Determining the rotation rate of Earth’s three rocky terrestrial planet sisters — Mercury, Venus, and Mars — is a relatively easy task for planetary scientists to undertake. However, gas-giant planets, such as Jupiter and Saturn, are much more problematic because they both lack measurable solid surfaces, and are also heavily shrouded by thick, obscuring cloud-layers that hinder direct visual measurements by space probes. In contrast, the solid, inner terrestrial planets only require the measurement of the time it takes for a surface feature to rotate back into view again. The enormous, beautiful, ringed gas-giant, Saturn — the second largest planet in our Solar System after the behemoth, Jupiter — presents a monumental challenge to planetary scientists because different parts of this lovely sphere of hydrogen and helium rotate at different speeds, whereas its rotation axis and magnetic pole are aligned. However, in March 2015, a team of planetary scientists in Israel announced that they have devised a new method that promises to determine Saturn’s rotation period, and also offers insight into the mysterious internal structure of this giant planet, the myriad secrets of its formation, and its strange weather patterns.
The new method was worked out by Dr. Ravit Helled of the Department of Geosciences at the University of Tel Aviv’s Raymond and Beverly Sackler Faculty of Exact Science and Dr. Eli Galanti and Dr. Yohai Kaspi of the Department of Earth and Planetary Sciences at the Weizmann Institute of Science. The new method is based on Saturn’s measured gravitational field and the very unusual fact that its east-west axis is shorter than its north-south axis. The research was published in the March 25, 2015 issue of the journal Nature.
The Saturn System
The giant planet Saturn is circled by a host of bewitching moonlets of sparkling ice, as well as 62 known moons. However, Saturn is most famous for its remarkable system of beautiful and magnificent rings, which are actually collections of a multitude of icy bits, fragments, and chunks, that range in size from smoke-size frozen particles to boulders as large as a hotel. These orbiting, twirling, frozen objects gravitationally interact with one another, and bounce around together to create a mesmerizing dance around their planet. The frigid chunks of ring-material are also influenced by Saturn’s magnetosphere — which is defined as the region of a planet’s magnetic influence. The icy bits of ring-material are also influenced by Saturn’s larger moons.
Saturn’s rings are divided into 5 main components: the G, F, A, B, and C rings, that are listed from the outermost to the innermost. However, it is somewhat more complicated than this because the primary divisions must be further subdivided into thousands of separate ringlets. The A, B, and C rings can be easily observed because they are very wide. However, the F and G rings are slender, thin, and ethereal — which makes them very difficult to observe. There is also a sizeable gap between the A ring and the B ring, which is called the Cassini Division.
The many moons of Saturn are fascinating small worlds. Titan, Saturn’s largest moon, is the second largest moon in our entire Solar System — after Ganymede of Jupiter. Veiled in a dense and obscuring blanket of orange fog, Titan is known for its bizarre clouds of methane, and its hydrocarbon seas and lakes. Titan’s thick, strange rust-colored atmosphere is composed of a remarkable icy concoction of compounds that are thought to be similar to those that were present in Earth’s primordial atmosphere. Although Titan’s atmosphere is much thicker than that of our own planet, it is composed primarily of nitrogen — just like Earth’s. However, Titan’s atmosphere also contains greater amounts of “smoggy” chemicals such as methane and ethane — and the smog on Titan is so dense that it actually rains “gasoline” down on to the surface of this weird and tormented moon-world. Some of the chemicals present in Titan’s atmosphere might nurture primitive, simple tidbits of methane-based life. Life as we do not know it may inhabit this strange and alien distant moon. However, there is currently no evidence for this — Earth is the only planet that we know of that hosts life.
In addition, there are other icy, mid-sized moons that circle around this enormous ringed gas giant planet. Rhea is Saturn’s largest icy moon — the second-largest moon inhabiting the Saturn system, after Titan. Iapetus, the third largest of Saturn’s moons, is known for being two-faced — one side of Iapetus is composed of sparkling, gleaming, very bright and highly reflective ice, while the other side is dark and non-reflective — a black region that contrasts markedly with the glistening, pristine white ice on the other side of the moon. Iapetus is larger than Mimas and Enceladus, which are two other mid-sized moons of Saturn. There is a huge impact crater on Mimas, and it is certainly the most prominent feature on what is apparently a mercilessly pelted and heavily cratered little moon-world. The gigantic impact crater dubbed Herschel on this 400-kilometer moon was blasted out by a crashing object from space composed of rock, ice, or both, that almost powdered it. Enceladus also is an intriguing little moon-world, 500-kilometers in diameter, that likely contains a global subsurface ocean of water beneath its frozen, frigid crust of ice. Where there is liquid water, there is the possibility — though by no means the promise — of life as we do know it to exist. Enceladus also shows the highest albedo of any other moon in our entire Solar System — meaning that it has the brightest and most highly reflective surface. It also has a very active geology, which has caused it to be almost entirely free of craters. This is because it is being constantly resurfaced by the material that erupts from its gushing icy geysers that are responsible for a fresh snow that keeps the surface of the little moon smooth and bright!
Some theories propose that the Saturn system started out with several large moons that were similar to the four large Galilean moons of Jupiter — Io, Europa, Ganymede, and Callisto. Alas, some highly destructive and weird occurrences disrupted the Saturn system that shot its large moons onto a collision course, whereby they blasted into one another, wreaking havoc. According to this scenario, there were a few dramatic mergers between moons, forming the large, tortured moon Titan — however, there was also a large amount of moon-material left over from the collisions to form the icy mid-sized satellites of Saturn — Rhea, Dione, Tethys, Enceladus, Iapetus, and Mimas.
Until 2004, no spacecraft had paid a visit to the gigantic ringed-planet for more than twenty years. Pioneer II had obtained the very first up-close and personal images of Saturn when it zipped past it back in 1979. Voyager 1 had its own close encounter about a year later, and in August 1981 Voyager 2 had its fleeting — but successful — meet-up with Saturn.
At last, on July 1, 2004, NASA’s Cassini spacecraft entered Saturn-orbit, and started collecting some truly breathtaking and revealing images of that planet and its many mysterious and bewitching moons. Although Saturn superficially seems to be a calm planet, Cassini revealed that looks can be deceiving. In fact, Cassini managed to successfully image the “Great Springtime Storm” that raged through Saturn in early 2011. NASA announced the discovery of this huge, whirling tempest on October 25, 2012. The ferocious vortex showed an enormous cloud cover as large as our entire planet.
Shedding New Light On Saturn’s Strange Spin
According to the new method devised by the Israeli scientists, Saturn’s day is precisely 10 hours, 32 minutes and 44 seconds long. When the astronomers applied their new method to Jupiter, whose rotation period is already well-known, their results proved to be identical to the conventional measurement — thereby showing that their new method is both consistent and accurate.
“In the last two decades, the standard rotation period of Saturn was accepted as that measured by Voyager 2 in the 1980s: 10 hours, 39 minutes, and 22 seconds. But when the Cassini spacecraft arrived at Saturn 30 years later, the rotation period was measured as eight minutes longer. It was then understood that Saturn’s rotation period could not be inferred from the fluctuations in radio radiation measurements linked to Saturn’s magnetic field, and was in fact still unknown,” explained Dr. Helled in a March 25, 2015 Tel Aviv University Press Release. The Cassini spacecraft had measured a signal linked to Saturn’s magnetic field with a periodicity of 10 hours, 47 minutes, and 6 seconds long. This proved to be slower than earlier recordings.
“Since then, there has been this big open question concerning Saturn’s rotation period. In the last few years, there have been different theoretical attempts to pin down an answer. We came up with an answer based on the shape and gravitational field of the planet. We were able to look at the big picture, and harness the physical properties of the planet to determine its rotational period,” Dr. Helled continued to explain to the press.
Dr. Helled’s new method depends on a statistical optimization method that involves several solutions. First, the solutions needed to reproduce the ringed-planet’s properties (within their uncertainties): its gravitational field and mass. Then the planetary scientists studied this new information in order to determine the rotation period on which most of the collected solutions converged.
The mass of Saturn’s core and the mass of the heavy elements that make up its composition, such as water and rocks, are influenced by the rotation period of the planet.
“We cannot fully understand Saturn’s internal structure without an accurate determination of its rotation period,” Dr. Helled noted in the March 25, 2015 Tel Aviv University Press Release. This is because an understanding of Saturn’s composition provides important information concerning the ringed-planet’s birth and evolution in general, as well as on the physical and chemical properties of the solar nebula from which our entire Solar System emerged about 4.56 billion years ago. This swirling disk of gas and dust surrounded our Sun in its infancy, and from it all of the planets, moons, asteroids, comets, and assorted smaller bodies that compose our Sun’s familiar family were ultimately born.
“The rotation period of a giant planet is a fundamental physical property, and its value affects many aspects of the physics of these planets, including their interior structure and atmospheric dynamics. We were determined to make as few assumptions as possible to get the rotational period. If you improve your measurement of Saturn’s gravitational field, you narrow the error margin,” Dr. Helled added.
The planetary scientists want to apply their newly devised technique to other gaseous planets in the Solar System, such as Uranus and Neptune. Their new method could also be used in the future to study faraway gaseous alien worlds in orbit around distant stars beyond our Sun.