Saturn’s sixth largest moon Enceladus is, some think, the most exciting object in the Solar System. It has a warm, salty ocean 12 miles/20 kilometers below its icy surface that appears to be potentially habitable.
Could there be some kind of alien aquatic life at Enceladus that has evolved completely separately to life on Earth?
It’s a question of the utmost importance in the planetary science and astrobiology communities. So much so that pressure is building for NASA to send a flagship mission to Enceladus to find out exactly what’s going on under the icy crust of the tiny moon.
It’s a matter currently being pored over as part of the Decadal Survey for Planetary Science and Astrobiology, a report compiled by the National Academy of Sciences that will set out the priorities for NASA for the next 10 years. It will be published on April 19, 2022.
Will the incredible “Orbilander” mission be on that list?
A concept for a flagship NASA mission costing about $3 billion, Orbilander would see a single spacecraft first orbit Enceladus, then land on it.
“Enceladus hosts the best-characterized ocean in the Solar System, second only to Earth’s,” said Shannon M. MacKenzie, a planetary scientist at the Johns Hopkins University Applied Physics Laboratory and lead author of the Enceladus Orbilander Mission Concept. Enceladus has a heat source and liquid water and we know that those two things combine to create fantastically intriguing diversity of life on our planet. So why not on Enceladus? “There’s liquid water there and the chemical ingredients that we think that biochemistry needs, but it's very hard to use the typical tools that a satellite would have to interrogate that ocean because it’s under kilometers of ice,” said MacKenzie.
Ah yes, the ice.
Scientists have known Enceladus has a 19 miles/30 kilometers-deep subsurface ocean since 2014 when data from NASA’s spacecraft Cassini revealed that the tiny moon has Yellowstone-style geysers that erupt through fissures in the ice. “There are these long cracks 10s of kilometers long at the south pole that have these individual jets within them that spew out material from the ocean into space,” said MacKenzie.
Scientists call them “tiger stripes” and they look like this:
Those geysers make it theoretically possible for a spacecraft to sample its subsurface ocean without actually having to go through its ice crust.
“Cassini’s incredible images of Enceladus helped peel back the layers of what's going on,” said MacKenzie. “We’re now able to make a really compelling case for Enceladus being a potentially habitable environment.”
That ocean is probably heated by the moon’s core; cue theories about hydrothermal vents—like those found on Earth—that could support life.
One recent study suggests that the subsurface ocean on Enceladus appears to be churning with currents like those in Earth’s oceans. Another that it hosts icequakes. There’s also intriguing evidence for water-rock interactions happening at the base of the ocean.
“If I had to put my money on something in the Solar System that will be a slam dunk for getting the information we need and say life exists it would be Enceladus,” said Dr. Jackie Faherty, Senior Scientist and Senior Education Manager jointly in the Department of Astrophysics and the Department of Education at the American Museum of Natural History. “You don’t have to dig a hole—those geysers going off are shooting that material up for a spacecraft to stick out its tongue to take a lick ... Enceladus is the most exciting object in the Solar System.”
However, Orbilander would land on Enceladus. “Orbilander is going to go and sample those plumes—actually ice particles in space that have come out of those cracks—about twice per day for 200 days, then it will land,” said MacKenzie. “That’s because the largest particles aren't going to make it high up—they don't have enough kinetic energy—so they fall back down onto the surface.”
Actually landing on Enceladus will be easier than on, say, Mars because as a much smaller body there's a lot less gravity—about a hundredth of Earth’s.
The main problem with the Orbilander mission concept is that Enceladus is so very tiny. It’s just 311 miles/500 kilometers in diameter—about a seventh the size of our Moon—so physically getting out of Saturn orbit and into orbit around Enceladus won’t be easy. “One way is to carry a lot of fuel, but that’s expensive,” said MacKenzie. In dollars, yes, but also in terms of mass—the bigger the spacecraft the slower it will travel. “Another is to take advantage of gravity assists and do a “moon tour” in the Saturn system,” said MacKenzie.
Any mission to Enceladus won’t happen tomorrow, whatever the Decadal Survey recommends NASA work on. The Orbilander concept suggests a launch in 2038 and an arrival in 2050 to begin a 200-day orbit. After samples of the plumes have been taken for “life detection” a suitable landing place can be found.
Is 2050 too far away for anyone to get excited about Enceladus? Perhaps, but there are good reasons to wait until then. “That’s when the south pole of Enceladus will be coming into southern summer, which means more of it is going to be illuminated as the mission goes on,” said MacKenzie.
Though it was Cassini that unveiled its subsurface ocean, NASA’s Voyager images back in the 1980s revealed Enceladus to be both very bright and devoid of craters; clues that it was covered in highly reflective ice and that it’s surface keeps renewing itself. In short, Enceladus is geologically active—it’s warm and it’s wet ... that’s what where life as we know it can exist.
Enceladus will always be a world of interest in the search for life beyond Earth. It’s now surely only a matter of time before NASA visits it.
Wishing you clear skies and wide eyes.
