In his seminal book and televisions series, Cosmos, Carl Sagan eulogized what he called the “travelers tales” sent back by robotic visitors to the outer planets. Launched in 1977 and carrying less computing power than some modern microwave ovens, the probes Voyager I and Voyager II followed in a similar path to Pioneer 10 and Pioneer 11 on a mission to explore the planets beyond the orbit of Mars. Taking advantage of a once-in-175-years alignment of the planets that allowed the probes to get a gravitational assist surfing from one planet to the next, they sent back stunning images and details of the giant planets Jupiter, Saturn, Uranus and Neptune, plus their menagerie of moons.
The majestic rings of Saturn, first seen by Galileo through his telescope in 1610, were eventually explained as bands of chunks water ice encircling the planet in unimaginably huge quantities. Even as we speak, long after they have passed the orbit of Pluto never to return, the venerable probes continue to educate us about the extent of the heliosphere, the vast bubble of energy being pushed outwards by the sun’s radiation, protecting us from the violent solar winds of interstellar space, and how far one has to travel before the influence of our star begins to wane.
However because the Voyagers and Pioneers were designed to fly past the planets, they only got to spend a few weeks near each one. They will now spend the rest of eternity making their way across the vast distances between stars, but a younger probe is staying closer to home and keeps on feeding us with juicy information about Saturn and its moons. Cassini was launched in 1997 and has been orbiting the ringed giant since 2004. The information sent back on a regular basis has been staggering, steadily accumulating more into the invaluable store of human knowledge.
The latest discovery is an embryonic moon beginning to take shape in Saturn’s A ring, as the rocky material clumps under a slow, natural process driven by gravity, just like it happened during the formation of the planets 4.6 billion years ago. This nebular theory of the origin of the planets has plenty of evidence to back it up in the form of radiometric dating of rocks on Earth and meteorites, but this is the first time this process of accretion has ever been observed.
We now know that Saturn has at least 53 moons. We have learned that its largest moon, Titan, is more similar to Earth than any other world in the solar system. Thanks to the Huygens probe that Cassini parachuted onto Titan’s surface in 1995, we know that it has lakes of liquid methane, and an active atmosphere with wind, occasional rain, and a history of erosion on the surface, below which there may even be salt water oceans. The icy moon Enceladus has been shown to be the most likely candidate for harboring primitive life, with its vast geysers of salt water erupting from its south pole.
This is not to say that everything we know about Saturn has been learned through Cassini. The composition of the planet’s rings as millions of independently orbiting objects was confirmed by spectroscopic observations in 1895. As recently as 2013, ground-based studies at the Keck Observatory in Hawaii found that Saturn’s magnetic field is causing material to “rain” from the rings into the planet’s upper atmosphere. Indeed the dark bands in the atmosphere that were first seen by the Voyager mission were not seen again until the Keck Observatory made its spectroscopic observations in near infrared wavelengths.
Nonetheless, Cassini has been one of the most successful space missions ever, and will conclude in spectacular fashion with a series of maneuvers beginning in 2016. The probe will make repeated dives between Saturn and its innermost ring through the course of a year, before swinging over the planet’s north pole and then taking a close pass by the plumes ejecting from Enceladus. Finally, in September 2017, Cassini will conclude its grand finale by plunging into Saturn’s atmosphere.
Cassini’s discoveries raise a possibility that is as profound as it is thought provoking. If primitive life is found to exist on these icy worlds far from the sun, it changes our view of what is meant by the “habitable zone.” The small, rocky and metallic worlds within 142 million miles (225 million km) of the sun have long been thought of as being the only planets in the solar system with a chance of supporting life. Earth is traditionally thought of in Goldilocks terms. To put it crudely, the perception is that Mars is too far away from the sun and is too cold, while Venus is too close and is too hot. Of course there are more complex reasons for those planets not being more Earth-like. In any case Cassini has given us the possibility that our geocentric ideas about a “habitable zone” may be way off.
Indeed any discovery of life elsewhere in the solar system, no matter how primitive, would be a turning point for the human race, much more so than the first moon landing. The idea that Earth is the only world capable of supporting life is central to many people’s ideas about our place in the universe. Are we the one and only, or one among many? The answers that were thought to lie on Mars could well lie on the icy worlds around Saturn, now under intense scrutiny from a very curious observer.