A photo of a mysterious feature on Saturn’s moon that has baffled scientists is now going viral.
The planet is some 793 million miles (1.3 billion km) from Earth, yet its stark distance didn't stop NASA's Cassini spacecraft from capturing some incredible images of it back in 2007.
Named Iapetus, it's one of Saturn’s 274 moons, and orbits more than two million miles (3.22 million km) away from the planet.
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Despite this, it's still tidally locked to Saturn - so always shows it the same side, like how our moon always shows the same face to Earth.
When Iapetus was discovered in 1671 by Giovanni Cassini, he noticed that one side of it was much darker than the other.
While scientists have a couple of theories as to why that's the case, Iapetus - which has a radius around two-fifths of Earth's moon - features another unusual detail which has space experts stumped.
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Iapetus has a huge ridge of mountains that run along its equator. They show up clearly in a photo taken by the Cassini spacecraft and almost look like a seal where two halves of the moon have been assembled via a mold.
These tall mountains measure up at about six miles (10 kilometers) high.
On the side of Iapetus that faces away from Saturn, the ridge looks broken up into separate, bright mountains.
These were first seen by the Voyager I and II spacecraft in the 80s, so they’re sometimes called the Voyager Mountains.
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Taking to Reddit's R/SpacePorn community, one person shared a photo of the prominent ridges, writing: "A strange line along the equator of Saturn’s moon Iapetus."
The post has since received thousands of upvotes and hundreds of comments.
People were quick to share their own tongue-in-cheek theories about the ridges.
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"No no it just came out of the mold. They haven't trimmed the seam yet," one joked, as a second pointed out its striking resemblance to a bath bomb.
"A Kinder moon!" a third declared, as a fourth quipped: "I say we crack it open and find the toy inside!"
"It’s so obvious," someone else wrote: "It’s just a giant walnut."
What is the cause of Iapetus' ridge?
NASA explains there are two main theories as to how the ridges formed.
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"Some scientists think the ridge was formed at an earlier time when Iapetus rotated much faster than it does today," its website explained.
"Others think the ridge is made of material left from the collapse of a ring."
Another answer could be that the ridge is formed by icy material which welled up from beneath the moon's surface and then solidified.
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For now, though, the ridge's origin remains a mystery...
Ellie Kemp