In our review of the search for extraterrestrial life in the Solar System, we have journeyed outward from Earth, first to Mars and then to Jupiter’s moon Europa. Our next place to look is among the moons of Saturn. Fortunately, the ringed planet is one of the most explored destinations in the Solar System.
Explorers of Saturn
In 1979, Pioneer 11 was the first spacecraft to fly by Saturn. The Voyager spacecraft followed up, in 1980 and 1981, with flybys that provided more detailed observations of the gas giant and some of its moons. Between them, Voyager 1 and Voyager 2 sent back over 2,000 images of Saturn itself, its iconic system of rings, and its moons Titan, Enceladus, Tethys, and Dione.
In 2005, the Cassini-Huygens mission arrived on the scene. The Huygens probe descended to the surface of Titan, while Cassini continued an ambitious mission unlike any of its predecessors: a years-long tour of the Saturnian system that has collected an enormous amount of data, captured thousands of breathtaking images, and fueled analyses and hypotheses that are intriguing, to say the least.
Saturn’s 62 moons are as diverse in size, shape, and composition as their number suggests. Two of the moons – Titan (the subject of the next post in this series) and Enceladus – have drawn the special interest of scientists focused on the search for extraterrestrial life.
A Mysterious Little Egg Circling Saturn
First spied from Earth by William Herschel, in 1789, Enceladus was very nearly a complete mystery until the Voyager spacecraft flew by. This is partly because it is so small – only 310 miles (499 km) in diameter, less than one-sixth the size of Jupiter’s moon Europa. Images transmitted back to Earth by Voyager 2, in 1981, show a slightly egg-shaped white moon against a pitch black background. Intriguingly, in one picture (below) we see a region that is pockmarked by craters, while another larger area is almost devoid of craters.
Like Europa, Enceladus is covered in a crust composed of water-ice. Its surface is as diverse as any that has been observed in the Solar System thus far. Some areas have been cratered by impacts dating back billions of years. Other regions have few, if any, craters, but they display an array of surface features such as ridges, troughs, and canyons that suggest powerful tectonic forces are continually molding the terrain.
The search for signs of life on Enceladus was given a powerful boost when, in 2005, Cassini returned images that showed plumes of water vapor erupting from the south polar region. The geysers are composed of both salty and “fresh” particles, with the heavier salty ones returning to the surface and the fresh particles escaping into Saturn’s E-ring. Scientists deduced the source of the plumes to be a subsurface ocean. At first, this reservoir of water was thought to be limited to the south polar region of the moon, but later observations of distinct “wobbles” in Enceladus as it orbits Saturn suggest that the subsurface ocean is, in fact, global in scale.
The subsurface ocean of Enceladus makes it a prime candidate in the search for life in the Solar System. What is the source of heat for the vast reservoir of liquid water? Are there hydrothermal vents deep in the ocean where microbial life could have gotten a foothold? Could the subsurface ocean provide a habitat for microorganisms or even algae-like life to cling to the inner surface of the icy crust? Only a future mission to Enceladus with a primary goal of searching for life can answer these questions.
Hopes Dim for Future Exploration
The Cassini mission is scheduled to end with the spacecraft destroying itself by plunging into Saturn’s turbulent atmosphere, in September 2017. Three proposals focusing on Enceladus were submitted to NASA’s Discovery Program; none was included in the program’s September 2015 announcement of finalists for the Discovery 13 Mission. Unfortunately, as of now, there are no accepted plans to send another spacecraft to Enceladus.