Hi there! You know the thunderstorms on our Earth. In fact, you can have thunderstorms once you have an atmosphere. And you have many atmospheres in our Solar System, particularly on the giant planets. Today we describe a thunderstorm on Saturn, which happened between November 2007 and July 2008, and was observed by Cassini. This thunderstorm is described in Analysis of a long-lived, two-cell lightning storm on Saturn, by G. Fischer et al. This study will be published soon in Astronomy and Astrophysics.
Physics of a thunderstorm
Basically, a thunderstorm results from the encounter between cool and hot air. For instance, after a hot summer day, you have hot air in the low atmosphere, while colder air is brought by the wind. Then the hot air, which is lighter, gains altitude. This convective motion induces displacements of electric charges, and so a difference of electrostatic potential between the ground and the top atmosphere. This difference in electrostatic potential creates electric lightning, which actually balances the charges between the sky and the ground. All this results in unstable weather conditions, in particular rain and strong wind. The rain is due to the moist contained in the hot air, which coalesced as clouds while gaining altitude. Thunderstorms are among the most dangerous natural phenomena.
As I said, you can have thunderstorms on any planet with an atmosphere. Today, we are on Saturn.
The atmosphere of Saturn
The radius of Saturn is about 60,000 km, which corresponds to the distance to the center, where the atmospheric pressure reaches 1 bar. At its center Saturn has probably a rocky core, which radius is about 25,000 km. This leaves room for a very thick atmosphere, i.e. what I would call the Saturnian air, mainly composed of molecular hydrogen and helium. Interestingly for us, there are clouds in the atmosphere of Saturn, which composition depend on the altitude, itself correlated with the pressure. The less dense clouds (up to 2 bars), in the upper atmosphere, mainly consist of ammonia ice, while denser clouds contain water ice. The densest clouds, which pressure exceeds 9.5 bars, contain water droplets with ammonia in aqueous solution.
The winds on Saturn are very strong, i.e. up to 1,800 km/h, or 1,120 mph, which of course facilitates the encounters between different air masses (with different temperatures). Moreover, the atmosphere of Saturn is organized into parallel bands, as is the atmosphere of Jupiter. These bands rotate at slightly different rates, which prompted the International Astronomical Union to define 3 reference systems for the rotation of Saturn:
- System I: spin period of 10 h14 min for the equatorial bands,
- System II: spin period of 10 h 39 min 24 s, at the other latitudes,
- System III: spin period of 10 h 39 min 22.3 s, for the radio emissions.
The detected episodes
To be honest with you, I did not manage to get an exhaustive list of the detected events. By the way, if you have some information, I would be glad to get it. You can comment at the end of this article.
You can find below a list of thunderstorms, which have been detected by the Cassini spacecraft between 2004 and 2010. The study we discuss today is on the Storm F.
- Storm 0: May 26–31, 2004
- Storm A: July 13–27, 2004
- Storm B: August 3–15, 2004
- Storm C: Sept. 4–28, 2004
- Storm D: June 8–15, 2005
- Storm E: Jan. 23 – Feb. 23, 2006
- Storm F: Nov. 27, 2007 – July 15, 2008
- Storm G: Nov. 19 – Dec. 11, 2008
- Storm H: Jan. 14 – Dec. 13, 2009
- Storm I: Feb. 7 – July 14, 2010
These events were identified in detecting radio emissions, due to Saturn electrostatic discharges (SEDs for short). Before that, the Voyager spacecrafts have detected SEDs in 1980 and 1981, but attributed their origins to impacts in the rings. Since then, other events have been detected. In particular, Great White Spot events, i.e. huge disturbance encircling the planets, can be seen from the Earth. They seem to appear roughly every 30 years, which could be correlated with the duration of Saturn’s year (29.46 years). The last Great White Spot has been observed in 2010-11.
Radio and optical observations
As I said, these events are usually detected thanks to their radio emissions. For that, Cassini disposed of the Radio and Plasma Wave Science (RPWS) instrument, equipped with a High Frequency Receiver.
This receiver listened to Saturn in 3 different modes alternatively, allowing to cover a pretty wide range between 325 and 16025 kHz.
These radio measurements were supplemented by optical observations by the Cassini ISS (Imaging Science Subsystem), by optical observations from Earth, and even by Earth-based radiotelescopes, for the strongest discharges.
The detection of such events strongly depends on the location of the spacecraft with respect to the storm. When the spacecraft is opposite to the storm, you detect almost nothing. Almost, because measuring radio emissions permits over-the-horizon detection, especially when the SED storm is located on the night side (opposite the Sun) and Cassini on the day side. This could be due to a temporary trapping of the radio waves below Saturn’s ionosphere before they are released.
So, Cassini’s RPWS detects the discharges, ISS and the Earth-based telescopes see the storms… Let us see the results for the Storm F (November 2007 to July 2008).
The Storm F
RPWS detected about 277,000 SEDs related to this Storm F. But the analysis of the images revealed two phases.
One or two events?
From November 2007 to March 2008, ISS saw one convection cell, at the latitude of ~35° south. And in March 2008 a second cell appeared, at roughly the same latitude, and separated from the first cell by about 25° in longitude. These two cells drifted both of about 0.35° per day. The presence of these two cells with a correlated motion makes this event a very interesting one… and the authors also detected dark ovals.
A storm appears as a a bright spot, while a dark oval is a dark one. Several dark ovals were seen, the largest one, nicknamed S3 drifted by 0.92° per day, i.e. much faster than the storms. These dark ovals have probably no SED activity. Several explanations have been proposed to explain these features. They could either be clouds of carbon soot particles, produced by the dissociation of methane in the lightning channels, or remnants of convection cells, in which the ammonia particles have fallen deeper into the atmosphere, leaving darker spots.
So, this paper describes the event. The physics behind still needs some clarification, so you can be sure that devoted papers will follow. Stay tuned!
The study and its authors
You can find the study here. And now, the authors:
- The ResearchGate profile of Georg Fischer,
- the webpage of Philippe Zarka,
- the website of Marc Delcroix,
- the webpage of Ulyana A. Dyudina,
- the one of William S. Kurth,
- and the one of Donald A. Gurnett.