497 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
Eph-Ephrin Signaling and the Role of EFN-4 in Caenorhabditis Elegans Nervous Systems Development
Eph receptor tyrosine kinases and their ephrin ligands are required for multiple aspects of nervous system development including axon outgrowth, synaptic plasticity, and the formation of topographic maps in the visual system. The Caenorhabditis elegans ephrin-A, efn-4, has a defined role in hypodermal patterning but its role in nervous system development is not well understood. We find that loss-of-function mutations in efn-4 lead to suppression of axon branching in C. elegans model of X-linked Kallmann syndrome, a human genetic disorder that presents with loss of sense of smell and failure to undergo spontaneous puberty. In addition, efn-4 mutants have defects in AIY interneuron axon outgrowth. Tissue specific rescue experiments indicate that efn-4 is required non-cell autonomously in the hypodermis to promote axon extension. Also, non-cell autonomous expression in the body wall muscle is sufficient to rescue anosmin-dependent axon branching, suggesting that primary axon outgrowth is genetically distinct from axon branching. Previous genetic and biochemical analyses failed to establish whether the canonical C. elegans Eph receptor, vab-1, functions as the efn-4 receptor during embryonic development. We show via biolayer interferometry that VAB-1 binds with high-affinity to EFN-4. Furthermore, EFN-4 binds with promiscuity to both the canoncial VAB-1 Eph receptor in C. elegans and additionally to at least one other binding partner the L1CAM (L1 Cell Adhesion Molecule) LAD-2. VAB-1 may have additional functional roles in guiding AIY primary neurite outgrowth to the central plexus. Our findings suggest EFN-4 is a key player in cell-to-cell communications that guide AIY neuronal projections
Abiotic O Levels on Planets around F, G, K, and M Stars: Possible False Positives for Life?
In the search for life on Earth-like planets around other stars, the first
(and likely only) information will come from the spectroscopic characterization
of the planet's atmosphere. Of the countless number of chemical species
terrestrial life produces, only a few have the distinct spectral features and
the necessary atmospheric abundance to be detectable. The easiest of these
species to observe in Earth's atmosphere is O (and its photochemical
byproduct, O). But O can also be produced abiotically by photolysis
of CO, followed by recombination of O atoms with each other. CO is
produced in stoichiometric proportions. Whether O and CO can accumulate
to appreciable concentrations depends on the ratio of far-UV to near-UV
radiation coming from the planet's parent star and on what happens to these
gases when they dissolve in a planet's oceans. Using a one-dimensional
photochemical model, we demonstrate that O derived from CO
photolysis should not accumulate to measurable concentrations on planets around
F- and G-type stars. K-star, and especially M-star planets, however, may build
up O because of the low near-UV flux from their parent stars, in
agreement with some previous studies. On such planets, a 'false positive' for
life is possible if recombination of dissolved CO and O in the oceans is
slow and if other O sinks (e.g., reduced volcanic gases or dissolved
ferrous iron) are small. O, on the other hand, could be detectable at UV
wavelengths ( < 300 nm) for a much broader range of boundary
conditions and stellar types.Comment: 20 pages text, 9 figure
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
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
First Zipcar, Now Uber: Legal and Policy Issues Facing the Expanding “Shared Mobility” Sector in U.S. Cities
Innovations and technological disruptions in the “sharing economy” are shifting the contours of urban travel in the United States. Carsharing organizations such as car2go and Zipcar have grown exponentially over the past decade, expanding their memberships from 52,347 in 2004 to 1,181,087 in 2015. Ridesourcing companies like Lyft and Uber, which were entirely absent from most U.S. cities as recently as 2010, are now global powerhouses, each reportedly worth billions of dollars. Private investors, after avoiding investments in urban transit services for more than half a century, are now offering venture capital for Bridj, Chariot, and other companies. This Article explores the dynamics of “shared mobility” and the policy issues facing the participants in that sector through a review of the evolution of four prominent types of shared mobility providers: (1) carsharing organizations; (2) transportation network companies such as Lyft and Uber; (3) privately operated “microtransit” operators; and (4) crowdsourced intercity bus lines. The analytical portion of the study in Part I describes and critiques how these sectors have evolved and summarizes the notable legal and policy issues they face. Part II develops a typology that categorizes their services and shows how each has disrupted the transportation sector. The last section also offers conclusions and suggestions for further study
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
Modeling pN2 through Geological Time: Implications for Planetary Climates and Atmospheric Biosignatures
Nitrogen is a major nutrient for all life on Earth and could plausibly play a
similar role in extraterrestrial biospheres. The major reservoir of nitrogen at
Earth's surface is atmospheric N2, but recent studies have proposed that the
size of this reservoir may have fluctuated significantly over the course of
Earth's history with particularly low levels in the Neoarchean - presumably as
a result of biological activity. We used a biogeochemical box model to test
which conditions are necessary to cause large swings in atmospheric N2
pressure. Parameters for our model are constrained by observations of modern
Earth and reconstructions of biomass burial and oxidative weathering in deep
time. A 1-D climate model was used to model potential effects on atmospheric
climate. In a second set of tests, we perturbed our box model to investigate
which parameters have the greatest impact on the evolution of atmospheric pN2
and consider possible implications for nitrogen cycling on other planets. Our
results suggest that (a) a high rate of biomass burial would have been needed
in the Archean to draw down atmospheric pN2 to less than half modern levels,
(b) the resulting effect on temperature could probably have been compensated by
increasing solar luminosity and a mild increase in pCO2, and (c) atmospheric
oxygenation could have initiated a stepwise pN2 rebound through oxidative
weathering. In general, life appears to be necessary for significant
atmospheric pN2 swings on Earth-like planets. Our results further support the
idea that an exoplanetary atmosphere rich in both N2 and O2 is a signature of
an oxygen-producing biosphere.Comment: 33 pages, 11 figures, 2 tables (includes appendix), published in
Astrobiolog
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.
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