Saturn’s dynamic global energy budget, as revealed by Cassini
By Catherine Maggiori
Synopsis: A recent paper in Nature Communications discusses new revelations from Cassini data on Saturn’s global energy budget. Updated values for Saturn’s internal heat, planetary reflectance, and energy excesses are used to describe seasonal variances, giant storms, and planetary cooling.
Cassini is the gift that keeps on giving.
Launched in 1997, Cassini (or Cassini-Huygens) was a joint planetary exploration mission launched by NASA, the European Space Agency (ESA), and the Italian Space Agency (ASI), comprising the Cassini spacecraft and the Huygens probe. Over its near 20-year lifespan, Cassini produced key discoveries and astonishing observations of our Solar System, including (but not limited to):
The Huygens probe successful touchdown and data transmission on Titan, the first ever landing on an outer solar system moon
The discovery of six new moons in the Saturnian system, as well as several moonlets (including the adorably nicknamed “Peggy”)
Direct observations of icy plumes containing organic molecules erupting from Enceladus, strongly indicating the presence of an internal salty ocean, and placing Enceladus among the highest priority targets for astrobiology research
Confirming the presence of large liquid hydrocarbon lakes and seas on Titan, as well as a global liquid water ocean
Producing some of the best images of Saturn’s hexagon, a large cloud pattern around the north pole
Even after Cassini’s “Grand Finale” and final dive into Saturn in 2017, data from the mission is still yielding valuable results and discoveries, including a paper published in June 2024 in Nature Communications from Wang et al., describing new insights into Saturn’s global energy budget and energy imbalances from Cassini data.
To understand how a planet and its climate evolved over time, you have to understand the relationship between its emitted and absorbed radiant energy i.e. its global energy budget. The equilibrium of a planet’s global energy budget can affect its thermal structure, surface properties, atmospheric circulation, weather, and internal heat.
It’s difficult to gauge this on gas giants, especially those with large scale seasonal weather variations, like Saturn. Saturn’s rings make these discernments even more challenging; they can prevent physical observations, scatter solar radiance, and reflect additional solar energy onto Saturn’s atmosphere. Other oddities of Saturn, like its relatively elliptical orbit and high axial tilt, cause seasonal variations, which further impact its energy budget.
Long-term observations from Cassini are a boon to studying Saturn and Wang et al. used data from multiple Cassini instruments, including the Composite Infrared Spectrometer (CIRS), Imaging Science Subsystem (ISS), and the Visual and Infrared Mapping Spectrometer (VIMS), to generate a more complete picture of Saturn’s energy fluxes, better coverage of wavelength, viewing geometry, and observational period, than models or previous measurements alone.
Wang et al. extended results from Cassini’s Saturnian observations (2004 - 2017) to cover Saturn’s entire 30-year orbital period (1995 - 2025), producing higher values for internal heat flux (2.84 ± 0.20 W/m2) and Bond albedo (0.41 ± 0.02) than previously predicted. Saturn’s Bond albedo is a measurement of the amount of electromagnetic radiation it scatters back into space and it affects the amount of energy absorbed by Saturn from the Sun, nearby planets, and its reflected solar radiance.
These updated values indicate that Saturn has a radiant energy deficit, absorbing more energy than it emits. Saturn’s global energy budget is not stagnant and has significant seasonal fluctuations, indicating planetary cooling. Seasonal energy imbalances occur at both the global and hemispheric scale, helping to spawn to Saturn’s giant convective storms.
From 2010 - 2011, Cassini observed a huge increase in the emitted power from Saturn’s northern hemisphere: the result of one of Saturn’s recurring Great White Spot storms. This storm greatly affected the thermal structure of Saturn’s atmosphere, increasing the northern hemisphere's radiant energy output and partially balancing Saturn’s global radiant energy output.
Above: Figure 2 from Wang et al. depicting the global and hemispheric (NH = northern hemisphere; SH = southern hemisphere) averages of Saturn’s emitted power during the Cassini period.
Storms in Saturn’s northern hemisphere are common, especially during the Saturnian spring and summer; excess energy is produced and hemispheric energy imbalances become large. There may also be a connection between temporal energy excesses and storm formation on other planets, like Mars. Jupiter, Uranus, and Neptune likely exhibit similar energy imbalances.
These findings strongly suggest that global energy budgets are an important factor to consider when generating climate, planetary formation, and atmospheric models for Saturn and other planets in our Solar System.
Author info: Dr. Catherine Maggiori is an astrobiologist and microbiologist. You can find her at the bench, lurking on Twitter, or at the climbing gym.
Article info: Cassini spacecraft reveals global energy imbalance of saturn. Nature: https://www.nature.com/articles/s41467-024-48969-9