Today NASA’s Cassini probe will zip by Saturn’s moon Enceladus, just 50 kilometers (30 miles) above the icy moon’s surface, flying through a plume of water vapor, organic materials, and ice. The spacecraft has buzzed Enceladus in the past, even passing through its plume, but never before so close to the surface.
The plume erupts from what evidence suggests is a vast, salty ocean churning beneath 30–40 kilometers (19–25 miles) of ice in Enceladus’s southern polar region. Scientists suspect that Enceladus’s underlying ocean hosts some hydrothermal activity similar to that found in deep oceans on Earth. From today’s flyby, Cassini scientists expect to glean clues about what kinds of chemistry are going on beneath the shell of ice.
Eos: What is the plume, where does it come from, and why do scientists want to know more about it?
Spilker: The Enceladus plume is a continuous eruption of ice particles, water vapor, and organic molecules that sprays from long, linear fractures we call “tiger stripes” near the moon’s south pole. We’ve learned since Cassini first detected the plume in 2005 that the icy moon has a global ocean beneath the surface, with likely hydrothermal activity. These qualities have made Enceladus one of Cassini’s primary targets of interest because they suggest that this small moon could have the right ingredients to create habitable environments for simple forms of life.
Eos: When will Cassini enter and exit the plume, and when will key measurements or observations take place?
Spilker: The closest approach to Enceladus is scheduled for 11:22 a.m. Eastern Daylight Time (EDT) on 28 October [today]. Traveling at 30,600 kilometers per hour (19,000 miles per hour), Cassini will fly through the plume in tens of seconds. We’re also observing before and after closest approach with our cameras and other remote sensing instruments. This is our last opportunity to get this close to Enceladus for the remainder of Cassini’s mission, so it’s high-value science.
Eos: What are a couple of the most important measurements Cassini will make during the fly-through?
Spilker: We’ll collect the gas from the plume to analyze it for signs of molecular hydrogen. The confirmation of hydrogen in the plume would be an independent line of evidence that there’s hydrothermal activity taking place in the ocean. The amount of hydrogen will tell us how much hydrothermal activity is going on down at the seafloor. In addition, Cassini will directly sample the icy spray from Enceladus to learn about the chemistry of the ocean.
Eos: Will Cassini get wet or coated with a film from the plume? Could the plume push Cassini off course?
Spilker: Cassini won’t be coated in plume ice. It’s a small amount of material—like flying through a very thin fog. The gas density is quite low, not enough to have a significant effect on Cassini’s trajectory. The total amount of material we’ll collect is the volume of a small droplet of water, but it’s enough for our highly sensitive instruments to analyze to see if the ingredients for life are present in the ocean.
—JoAnna Wendel, Staff Writer
Citation: Wendel, J. (2015), Cassini probe dives through Enceladus plume, Eos, 96, doi:10.1029/2015EO038311. Published on 28 October 2015.