Of all of the planets in our photo voltaic system, Saturn is by far the mooniest. And that is saying lots. Sure, we’re right here in our nook with our single good friend, The Moon™, however Neptune wanders the universe with 16 recognized companions, Uranus boasts 28 of its personal, and there are a whopping 95 moons within the Jovian neighborhood. But Saturn? It’s in a special league. This ringed world has 146 of those pure satellites. Yet, chances are you’ll be stunned to know that even with such a beautiful Saturnian choice, scientists are largely pining over only one.
The golden baby in Saturn’s system is called Enceladus, and it is so particular as a result of scientists consider it to be a major location to seek for life past Earth. That perception stems from a number of discoveries made over time, most clearly the truth that Enceladus appears to have a subsurface ocean that will host molecules recognized to assist produce life as we all know it. Better but, it additionally seems to have big plumes of water ice deposits (suppose icy geysers capturing into area) related to that ocean, which implies spacecraft orbiting the world may theoretically catch proof of these molecules in the event that they’re truly there.
Thus, when learning Enceladus, each element actually issues — which brings us to a brand new, very unusual element that scientists have their eye on: A bizarre, disappearing darkish spot on this ice-capped moon. No one fairly is aware of what it’s but, however it might inform us one thing about these plumes that might maintain the dear constructing blocks of life we search.
This darkish spot was one of many intriguing subjects of debate in the course of the 2024 American Geophysical Union assembly in Washington, D.C. — the place scientists congregated to seek for the ultimate items in cosmic puzzles they have been engaged on all yr.
Awe stuffed the room as Cynthia B. Phillips, a planetary geologist at NASA’s Jet Propulsion Laboratory who introduced the analysis, went into super element about how she and her staff initially recognized the darkish spot. It was because of her crewmember Leah Sacks, who helped pore by a bulk of knowledge about Enceladus, collected by NASA’s Voyager and Cassini missions. The aim of the evaluation was to check photos of the identical area taken by these spacecraft as a way to determine any modifications on the moon’s floor.
Possible modifications may reveal superior details about geologic exercise on the world, however we’ll get to that shortly; first let’s dive into the mysteries of the darkish spot.
“After looking at dozens and dozens of picture pairs — she discovered one thing attention-grabbing,” Phillips mentioned in the course of the convention. “It’s somewhat darkish spot; it is a few kilometer throughout. She noticed it in a picture from 2009 and seemed once more in 2012 and it appeared to be gone.”
The darkish spot was slowly fading away and getting smaller because the years progressed, and it by no means turned extra pronounced once more. How bizarre, and particularly so as a result of Enceladus has what’s known as a excessive “albedo.” That principally means the world is basically vibrant — it is subsequently sudden to discover a darkish spot on it in any respect, not to mention one which’s fading away. But earlier than getting too excited, the scientists made positive to second guess themselves as a lot as attainable to rule out the apparent caveats.
“First our query was,” Phillips mentioned, “‘properly, is it simply that in a few of these low decision photos, we’re not seeing it, nevertheless it’s actually there?'” In brief, the reply was a easy “no, in all probability not.” For instance, a direct comparability of a 2010 picture and a 2011 picture reveals the darkish spot smaller within the 2011 picture, though the 2011 picture had the next decision.
The subsequent query was: Is this a shadow of some kind? Well, nope. Doesn’t appear like it.
The staff pulled out some photos with lighting coming from totally different instructions, and the placement of the spot appeared constant. The researchers even discovered a sequence of photos with the darkish spot the place the sunshine’s angle of incidence (aka, the angle at which mild strikes a floor) will get increased and better. If the spot had been a shadow, you’d count on it to turn out to be extra distinguished with the next incidence angle. This wasn’t the case — it nonetheless turned much less distinct as time went on. “We do not suppose it is topography; we do not suppose it is only a shadow,” Phillips instructed Space.com.
And it did not finish right here — the staff additionally checked out photos taken in UV mild and shade (the latter of which apparently recommended that the darkish spot is a reddish brown, in contrast to the standard blueish darker areas on different sections of the moon). None of this recommended a straightforward clarification for the characteristic.
So, what’s it?
“I believe the extra probably [case] is that it’s some form of a crater,” Phillips instructed Space.com. “And the explanation why it is darkish is possibly it is a chunk of some form of darkish materials that landed on the floor, and also you’re both seeing a few of that impactor left behind, and that is why it has that bizarre shade, otherwise you’re seeing that when it impacted, it uncovered some form of bedrock of ice that was a special shade.”
But for nearly each probably and mundane situation in area analysis, there tends to exist a uncommon and thrilling one serving as a counterpoint.
“The actually cool clarification could be if it was truly arising from beneath, one way or the other; if that reddish shade was truly an indication of the inside composition of Enceladus,” she mentioned. “That’s unlikely, however that’d be actually attention-grabbing.”
Still, though we do not know what the darkish spot is, Phillips factors out that there’s certainly one thing fairly main we are able to derive from its presence: “‘What is it?’ I do not know the reply to that — however what I can say is: ‘What can we use it for?'”
Remember the plumes
In a nutshell, the researchers suppose the darkish spot seemed to be fading progressively as a result of deposits from these icy Enceladus plumes may need coated it up. “We know the entire floor is roofed by plume deposits — like little layers of ice build up over time,” Phillips mentioned.
Alas, in principle, this makes a variety of sense. But once you actually give it some thought, there are some excellent points right here.
For instance, the staff noticed the darkish spot fading over just some years — this may indicate that just some years is lengthy sufficient for ice plume deposits to create a sheet of ice thick sufficient to cowl such a distinguished spot. After all, it is seen from area! Yet, in line with varied calculations of the darkish spot and fashions of the moon’s plumes, Phillips says it ought to take one thing like 100 years to create a layer thick sufficient to cowl this sort of spot.
“What this might imply, although, is that the plume deposition mannequin, not less than on this location, is an underestimate,” she mentioned. “One factor we’ve not taken under consideration, although, is deposition from collisions with E ring particles.”
E ring particles check with the tremendous small water ice particles in Saturn’s rings. Potentially, the staff causes, a few of these particles could possibly be serving to construct the sheet overlaying the darkish spot. But the story of this spot’s origin and evolution, at this level, is mirrored by the abrupt ending of our story of its discovery.
There are just too many unanswered questions.
“What would the deposition fee wanted to cowl the black spot on this time-frame point out about deposition charges? Is the E ring contributing to cowl that spot? Is there possibly one other mechanism?” Phillips contemplated.
“And, you understand, what’s the black spot?”
