4,775 research outputs found
Kinetic conversion of CO to CH4 in the Solar System
Some of the most interesting chemistry in the Solar System involves changes in the oxidation state of the simple carbon species. The chemical pathways for the conversion of CH4 to CO and CO2 are for the most part known. The reverse process, the reduction of CO to CH4, is, however, poorly understood. This is surprising in view of the importance of the reduction process in the chemistry of the Solar System. Recently we investigated the chemical kinetics of a hitherto unsuspected reaction. It is argued that the formation of the methoxy radical (CH3O) from H+H2CO may play an essential role in the reduction of CO to CH4. The rate coefficient for this reaction has been estimated using the approximate theory of J. Troe and transition state theory. We will discuss the implications of this reaction for the chemistry of CO on Jupiter, in the solar nebula, for interpreting the laboratory experiments of A. Bar-Nun and A. Shaviv and A. Bar-Nun and S. Chang, and for organic synthesis in the prebiotic terrestrial atmosphere. The possible relation of CO reduction in the solar nebula and polyoxymethylene observed in comet Halley will be discussed
Influence of Doubled CO2 on Ozone via Changes in the BrewerâDobson Circulation
In this short note, the effect of enhanced circulation due to doubling CO2 on ozone is investigated. The difference of BrewerâDobson circulation (BDC) between the doubled CO2 and control run from an idealized atmospheric general circulation model is added to the BDC climatology derived from National Centers for Environmental PredictionâDepartment of Energy Reanalysis 2 (NCEP2) from 1979 to 2002. Then it is used to drive the California Institute of Technology/Jet Propulsion Laboratory (Caltech/JPL) two-dimensional chemistry and transport model. The results reveal that the total ozone increases by 7 and 3.5 Dobson units (DU) in the high latitudes of the Northern and Southern Hemispheres, respectively, and decreases by 4 DU in the Tropics as a result of the increase in BDC associated with doubled CO2. If the change of eddy mixing coefficients after doubling CO2 is also considered, the total ozone will increase by 6.5 and 3 DU in the high latitudes of the Northern and Southern Hemispheres after combining both effects from the change in BDC and eddy mixing coefficients
Atmospheric pressure as a natural climate regulator for a terrestrial planet with a biosphere
Lovelock and Whitfield suggested in 1982 that, as the luminosity of the Sun increases over its life cycle, biologically enhanced silicate weathering is able to reduce the concentration of atmospheric carbon dioxide (CO_2) so that the Earth's surface temperature is maintained within an inhabitable range. As this process continues, however, between 100 and 900 million years (Ma) from now the CO_2 concentration will reach levels too low for C_3 and C_4 photosynthesis, signaling the end of the solar-powered biosphere. Here, we show that atmospheric pressure is another factor that adjusts the global temperature by broadening infrared absorption lines of greenhouse gases. A simple model including the reduction of atmospheric pressure suggests that the life span of the biosphere can be extended at least 2.3 Ga into the future, more than doubling previous estimates. This has important implications for seeking extraterrestrial life in the Universe. Space observations in the infrared region could test the hypothesis that atmospheric pressure regulates the surface temperature on extrasolar planets
Convective flow in the presence of a small obstacle: Symmetry breaking, attractors, hysteresis, and information
This work explores the stability and hysteresis effects that occur when a small sink of momentum is introduced into a heat-driven, two-dimensional convective flow. As per standard fluid mechanical intuition, the system minimizes work generation and dissipation when one component of momentum is extracted. However, when the sink absorbs all incoming momentum, the system configures itself such that one of the convection plumes aligns directly with the sink. This state is the most hydrodynamically stable, but it maximizes, rather than minimizes extracted mechanical work. Furthermore, in the case of only vertical momentum extraction, there are two attractors, with different stabilities. Numerical experiments involving slow variations of the horizontal momentum extraction show a clear history dependence. This hysteresis preserves information about the system's past states, and hence represents a primitive memory. The momentum sink can also be used to manipulate the horizontal position of the flow field, with potential applications in microfluidics and laminar convection systems. This simple system exhibits the phenomena of autocatalysis (during the initial growth of the convection plumes), negative feedback (the attractors are either fully or quasistable), memory, and elementary computation
Triton: Topside ionosphere and nitrogen escape
The principal ion in the ionosphere of Triton is N^+. Energetic electrons of magnetospheric origin are the primary source of ionization, with a smaller contribution due to photoionization. To explain the topside plasma scale height, we postulate that N^+ ions escape from Triton. The loss rate is 3.4 Ă 10^7 cm^(â2) s^(â1) or 7.9 Ă 10^(24) ions s^(â1). Dissociative recombination of N^+_2 produces neutral exothermic fragments that can escape from Triton. The rate is estimated to be 8.6 Ă 10^6 N cm^(â2) s^(â1) or 2.0 Ă 10^(24) atoms s^(â1). Implications for the magnetosphere of Neptune and Triton's evolution are discussed
Observed Tightening of Tropical Ascent in Recent Decades and Linkage to Regional Precipitation Changes
Climate models predict that the tropical ascending region should tighten under global warming, but observational quantification of the tightening rate is limited. Here we show that the observed spatial extent of the relatively moist, rainy and cloudy regions in the tropics associated with largeâscale ascent has been decreasing at a rate of â1%/decade (â5%/K) from 1979 to 2016, resulting from combined effects of interdecadal variability and anthropogenic forcings, with the former contributing more than the latter. The tightening of tropical ascent is associated with an increase in the occurrence frequency of extremely strong ascent, leading to an increase in the average precipitation rate in the top 1% of monthly rainfall in the tropics. At the margins of the convective zones such as the Southeast Amazonia region, the contraction of largeâscale ascent is related to a longâterm drying trend about â3.2%/decade in the past 38 years
A Multiple Scattering Polarized Radiative Transfer Model: Application to HD 189733b
We present a multiple scattering vector radiative transfer model which
produces disk integrated, full phase polarized light curves for reflected light
from an exoplanetary atmosphere. We validate our model against results from
published analytical and computational models and discuss a small number of
cases relevant to the existing and possible near-future observations of the
exoplanet HD 189733b. HD 189733b is arguably the most well observed exoplanet
to date and the only exoplanet to be observed in polarized light, yet it is
debated if the planet's atmosphere is cloudy or clear. We model reflected light
from clear atmospheres with Rayleigh scattering, and cloudy or hazy atmospheres
with Mie and fractal aggregate particles. We show that clear and cloudy
atmospheres have large differences in polarized light as compared to simple
flux measurements, though existing observations are insufficient to make this
distinction. Futhermore, we show that atmospheres that are spatially
inhomogeneous, such as being partially covered by clouds or hazes, exhibit
larger contrasts in polarized light when compared to clear atmospheres. This
effect can potentially be used to identify patchy clouds in exoplanets. Given a
set of full phase polarimetric measurements, this model can constrain the
geometric albedo, properties of scattering particles in the atmosphere and the
longitude of the ascending node of the orbit. The model is used to interpret
new polarimetric observations of HD 189733b in a companion paper.Comment: 13 pages, 13 figures. Accepted for publication in Ap
Perfect State Transfer, Effective Gates and Entanglement Generation in Engineered Bosonic and Fermionic Networks
We show how to achieve perfect quantum state transfer and construct effective
two-qubit gates between distant sites in engineered bosonic and fermionic
networks. The Hamiltonian for the system can be determined by choosing an
eigenvalue spectrum satisfying a certain condition, which is shown to be both
sufficient and necessary in mirror-symmetrical networks. The natures of the
effective two-qubit gates depend on the exchange symmetry for fermions and
bosons. For fermionic networks, the gates are entangling (and thus universal
for quantum computation). For bosonic networks, though the gates are not
entangling, they allow two-way simultaneous communications. Protocols of
entanglement generation in both bosonic and fermionic engineered networks are
discussed.Comment: RevTeX4, 6 pages, 1 figure; replaced with a more general example and
clarified the sufficient and necessary condition for perfect state transfe
Enceladus: Cassini observations and implications for the search for life
Aims. The recent Cassini discovery of water vapor plumes ejected from the south pole of the Saturnian satellite, Enceladus, presents a unique window of opportunity for the detection of extant life in our solar system.
Methods. With its significant geothermal energy source propelling these plumes >80 km from the surface of the moon and the ensuing large temperature gradient with the surrounding environment, it is possible to have the weathering of rocks by liquid water at the rock/liquid interface. For the cases of the putatively detected salt-water oceans beneath the ice crusts of Europa and Callisto, an isolated subsurface ocean without photosynthesis or contact with an oxidizing atmosphere will approach chemical equilibrium and annihilate any ecosystems dependent on redox gradients unless there is a substantial alternative energy source. This thermodynamic tendency imposes severe constraints on any biota that is based on chemical energy. On Enceladus, the weathering of rocks by liquid water and any concomitant radioactive emissions are possible incipient conditions for life. If there is CO, CO2 and NH3 present in the spectra obtained from the plume, then this is possible evidence that amino acids could be formed at the rock/liquid interface of Enceladus. The combination of a hydrological cycle, chemical redox gradient and geochemical cycle give favorable conditions for life.
Results. We discuss the search for signatures of these species and organics in the Cassini UVIS spectra of the plume and implications for the possible detection of life
Spin Star as Switch for Quantum Networks
Quantum state transfer is an important task in quantum information
processing. It is known that one can engineer the couplings of a
one-dimensional spin chain to achieve the goal of perfect state transfer. To
leverage the value of these spin chains, a spin star is potentially useful for
connecting different parts of a quantum network. In this work, we extend the
spin-chain engineering problem to the problems with a topology of a star
network. We show that a permanently coupled spin star can function as a network
switch for transferring quantum states selectively from one node to another by
varying the local potentials only. Together with one-dimensional chains, this
result allows applications of quantum state transfer be applied to more general
quantum networks.Comment: 10 pages, 2 figur
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