The ring leader

Rohit Gupta | Updated on August 22, 2014 Published on April 18, 2014

Sunlit side: Saturn is surrounded by numberous thin ringlets. Pic- wikicommons


In Saturn, we see an atom, the nebula, the entire universe

Often the history of a single object can reflect, when magnified a much larger ongoing saga in nature. A lonely piece of rock may contain the imprints of earth’s magnetic field recorded over millennia; the rings inside an old tree may contain the annual auguries of climate; and oceanic tides echo forever in the memories of a seashell.

Consider the planet Saturn, which resembles a minor solar system itself, a gaseous sun orbited by many small moonlets and debris. The famous rings of Saturn are clearly visible to astronomers today like a halo, but only with the help of a modern telescope. In 1610, when Galileo trained his newly fashioned instrument on the planet, the primitive device showed him the “ears of Saturn” as two moons. Apart from the fact that rings around a planet were unheard of, what further perplexed the Italian scientist was their periodic appearance and disappearance over time.

The first person to identify the appendages as a ring, with a more penetrating (approximately 50 times magnification) instrument was Christiaan Huygens in 1655, who explained the waxing and waning phases of the rings thus: “...the ring, which I had imagined was inclined at about the same angle to the plane of the ecliptic — with a permanent and unchanging inclination, be it understood, as is known to be the case on this Earth of ours with the plane of the equator. From this inclination it necessarily followed that in its different aspects the same ring showed to us at one time a rather broad ellipse, at another time a narrower ellipse, and sometimes even a straight line. As regards the handle-like formations, I understand that this phenomenon was due to the fact that the ring was not attached to the globe of Saturn, but was separated from it the same distance all around.”

This deceptively simple advance by Huygens involved a superior use of metallurgy, engineering, an improved theory of optics and the construction of machines for grinding telescopic lenses from glass. The same knowledge would also enable him to design other optical instruments, using which Huygens “caused a stir in Paris in 1678 with the ‘infinity of little animals’ discovered through his own new microscopes.” These biological innovations were in turn influenced by and had an impact on figures such as Antonie Van Leeuwenhoek and Robert Hooke. “After Hooke had nudged Leeuwenhoek towards observations of the frog embryo that led to his discovery of the capillary circulation in the first place, it was Huygens who early encouraged him to extend his efforts to warm-blooded animals,” writes author Edward G. Ruestow.

For a long time, the rings of Saturn were thought to be a continuous solid disc, a rather strange shape which Huygens justified, “… some people have imagined that, if it were possible to construct a continuous arch all the way around the Earth, it would sustain itself without any support. Therefore, let them not consider it absurd if a similar thing has happened of itself in the case of Saturn.”

This view of one solid ring prevailed until Giovanni Domenico Cassini’s observations in 1675, when he discovered dark gaps separating multiple rings. Almost a century later, in the aftermath of Newton’s theory of gravity — Pierre-Simon Laplace showed in 1785 that this ring structure could not be mechanically stable under gravity, and therefore Saturn was surrounded by not a few but numerous very thin ringlets, although he too insisted that the rings were solid.

During the late 18th century, Immanuel Kant and Laplace developed their nebular model for the origin of the solar system. The analogy of the rings entered prominently into the development of this theory, according to which the Sun and planets formed out of a primeval rotating nebula. Upon its contraction, the nebula broke into a series of rings that subsequently gravitationally condensed into the Sun and planets. The rings of Saturn were viewed as a fossil record of this process, carried out only part of the way.

The concept that the rings were not solid, but consisted of many independent tiny satellites was first put forward by James Clerk Maxwell in an essay that won the Adams prize at Cambridge University in 1859. In a way, the solid rings of Saturn had finally been broken down into particles, almost in parallel with the rise of atomic theory.

This triumphant paper by Maxwell was adopted by Hantaro Nagaoka, a Japanese physicist in 1903 to create a model of the atom based on Saturn. Historians of physics tell us, “Nagaoka assumed that the atom is a large, massive, positively charged sphere, encircled by very many (in order of magnitude: hundreds) light-weight, negatively charged electrons, bound by electrostatic forces analogous to Saturn’s ring, which is stabilised and attracted to the heavy planet by gravitation and consists of a myriad of small fragments.”

It is a statement to its uniqueness that the same planet has served as a metaphor for models of both an atom and a nebula. Not to mention the mysterious hexagon over its North Pole, on a sphere otherwise having an almost featureless atmospheric surface. Due to its wafer-thin rings and multitude of moonlets, Saturn also represents the most complex gravitational laboratory in the solar system.

(Rohit Gupta explores the history of science as Compasswallah. Follow Rohit on Twitter >@fadesingh)

Published on April 18, 2014

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