Pluto's 'Icy Heart' May Have Tilted the Dwarf Planet Over (theverge.com)

An anonymous reader quotes a report from The Verge: Pluto's most iconic feature -- its "icy heart" -- may have been responsible for tipping the dwarf planet over. Scientists believe the 600-mile-wide region of frozen plains known as Sputnik Planitia gained enough mass over the years, causing Pluto to tilt to its current orientation. And that could mean there's a subsurface ocean lurking underneath the dwarf planet. The cracks and faults on Pluto's surface tell the story of its rollover, according to two new studies published today in Nature. Researchers used computer models to simulate Pluto's reorientation, which would have put a lot of stress on the crust and created these cracks. Those models match up pretty well with the patterns of canyons and mountains that NASA's New Horizons spacecraft saw when it flew by Pluto last year. As for how the flip occurred, the two Nature studies offer complementary arguments. Isamu Matsuyama's study says that the low-lying Sputnik Planitia filled up with a bunch of nitrogen ice, gaining mass that pushed Pluto over. But the second study says the nitrogen ice wasn't enough to completely change Pluto's orientation. Even more weight was needed, and a dense ocean lurking just underneath Sputnik Planitia would have been enough to do the trick. Nimmo's study is just further evidence that liquid may be teaming underneath Pluto, making this dwarf planet one of a growing group of objects in our Solar System that harbor oceans. Sputnik Planitia is located in a very special place on Pluto, right next to something called a tidal axis -- the imaginary line that connects Pluto and its largest moon Charon. This axis dictates how Pluto moves if its mass changes. If you were to add extra weight to a certain point on Pluto, the entire dwarf planet would reorient itself so that the weighted point would end up next to this axis.

Comments

"Planet?!"

[hattable]

Are you trying to start a flame war?!

Re:"Planet?!"

[Rei]

The funny thing is, it's the popular press are the ones who always jump over backwards to make sure they use the term "dwarf planet" when describing Pluto, as people like Tyson always give anyone who doesn't use the IAU's term a "Scientists Have Decided, Anyone Who Doesn't Accept This Is Rejecting Science" dressing down. You'll notice that the linked popular press articles were very careful to always say "dwarf planet". Yet in the scientific press people still use "planet" to describe Pluto as much as they do dwarf planet. Including in the linked Nature article, which uses both (ex: "... can substantially alter Pluto’s inertia tensor, resulting in a reorientation of the dwarf planet .." but "... loading and global expansion due to the freezing of a possible subsurface ocean generates stresses within the planet’s lithosphere..").

The IAU is overwhelmingly a group of astronomers. Aka, people who study stars, not planets. Planetologists never wanted this redefinition; to a planetologist, if it's in hydrostatic equilibrium and not fusing, it's a planet - regardless of whether it means some (ill defined and based on a false premise) "cleared the neighborhood" test. Hydrostatic equilibrium is the meaningful characteristic for studying planets. If the body is too small to deform to hydrostatic equilibrium, then you're looking at primitive material; it's the sort of place you'd go to study accretion, the origins of the solar system, the building blocks of life that planets were seeded with, etc. If the body is large enough to deform to hydrostatic equilibrium, then you're looking at altered material, the release of heat, generally fluids, often liquid water and atmospheres (even if the body has since "died" and had its atmosphere stripped), etc. They're the sort of places you go to learn about tectonics, vulcanism, geochemistry, prebiotic chemistry, searches for current or past life, etc. There's a very big difference between the two.

"Cleared the neighborhood" has zero significance to a planetologist. An exact copy of Earth, with all of its life, receiving the same amount of light, etc, would not have "cleared its neighborhood" if it were located in the habitable zone of a young star or a very large star. Not that an exact copy of Earth *anywhere* would be classified as a planet by the IAU, as they explicitly prevent extrasolar planets from being classified as planets. Despite the fact they have an "extrasolar planets" working group. Of course, internal consistency sure never seemed to be a concern of the IAU, as to them "dwarf stars" are stars and "dwarf galaxies" are galaxies, but apparently "dwarf planets" aren't planets. Just terrible terminology. Of course, any terminology that says "Mars and Jupiter are part of the same group, but Mars and Pluto aren't" is pretty ridiculous. If any line needed to be drawn in the "planet" group, it was to separate the gas giants, ice giants and rocky planets from each other.

To me, the worst parts of it are twofold. One, it's based entirely on a false premise: that planets each cleared their own neighborhood. But this is not at all what solar system formation models say. The inner planets had significant sweeping from Jupiter; Mars's neighborhood was almost entirely swept by Jupiter, not by itself. Some people might respond, "Well, okay, yes, that's true, but Mars could have swept its neighborhood, according to its Stern-Levison parameter." That's not what the Stern-Levison parameter says. The parameter is based around the ability of a body to scatter asteroids, with our current asteroid belt distribution. Not protoplanets. Not to mention, Stern is one of the biggest opponents of the "Cleared the Neighborhood" concept (I've never heard Levison's comments on the issue).

My other biggest issue with the decision is timing. And not even the "let's do this on the last day of a conference that most of our membership didn't even attend, after lots of people have left, including most of the opponents of the

Re:"Planet?!"

[stealth_finger]

"Planet?!" Are you trying to start a flame war?!

Would mod you up as funny if I could. Now... let's get the flame war started. The headline did call Pluto a 'Dwarf Planet'. Never liked that name, too clumsy. What's wrong with 'planetoid'? ... too colloquial?

Mass challenged planet shaped object?

Re:"Planet?!"

[rgbatduke]

You forgot to point out that referring to Pluto as a Dwarf planet is bound to make it feel bad. Don't blame me if it retaliates by knocking some comet out of the Oort cloud to come slam into the Earth in a few more decades. That'll show 'em who is a Dwarf and who isn't. Dwarf on this!

Re:"Planet?!"

[Applehu Akbar]

Personally I think that Pluto being the nearest of the KBOs is much more interesting scientifically than if it were merely the most distant planet. Now that we have detailed imagery of it, let's hope that the retargeted New Horizons is able to reach Eris so we can compare the two bodies.

Re:"Planet?!"

[Rei]

Pluto isn't the nearest KBO, although it's the nearest large one. FYI, it's currently estimated that we've only discovered about 1% of KBOs with diameters of 100km or greater. The further out you go, the easier it gets to hide things. In the "inner Oort cloud" (ill defined) you could hide entire ice giants.

NH will not go anywhere close to Eris; we're lucky that there's anything it's able to intercept at all. It'll be passing a small, undifferentiated KBO, so it should be more like Pluto's small moons than Pluto or Charon (or Eris). Eris, and the many other interesting large KBOs, will have to wait for a later date. IMHO I'd love to see Haumea - a body that's spinning so fast that its equilibrium shape is nearly twice as wide at the equator as it is at the poles. Its day length is only 3,9 hours. It has an unusually high rock concentration for a KBO, meaning it might be mostly comprised of the mantle of what it was before a large impact sent it spinning nearly into obliteration. Even weirder, the spectra we've been able to get it are indicative of a body that's being recently (continuously or just once?) resurfaced with fresh water ice... but its equilibrium temperature is only 50K, so what gives with that? There's also some implication that it may have cyanide compounds on its surface, which have been suggested in Titan research as a possible backbone for life at cryogenic temperatures.

There's a good launch window that opens in 2025. I hope we take it. There's a "backup" possibility in 2026. After that, there's no good windows until 2037 (if you want a Jupiter assist, that is)

Re:"Planet?!"

[budgenator]

It's a binary whatchamacallit you insensitive clod!

Re:"Planet?!"

[KiloByte]

Another issue: why is "orbiting a fusor" even a requirement? Two identical balls of rock, with same atmosphere and what not, one orbiting the star directly, one orbiting a big chunk of gas, makes one a "planet" and the other one a mere "moon". Same for a freestanding planet going through the galaxy or even outside.

Re:"Planet?!"

[Rei]

Exactly. "Moon" should just be seen as another optional modifier term that one may or may not add when describing a body in space, not exclusive of other terms.

One thing I've noticed is that people are constantly correcting themselves when describing large moons - they often accidentally call them "planets", and then having to back up and correct that to "moon". The reason we instinctively want to call them planets is because, well, they are like planets. Not just in appearance, but structure too. I think it would be quite reasonable to call, say, Titan a "planetary moon".

We need a wide variety of orbital and compositional terms, and not to insist that you can't use X with Y when both X and Y are clearly applicable. This approach of trying to make just a couple categories that everything has to be jammed into is clearly unsustainable. The more types of bodies we discover in the universe, the worse it's going to get.

Re:Fascinating stuff...

[michelcolman]

Seems to make sense, except for the fact that there's no up and down in space. Pluto is only experiencing significant gravity from the sun and from its largest moon Charon. So I'm having trouble understanding just what forces made it "tip over". What happened to conservation of momentum? You can't just make something spin in a different direction by shifting its mass around a little, unless some external force is involved. Was it tidal forces from Charon? That would mean the rotational momentum was transfered to Charon in some way, altering its orbit or its axis of rotation. How did that work?

Would love to hear more details if anyone understands what actually (supposedly) happened.

So what?

[Black Parrot]

Lots of women with icy hearts have tilted me over.

Re:So what?

[Tablizer]

She found out, and that's where the dwarf status came from.

Angular momentum

[Viol8]

I would assume. You move a mass around on something thats spinning and the acis of rotation (and often speed) at which it spins will change.

Re:Angular momentum

[michelcolman]

Well, no, that doesn't happen. You can spin faster and slower by moving mass closer to the axis and further out (like an ice skater stretching her arms), but you can't change the total rotational momentum (which remains constant in the ice skater example) nor its direction without an external force. That external force, in this case, is probably gravity (and more particularly tidal forces since a constant gravitational force does not cause rotations). I'm just having trouble understanding how it can tip the axis of an entire planet.

Re:Angular momentum

[Hognoxious]

I was thinking the same. And movement parallel to the axis doesn't cause them to tip over either, for example when they drop into that crouching spin with one leg out in front.

Re:Angular momentum

[rgbatduke]

Not to go into this in complete detail, but:

a) The moment of intertia is a tensor, not a scalar like you learned (maybe) in Intro physics. So lots of motions -- like a skater extending just ONE arm out well above her center of mass -- are going to alter the axis of rotation. This is why it is important to balance your tires -- to keep the 'natural' axis of rotation identical to the physical axis of your car's bearings.

b) A moon exerts a torque on the planet it orbits. If the moon isn't perfectly in the ecliptic plane with the sun or if there is any bobble, not just the magnitude of the angular momentum varies, but the direction too. This is why the Earth's axis precesses -- it is tilted relative to the average direction of these torques.

So maybe the physicists who published the article(s) and their referees aren't actually incompetent. Just sayin'...

rgb

Re:Angular momentum

[michelcolman]

I never said they were incompetent, just that I didn't understand.

Meanwhile, after skimming through the original paper, I figured out that the actual axis of rotation (the vector of angular momentum) hasn't changed. The surface has just realigned itself. So even though the axis of rotation is still angled the same way relative to Pluto's orbit around the sun, there are now different bits of the surface at the poles. That makes a lot more sense.

(Yes, tidal effects do tend to affect spins, like causing tidal lock, and in fact Pluto and Charon are tidally locked, but that's not what this is about).

Re:Fascinating stuff...

[Rei]

Gravity isn't a constant force; it decreases with distance. Having heavier mass nearer and lighter mass further is a lower energy state, and is thus thermodynamically favoured.

Re:Fascinating stuff...

[michelcolman]

But with flywheels, the momentum does not disappear. You make the spacecraft turn one way by spinning the flywheel the other way. The total momentum remains the same. Due to all sorts of forces trying to make a spacecraft turn (slight drag from fringes of atmosphere, tidal forces, even forces caused by light), the momentum from all those corrections builds up in the flywheel and they have to get rid of it with special momentum dumping procedures, for example with thrusters or using electromagnets against the earth's magnetic field. It's not that simple, momentum doesn't just go away. Certainly not by moving some mass around, otherwise that's what the sattellites would do. You can speed up the rate of rotation by bringing mass closer to the axis (like an ice skater) but the momentum remains the same and you can't just decide to start spinning around a different axis without some kind of external force.

Also, there's a big difference between changing orientation and changing the speed of rotation. Making a spacecraft turn to point in a different direction is simple. But if it's spinning in one direction and you want to make it spin in a different direction, it's quite a bit more complicated.

Re:Fascinating stuff...

[michelcolman]

I don't think that's quite accurate. It is true that objects may line themselves up with the direction of the gravitational gradient, but not because the heaviest part wants to be closer. It may well end up pointing exactly the other way. Take an ordinary stick in orbit, for example. If it is lined up with the gravitational vector, its lowest end will be attracted to the body more than its further end. If it turns slightly away from vertical, the tidal force will tend to bring it back to vertical because gravity is still pulling the lowest end more strongly. The weights don't really matter, you may have a stick with a heavy end pointing away from the planet and it will be perfectly stable that way (the heavy bit receives less pull if it's further away and therefore tends to stay away from the planet).

Anyway, tidal forces are indeed responsible for things like tidal locking of the moon (where the moon's slowing down was compensated by a change in its orbital distance and smaller effects on the axial rotation of the earth) but I don't see how it can tilt the rotational axis of a planet. Certainly the sun's tidal force at that distance is nowhere nearly strong enough, or is it?

I'm not saying those scientists are wrong, by the way. Just that it needs to be explained properly because the simplified explanation does not make sense.

"Teaming"?

[Anonymous Coward]

Learn to English, submitters and editors. You're looking for the version of "teem" that implies abundance and fecundity, not collaboration.

Re:Fascinating stuff...

[stealth_finger]

I think it's just badly written and not flipping up or down, rather it's saying that pluto re-orientated to have the most massive part which is this heart bit apparently, on the axis of rotation with Charon, so always facing away from it, making them tidally locked I guess. At least that's the understanding I took from it and another article somewhere I can't find again.

Re:Global warming

[stealth_finger]

Nah, it's caused by right wingers blowing steam.

Re:Fascinating stuff...

[rgbatduke]

Anyway, tidal forces are indeed responsible for things like tidal locking of the moon (where the moon's slowing down was compensated by a change in its orbital distance and smaller effects on the axial rotation of the earth) but I don't see how it can tilt the rotational axis of a planet. Certainly the sun's tidal force at that distance is nowhere nearly strong enough, or is it?

Sheesh!

https://en.wikipedia.org/wiki/...

I guess that together, they are! The moon and sun exert an AVERAGE TORQUE on the Earth, and it is large enough to cause the axis to precess! This effect is also very dependent on things like mass distribution -- ice ages rearrange substantial surface mass and can indeed alter the tensor moment of inertial and hence cause the axis of rotation itself to move relative to the not-terribly-solid earth, with interesting consequences, see:

https://en.wikipedia.org/wiki/...

especially: "The redistribution of ice-water on the surface of the Earth and the flow of mantle rocks causes changes in the gravitational field as well as changes to the distribution of the moment of inertia of the Earth. These changes to the moment of inertia result in a change in the angular velocity, axis, and wobble of the Earth's rotation.

The weight of the redistributed surface mass loaded the lithosphere, caused it to flex and also induced stress within the Earth. The presence of the glaciers generally suppressed the movement of faults below.[72][73][74] However, during deglaciation, the faults experience accelerated slip triggering earthquakes. Earthquakes triggered near the ice margin may in turn accelerate ice calving and may account for the Heinrich events.[75] As more ice is removed near the ice margin, more intraplate earthquakes are induced and this positive feedback may explain the fast collapse of ice sheets."

rgb

Re:Fascinating stuff...

[michelcolman]

After having given the actual original article a quick 3 minute read, I was just coming back here to post exactly that. Pluto's axis of rotation hasn't changed, its surface has just reoriented itself so that different bits of the surface are at the poles.

It's as if you would move the surface of the earth around so that Belgium is at the north pole, The planet would still be spinning around an axis angled 23.4 from "vertical" relative to its orbit around the sun, but the map of the earth would look different. And it would be a hell of a lot colder here.

Re:Fascinating stuff...

[michelcolman]

You are quite right about tidal effects causing precession, and your links were very informative, but meanwhile I figured out that this is not what the actual paper was about.

Yes, Pluto and Charon are tidally locked because of the mechanisms you described. That happened very long ago.

However, more recently, the redistribution of mass on Pluto's surface caused the whole surface to realign itself with the axis of rotation (which did not change). So Pluto's axis did not tilt (at least not due to the described effect), it's still pointing in the same direction in space, but it's now going through a different part of the surface (if I understood correctly).

Poor planetoid

[Dunbal]

Are we DONE ALREADY with teasing Pluto? Poor little planetoid. Had its planetary title taken away and now people are making fun of it for wearing its heart on its sleeve. LEAVE PLUTO ALONE LEAVE IT ALONE!

Re:Global warming

[budgenator]

This is more evidence that Pluto is getting colder. That the Earth is warming while other planets are getting colder is yet more proof that we are responsible for global warming. I'm sure the deniers will be out in force to accuse me of being a shill, but global warming is an undeniable fact.

Actually Pluto is getting Warmer like most of the solar system. Now when this gets ugly, I want everybody to notice that not only did I not start this, I didn't say anything about terrestrial anthropogenic warming either.