Mystery dark spot on Enceladus intrigues scientists
- Saturn’s moon Enceladus has a bright surface of ice above an inner ocean. Recently, scientists noticed an odd dark reddish-brown spot on this surface in old spacecraft images. What was it?
- The dark spot faded away over about three years. It was seen in images taken in 2009, but had disappeared in later images from 2012.
- The spot might have been covered over by material from the icy plumes emanating from the little moon’s interior. Or here’s another possibility. The dark spot might have been a crater with dark material inside it.
Mystery dark spot on Enceladus
Just a few earthly years ago, a dark spot showed up on the icy surface of Saturn’s moon Enceladus. Now scientists are trying to figure out why. Researchers from NASA’s Jet Propulsion Laboratory (JPL) noticed the incident while examining Cassini spacecraft images from between 2009 and 2012. They spotted a half-mile-wide dark spot in the images, which seemed to fade away and then disappeared entirely three years later. What was it? The surface of Enceladus is changeable. But this unusual sort of feature hadn’t been seen before. It was an intriguing new mystery.
The lead author of the study, Cynthia Phillips, presented the findings at the 2024 American Geophysical Union meeting (AGU24) in Washington, D.C in December 2024. You can read the abstract here. There’s also a previous abstract from the Lunar and Planetary Science Conference (LPSC2024). And Monisha Ravisetti of Space.com wrote about the discovery, too, on December 15, 2024.
Combing through old images
Cynthia Phillips told Space.com that her colleague Leah Sacks had been going through data about Enceladus from the Voyager and Cassini missions, when she discovered the dark spot. Voyager 1 flew past Saturn and its moons in November 1980. Voyager 2 then followed in August 1981. Since then, the Cassini also visited the Saturn system, orbiting Saturn and weaving among its moons, from July 2004 to September 2017.
The researchers were looking for changes that might have occurred on Enceladus’ surface, when they found the dark spot. It was in images sent back to Earth by Cassini in 2009. But in later images from 2012, it was gone. Space.com quoted Phillips as saying:
After staring at dozens and dozens of image pairs, she [Sacks] found something interesting. It’s a little dark spot; it’s about a kilometer across. She spotted it in an image from 2009 and looked again in 2012 and it seemed to be gone.
Images from between 2009 and 2012, both high and low resolution, appeared to show the spot gradually fading and getting smaller. The dark spot stood out since Enceladus’ icy surface is overall quite bright, with a high albedo. So what could have caused it and why did it fade away?
Searching for the answer
The research team wondered if the spot might be still there in some of the lower-resolution images, but perhaps just not so visible. But that possibility was pretty much ruled out, because an image from 2011 showed the spot as smaller than in the 2009 images, even though this 2011 image had higher resolution. Phillips told Space.com:
First our question was, well, is it just that in some of these low resolution images, we’re not seeing it, but it’s really there? In short, the answer was a simple no, probably not.
Could the spot have just been a shadow? The researchers looked at images with the light coming from various directions. But the spot remained the same in appearance. There was even a sequence of images where the incidence – the angle – of sunlight hitting Enceladus’ surface became progressively higher. But the spot didn’t become more prominent, like it would be expected to if it was a shadow. Indeed, in the images it still continued to fade over the years.
Interestingly, the spot was also different in color from the surrounding terrain. While the icy surface is whitish with some areas of darker blue, the spot was reddish-brown.
Enceladus has active geysers
Enceladus only has an extremely thin, tenuous atmosphere, composed mostly of water vapor. But its surface is dynamic, due at least in part to the active geysers at the little moon’s south pole. These huge geysers bring up plumes of water and other material from the global ocean below Enceladus’ surface. In other words, the researchers think the spot might have been covered up by deposits of water-ice particles that fall back to the surface. Phillips said:
We know the whole surface is covered by plume deposits, like little layers of ice building up over time.
It might not be quite that simple, though. The spot faded over a period of about three years. And some calculations suggest it could take up to about 100 years for enough plume material to be deposited to cause the amount of fading observed by these scientists.
What else might have covered the spot?
Material from Saturn’s E-ring?
What about additional material from Saturn’s E-ring? It seems the plumes of Enceladus also inject material into the E-ring. Later, some of this material also falls back to the surface. Phillips added:
What this could mean, though, is that the plume deposition model, at least in this location, is an underestimate. One thing we haven’t taken into account, though, is deposition from collisions with E-ring particles.
What would the deposition rate needed to cover the black spot in this time frame indicate about deposition rates? Is the E-ring contributing to cover that spot? Is there maybe another mechanism? And, you know, what is the black spot?
That, of course, is the big question. And are there other spots like it that haven’t been found yet?
Impact crater, or material from below?
Right now, Phillips and her team think the most likely scenario is that something new struck Enceladus, meaning the spot is a crater. As she explained to Space.com:
I think the more likely [case] is that it is some kind of a crater. And the reason why it’s dark is maybe it’s a chunk of some kind of dark material that landed on the surface, and you’re either seeing some of that impactor left behind, and that’s why it has that weird color. Or you’re seeing that, when it impacted, it exposed some kind of bedrock of ice that was a different color.
Another, more exciting possibility is that the spot is material that came up to the surface from below, perhaps even from the subsurface ocean itself. Phillips said:
The really cool explanation would be if it was actually coming up from underneath, somehow; if that reddish color was actually a sign of the interior composition of Enceladus. That’s unlikely, but that’d be really interesting.
Other changes on Enceladus’ surface
The dark spot is one of several changes the researchers found in the images. Others include long straight dark streaks. These faded or brightened in a manner similar to the dark spot. The LPSC paper abstract said:
At this stage of the project, we have detected several possible surface morphological brightness changes at Enceladus. Since the satellite has a near-uniform high albedo [e.g. 8, 9], we anticipate any new dark surface features to brighten with time to match the overall brightness.
The first potential change relates to a dark streak that stretches for over 100 km on the leading/sub-Saturn hemispheres. This linear dark streak is surprisingly straight, with a consistent width (<5 km) and brightness across its length. It is visible on ~50 Cassini images spread across several imaging sequences. Near the eastern edge of the streak, we found a brightening of the dark streak between images separated by ~5 years. This change appears to be happening in segments, with portions of the dark material remaining dark, while sections on either side appear to become brighter. We believe this is evidence for a real observed brightness change and not an artifact of differing lighting conditions.
The researchers will continue to examine older images of Enceladus to try to find other similar anomalies. This will include comparisons between Voyager and Cassini images. What else might be waiting to be discovered?
Bottom line: Scientists found an unusual dark spot on Enceladus while looking at old images sent back by Cassini. The odd reddish-brown patch faded away over 3 years.
Source: Surface Appearance Changes Over Time on Enceladus
Source: Searching for Geological Changes at Enceladus
Read more: Complex organics on Enceladus: A clue to possible life?
Read more: Enceladus hosting cell-sized particles, a hint of life?
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