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?!"

[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)

So what?

[Black Parrot]

Lots of women with icy hearts have tilted me over.

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