent Rayleigh scattering, while ozone absorbs visible
light, creating the bottom of the 'U'. Whether such uniqueness has practical
utility depends on observational noise. If observations are photon limited or
dominated by astrophysical sources (zodiacal light or imperfect starlight
suppression), then the use of broadband visible wavelength photometry to
identify Earth twins has little practical advantage over obtaining detailed
spectra. However, if observations are dominated by dark current then optimized
photometry could greatly assist preliminary characterization. We also calculate
the optimal photometric bins for identifying extrasolar Archean Earths, and
find that the Archean Earth is more difficult to unambiguously identify than a
modern Earth twin.Comment: 10 figures, 38 page
The Increasing Rotation Period of Comet 10P/Tempel 2
We imaged comet 10P/Tempel 2 on 32 nights from 1999 April through 2000 March.
R-band lightcurves were obtained on 11 of these nights from 1999 April through
1999 June, prior to both the onset of significant coma activity and perihelion.
Phasing of the data yields a double-peaked lightcurve and indicates a nucleus
rotational period of 8.941 +/- 0.002 hr with a peak-to-peak amplitude of ~0.75
mag. Our data are sufficient to rule out all other possible double-peaked
solutions as well as the single- and triple- peaked solutions. This rotation
period agrees with one of five possible solutions found in post-perihelion data
from 1994 by Mueller and Ferrin (1996, Icarus, 123, 463-477), and unambiguously
eliminates their remaining four solutions. We applied our same techniques to
published lightcurves from 1988 which were obtained at an equivalent orbital
position and viewing geometry as in 1999. We found a rotation period of 8.932
+/- 0.001 hr in 1988, consistent with the findings of previous authors and
incompatible with our 1999 solution. This reveals that Tempel 2 spun-down by
~32 s between 1988 and 1999 (two intervening perihelion passages). If the
spin-down is due to a systematic torque, then the rotation period prior to
perihelion during the 2010 apparition is expected to be an additional 32 s
longer than in 1999.Comment: Accepted by The Astronomical Journal; 22 pages of text, 3 tables, 6
figure
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
Detecting and Constraining N Abundances in Planetary Atmospheres Using Collisional Pairs
Characterizing the bulk atmosphere of a terrestrial planet is important for
determining surface pressure and potential habitability. Molecular nitrogen
(N) constitutes the largest fraction of Earths atmosphere and is likely
to be a major constituent of many terrestrial exoplanet atmospheres. Due to its
lack of significant absorption features, N is extremely difficult to
remotely detect. However, N produces an N-N collisional pair,
(N), which is spectrally active. Here we report the detection of
(N) in Earths disk-integrated spectrum. By comparing spectra from
NASAs EPOXI mission to synthetic spectra from the NASA Astrobiology
Institutes Virtual Planetary Laboratory three-dimensional spectral Earth
model, we find that (N) absorption produces a ~35 decrease in flux
at 4.15 m. Quantifying N could provide a means of determining bulk
atmospheric composition for terrestrial exoplanets and could rule out abiotic
O generation, which is possible in rarefied atmospheres. To explore the
potential effects of (N) in exoplanet spectra, we used radiative
transfer models to generate synthetic emission and transit transmission spectra
of self-consistent N-CO-HO atmospheres, and analytic N-H
and N-H-CO atmospheres. We show that (N) absorption in the
wings of the 4.3 m CO band is strongly dependent on N partial
pressures above 0.5 bar and can significantly widen this band in thick N
atmospheres. The (N) transit transmission signal is up to 10 ppm for an
Earth-size planet with an N-dominated atmosphere orbiting within the HZ of
an M5V star and could be substantially larger for planets with significant
H mixing ratios.Comment: Accepted for publication in The Astrophysical Journal. 46 pages, 12
figures, 3 table
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