21 research outputs found
Abiotic Ozone and Oxygen in Atmospheres Similar to Prebiotic Earth
The search for life on planets outside our solar system will use
spectroscopic identification of atmospheric biosignatures. The most robust
remotely-detectable potential biosignature is considered to be the detection of
oxygen (O_2) or ozone (O_3) simultaneous to methane (CH_4) at levels indicating
fluxes from the planetary surface in excess of those that could be produced
abiotically. Here, we use an altitude-dependent photochemical model with the
enhanced lower boundary conditions necessary to carefully explore abiotic O_2
and O_3 production on lifeless planets with a wide variety of volcanic gas
fluxes and stellar energy distributions. On some of these worlds, we predict
limited O_2 and O_3 build up, caused by fast chemical production of these
gases. This results in detectable abiotic O_3 and CH_4 features in the
UV-visible, but no detectable abiotic O_2 features. Thus, simultaneous
detection of O_3 and CH_4 by a UV-visible mission is not a strong biosignature
without proper contextual information. Discrimination between biological and
abiotic sources of O_2 and O_3 is possible through analysis of the stellar and
atmospheric context - particularly redox state and O atom inventory - of the
planet in question. Specifically, understanding the spectral characteristics of
the star and obtaining a broad wavelength range for planetary spectra should
allow more robust identification of false positives for life. This highlights
the importance of wide spectral coverage for future exoplanet characterization
missions. Specifically, discrimination between true- and false-positives may
require spectral observations that extend into infrared wavelengths, and
provide contextual information on the planet's atmospheric chemistry.Comment: Accepted for publication in The Astrophysical Journal. 43 pages, 6
figure
Spectral signatures of photosynthesis II: coevolution with other stars and the atmosphere on extrasolar worlds
As photosynthesis on Earth produces the primary signatures of life that can
be detected astronomically at the global scale, a strong focus of the search
for extrasolar life will be photosynthesis, particularly photosynthesis that
has evolved with a different parent star. We take planetary atmospheric
compositions simulated by Segura, et al. (2003, 2005) for Earth-like planets
around observed F2V and K2V stars, modeled M1V and M5V stars, and around the
active M4.5V star AD Leo; our scenarios use Earth's atmospheric composition as
well as very low O2 content in case anoxygenic photosynthesis dominates. We
calculate the incident spectral photon flux densities at the surface of the
planet and under water. We identify bands of available photosynthetically
relevant radiation and find that photosynthetic pigments on planets around F2V
stars may peak in absorbance in the blue, K2V in the red-orange, and M stars in
the NIR, in bands at 0.93-1.1 microns, 1.1-1.4 microns, 1.5-1.8 microns, and
1.8-2.5 microns. In addition, we calculate wavelength restrictions for
underwater organisms and depths of water at which they would be protected from
UV flares in the early life of M stars. We estimate the potential productivity
for both surface and underwater photosynthesis, for both oxygenic and
anoxygenic photosynthesis, and for hypothetical photosynthesis in which longer
wavelength, multi-photosystem series are used.Comment: 59 pages, 4 figures, 4 tables, forthcoming in Astrobiology ~March
200
The MUSCLES Treasury Survey. V. FUV Flares on Active and Inactive M Dwarfs
M dwarf stars are known for their vigorous flaring. This flaring could impact
the climate of orbiting planets, making it important to characterize M dwarf
flares at the short wavelengths that drive atmospheric chemistry and escape. We
conducted a far-ultraviolet flare survey of 6 M dwarfs from the recent MUSCLES
(Measurements of the Ultraviolet Spectral Characteristics of Low-mass
Exoplanetary Systems) observations, as well as 4 highly-active M dwarfs with
archival data. When comparing absolute flare energies, we found the
active-M-star flares to be about 10 more energetic than inactive-M-star
flares. However, when flare energies were normalized by the star's quiescent
flux, the active and inactive samples exhibited identical flare distributions,
with a power-law index of - (cumulative distribution). The
rate and distribution of flares are such that they could dominate the FUV
energy budget of M dwarfs, assuming the same distribution holds to flares as
energetic as those cataloged by Kepler and ground-based surveys. We used the
observed events to create an idealized model flare with realistic spectral and
temporal energy budgets to be used in photochemical simulations of exoplanet
atmospheres. Applied to our own simulation of direct photolysis by photons
alone (no particles), we find the most energetic observed flares have little
effect on an Earth-like atmosphere, photolyzing 0.01% of the total O
column. The observations were too limited temporally (73 h cumulative exposure)
to catch rare, highly energetic flares. Those that the power-law fit predicts
occur monthly would photolyze 1% of the O column and those it
predicts occur yearly would photolyze the full O column. Whether such
energetic flares occur at the rate predicted is an open question.Comment: Accepted to ApJ. v2 fixed some transposed errors, added PDF To
Biosignatures from Earth-Like Planets Around M Dwarfs
Coupled one-dimensional photochemical-climate calculations have been
performed for hypothetical Earth-like planets around M dwarfs. Visible,
near-infrared and thermal-infrared synthetic spectra of these planets were
generated to determine which biosignature gases might be observed by a future,
space-based telescope. Our star sample included two observed active M dwarfs,
AD Leo and GJ 643, and three quiescent model stars. The spectral distribution
of these stars in the ultraviolet generates a different photochemistry on these
planets. As a result, the biogenic gases CH4, N2O, and CH3Cl have substantially
longer lifetimes and higher mixing ratios than on Earth, making them
potentially observable by space-based telescopes. On the active M-star planets,
an ozone layer similar to Earth's was developed that resulted in a
spectroscopic signature comparable to the terrestrial one. The simultaneous
detection of O2 (or O3) and a reduced gas in a planet's atmosphere has been
suggested as strong evidence for life. Planets circling M stars may be good
locations to search for such evidence.Comment: 34 pages, 10 figures, Astrobiology, in pres
La Tierra vista como exoplaneta
Earth, as the only example of a habitable world, offers the first elements to characterize the spectra
of terrestrial planets around other stars. Those planets may be detected in the next decade by missions like
CoRoT and Kepler, and characterized by Terrestrial Planet finder and Darwin. In this paper, I reviewed
the research that uses Earth to determine the possible characteristics of habitable worlds around other
stars. Comparing Earth�s characteristics with those of the terrestrial planets in the Solar System, the
main properties of a habitable world have been determined. A habitable planet must have atmosphere,
liquid water and the right size to keep that atmosphere and to maintain tectonic activity for long geologic
periods. A habitable world could be recognized as such by the detection of biosignatures on its spectrum.
Simulations of past and present Earth-like atmospheres and the knowledge of the geological evolution of
our planet indicate that oxygen (O2) is an excellent signature of life, in particular if it comes along with
compounds like methane and nitrous oxide. Also, the pigments used by photosynthetic organisms could
generate a signature in a planet�s spectrum. This signature may be similar to the chlorophyll absorption
on Earth. Earthshine observations help to analyze the disk average spectrum of our planet and to determine
the changes of the biosignatures given certain conditions of illumination and geometry. From such
observations and models that generate disk averaged spectra of Earth it has been found that clouds are
the biggest challenge to identify biosignatures and characteristics of the planetary surface in general. The
atmospheric abundance of the compounds produced by life depends on the amount of ultraviolet radiation
received by the planet as it drives most of the atmospheric chemistry. This radiation depends on the stellar
type of the planet�s parent star. The characterization of terrestrial planets requires the knowledge of the
target star properties (age, effective temperature, radiation emitted from the ultraviolet to the infrared),
as well as to build spectra libraries that allow recognizing habitable worlds from those that are not.La Tierra, como único ejemplo de planeta habitable, nos da los primeros elementos para caracterizar
el espectro de planetas de tipo terrestre alrededor de otras estrellas que podrían ser detectados en
el transcurso de la próxima década gracias a misiones como CoRoT y Kepler y caracterizados por las
misiones Terrestrial Planet Finder y Darwin. En este artículo se compendian los estudios que utilizan a
la Tierra para determinar las posibles características de mundos habitables alrededor de otras estrellas.
A partir de comparar las características de la Tierra con las de los demás planetas terrestres del Sistema
Solar se ha determinado que, en principio, un planeta habitable debe tener atmósfera, agua líquida y el
tamaño adecuado para retener dicha atmósfera y mantener actividad tectónica por periodos de tiempo
geológicamente largos. Un planeta habitado podría ser reconocido como tal a partir de la detección de
bioseñales en su espectro. Simulaciones de atmósferas similares a la Tierra presente y pasada, así como
el conocimiento de la evolución geológica de nuestro planeta indican que el oxígeno (O2) resulta una
excelente señal de vida, en especial si está acompañado de compuestos como el metano o el óxido nitroso.
Los pigmentos usados por organismos fotosintéticos también pueden generar una señal en el espectro
de un planeta, la cual sería similar a la absorción de la clorofila en la Tierra. Las observaciones del
brillo de la Tierra permiten analizar el espectro promediado del disco de nuestro planeta y determinar
los cambios de las bioseñales dadas diferentes condiciones de iluminación y geometría. A partir de estas
observaciones y modelos que generan espectros promediados del disco terrestre, se ha encontrado que
la presencia de nubes en una atmósfera es el mayor reto para reconocer bioseñales y, en general, las
características de la superficie planetaria. La concentración atmosférica de los compuestos producidos
por la vida depende de la cantidad de radiación ultravioleta que recibe el planeta, pues ésta controla
buena parte de la química atmosférica. Esta radiación depende del tipo de estrella alrededor de la cual
gira el planeta. La caracterización de planetas de tipo terrestre requiere conocer las propiedades de la
estrella alrededor de la cual se buscan planetas (edad, temperatura superficial, radiación emitida del
ultravioleta al infrarrojo), así como construir bibliotecas de espectros planetarios que permitan reconocer
los mundos habitables de los que no lo son