A new study has shed light on Enceladus’ iconic "tiger stripe" fissures, which spew vast quantities of mineral-rich water into space. The material emerging from these rents in the icy moon’s surface is thought to be drawn from a subsurface ocean, which could play host to extraterrestrial life.
Our solar system plays host to a diverse and fascinating collection of moons that sport a plethora of strange and often visually spectacular characteristics. There are bizarre spongy moons, flying saucer-shaped moons, hellish lava moons, and even a death-star shaped moon.
Few however, are as majestic and fascinating as Enceladus. This frozen ball treads an eccentric orbit around everyone’s favorite ringed giant Saturn, and has been imaged extensively by the now retired Cassini spacecraft.
Cassini revealed a desolate tortured landscape riven with monumental divides. Yet, contrary to its outwardly icy, barren appearance, Enceladus was found to be very much alive. Geologically speaking at least.
Monumental geysers were seen pouring from vast chasms that scarred the strange moon’s south polar region. Each of these cracks have been named after cities that featured in the famous collection of Middle Eastern folk tales, One Thousand and One Nights.
Scientists concluded that the source of the matter erupting from the tiger stripes was a vast subsurface ocean, which may even be suitable for the evolution of extraterrestrial life.
Many questions remain regarding the nature of the tiger stripes. For example, why do these distinctive formations only exist at Enceladus’ south pole? Why they are so evenly spaced, and why they haven’t simply closed up or frozen over?
A new study carried out by a team of scientists working in the US sought answers to these questions by using computer models to simulate the complex physical processes governing the Saturnian moon.
“Since it is thanks to these fissures that we have been able to sample and study Enceladus’ subsurface ocean, which is beloved by astrobiologists, we thought it was important to understand the forces that formed and sustained them,” says lead author of the study Doug Hemingway, of the Carnegie Institution for Science. “Our modeling of the physical effects experienced by the moon’s icy shell points to a potentially unique sequence of events and processes that could allow for these distinctive stripes to exist.”
According to the paper, the tiger stripes owe their existence to Enceladus' eccentric orbit. As the moon travels periodically closer to, and further from Saturn, it is placed under enormous stress by the nearby gas giant’s powerful gravitational influence.
This is what stops the moon from freezing solid. The ice sheets covering the polar regions experience the greatest amount of stress as the moon flexes over the course of its orbit. Because of this, they are thinner than the ice sheets covering the equator and during cooling periods, some of the subsurface ocean below the poles begins to freeze and expand, placing stress on these relatively weak regions.
It's believed that the first tiger stripe, which scientists now refer to as the Baghdad fissure, was created when the pressure built to the point where the south polar ice sheet burst a seam. The eruption of material through Baghdad relieved the pressure on the northern ice sheet. According to the authors of the paper it could easily have happened the other way around, with the north pole sporting the stripes.
Once open, Saturn’s gravitational influence prevented the fissure from closing, in a similar way to how flexing an open wound can prevent it from healing. However, the models predict that once Baghdad burst open, there wouldn’t have been enough stress-induced pressure to create further fissures, so how did they form?
According to the researchers, a portion of the material expelled from the original crack settled on its edges. This extra weight may have caused the ice sheet to strain and deform. Eventually, the pressure caused parallel tiger stripes to form roughly 35 km (21 miles) away from Baghdad.
Enceladus remains one of the most exciting prospects for the discovery of extraterrestrial life in our solar system, and numerous robotic explorers have been proposed to probe its icy depths.
The paper has been published in the journal Nature Astronomy.
Source: Carnegie Institution for Science
