NASA’s Saturn-orbiting Cassini spacecraft will get its final look at two icy moons of Saturn— one famous and one shrouded in mystery. It will fly by the erupting moon Enceladus three times, checking out its plumes in the best detail to date. And it will take one last look at the moon Dione.
While Pluto may be taking all the spotlight away from other planets, astronomers are still keenly interested in Saturn’s moons. One reason is they represent some of the best chances of finding life in our solar system. Many of these icy moons are believed to host global oceans underneath. Warmed by gravitational interactions with massive Saturn, there could be potential microbes floating under the surface waiting for humans to stumble upon them. Or possibly aliens hiding from sight?
To many astronomers, the orbits of Uranus and Neptune seemed to indicate the existence of another body—something altering their movements around the sun. Between 1877 and 1930, a number of theories were proposed. They predicted the existence of anywhere from one to five planets beyond Neptune. However, one astronomer, Jean Baptiste Aimable Gaillot, reran calculations on the planetary orbits, and determined that these theories were put forth in error. The astronomy community largely abandoned the idea of another planet beyond Neptune.
However, there were lingering optimists. One such astronomer was American William H. Pickering, who predicted a Planet O far out beyond Neptune, as well as Planets P through U, all of which were debunked. Another American, Percival Powell, theorized the existence of celestial body he called Planet X.
The long last look at Dione will occur on August 17, when Cassini plans on making gravitational measurements which will help gather more data about the icy shell and mysterious interior.
“There are intriguing hints that perhaps there’s something similar going on on Dione that we might have on Enceladus, but we haven’t found the equivalent of a smoking gun,” Linda Spilker, the Cassini project scientist at NASA’s Jet Propulsion Laboratory, told Discovery News.
The plumes on Enceladus were first found from measurements by Cassini’s magnetometer, which recorded evidence that the magnetic field lines do not go down to the surface — as is what would be expected on an airless body.
However, similar evidence is so far lacking at Dione. Perhaps it’s because the moon is bigger, Spilker says, meaning the plumes are pulled down by gravity and are harder to see. Scientists will also look at Dione from afar in 2017, when the moon passes in front of a background star. If there are plumes erupting from Dione, it would dim the starlight.
As for Enceladus itself, three final flybys are planned in 2015 to learn more about its environment: a view of the north pole on Oct. 14, a “plunge” into the known location of a plume Oct. 28, and an attempt to look at the thermal environment of the south pole Dec. 19.
It’s now winter time in the south pole of Enceladus, providing a unique opportunity to look at any heat coming from the “tiger stripes” in that region. These zones are associated with the icy plumes that are coming from Enceladus.
The north pole flyby will allow them to use a “high phase angle” to examine the plumes in more detail; it’s sort of the equivalent of driving into the sun with the dust on the windshield, Spilker said, because the sun reflects off the dust. This allows them to measure how the plumes have changed.
And the most spectacular of the trio of flybys will be going deep into a known plume region itself, to measure the gas and particles and learn more about their source. The working theory is that water from the sea floor of Enceladus goes through the cracks below the surface, interacts with the rocky core and heats up, then erupts back above the surface and interacts with colder water. Nanosilica particles have been detected in the plumes, which occur in the presence of hot water.
These regular flybys of moons are possible through interactions with the massive Titan, one of the larger moons in the solar system (almost as big as the planet Mercury) and also of interest because it has an ecosystem full of ethane and methane. Its liquid lakes make it unique in the solar system. So Cassini does observations of this intriguing world while taking advantage of its gravity to slingshot to other locations.
Perhaps the most spectacular example of this will take place close to the end of the mission, when Cassini uses Titan’s gravity to “hop” to a spot in the inner part of Saturn’s rings. There, the spacecraft will spend 22 orbits before plunging into Saturn’s atmosphere on Sept. 15, 2017.
“We do that to protect those worlds that have an ocean,” Spilker explained. When Cassini makes its suicidal plunge, it will point its high-gain antenna at Earth and transmit measurements on Saturn’s atmosphere until it begins tumbling, moving it out of contact from Earth in its final moments before destruction.