Enceladus and its endless secrets

The possibility of worlds other than our own, where life could exist, has long fascinated us. Often seen in the realm of science fiction, we could get even closer to reality as scientists have identified several worlds that contain some of the ingredients necessary for life. One of them is Enceladus, the icy moon, the brightest in the solar system.

Enceladus was discovered on August 28, 1789 by the British astronomer William Herschel, better known for his discovery of the planet Uranus. Little is known about how William went about it and made his discovery.

A dwarf named after a giant

What we do know, however, is that it was William’s son, John Hersel, who gave the moon his name Enceladus, after the giant Enceladus from Greek mythology. In his 1847 publication The Results of Astronomical Observations Made at the Cape of Good Hope, John proposed names for the first seven discovered moons of Saturn, including Enceladus. He chose these names because Saturn, known in Greek mythology as Kronos, was the leader of the Titans.

For almost two centuries, very little was known about Enceladus. That changed in the 1980s, when the US spacecraft Voyager 1 and Voyager 2 flew over the moon, taking pictures. The pictures show that the icy surface of this small moon is very smooth and completely white in some places.

Enceladus is, in fact, the most reflective body in the solar system. Scientists, however, did not know why this happened for several decades. The reflectivity of Enceladus means that it reflects nearly all of the sunlight that hits it, resulting in extremely low surface temperatures, on the order of -200 degrees Celsius.

E ring and tiger stripes

Shortly after NASA’s Cassini spacecraft began studying the Saturn system in 2004, Enceladus began to reveal its secrets. After spending more than a decade in close proximity to the small moon, including flying as close as 50 km away, Cassini was able to unearth a wealth of information about Enceladus.

Cassini found that particles of icy water and gas are ejected from the surface of the moon at a speed of about 400 meters per second. These continuous eruptions create a halo of fine dust around the Moon that supplies material for Saturn’s E ring. While a small portion of this remains in the ring, the rest falls like snow back onto the Moon’s surface, turning it bright white. Scientists informally call the warm faults on the crust of Enceladus, from which jets of water emanate, “tiger stripes.”

A publicity image captured by the camera of the Cassini spacecraft Enceladus, one of Saturn’s moons, on the dark side of Saturn’s rings, taken October 27, 2007 | Photo: NASA/JPL/INSTITUTE OF SPACE SCIENCES

By measuring the moon’s slight wobble as it orbits Saturn, and from gravity measurements based on the Doppler effect, scientists were able to determine that these jets were generated by the global ocean inside the moon. Because this ocean supplies the jet, which in turn creates Saturn’s E ring, it follows that studying E ring material is akin to studying Enceladus’s ocean.

While the E Ring is primarily made up of ice droplets, silica nanograins are also present, which can only form where liquid water and rock interact at temperatures above 90 degrees Celsius. Along with other evidence collected, this suggests the existence of hydrothermal vents deep under the shell of this moon, similar to those found at the bottom of the Earth’s oceans.

orbital resonance

Enceladus takes 33 hours to complete its journey around Saturn, nearly half the time taken by the more distant moon Dione. Thus, Enceladus is trapped in orbital resonance with Dione, whose gravity pulls Enceladus’ orbit into an elliptical shape. This means that Enceladus is sometimes closer to Saturn and sometimes farther away, resulting in tidal heating inside the moon.

At just over 500 km across, Enceladus is small enough to fit within the Indian state of Maharashtra, which stretches about 700 km north to south and 800 km east to west. What it lacks in size it more than makes up for in growth, as Enceladus has a global ocean, unique chemistry, and internal heat. All of this means that while we still have a lot of data on the Moon, researchers eventually plan to return to Enceladus to learn more about its secrets.