In news that reminds us it's definitely worth dusting off old photos once in a while, one amateur astronomer thinks he's spotted geysers erupting from the south pole of Saturn's moon Enceladus....in images taken by the Voyager 1 probe in 1980.
Sure, those geysers are little more than a handful of stray grey pixels trailing off the bottom of a misshapen white blob. But if the finding holds up, it would extend our baseline data for geyser activity on Enceladus way, way, back -- more than 25 years before the Cassini probe's first ice geyser sighting in 2005. That, according to Ted Stryk, the philosophy professor, space blogger, and planetary data hobbyist who analysed the images last fall and wrote up his findings this week, is "very good news" for the folks who want to send a spacecraft to Saturn's icy moon to hunt for signs of life.
"Many [NASA] New Frontiers proposals now are about exploring these plumes," Stryk told Gizmodo. "One of the biggest concerns is: what if they're ephemeral? The fact that the one time we've looked back 25 years earlier now [and seen the plumes] is a good sign."
Enceladus' south polar plume is shown spraying water vapour and icy particles off into space in this Cassini photo. Image: NASA/JPL/Space Science Institute
Since NASA's Saturn-orbiting Cassini probe started snapping high-resolution photos of Enceladus in the early 2000s, it's become one of the most fascinating objects in the solar system for astrobiologists and planetary scientists alike. Despite being a mere 499km in diameter, Enceladus is a complex, and surprisingly Earth-like world. Beneath the moon's shiny, ice-covered surface lies a global liquid water ocean, kept warm by a rocky core that gets its heat from the gravitational tug of another Saturnian moon, Dione.
So far, all signs have pointed to Enceladus' subsurface ocean having an Earth-like composition -- and excitingly, we don't need to pull a Europa Report and drill through the surface to find out if that's true. Enceladus' south pole features a complex "tiger-stripe" geyser system; fissures that appear to open and close as the moon's crust flexes under gravitational stress. These geysers are essentially offering free samples to any spacecraft that happen to be in the neighbourhood.
It's for these reasons that Enceladus has become a prime target in the search for alien life in our solar system.
Enceladus, seen over Saturn in a photo snapped by Voyager 1 on November 13th, 1980. Image: NASA / JPL-Caltech / Ted Stryk
12 years of Cassini data showing continual geyser activity on Enceladus is promising, but it sure would be a bummer if those geysers turned off say, every 25 years, and we didn't know about it before launching another space probe. Stryk, who was involved with image processing for NASA's New Horizons mission to Pluto, and who once discovered an unknown moon of Neptune in old Voyager 2 images (no biggie), has wondered for years whether more information on Enceladus' activity could be gleaned from Voyager 1.
"Soon after Cassini initially announced the data [on Enceladus' geysers], I looked through the Voyager images tagged in the data archives for Enceladus," Stryx told Gizmodo. "But I didn't find anything at the right illumination angle."
It wasn't until the summer of 2015, while sitting in a meeting on processing very high phase, or backlit, images of Pluto from the New Horizons flyby, that Stryk got a new idea: maybe Enceladus was present in some of the high-phase images from Voyager 1, which would have offered a better lighting angle for spotting plumes.
Last fall, he got to work scouring high phase images from the spacecraft's Saturn flyby, in the fall of 1980. And there, hidden in the corner of eight wide-angle, black-and-white photos Voyager 1 snapped on November 13th, he saw it: Enceladus, itself little more than a smudge, with a telltale smear emanating from its south pole.
Enceladus, from Voyager 1, on November 13th, 1980. The smear emanating from the moon's south pole is consistent with the tiger stripe geysers captured in much higher resolution by Cassini. Image: NASA / JPL-Caltech / Ted Stryk
"In stacking [the images], I was pretty confident that I had pulled out the plumes," Stryk said. "It's the right illumination, the right direction." Stryk, who wrote up a Lunar and Planetary Sciences conference abstract on his findings, says that most of the astronomers he's discussed his work with have been "very positive about the analysis."
That doesn't mean it's an open-and-shut case. Cornell astronomer Jonathan Lunine told Gizmodo that while Stryk's abstract generated "a great deal of fun discussion" at a Cassini Project Science group meeting last week, he needs to see more details to rule out the possibility of image processing artifacts.
"If further analysis proves out Stryk's analysis, then it is direct evidence that the plume has a longevity of at least decades," Lunine said. "And, now I can stop telling audiences that the Voyager cameras were unable to detect the presence of a plume."
Stryk, for his part, plans to continue sifting through Voyager images to find more evidence of the plume. If one thing's clear, it's that Enceladus is shaping up to be a more tantalising target than ever -- and that the Voyager 1 probe, which is still sending back data and making discoveries from the edge of interstellar space, was well worth the investment.