How does an entire planet change the speed of its rotation in 20 years? That’s the sort of change that takes hundreds of millions of years. Even more mysterious was the Cassini Mission’s detection of electromagnetic patterns that suggested that Saturn’s rotation is different in the northern and southern hemispheres. “For a long time, I assumed there was something wrong with the data interpretation,” said astrophysicist Duane Pontius. “It’s just not possible.”
Unveiling the gas giant’s trick for hiding its rotation
For decades, Saturn has defied attempts to pin down its exact rotation period. Now a new study in the American Geophysical Union’s Journal of Geophysical Research: Space Physics may have finally unveiled the gas giant’s trick for hiding its rotation, and provide the key to giving up its secret. The new research shows how seasonal changes on Saturn may be confusing attempts by scientists to calculate its exact rotation period.
Discovering the length of a day on any planet seems like a straightforward task: find some feature on the planet and clock it as it rotates around once. Or, if it’s a gas giant like Jupiter, which has no solid surface features, scientists can listen for periodic modulations in the intensity of radio signals created within the planet’s rotating magnetic field.
A planet’s rotation period is one of the fundamental facts about a planet, along with its size, composition, orbital period, and other facts that not only describe a planet but help to explain its behavior, history and even provide clues to its formation.
Cassini Discovers Something Astonishing
Saturn emits only low-frequency radio patterns that are blocked by Earth’s atmosphere, making it difficult to study Saturn’s rotation from the Earth’s surface. In contrast, Jupiter emits radio patterns at higher frequencies that allowed radio astronomers to work out its rotation period before the space age got well under way.
It wasn’t until spacecraft were sent to Saturn that scientists were able to collect data about its rotation. Voyagers 1 and 2 sent home the first hints of Saturn’s rotation in 1980 and 1981. They detected a modulation of radio intensity that suggested the planet rotates once every 10 hours and 40 minutes.
“So that was what was called the rotation period,” said Pontius of Birmingham-Southern College in Alabama and a co-author of the new study.
When the Cassini spacecraft arrived at Saturn 23 years later to study the planet for 13 years, it found something astonishing. “In about 2004 we saw the period had changed by 6 minutes, about 1 percent,” Pontius said.
The Plasma “Brake”
Ionized gas – called plasma – in the upper atmosphere of Saturn is coupled to its magnetic fields. As the charged particles comprising the plasma move upward along the magnetic field lines, they drain angular momentum from the lower levels of the atmosphere. This “magnetic braking” is the slowing of plasma as it flies further from the planet, in the same way a spinning dancer’s arms move slower when they are outstretched than when they are held close to the body. Shortly after our Sun first formed, it was spinning about five times faster than its current rotation rate, but has since slowed down due to billions of years of magnetic braking. However, in the case of Saturn, a continuous 1% change in the rotation rate every decade is incompatible with its age. Some other process must also be affecting the apparent changes in the rotation rate of Saturn.
To find out what was really going on, Pontius and his co-authors started by looking at how Saturn is different from its closest sibling, Jupiter.
“What does Saturn have that Jupiter lacks, beside the obvious rings?” Pontius asked. The answer: seasons. Saturn’s axis is tilted about 27 degrees, similar to Earth’s 23-degree tilt. Jupiter has barely any tilt at all—just 3 degrees. The tilt means the northern and southern hemispheres of Saturn receive different amounts of radiation from the Sun depending on the season. The different doses of ultraviolet light affect the plasma at the edge of Saturn’s atmosphere.
According to the model being proposed by Pontius and his colleagues, the variations in UV from summer to winter in the different hemispheres affects the plasma so that it creates more or less drag at the altitudes where it encounters the planet’s gaseous atmosphere.
That difference in drag makes the atmosphere slow down, which is what sets the period seen in the radio signals. Change the plasma seasonally, and you change the period of the radio emissions, which is what is seen on Saturn.
The new model provides a solution to the puzzle of Saturn’s impossible changing rotation periods. It also shows that the observed periods are not the rotation period of Saturn’s core, which remains unmeasured.
Summary of the Discovery
“The interaction between the magnetosphere and the atmosphere is rather complex but can be explained with a simple model,” explains Pontius in an email to The Daily Galaxy. “Electrical currents are generated by particle collisions in the vicinity of Saturn’s moon Enceladus, and those currents can flow freely along magnetic field lines all the way to the planet’s atmosphere. Once they reach its electrically conducting layer, they can travel horizontally across field lines to close the circuit. This is like a standard experiment in introductory physics where a wire carrying a current in the presence of a magnetic field feels a transverse force. That’s what happens here: the current exerts a force on the atmosphere.
“This force is relatively small,” Pontius continues, “so its immediate effect on the immensely more massive atmosphere is small. However, it is persistent and always in the same direction–opposite to the direction of planetary rotation–so its influence accumulates over time. As an analogy, picture a circular vat of motionless water. Imagine that you insert a thin wooden stick vertically into it about halfway between the center and the edge, then move the stick in a complete circle about the center. When done, the water will have been barely perturbed, and if you release the stick it will just float in place. However, imagine steadily moving the stick in that circle repeatedly, for days, months, or even years. The repeated influence of the stick will eventually cause all the water to move with the stick, so once released the stick should continue moving with the circulating water. (The analogy isn’t perfect because of drag forces between the water and the inner surface of the vat, but it captures the essence of the idea.)
“Our model proposes that the electrical currents that exert forces in the two hemispheres are unequal because the electrical conductivities vary with exposure to the Sun, i.e., the seasons. When one hemisphere has summer conditions and the other has winter, solar irradiation increases the conductivity in the summer, which causes currents to flow there preferentially. This is exactly what happens with ordinary, unequal resistors in parallel: the resistor with the lower resistivity/higher conductivity draws more current than the resistor with the higher resistivity/lower conductivity. More current strengthens the drag on the atmosphere and slows it.”