arXiv subjects

Juan Alday

Publications and source records attributed to Juan Alday.

7 recordsLinked to original sources

arXiv2026

Oxygen isotope fractionation in the Martian atmosphere induced by CO$_2$ photolysis and O$_3$ formation

The enrichment of heavy isotopes of volatile elements in the Martian atmosphere indicates that Mars lost a large portion of its atmosphere through escape to space. Recent atmospheric measurements by ExoMars Trace Gas Orbiter (TGO) have suggested that the vertical profiles of oxygen isotopic compositions are influenced by chemical reactions involving isotopic fractionation. However, their quantitative impacts have not yet been fully evaluated. In this study, we develop a 1D photochemical model that incorporates oxygen isotopic fractionation associated with CO$_2$ photolysis and O$_3$ formation to investigate the vertical profiles of oxygen isotopic compositions. Our calculations show that CO is depleted in heavy oxygen isotopes relative to CO$_2$, reaching $\delta ^{18}$O $\sim -25$ per mil and $\delta ^{17}$O $\sim -15$ per mil, primarily due to isotopic fractionation during CO$_2$ photolysis. The vertical profiles of oxygen and carbon isotopic compositions are in good agreement between our model and the TGO measurements. O$_3$ is strongly enriched in $^{18}$O and $^{17}$O, reaching $\delta ^{18}$O $\sim 100$ per mil and $\delta ^{17}$O $\sim 50$ per mil as a consequence of the isotopic fractionation during its formation, whereas atomic oxygen is highly depleted in the heavy oxygen isotopes with $\delta ^{18}$O $\lesssim -100$ per mil and $\delta ^{17}$O $\lesssim -50$ per mil so as to compensate for their enrichment in O$_3$. These chemical fractionation processes can deplete the heavy oxygen isotopes in species that escape from the upper atmosphere, and thereby enhance the isotopic fractionation associated with oxygen escape to space. Such fractionated isotopic compositions of escaping oxygen may be detectable by the Martian Moons eXploration (MMX) mission.

astro-ph.EP↗

arXiv2025

Using SOFIA's EXES to search for C$_6$H$_2$ and C$_4$N$_2$ in Titan's atmosphere

In Titan's atmosphere, the chemistry of small hydrocarbons and nitriles represent an important link from molecular species to the ubiquitous organic haze that gives Titan its characteristic yellow color. Here we present a new search for two previously undetected molecules, triacetylene (C$_{6}$H$_{2}$) and the gas phase dicyanoacetylene (C$_{4}$N$_{2}$), using the Echelon-Cross-Echelle Spectrograph (EXES) instrument aboard the SOFIA (Stratospheric Observatory For Infrared Astronomy) aircraft. We do not detect these two molecules but determine upper limits for their mixing ratios and column abundances. We find the $3\sigma$ upper limits on the uniform volume mixing ratio (VMR) above 100 km for C$_{6}$H$_{2}$ to be $4.3\times10^{-11}$ which is lower than the photochemical model predictions. This new upper limit suggests that the growth of linear molecules is inhibited. We also put a strict upper limit on the uniform VMR for gas phase C$_{4}$N$_{2}$ above 125 km to be $1.0\times10^{-10}$. This upper limit is well below the saturation mixing ratio at this altitude for C$_{4}$N$_{2}$ and greatly limits the feasibility of C$_{4}$N$_{2}$ forming ice from condensation.

astro-ph.EP↗

arXiv2025

archNEMESIS: an open-source Python package for analysis of planetary atmospheric spectra

ArchNEMESIS is an open-source Python package developed for the analysis of remote sensing spectroscopic observations of planetary atmospheres. It is based on the widely used NEMESIS radiative transfer and retrieval tool, which has been extensively used for the investigation of a wide variety of planetary environments. The main goal of archNEMESIS is to provide the capabilities of its Fortran-based predecessor, keeping or exceeding the efficiency in the calculations, and benefitting from the advantages Python tools provide in terms of usability and portability. The code, stored in a public GitHub repository under a GPL-v3.0 license, is accompanied by detailed documentation available at https://archnemesis.readthedocs.io/.

astro-ph.IM↗

arXiv2024

NEMESISPY: A Python package for simulating and retrieving exoplanetary spectra

NEMESISPY is a Python package developed to perform parametric atmospheric modelling and radiative transfer calculation for the retrievals of exoplanetary spectra. It is a recent development of the well-established Fortran NEMESIS library (P. G. J. Irwin et al., 2008), which has been applied to the atmospheric retrievals of both solar system planets and exoplanets employing numerous different observing geometries. NEMESISPY can be easily interfaced with Bayesian inference algorithms to retrieve atmospheric properties from spectroscopic observations. Recently, NEMESISPY has been applied to the retrievals of Hubble and Spitzer data of a hot Jupiter (Yang et al., 2023), as well as to JWST/Mid-Infrared Instrument (JWST/MIRI) data of a hot Jupiter (Yang et al., 2024).

astro-ph.EP↗

arXiv2022

Callisto's atmosphere: First evidence for H2 and constraints on H2O

We explore the parameter space for the contribution to Callisto's H corona observed by the Hubble Space Telescope (Roth et al. 2017a) from sublimated H2O and radiolytically produced H2 using the Direct Simulation Monte Carlo (DSMC) method. The spatial morphology of this corona produced via photo- and magnetospheric electron impact-induced dissociation is described by tracking the motion of and simulating collisions between the hot H atoms and thermal molecules including a near-surface O2 component. Our results indicate that sublimated H2O produced from the surface ice, whether assumed to be intimately mixed with or distinctly segregated from the dark non-ice or ice-poor regolith, cannot explain the observed structure of the H corona. On the other hand, a global H2 component can reproduced the observation, and is also capable of producing the enhanced electron densities observed at high altitudes by Galileo's plasma-wave instrument (Gurnett et al., 1997, 2000), providing the first evidence of H2 in Callisto's atmosphere. The range of H2 surface densities explored, under a variety of conditions, that are consistent with these observations is ~(0.4-1)x10^8 cm^-3. The simulated H2 escape rates and estimated lifetimes suggest that Callisto has a neutral H2 torus. We also place a rough upper limit on the peak H2O number density (<~10^8 cm^-3), column density (<~10^15 cm^-2), and sublimation flux (<~10^12 cm^-2 s^-1), all of which are 1-2 orders of magnitude less than that assumed in previous models. Finally, we discuss the implications of these results, as well as how they compare to Europa and Ganymede.

astro-ph.EP↗

arXiv2022

A New UV Spectral Feature on Europa: Confirmation of NaCl in Leading-hemisphere Chaos Terrain

Recent visible-wavelength observations of Europa's surface obtained with the Hubble Space Telescope revealed the presence of an absorption feature near 450 nm that appears spatially correlated with leading-hemisphere chaos terrain. This feature was interpreted to reflect the presence of irradiated sodium chloride ultimately sourced from the interior. Here, we use ultraviolet spectra also collected with the Hubble Space Telescope to detect an additional previously unseen absorption near 230 nm, which spatially correlates with the 450 nm feature and with the same leading-hemisphere chaos terrain. We find that the new ultraviolet feature is also well-matched by irradiated sodium chloride at Europa-like conditions. Such confirmation of sodium chloride within geologically young regions has important implications for Europa's subsurface composition.

astro-ph.EP↗

arXiv2020

First detection of ozone in the mid-infrared at Mars: implications for methane detection

The ExoMars Trace Gas Orbiter (TGO) was sent to Mars in March 2016 to search for trace gases diagnostic of active geological or biogenic processes. We report the first observation of the spectral features of Martian ozone (O3) in the mid-infrared range using the Atmospheric Chemistry Suite (ACS) Mid-InfaRed (MIR) channel, a cross-dispersion spectrometer operating in solar occultation mode with the finest spectral resolution of any remote sensing mission to Mars. Observations of ozone were made at high northern latitudes (>65N) prior to the onset of the 2018 global dust storm (Ls = 163-193). During this fast transition phase between summer and winter ozone distribution, the O3 volume mixing ratio observed is 100-200 ppbv near 20 km. These amounts are consistent with past observations made at the edge of the southern polar vortex in the ultraviolet range. The observed spectral signature of ozone at 3000-3060 cm-1 directly overlaps with the spectral range of the methane (CH4) nu3 vibration-rotation band, and it, along with a newly discovered CO2 band in the same region, may interfere with measurements of methane abundance.

astro-ph.EP↗