7,837 research outputs found
Which Way to Go? Observations Based on Discussion on Global Perspectives and Energy Strategies
This short paper is a glimpse of the recent lively discussion on securing future energy demand. The debate, which resorts to technological and economic arguments, uses mutually exclusive concepts, such as "hard" and "soft" technology, in order to differentiate between opposing approaches to the evaluation of technological benefits and risks.
This is a summary of some conclusions that emerged from such discussions of D. Meadows, co-author of the Club of Rome study "Limits to Growth," Amory Lovins of "Friends of the Earth," and members of the Energy Systems Program. The paper concludes with a short list of research topics which could help clarify the implications of alternate paths
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
Identifying Planetary Biosignature Impostors: Spectral Features of CO and O4 Resulting from Abiotic O2/O3 Production
O2 and O3 have been long considered the most robust individual biosignature
gases in a planetary atmosphere, yet multiple mechanisms that may produce them
in the absence of life have been described. However, these abiotic planetary
mechanisms modify the environment in potentially identifiable ways. Here we
briefly discuss two of the most detectable spectral discriminants for abiotic
O2/O3: CO and O4. We produce the first explicit self-consistent simulations of
these spectral discriminants as they may be seen by JWST. If JWST-NIRISS and/or
NIRSpec observe CO (2.35, 4.6 um) in conjunction with CO2 (1.6, 2.0, 4.3 um) in
the transmission spectrum of a terrestrial planet it could indicate robust CO2
photolysis and suggest that a future detection of O2 or O3 might not be
biogenic. Strong O4 bands seen in transmission at 1.06 and 1.27 um could be
diagnostic of a post-runaway O2-dominated atmosphere from massive H-escape. We
find that for these false positive scenarios, CO at 2.35 um, CO2 at 2.0 and 4.3
um, and O4 at 1.27 um are all stronger features in transmission than O2/O3 and
could be detected with SNRs 3 for an Earth-size planet orbiting a
nearby M dwarf star with as few as 10 transits, assuming photon-limited noise.
O4 bands could also be sought in UV/VIS/NIR reflected light (at 0.345, 0.36,
0.38, 0.445, 0.475, 0.53, 0.57, 0.63, 1.06, and 1.27 um) by a next generation
direct-imaging telescope such as LUVOIR/HDST or HabEx and would indicate an
oxygen atmosphere too massive to be biologically produced.Comment: 7 pages, 4 figures, accepted to the Astrophysical Journal Letter
Attacking Group Protocols by Refuting Incorrect Inductive Conjectures
Automated tools for finding attacks on flawed security protocols often fail to deal adequately with group protocols. This is because the abstractions made to improve performance on fixed 2 or 3 party protocols either preclude the modelling of group protocols all together, or permit modelling only in a fixed scenario, which can prevent attacks from being discovered. This paper describes Coral, a tool for finding counterexamples to incorrect inductive conjectures, which we have used to model protocols for both group key agreement and group key management, without any restrictions on the scenario. We will show how we used Coral to discover 6 previously unknown attacks on 3 group protocols
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
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
Discrete Model of Ideological Struggle Accounting for Migration
A discrete in time model of ideological competition is formulated taking into
account population migration. The model is based on interactions between global
populations of non-believers and followers of different ideologies. The complex
dynamics of the attracting manifolds is investigated.
Conversion from one ideology to another by means of (i) mass media influence
and (ii) interpersonal relations is considered. Moreover a different birth rate
is assumed for different ideologies, the rate being assumed to be positive for
the reference population, made of initially non-believers. Ideological
competition can happen in one or several regions in space. In the latter case,
migration of non-believers and adepts is allowed; this leads to an enrichment
of the ideological dynamics. Finally, the current ideological situation in the
Arab countries and China is commented upon from the point of view of the
presently developed mathematical model. The massive forced conversion by
Ottoman Turks in the Balkans is briefly discussed.Comment: 24 pages, with 5 figures and 52 refs.; prepared for a Special issue
of Advances in Complex System
Detection of Ocean Glint and Ozone Absorption Using LCROSS Earth Observations
The Lunar CRater Observation and Sensing Satellite (LCROSS) observed the
distant Earth on three occasions in 2009. These data span a range of phase
angles, including a rare crescent phase view. For each epoch, the satellite
acquired near-infrared and mid-infrared full-disk images, and partial-disk
spectra at 0.26-0.65 microns (R~500) and 1.17-2.48 microns (R~50). Spectra show
strong absorption features due to water vapor and ozone, which is a
biosignature gas. We perform a significant recalibration of the UV-visible
spectra and provide the first comparison of high-resolution visible Earth
spectra to the NASA Astrobiology Institute's Virtual Planetary Laboratory
three-dimensional spectral Earth model. We find good agreement with the
observations, reproducing the absolute brightness and dynamic range at all
wavelengths for all observation epochs, thus validating the model to within the
~10% data calibration uncertainty. Data-model comparisons reveal a strong ocean
glint signature in the crescent phase dataset, which is well matched by our
model predictions throughout the observed wavelength range. This provides the
first observational test of a technique that could be used to determine
exoplanet habitability from disk-integrated observations at visible and
near-infrared wavelengths, where the glint signal is strongest. We examine the
detection of the ozone 255 nm Hartley and 400-700 nm Chappuis bands. While the
Hartley band is the strongest ozone feature in Earth's spectrum, false
positives for its detection could exist. Finally, we discuss the implications
of these findings for future exoplanet characterization missions.Comment: Accepted to The Astrophysical Journal; recalibration data for LCROSS
VSP can be found at:
https://sites.google.com/site/tdrobinsonscience/science/moo
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