NASA’s New Horizons spacecraft is less than three weeks from sailing through the Pluto system. What it will find there is still anyone’s guess, but images the spacecraft has already sent home are every bit as puzzling as we’d hoped.
The newest photos reveal a possible bright spot at Pluto’s polar cap. And Charon, the dwarf planet’s oversize moon, has a curious dark spot (“Who ordered that?” asked Alan Stern, New Horizons principal investigator). From Hubble images, we’ve also recently learned that Pluto’s small moons are chaotically rotating, and one of them is way too dark.
Before the spacecraft reveals any more about the system, I’d like to spend a little time discussing the feature I’d most love to see rising from the Plutoscape—a landform I’ve been assured could, in fact, exist on the dwarf world.
So far, this feature only exists in one place in the solar system. It’s a feature that isn’t fully understood. A feature that, if you were standing next to it, would be among the more awe-inspiring things you could ever hope to see.
It lives on Saturn’s strange moon Iapetus, a world with a two-toned surface that is quite like the wildly varied surface of Pluto.
(drumroll) It’s the enormous mountain range that runs straight as an arrow along three-quarters of the Iapetian equator. Why only three-quarters of the way around? That’s one of the mysteries that isn’t solved yet.
Rising roughly 20 kilometers high in some parts, Iapetus’ mountain ridge is, to the best of our knowledge, one of a kind. No one knew it was there until the Cassini spacecraft took a good look at the walnut-shaped moon in 2004 and discovered mountains that dwarf the tallest peaks on Earth.
It’s not clear how the ridge formed, but the most likely possibility links it to a ring that used to encircle the moon.
One story suggests that once upon a time, Iapetus had a moony friend. A moon-moon, if you will. But over time, that moon wandered too close to Iapetus and got shredded by the larger moon’s gravity. The resulting debris ring began raining onto Iapetus, depositing material along the moon’s equator.
Another story suggests an early collision blasted both a debris ring and a moonlet into orbit around Iapetus. Eventually, the moonlet fell back to the surface, creating a large impact basin, and the ring again left its mark around the moon’s middle.
Why the mountainous ridge only wraps around part of Iapetus is not yet understood, but there are few other suggestions that even come close to describing the origin of this oddity. (These include some kind of impact-induced deformation as well as geologic activity within the moon itself).
The sheer strangeness and unsolved mysteries surrounding this massive equatorial mountain range continue to captivate my curiosity, years after I first heard about it.
So, imagine my surprise when, back in December, Iapetus came up during a conversation with chief Plutophile Marc Buie.
“The crazy thing about that one is that equatorial ridge, and how weird is that?” asked Buie, of the Southwest Research Institute.
“What if we found an equatorial ridge around Pluto or Charon?” I said, realizing I might have just asked a really dumb question.
“That would be so cool,” Buie said. “If either of those objects has this equatorial ridge like Iapetus, our collective brains would just explode.”
Neuronal detonation aside, it would indeed be a discovery. As it turns out, it’s not entirely unlikely.
Early in Pluto’s history, the dwarf planet was dealt a violent blow by the body that would become Charon. Though both worlds survived the collision, the impact flung debris throughout the fledgling Plutonian system. Some of that debris, scientists think, ended up forming the four smaller moons (though recent observations may edit that story somewhat).
The rest of it may have temporarily coalesced into a debris ring around Pluto.
“One could imagine that early in the Pluto system, it almost certainly had a disk of material—or for lack of a better term, rings,” says NASA’s Jeff Moore, who specializes in the geology of icy moons. “It’s not impossible that a lot of the dust could settle down and form an equatorial ridge.”
If Moore and Buie are on board, that’s good enough for me to get my hopes up just a little bit. But it gets better.
“I think there’s a lot of us on the science team that wouldn’t be particularly surprised if we find an equatorial ridge around Pluto,” Buie said recently. “I would love to see it just sort of shaking everything up.”
(Learn more about what we might find on Pluto in July’s National Geographic cover story.)
(The video at the top of this post beautifully places the New Horizons mission within the context of a half-century of planetary exploration. Iapetus also makes a brief appearance! National Space Society/YouTube.)