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ASTROPHYSICS

Fireflies around Saturn

A Brazilian research team proposes a model to explain the origin of a tenuous ring around the second largest planet in the Solar System

In a telegram of just 15 lines to the International Astronomical Union (IAU), American astronomer Carolyn Porco announced on October 11, 2006 the discovery of four new rings around Saturn. A researcher at the University of Colorado Boulder, she led the imaging science team for the Cassini spacecraft, which two years prior had entered into orbit around the second largest planet in the Solar System and begun its mission to collect data about Saturn, its moons, and rings. Twelve years later, in an article published in The Astrophysical Journal in January this year, Brazilian physicist Othon Cabo Winter and his team from São Paulo State University (UNESP) in Guaratinguetá, São Paulo, have provided the most detailed description to date of the largest of these four rings.

With collaboration from astrophysicist Dietmar Foryta of the Federal University of Paraná (UFPR), the group has also proposed a mechanism to explain how the structure, which is possibly formed by microscopic ice particles, can persist despite its being entirely swept up by Janus and Epimetheus—two co-orbital moons—in just a few years. “The discovery of a ring of particles in the same orbit as these satellites makes this system complex and interesting,” says Winter, a specialist in Solar System dynamics.

Still to be officially named, the ring is at a distance of about 150,000 kilometers (km) from Saturn, just wide of the five innermost and easily visible rings—D, C, B, A, and F—which were named in the order they were discovered (see Pesquisa FAPESP, issue no. 108). In his master’s studies under Winter, physicist Alexandre dos Santos Souza analyzed a sequence of images captured by Cassini’s cameras over a period of nearly six hours on September 15, 2006. The images confirmed that the ring was complete and not just a discontinuous series of arcs as the researchers had initially suspected. From measurements of the light flux from the ring, Souza and Winter determined its width to be 7,000 to 8,000 km—larger than the Earth’s radius of 6,400 kilometers—and almost 50% wider than announced in 2006 by Carolyn Porco.

Microscopic particles
Being extremely tenuous, the ring was almost invisible even to Cassini’s cameras. The probe could only detect it from very specific angles, leading the UNESP group to liken its behavior to fireflies which, though present, are also only visible when intermittently glowing. This characteristic also led Winter and colleagues to infer that the ring is composed of microscopic particles of the order of magnitude of one micrometer—1 micrometer equals 1 thousandth of a millimeter. The rationale for their conclusion is explained by physics. Very small particles become visible when backlit because they scatter the light that strikes them. This phenomenon, called Mie scattering after German physicist Gustav Mie (1868–1957), occurs only when the size of a particle is equivalent to that of the wavelength of the incident light, which in the visible spectrum ranges from 0.37 to 0.75 micrometers. Nearly all of Saturn’s innermost rings are more readily visible because they not only are wider, but are formed by larger, centimeter- to meter-sized particles that reflect the light of the Sun.

After coming to this conclusion, and because the Cassini images indicate the firefly ring is not temporary, Winter and his collaborators set out to find an explanation for how it is maintained. Wouldn’t such tiny particles be swept up by Janus and Epimetheus, the two moons sharing the same orbit as the ring?

The researchers concluded it is likely the ring is indeed removed—but not permanently. As they orbit around Saturn, Janus—measuring 190 km in diameter—and Epimetheus—measuring almost 140 km—collide with and capture by gravity a large portion of the particles in their path. Mathematical simulations by physicists Daniela Mourão and Rafael Sfair indicate the particles in the firefly ring have a very short lifetime. According to their calculations, which took into account the gravitational pull from Saturn and adjacent moons (Mimas, Tethys, Enceladus, Dione, and Titan) and solar radiation pressure, the firefly particles have an average lifetime of 20 years. “That’s a very short life span,” says Winter. “The ring would disappear if not constantly replenished.”

The ring must be very young and is thought to be nearly entirely formed of ejecta from Saturn’s moons

Dust meets ice
Previous evidence that dust particles in space continually clash with celestial bodies—and with satellites in Earth’s orbit—suggested to the researchers a possible mechanism for replenishing the ring. In other simulations, physicist Silvia Giuliatti Winter, a specialist in Saturn’s rings, and her colleague, Sfair, calculated the rate at which micrometric particles are produced form the impingement of interplanetary dust particles on the surfaces of Janus and Epimetheus. According to the data, the collision of these larger particles (about 100 micrometers in diameter) with ice on the surface of the two moons produces almost 1 metric ton per day of smaller particles about 1 to 13 micrometers in diameter. “This would be enough to maintain the ring,” says Othon Winter. “According to our model, the smaller grains are produced in greater quantity.”

“The paper is significant in that it demonstrates the ring’s glow is consistent with material being released from the surface of Janus and Epimetheus as already suspected,” says physicist Matthew Hedman, a professor at the University of Idaho. A specialist in planetary dynamics, he is one of the researchers on the 13-year Cassini mission to explore Saturn’s rings and moons, and is currently analyzing much of the imagery sent from the spacecraft to Earth. “This means the ring is probably mostly composed of ejecta from the two moons, and this material is very young,” says Hedman. “Rings like these can undergo very rapid change in response to their changing environment.”

Shortly before completing their studies on the firefly ring, Winter, Daniela, and the German astronomer Lucas Treffenstädt, who had spent a season in Guaratinguetá a few years past, attempted to explain how two moons of similar size and mass, such as Janus and Epimetheus, could form in the same orbit. According to their model, which they presented in Astronomy and Astrophysics in 2015, it is likely that each of these satellites—Saturn has 62 in all—arose as the result of the collision of two larger bodies.

Project
On the relevance of small bodies in orbital dynamics (No. 16/24561-0); Grant Mechanism Thematic Project; Principal Investigator Othon Cabo Winter (UNESP); Investment R$1,009,436.80.

Scientific articles
WINTER, O. C. et al. Particles co-orbital to Janus and to Epimetheus: A firefly planetary ring. The Astrophysical Journal. Vol. 852 (14). Jan. 1, 2018.
TREFFENSTÄDT, L. L.; MOURÃO, D. C.; and WINTER, O. C.
Formation of the Janus-Epimetheus system through collisions. Astronomy and Astrophysics. Sept. 23, 2015.

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