Meet Rhea, a moon with an oxygen atmosphere

Discovered by Giovanni Cassini on December 23, 1672, Rhea is one of the many moons of Saturn. Centuries later, Cassini – a spacecraft named in honour of the Italian astronomer, mathematician and engineer – detected a very thin oxygen atmosphere around this moon. A.S.Ganesh takes a look at this moon and what we know about it…

One of the many things that makes our Earth really special is the presence of an oxygen atmosphere. This oxygen richly contributes to various processes happening here on Earth, making the planet a cradle of life.

Even though we have taken an interest in celestial bodies and explored space, near and far, Earth remained the only one with an oxygen atmosphere for long. That changed in 2010 when a spacecraft for the first time directly captured molecules of an oxygen atmosphere at a world other than our Earth. That world was one of Saturn’s many moons, Rhea.

The stars of Louis

The discovery of Rhea took place on December 23, 1672 by Italian astronomer, mathematician and engineer Giovanni Cassini. Rhea was the second of four Saturn satellites that Cassini discovered and the third to be found orbiting the ringed planet. Cassini named the four moons that he discovered – Iapetus, Tethys and Dione in addition to Rhea – Sidera Lodoicea (“the stars of Louis”), in honour of King Louis XIV of France.

This name, however, never picked up outside of France. The names that we recognise them today with were given over a century later by English astronomer John Herschel. Son of astronomer William Herschel, the discoverer of Uranus, John suggested the name Rhea in 1847.

Saturn’s second largest

Saturn’s second largest satellite after Titan, Rhea has a mean radius of 764 km – equivalent in size to a little over a 10th of our Earth. Like Tethys and Dione, Rhea is also tidally locked in phase with its parent, meaning that one side of the moon always faces Saturn.

For centuries after its discovery, Rhea remained merely a tiny dot to astronomers. It was the Voyager (1 and 2) encounters in 1980 and 1981 that finally threw some light on it. Images from these encounters suggested that Rhea’s features could be divided into two regions. While the heavily cratered terrain has craters larger than 40 km across, parts of the equatorial and polar region have craters less than 40 km across.

Following the Voyager encounters, there were no missions to the region until the Cassini spacecraft made its way to orbit around Saturn in 2004. Cassini then made a number of targeted close flybys of Rhea in the decade that followed.

Tenuous atmosphere

In 2010, Cassini was able to detect around Rhea a very thin atmosphere, known as an exosphere, that was infused with oxygen and carbon dioxide. Even though the source of carbon dioxide isn’t certain, the oxygen is believed to arise when Saturn’s magnetic field rotates over Rhea. The moon’s water-ice surface is peppered with energetic particles that were trapped in Saturn’s magnetic field, and the chemical reactions that ensue decompose the surface and release oxygen.

When compared to what we have at Earth, oxygen at Rhea’s surface is estimated to be about 5 trillion times less dense. Despite the current minuscule composition, the formation of oxygen and carbon dioxide could well lead to complex chemistry.

The average age of Rhea’s plains is theorised to be around four billion. We’ve known this moon, which has been around for so long, for a few centuries now. During this time, we have learned a little bit about it, making it more than just a dot for us. There is, however, plenty still left to find out.

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