A Hexagon in Saturn's Northern Stratosphere Surrounding the Emerging Summertime Polar Vortex

TL;DR

This study uses Cassini infrared data to analyze the formation and characteristics of Saturn's northern polar vortex, revealing a hexagonal boundary influenced by Rossby waves, with implications for understanding planetary wave dynamics.

Contribution

It provides new insights into the timescales and structure of Saturn's northern polar vortex, including the discovery of a hexagonal boundary in the stratosphere linked to Rossby waves.

Findings

The NPSV formed during late northern spring.

The NPSV boundary is hexagonal, influenced by Rossby waves.

The vortex's temperature and composition contrasts develop after summer solstice.

Abstract

Saturn's polar stratosphere exhibits the seasonal growth and dissipation of broad, warm, vortices poleward of $\sim75^\circ$ latitude, which are strongest in the summer and absent in winter. The longevity of the exploration of the Saturn system by Cassini allows the use of infrared spectroscopy to trace the formation of the North Polar Stratospheric Vortex (NPSV), a region of enhanced temperatures and elevated hydrocarbon abundances at millibar pressures. We constrain the timescales of stratospheric vortex formation and dissipation in both hemispheres. Although the NPSV formed during late northern spring, by the end of Cassini's reconnaissance (shortly after northern summer solstice), it still did not display the contrasts in temperature and composition that were evident at the south pole during southern summer. The newly-formed NPSV was bounded by a strengthening stratospheric thermal gradient near $78^\circ$N. The emergent boundary was hexagonal, suggesting that the Rossby wave responsible for Saturn's long-lived polar hexagon - which was previously expected to be trapped in the troposphere - can influence the stratospheric temperatures some 300 km above Saturn's clouds.