2,068 research outputs found
Abiotic Input of Fixed Nitrogen by Bolide Impacts to Gale Crater During the Hesperian: Insights From the Mars Science Laboratory
Molecular hydrogen (H2) from volcanic emissions is suggested to warm the Martian surface
when carbon dioxide (CO2) levels dropped from the Noachian (4100 to 3700 Myr) to the Hesperian (3700
to 3000 Myr). Its presence is expected to shift the conversion of molecular nitrogen (N2) into different forms
of fixed nitrogen (N). Here we present experimental data and theoretical calculations that investigate the
efficiency of nitrogen fixation by bolide impacts in CO2‐N2 atmospheres with or without H2. Surprisingly,
nitric oxide (NO) was produced more efficiently in 20% H2 in spite of being a reducing agent and not likely to
increase the rate of nitrogen oxidation. Nevertheless, its presence led to a faster cooling of the shock wave
raising the freeze‐out temperature of NO resulting in an enhanced yield. We estimate that the nitrogen
fixation rate by bolide impacts varied from 7 × 10−4 to 2 × 10−3 g N·Myr−1·cm−2 and could imply fluvial
concentration to explain the nitrogen (1.4 ± 0.7 g N·Myr−1·cm−2) detected as nitrite (NO2
−) and nitrate
(NO3
−) by Curiosity at Yellowknife Bay. One possible explanation is that the nitrogen detected in the
lacustrine sediments at Gale was deposited entirely on the crater's surface and was subsequently dissolved
and transported by superficial and ground waters to the lake during favorable wet climatic conditions. The
nitrogen content sharply decreases in younger sediments of the Murray formation suggesting a decline of H2
in the atmosphere and the rise of oxidizing conditions causing a shortage in the supply to putative
microbial life.We acknowledge the NASA Mars
Science Laboratory Program, Centre
National d'Études Spatiales, the
Universidad Nacional Autónoma de
México (PAPIIT IN109416, IN111619,
and PAPIME PE103216), and the
Consejo Nacional de Ciencia y
Tecnología de México (CONACyT
220626) for their support
The Role of Hydrogen Cyanide in Chemical Evolution
Two major research areas are investigated: The
electrosynthesis of hydrogen cyanide; and the role of
cyanocomplexes in the free - radical oligomerization of
hydrogen cyanide.
The electric discharge production of hydrogen
cyanide from a simulated primitive atmosphere composed
of methane, nitrogen and water vapor was investigated.
The radiation chemical yield (G) of formation of HCN was determined to be 0.26. A free radical mechanism
was proposed to account for the observed chemical
changes. Computer simulations of the reaction
mechanism could effectively model the early stages of
electrolysis of the gas mixture, and permitted the
estimation of the rate of electrosynthesis of hydrogen
cyanide under various atmospheric conditions .
The possible role of cyanocomplexes of transition
elements on the free- radical oligomerization of
hydrogen cyanide was investigated. Aqueous, oxygenfree,
dilute solutions of hydrogen cyanide and hexacyanoferrate(II) or (III) were submitted to various
doses of gamma irradiation. The presence of either
cyanocomplex led to a significant decrease in the rate
of decomposition of hydrogen cyanide. The major
products were ammonia and carbon dioxide . Computer
simulations of these systems permitted the elucidation
of the reaction mechanism and the derivation of rates
of reactions of free- radicals with the cyanocomplexes.
The results obtained provide an insight into the possible
role of cyanocomplexes of transition elements in
chemical evolution
Effect of pupil size on wavefront refraction during orthokeratology
Purpose. It has been hypothesized that central and peripheral refraction, in eyes treated with myopic overnight orthokeratology,
might vary with changes in pupil diameter. The aim of this work was to evaluate the axial and peripheral refraction and optical
quality after orthokeratology, using ray tracing software for different pupil sizes.
Methods. Zemax-EE was used to generate a series of 29 semi-customized model eyes based on the corneal topography
changes from 29 patients who had undergone myopic orthokeratology. Wavefront refraction in the central 80 degrees of the
visual field was calculated using three different quality metrics criteria: Paraxial curvature matching, minimum root mean
square error (minRMS), and the Through Focus Visual Strehl of the Modulation Transfer Function (VSMTF), for 3- and 6-mm
pupil diameters.
Results. The three metrics predicted significantly different values for foveal and peripheral refractions. Compared with the
Paraxial criteria, the other two metrics predicted more myopic refractions on- and off-axis. Interestingly, the VSMTF predicts
only a marginal myopic shift in the axial refraction as the pupil changes from 3 to 6 mm. For peripheral refraction, minRMS and
VSMTF metric criteria predicted a higher exposure to peripheral defocus as the pupil increases from 3 to 6 mm.
Conclusions. The results suggest that the supposed effect of myopic control produced by ortho-k treatments might be dependent
on pupil size. Although the foveal refractive error does not seem to change appreciably with the increase in pupil diameter (VSMTF
criteria), the high levels of positive spherical aberration will lead to a degradation of lower spatial frequencies, that is more
significant under low illumination levels.This study has been funded by FEDER through the COMPETE Program and
by the Portuguese Foundation for Science and Technology (FCT) in the
framework of projects PTDC/SAU-BEB/098391/2008, PTDC/SAU-BEB/
098392/2008, and the Strategic Project PEST-C/FIS/UI607/2011. Dr.
Navarro acknowledges funding by the Spanish Ministery of Economy and
Competivity, grant FIS2014-58303-P. The authors have no proprietary
interests in the methods and devices described in this manuscript
Gibrat’s law for countries
A re-assessment of Gibrat's Law in the context of country size is carried out in this paper. In addition, how similarly population is distributed in cities and countries is analyzed from a temporal perspective. Although evidence of Gibrat's Law is found, it is weaker than that previously established in Rose (2006). This is due to the methodology applied and is especially appreciable in very small countries. Nonetheless, we observe that the population growth process in countries is similar to that of cities. As a result, the similarities between how the population is distributed in these two geographical categories have increased over time
Gibrat’s law for countries
If the population of a given country evolves according to Gibrat'�s Law, its growth rate will be independent of its initial size. This short paper further investigates this empirical regularity by the application of a suitable panel unit root test and non-parametric methods. The evidence regarding its fulfilment is weaker than that previously found
Depth-of-field after orthokeratology: a theoretical study
Trabajo presentado al 8th European Meeting on Visual and Physiological Optics, celebrado en Antwerp (Bélgica) del 22 al 24 de agosto de 2016.We evaluated the possible effect of orthokeratology on accommodative response. Using optical modelling we computed the negative half of the depth-of-field (DoFi) for the range of target vengeances from –1.00 D to –3.00 D, of two eye models designed to mimic the levels of primary and secondary spherical aberration found in 24 patients before and after undergoing orthokeratology (ortho-k). Five trained observers were subjected to a resolution task to identify the negative threshold of the depth-of-field of these model eyes by viewing a set of computed images representative of the model eyes trough focus retinal image quality for five target vengeances (TV), from –1.00 to – 3.00 D. The differences in the DoFi estimated by the five observers were maximum for a –3.00 D TV (0.21 D), with the post ortho-k model presenting a higher DoFi compared to the pre ortho-k model. Differences were consistent for all five observers and all TV’s. In conclusion, the increase in spherical aberration after ortho-k seems to contribute to a small increase in the DoFi. Although small, the benefits might be sufficient to improve retinal image quality in eyes with high accommodative lag.Peer reviewe
Orthogonal basis for the optical transfer function
We propose systems of orthogonal functions qn to represent optical transfer functions (OTF) characterized by including the diffraction-limited OTF as the first basis function q0 OTF perfect. To this end, we apply a powerful and rigorous theoretical framework based on applying the appropriate change of variables to well-known orthogonal systems. Here we depart from Legendre polynomials for the particular case of rotationally symmetric OTF and from spherical harmonics for the general case. Numerical experiments with different examples show that the number of terms necessary to obtain an accurate linear expansion of the OTF mainly depends on the image quality. In the rotationally symmetric case we obtained a reasonable accuracy with approximately 10 basis functions, but in general, for cases of poor image quality, the number of basis functions may increase and hence affect the efficiency of the method. Other potential applications, such as new image quality metrics are also discussed.This research was supported by the Spanish Ministry of Economía y Competitividad and the European Union MTM2014-52859 and FIS2014-58303
Morphology, topography, and optics of the orthokeratology cornea
The goal of this work was to objectively characterize the external morphology, topography, and optics
of the cornea after orthokeratology (ortho-k). A number of 24 patients between the ages of 17 and 30 years
(median ¼ 24 years) were fitted with Corneal Refractive Therapy® contact lenses to correct myopia between
−2.00 and −5.00 diopters (D) (median ¼ −3.41 D). A classification algorithm was applied to conduct an automatic
segmentation based on the mean local curvature. As a result, three zones (optical zone, transition zone,
and peripheral zone) were delimited. Topographical analysis was provided through global and zonal fit to a
general ellipsoid. Ray trace on partially customized eye models provided wave aberrations and retinal
image quality. Monozone topographic description of the ortho-k cornea loses accuracy when compared with
zonal description. Primary (C0
4) and secondary (C0
6) spherical aberration (SA) coefficients for a 5-mm pupil
increased 3.68 and 19 times, respectively, after the treatments. The OZ area showed a strong correlation
with C0
4 (r ¼ −0.49, p < 0.05) and a very strong correlation with C0
6 (r ¼ 0.78, p < 0.01). The OZ, as well as
the TZ, areas did not correlate with baseline refraction. The increase in the eye’s positive SA after ortho-k
is the major factor responsible for the decreased retinal optical quality of the unaccommodated eyeThis work was funded in part by European Fund for Regional
Development (FEDER) through the COMPETE Program and
by the Portuguese Foundation for Science and Technology
(FCT). FCT provided financial support in the framework of
projects PTDC/SAU-BEB/098391/2008, PTDC/SAU-BEB/
098392/2008 and the Strategic Project PEST-C/FIS/UI607/
2011. The authors have no proprietary interest in the methods
and devices described in this manuscript
Plauciano: la amenaza de la domus severiana.
En el trabajo presentamos una visión sintética del ascenso
y caída de un personaje no demasiado tratado por la historiografía
moderna, Plauciano. Sin embargo, tuvo un papel esencial en el acceso
al poder de Septimio Severo y también durante los años en que fue
prefecto del pretorio, desde 197 hasta enero de 205, cuando murió de
forma violenta. De él, dicen las fuentes antiguas que era pariente, pero
sobre todo, amigo muy próximo del emperador. Su cercanía a este, más
que el propio cargo, le permitió obtener un poder político y económico
casi omnímodo, por lo que fue considerado en su tiempo como un cuarto
césar. Incluso casó a su hija Plautilla con el heredero al trono, Caracalla.
Precisamente sus cotas de poder y su proximidad a Septimio suscitaron
recelos y envidias principalmente en el entorno imperial que finalmente
provocaron su caída y su muerte inmediata, aunque las fuentes antiguas
que escriben estos acontecimientos no coinciden en la narración de los
hechos.This paper presents a synthetic view of the rise and fall of a
little-treated character in modern historiography, Plautian. However, this
person played a key role in the accession to power of Septimius Severus
and during the years he was Praetorian Prefect, from 197 to January
205, when he died violently. Ancient sources state that he was a patient
person, and above all, a very close friend of the Emperor. His proximity
to the Emperor, rather than his position proper, enabled him to obtain
almost an absolute political and economic power, being considered at
the time as a fourth Caesar. He even managed that his daughter Plautilla
married the heir of the Caracalla throne. It is precisely his power and
his proximity to Septimius that raised suspicions and jealousies among
the Imperial community, leading ultimately to his downfall and his
immediate death; though the ancient sources that describe these events
do not necessary coincide
- …