arXiv subjects

Rafael Rianço-Silva

Publications and source records attributed to Rafael Rianço-Silva.

4 recordsLinked to original sources

arXiv2026

Detection of hydrocarbons in Titan using high-resolution cross-correlation spectroscopy

High-resolution cross-correlation spectroscopy (HRCCS) is a powerful technique for detecting molecules whose individual spectral lines are too weak to be identified directly, but its sensitivity is limited by the availability and quality of high-resolution opacity data. Many molecules of atmospheric and astrobiological interest lack complete line lists, restricting traditional template-based searches. In this work, we use Titan as a controlled testbed to develop and validate a new cross-section-based methodology for HRCCS template construction. We analyse K-band CRIRES+ observations of Titan (1.99 - 2.48 μm), and compute cross-correlation functions using both line-by-line and cross-section-based templates. Our analysis recovers known hydrocarbons such as methane (CH4) and acetylene (C2H2), and yields the first HRCCS detection of ethane (C2H6) with a significance of SNRpeak = 5.17 {\pm} 0.07. The ethane detection was made possible exclusively through cross-section-based templates, as no high-resolution line list currently exists for this molecule. These results demonstrate that cross-section-based template construction is a practical and powerful strategy for extending HRCCS to molecules that currently lack reliable line lists, and establish Titan as a benchmark for calibrating molecular detection techniques that can be applied to both solar system and exoplanet atmospheres. Future applications of this approach to other ground-based high-resolution spectrographs, as well as to JWST's highest-resolution modes and next-generation facilities such as the ELT, could significantly expand the inventory of molecules detectable in planetary atmospheres.

astro-ph.EP↗

arXiv2026

High Resolution Optical Methane Linelist from observations of Titan for Cross-Correlation studies

Exoplanet atmosphere characterization heavily relies on molecular spectroscopic data. Despite efforts to obtain comprehensive spectral libraries for the chemical characterization of exoplanet atmospheres, large gaps remain, particularly for larger molecules and higher frequencies at high spectral resolution. One key example is the methane (CH4) optical spectrum. CH4, the simplest hydrocarbon, is a crucial species for exoplanet atmosphere characterization and a possible biosignature. However, until now, high-resolution linelists at optical wavelengths for CH4 have been very challenging to obtain either experimentally or computationally, leaving the high resolution spectrum of CH4 uncharacterised across most of the visible spectrum. This restricts exploration of CH4 absorption in the optical regime, as upcoming instruments such as ELT-ANDES and VLT-RISTRETTO will start probing the atmospheres of ever smaller exoplanets in optical wavelengths. To address this spectroscopic data limitation, we observed Titan's optical spectrum, dominated by CH4 absorption, at the highest spectral resolution to date with VLT-ESPRESSO. From it, we produced an empirical, low-temperature high-resolution (R ~ 190000) linelist of CH4 in optical wavelengths which we present here, with thousands of previously unidentified lines. We employ this CH4 linelist (RRS-2026) to build a template suitable for high resolution cross-correlation spectroscopy (HRCCS) studies, a first for CH4 in optical wavelengths. With this new linelist, we performed the first HRCCS detection of CH4 in the atmospheres of Titan and Jupiter using optical high resolution spectra. This work sets the stage for the search for CH4 in exoplanet atmospheres through HRCCS with current and future ground-based high-resolution optical spectrographs, showcasing how Solar System observations provide useful products for exoplanet research.

astro-ph.EP↗

arXiv2026

Global Abiotic Sulfur Cycling on Earth-like Terrestrial Planets

Sulfur is a redox active element that may have helped mediate an electron flow that kickstarted life and which presently is an essential element for all life on Earth. Despite current uncertainties in global sulfur fluxes, modeling sulfur's abiotic cycling through Earth's deep history is important for understanding the impact of a planet wide biosphere on sulfur geochemical cycling and availability and vice versa. We present here an open-source, dynamical box model for estimating global sulfur fluxes and concentrations among surface and deep Earth reservoirs over Earth history, allowing tracking and estimation of the sulfur distribution in planetary reservoirs over deep time in the absence of life. While the main model presented here does not take into account the abrupt evolution of redox-shunting biosynthetic pathways such as oxygenic photosynthesis, we also modeled the abiotic sulfur cycle before and after a Great Oxidation Event-like transition on Earth-like planets. Our results suggest a considerably distinct chemical makeup of sulfur content in marine sediments in the absence of life on an Earth-like planet, leading to a marine sediment sulfate content two orders of magnitude larger than on present-day Earth and a marine sediment sulfide content 4 orders of magnitude lower than on present day Earth, attributable to the lack of microbial sulfur metabolism. This model could be useful for understanding sulfur cycling on potentially habitable exoplanets.

astro-ph.EP↗

arXiv2026

Detection of C3 in Titan with VLT-ESPRESSO

Titan is regarded as a natural laboratory in the Solar System for studying atmospheric photochemistry and the abiotic production of organic molecules on cold small exoplanets. Since the end of the Cassini-Huygens mission, telescope observations have enabled new detections of increasingly complex carbon-based molecules at infrared and sub-millimetre wavelengths, while the optical regime has been largely overlooked. Following a recent tentative detection of the 405 nm absorption band of C3 in Titan in archived optical VLT UVES spectra at resolving power R = 60000, this work reports an eight sigma detection of the C3 405 nm absorption band in Titan using dedicated ultra high resolution VLT ESPRESSO observations at R = 190000, the highest spectral resolution optical observations of Titan to date. The VLT ESPRESSO spectrum is compared to model spectra of Titan with varying C3 abundances. A chi squared analysis is used to assess the agreement between non solar spectral features and C3 absorption as the C3 abundance is varied, and a Bayesian Markov Chain Monte Carlo fit between model and observed spectra is performed. The chi squared analysis yields an eight sigma detection of C3, consistent with a C3 column density of approximately 1.5E13 cm-2, while the MCMC fit retrieves a C3 column density of 1.47E13 cm-2 at five sigma. These values are consistent with the order of magnitude predicted by photochemical models, which reach parts per million levels in the Titan mesosphere. This work demonstrates the usefulness of instruments and techniques originally developed for exoplanet research when applied to Solar System targets.

astro-ph.EP↗