Earth and Planetary Astrophysics (astro-ph.EP)

Abstract: Saturn's northern summertime hemisphere was mapped by JWST/MIRI (4.9-27.9 $\mu$m) in November 2022, tracing the seasonal evolution of temperatures, aerosols, and chemical species in the five years since the end of the Cassini mission. The spectral region between reflected sunlight and thermal emission (5.1-6.8 $\mu$m) is mapped for the first time, enabling retrievals of phosphine, ammonia, and water, alongside a system of two aerosol layers (an upper tropospheric haze $p<0.3$ bars, and a deeper cloud layer at 1-2 bars). Ammonia displays substantial equatorial enrichment, suggesting similar dynamical processes to those found in Jupiter's equatorial zone. Saturn's North Polar Stratospheric Vortex has warmed since 2017, entrained by westward winds at $p<10$ mbar, and exhibits localised enhancements in several hydrocarbons. The strongest latitudinal temperature gradients are co-located with the peaks of the zonal winds, implying wind decay with altitude. Reflectivity contrasts at 5-6 $\mu$m compare favourably with albedo contrasts observed by Hubble, and several discrete vortices are observed. A warm equatorial stratospheric band in 2022 is not consistent with a 15-year repeatability for the equatorial oscillation. A stacked system of windshear zones dominates Saturn's equatorial stratosphere, and implies a westward equatorial jet near 1-5 mbar at this epoch. Lower stratospheric temperatures, and local minima in the distributions of several hydrocarbons, imply low-latitude upwelling and a reversal of Saturn's interhemispheric circulation since equinox. Latitudinal distributions of stratospheric ethylene, benzene, methyl and carbon dioxide are presented for the first time, and we report the first detection of propane bands in the 8-11 $\mu$m region.

Abstract: The overlapping of mean-motion resonances is useful for low or zero-propellant space mission design, but while most related prior work uses a planar CRTBP model, tours of multi-moon systems require using resonances affected by two moons. In this work, we investigate Jupiter-Ganymede unstable 4:3 resonant orbits in a concentric circular restricted 4-body model for the Jupiter-Europa-Ganymede system. We show that despite their high order, secondary resonances between these orbits and Europa have a large effect, especially 11/34, 12/37, 23/71, 25/77, 34/105, and 35/108; we also find strong evidence that the associated resonant islands overlap. We then compute many of the new objects which appear inside the secondary resonances, which gives final confirmation of the secondary resonance overlap.

Abstract: The first step toward planet formation is grain growth from (sub-)micrometer to millimeter/centimeter sizes. Grain growth has been reported not only in Class II protoplanetary disks but also in Class 0/I protostellar envelopes. However, early-stage grain growth occurring in Class 0/I stages has rarely been observed on the protostellar disk scale. Here we present the results from the ALMA Band 3 ($\lambda$ = 3.1 mm) and 7 ($\lambda$ = 0.87 mm) archival data of the Class I protostellar disk WL 17 in the $\rho$ Ophiuchus molecular cloud. Disk substructures are found in both bands, but they are different: while a central hole and a symmetric ring appear in Band 3, an off-center hole and an asymmetric ring are shown in Band 7. Furthermore, we obtain an asymmetric spectral index map with a low mean value of $\alpha$ = 2.28 $\pm$ 0.02, suggestive of grain growth and dust segregation on the protostellar disk scale. Our radiative transfer modeling verifies these two features by demonstrating that 10 cm-sized large grains are symmetrically distributed, whereas 10 $\mu$m-sized small grains are asymmetrically distributed. Also, the analysis shows that the disk is expected to be massive and gravitationally unstable. We thus suggest a single Jupiter-mass protoplanet formed by gravitational instability as the origin of the ring-like structure, grain growth, and dust segregation identified in WL 17.