arXiv subjects

C. Lefour

Publications and source records attributed to C. Lefour.

3 recordsLinked to original sources

arXiv2026

New 12C/13C and 14N/15N isotopic ratio measurements in Jupiter's stratosphere revealed by ALMA

The collision of comet SL9 with Jupiter in 1994 changed the chemical composition of the Jovian stratosphere for decades. New molecules were detected minutes after the impacts (HCN, CO, CS, etc.) and some are still present today. They were deposited in the stratosphere at pressures lower than 0.1 mbar and were most probably formed by shock-induced chemistry recombining Jovian and cometary material. However, the question of the origin of these molecules is still not completely understood. One way to address this open question is to determine the isotopic composition of the new molecules. Isotopic ratios have long been measured in the Solar System. They present a variety of values depending on the object or the molecule and therefore trace different reservoirs of material. Derivations of carbon and nitrogen isotopic ratios in HCN four years after SL9 showed atypical depletions in the heavier isotopes that had never been observed before in the Solar System. These results suggested an unusual cometary composition or an unknown fractionation mechanism in the hot and shocked air parcels. We aim to measure carbon and nitrogen isotopic ratios in HCN to shed light on the puzzling results of 1998. With Atacama Large Millimeter/submillimeter Array data from 2017 and radiative-transfer calculations, we derived the abundance of two HCN isotopologues, H13CN and HC15N, at pressures probed from 0.03 to 1.8 mbar. We find 12C/13C = $73\pm5$ and 14N/15N = $245\pm29$, respectively (0.76-0.87) and (0.80-1.00) times the terrestrial references, and (0.69-0.87) and (0.42-0.70) times the solar-Jovian bulk values. In contrast to the strong depletions reported in 1998, our values are instead compatible with an enrichment in the heavier isotopes relative to the Jovian bulk. We interpret these enrichments as the direct signature of the cometary contribution in HCN and/or as 23 years of chemical evolution.

astro-ph.EP↗

arXiv2026

Detection of stratospheric HCN and tropospheric CO in Uranus and the implication for their sources

Uranus belongs to the category of ice giants that are common in our Galaxy. However, Uranus is one of the least explored and understood planets in our Solar System. This work aims to constrain the deep oxygen abundance of Uranus to better understand its formation. Another goal concerns the origin of exogeneous species, such as CO and HCN, found in the upper stratospheres of giant planets. We used spectral mapping observations of the CO (J=3-2) and HCN (J=4-3) rotational submillimeter lines obtained with ALMA in 2022 and 2024. We combined them with radiative transfer and thermochemical modeling to determine the tropospheric abundance of CO and the deep O/H ratio of Uranus. We used radiative transfer simulations with physical models of various sources of external CO and HCN to constrain the vertical and meridional distributions of these species and narrow down the nature of their external sources. We also applied a wind retrieval algorithm to search for zonal winds in the stratosphere of Uranus at the levels probed by the CO and HCN lines. We unambiguously detect tropospheric CO for the first time with a mole fraction of 5.8$\pm$0.3 ppb and stratospheric HCN with a mole faction of (1.8$\pm$0.2)$\times10^{-11}$ restricted to pressures lower than 0.2 mbar. Thermochemical calculations suggest that the deep interior of Uranus is enriched in oxygen with respect to the protoplanetary nebula by at least a factor of 52$^{+30}_{-20}$. We also find that the stratospheric CO is rather uniform over the observed latitudes and that the CO lines are best fit by an old comet impact model, in which a large comet hit the planet several centuries ago. CO therefore has a dual origin in Uranus. Finally, we do not detect stratospheric winds from these data, but the CO data indicate that zonal winds in the 10$^\circ$S-10$^\circ$N latitudinal range are likely retrograde at submillibar pressures.

astro-ph.EP↗

arXiv2026

Observations of the temporal evolution of Saturn's stratosphere following the Great Storm of 2010-2011. II. Latitudinal distribution of CO and stratospheric winds

Saturn's Great Storm of 2010-2011 has produced two stratospheric hot spots, the "beacons," that eventually merged to produce a gigantic one in April and May 2011. This beacon perturbed stratospheric temperatures, hydrocarbon, and water abundances for several years. We aim to assess whether the beacon induced any perturbation in another oxygen species, namely CO. A second goal is to measure how the vortex perturbed the stratospheric wind regime. We conducted interferometric observations of Saturn in the submillimeter range with SMA and ALMA to spatially resolve the CO (J=3-2) and (J=2-1) emissions, respectively. We used a previously determined CO vertical profile as a template, to search for (i) the meridional distribution of CO and (ii) variations of the CO abundance associated with the storm. The high spatial and spectral resolutions of the ALMA observations enabled us to retrieve the winds from the Doppler shifts induced by the winds on the lines. Despite limitations resulting from the removal of baseline ripples, we find a relatively constant meridional distribution of CO. The average CO mole fraction implied by the adopted and rescaled 220-year-old-comet-impact vertical profile is (1.7$\pm$0.7)$\times10^{-7}$ at 0.3\,mbar, i.e., where the contribution functions peak. We also find that the CO abundance has not been noticeably altered in the beacon. The winds measured at 1\,mbar show striking differences with those measured in 2018, after the demise of the beacon. We find the signature of the vortex as an anticyclonic feature. The equatorial prograde jet is 100 to 200 m.s$^{-1}$ slower, and broader in latitude, than in quiescent conditions. We also detect several prograde jets in the southern hemisphere. Finally, we detect a retrograde jet at 74$^\circ$N which could be a polar jet caused by the interaction of the Saturn magnetosphere with its atmosphere.

astro-ph.EP↗