Once again proving that the universe is not only curiouser than we suppose, but curiouser than we can suppose, the European Space Agency (ESA) announced last summer (July 26, 2011) that water expelled from one of Saturn’s moons rains onto Saturn.
ESA’s Herschel space observatory – a large infrared space telescope, stationed at the second Lagrange point of the sun-Earth system – helped make the discovery. It found that water from Enceladus forms a giant torus of water vapor around Saturn.
Here’s what we knew prior to 2011 about water from Enceladus. In 2009, the Cassini spacecraft narrow-angle camera found at least four distinct plumes of water ice spewing out from the south polar region of Saturn’s moon Enceladus, as shown in the awesome image below.
Cassini obtained this view of Enceladus in 2009 from a distance of approximately 617,000 kilometers (383,000 miles). In this image, light reflected off Saturn is illuminating the moon while the sun, almost directly behind Enceladus, is backlighting the plumes. This view looks toward the Saturn-facing side of Enceladus (504 kilometers across). North is up.
So since 2009 we’ve known there are plumes of water spewing from Enceladus. Meanwhile, it’s been known for 14 years that there is water in Saturn’s upper atmosphere.
Thanks to ESA’s Herschel space observatory, it became known in 2011 that Enceladus expels around 250 kg of water vapour every second, through a collection of jets from the south polar region known as the Tiger Stripes because of their distinctive surface markings. Computer models of these Herschel observations showed that the water from Enceladus creates a doughnut-shaped torus of vapor surrounding Saturn. It’s thought that about 3-5% of the water expelled by Enceladus ends up falling into Saturn.
The total width of the torus is thought to be more than 10 times the radius of Saturn, yet it is only about one Saturn-radius thick. Enceladus orbits the planet at a distance of about four Saturn radii, replenishing the torus with its jets of water as it moves in orbit.
Paul Hartogh, Max-Planck-Institut für Sonnensystemforschung, Katlenburg-Lindau, Germany, who led the collaboration on the analysis of the 2011 results, said:
There is no analogy to this behavior on Earth. No significant quantities of water enter our atmosphere from space. This is unique to Saturn.
He can say that again. Water from Enceladus ending up in Saturn’s atmosphere is truly bizarre and wonderful.
Although most of the water from Enceladus is lost into space, freezes on the rings or perhaps falls onto Saturn’s other moons, the small fraction that does fall onto Saturn is sufficient to explain the water observed in its upper atmosphere.
Bottom line: ESA’s Infrared Space Observatory found the water vapor in Saturn’s atmosphere in 1997. NASA/ESA’s Cassini/Huygens mission found jets of water spewing from Saturn’s moon Enceladus in 2009. In 2011, ESA’s Herschel space observatory was used to show that water from this moon of Saturn accounts for the water in Saturn’s upper atmosphere, making Enceladus the only moon in our solar system known to influence the chemical composition of its parent planet, and proving once again that nature … is cool.
Deborah Byrd created the EarthSky radio series in 1991 and EarthSky.org in 1994. Today, she serves as Editor-in-Chief of this website. She has won a galaxy of awards from the broadcasting and science communities, including having an asteroid named 3505 Byrd in her honor. A science communicator and educator since 1976, Byrd believes in science as a force for good in the world and a vital tool for the 21st century. "Being an EarthSky editor is like hosting a big global party for cool nature-lovers," she says.