A Mission-Long Radio Occultation View of Saturn's Atmosphere from Cassini

The Cassini-Huygens mission (2005-2017) produced the most extensive set of radio occultation observations yet obtained in the outer Solar System, providing a unique record of Saturn's atmosphere over more than a decade of seasonal change. During Cassini's 13-year orbital tour, 72 radio occultation experiments were conducted at Saturn, but only a small subset has so far been analyzed in detail. The remaining measurements represent a largely untapped resource for studying the planet's upper troposphere and lower stratosphere under different seasonal and operational conditions.

Radio occultation analysis exploits the bending of a spacecraft's radio signal as it passes behind the planetary limb. This technique yields highly sensitive vertical profiles of refractivity, density, pressure, and temperature, allowing the shallow visible atmosphere to be connected to deeper levels of circulation. Because radio occultations directly sample atmospheric structure with high vertical resolution, they provide an important complement to infrared measurements from Cassini's Composite Infrared Spectrometer (CIRS), which are affected by vertical smoothing and temperature-composition degeneracies.

Here we present a systematic analysis of Cassini's Saturn radio occultation dataset from 2005 to 2017. The dataset includes both the one-way observations enabled by Cassini’s ultra-stable oscillator (USO) and the later two-way observations acquired after the loss of USO capability in 2011. By processing the historical and previously unexplored measurements within a consistent framework, we construct a mission-long set of atmospheric profiles suitable for investigating Saturn's vertical thermal structure and its evolution over time.

Particular emphasis is placed on temperature variability near the 1-bar level and on the structure of the upper troposphere and lower stratosphere. Since the Cassini record spans the transition from northern winter toward northern summer, it offers a natural laboratory for examining how seasonal forcing is expressed in Saturn's atmosphere. The resulting profiles make it possible to investigate whether the observed temperature changes are mainly radiative in origin or shaped by dynamical variability in the stratosphere, including Saturn's stratospheric oscillation (SESO). Together, these measurements turn Cassini's radio occultation archive into a coherent atmospheric record, providing a new basis for the understanding of Saturn's climate and its seasonal evolution.