US scientists identify forces driving geological activity on Saturn moon

Parliamentary Yearbook reports on research by American scientists at Cornell University which provides evidence of Saturn’s gravitational pull on one of its moons, called Enceladus.  Saturn’s tidal gravitational forces are responsible for variations in the intensity of plumes of icy water particles erupting from geysers on the surface of Enceladus.

Enceladus is one of the inner moons orbiting the planet Saturn, which has over 60 moons in total. It was first discovered in 1789 by the German-born British astronomer, William Herschel.  Apart from the presence of water ice on its surface, very little was known about Enceladus until recently.  During the 1980s, the Voyager spacecraft mission to the Saturnian system revealed most of what we know about the moon today.

Enceladus is 500km wide and reflects almost all of the sunlight that strikes its surface, caused by a very smooth surface of fresh water ice. Consequently, the surface temperature is a chilly -201 degrees Celsius (-330 degrees Fahrenheit).  It has at least five different types of terrain.  In addition to areas containing craters (up to 21 miles wide), it has smooth plains, linear crack and ridges, fissures and distinctive crustal formations. According to experts, these features indicate that the interior of the moon may be liquid today, despite the fact that it should have frozen a very long time ago.

Following the Voyager mission, scientists proposed that Enceladus may be geologically active and believed that water venting through the moon’s surface could be responsible for the icy material in one of Saturn’s rings.

In 2005, the Cassini-Hyugens Mission to Saturn produced images of plumes of icy material rising from the surface of Enceladus.  Later images subsequently identified jets of icy particles resembling geysers or volcanoes erupting from the southern polar region of the moon.  These were identified as jets of water particles, freezing on contact with the cold space temperatures.

Since 2005, scientists have sought to understand the effects of Saturn’s gravity on Enceladus.  They have predicted that Saturn’s gravitational forces cause Enceladus to stretch and compress, creating heat and pressure which forces liquid through cracks in the moon’s icy surface.

In this latest research, American scientists from Cornell University have analysed 252 images of Enceladus during its orbit around Saturn. The images, taken by NASA’s Cassini spacecraft, provide evidence that the intensity of the icy plumes varies systematically depending on the proximity of Enceladus from Saturn.

The data show that the icy plume is dimmest when Enceladus is closest to Saturn, becoming increasingly brighter as it orbits away from the planet. At its most distant point from Saturn, the plume is three to four times times brighter than at its dimmest.  

According to Dr Matthew Ledman, lead researcher: “What this tells us is that Saturn’s tides are having a significant effect on how much material can escape from beneath Enceladus.”

Close up, Saturn’s gravitational squeeze partly closes up the polar fissures, nicknamed ‘tiger stripes’.  This limits how much material they release. As the moon moves further away from Saturn, the fissures open wider, releasing more material which results in bigger and brighter plumes.

Dr Ledman said: “Previous models predicted that when Enceladus was near the point most distant from Saturn, the cracks would be pulled open or widened, and the most amount of liquid would escape.  This is the first observational data we have that show quite clearly that is the case.”

He explained that the extent to which Enceladus responds to Saturn’s tidal forces may provide an important insight into the rigidity of Enceladus’ interior. The findings may help scientists understand what is happening beneath the moon’s surface and help to identify the source of the ice streams and water vapour.

NASA planetary scientist, Terry Hurford, was the first to predict Saturn’s tidal forces on Enceladus. He said: “On the model I used to predict this variability in the stresses, I assume there’s a global ocean and not just a local sea.”

www.parliamentaryyearbook.co.uk

Email: parliamentaryyearbook@blakemedia.org