3 Possible Models For Why Uranus Spins on Its Side

[Prof Mark R Smith]

TL;DR

Uranus spins on its side. Uranus has an obliquity (tilt) of 98º, making its axis of rotation closer to the ecliptic plane than any other planet. It is not known how it got this peculiar tilt.

Uranus spins on its side. Uranus has an obliquity (tilt) of 98º, making its axis of rotation closer to the ecliptic plane than any other planet. It is not known how it got this peculiar tilt.

Table of Contents
ToggleUranus Key PointsProblems with Current TheorySpin-Orbit Resonance What is It?Growing a Tilted Ice GiantModel...

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[TeethWhitener]

Thanks for this. The tilt of Uranus has been one of my favorite solar system mysteries ever since I was a little kid and we were just starting to get glimpses of the outer solar system from Voyager 2. It's nice to see some of the theories being debated.

 

[sophiecentaur]

Such an interesting topic and the article / work is well thought out. The argument against a collision is very convincing and, as @TeethWhitener says, puts a long lasting nagging worry to bed (for me at least). It could be applied to all sorts of tilt situations.

 

[alantheastronomer]

The impactor theory could always be ruled out for the simple reason that Uranus is not a solid body!

 

[TeethWhitener]

The impactor theory could always be ruled out for the simple reason that Uranus is not a solid body!

Wiki asserts that only 20% of Uranus’s radius is gaseous/superfluid atmosphere with no phase transition. The other 80% is icy mantle and rocky core. So there’s plenty of condensed phase matter for an impactor to hit.

The bigger difficulty with the impactor theory as I understand it is that Uranus’s tilt would almost certainly require multiple giant impacts, as opposed to a single one. This makes the scenario that much less likely.

 

[alantheastronomer]

Wiki asserts that only 20% of Uranus’s radius is gaseous/superfluid atmosphere with no phase transition. The other 80% is icy mantle and rocky core. So there’s plenty of condensed phase matter for an impactor to hit.

Those percentages are by volume, not mass - the "rocky" core is estimated to be between 0.5 - 4.5 Earth masses out of a total mass of 13.5. Apparently you didn't read the rest of the article...further down it states "The ice mantle is not in fact composed of ice in the conventional sense, but of a hot and dense fluid consisting of water, ammonia, and other volatiles..." In such a situation, all an impactor would accomplish is create turbulence that would dissipate over time...and it would still be unable to account for the moons and rings which orbit in the same plane as the planet's equator.
Another shortcoming of both this and the accretion theory, is that the material has to come from somewhere outside the orbital plane, which is just as much of a mystery as Uranus's axial tilt?

 

[TeethWhitener]

all an impactor would accomplish is create turbulence that would dissipate over time.

Explain to me how Uranus would violate angular momentum conservation please.

 

[alantheastronomer]

Explain to me how Uranus would violate angular momentum conservation please.

Who said anything about violating the conservation of angular momentum?! Since the mantle is not solid, the collision is inelastic - much of the kinetic energy goes into heating the material(which would also be true even if the mantle was made of solid ice) resulting in turbulence, which then subsides as it's energy gets radiated away due to friction.

 

[TeethWhitener]

Friction with what?

 

[TeethWhitener]

It looks like these folks have considered inelasticity and heating:
https://ui.adsabs.harvard.edu/abs/1990Icar...84..528K/abstract
Simulations have probably gotten better since 1990.

Edit: a more recent simulation from 2018:
https://iopscience.iop.org/article/10.3847/1538-4357/aac725

I’m happy to entertain peer reviewed papers that support your assertion that the impactor theory could be ruled out because Uranus is not solid.

 

[Greg Bernhardt]

Recent relevant news

Scientists found a secret in old Voyager 2 data. This is why we need to revisit Uranus and Neptune
https://www.cnn.com/2020/03/27/world/uranus-plasmoid-voyager-2-scn/index.html

 

[Dragrath]

Interesting so this model would only require one giant impact that said I'm not sure ifthat is a convincing reason enough that the model is better. After all exoplanet studies have been suggesting that planetary scale collisions might not as rare as we thought with one probable observation of such a collision based on huge quantities of transiting dust suggesting the break up of a planetary mass body and then other models suggest eccentric Jupiters probably are the result of giant planet collisions. Given the short window of observation of exoplanetary observations even one probable collision with our given sample sizes of observations somewhat suggests they shouldn't be too rare. The model also doesn't eliminate the need of giant impacts and we have very strong evidence for a giant impact and if not forcing for improbable glancing blows moreover probably at least two to give the proto Earth extremely high angular momentum needed to recreate all the compositional and orbital specifics of the Earth Moon system simultaneously. Not to mention solar system models have a consistent small but non negligible chance for Mercury to be perturbed out of its orbit within the next billion years and the Young age constraints on the formation of Saturn's rings and icy moons. Factoring in other indirect evidence such as the capture of Triton the structure of large asteroids and dwarf planets and the exotic menagerie of exoplanetary systems the expectation that planetary systems are orderly things does not seem to be supported to any degree.

The true benchmark to decide between models should be based off looking for differences in model predictions and finding out which better fits the observations. This inevitably means we need to get a mission going to Uranus. The planets moons, unusual magnetic field, and if possible gravity field harmonics to narrow down compositional differences and structure will ultimately be needed to ever get actual answers capable of constraining observations. Given what gravity harmonics have revealed about Jupiter and Saturn and even the Earth and Moon the only way this will be resolved is with direct observations.

 

[alantheastronomer]

It looks like these folks have considered inelasticity and heating:
https://ui.adsabs.harvard.edu/abs/1990Icar...84..528K/abstract
Simulations have probably gotten better since 1990.

Edit: a more recent simulation from 2018:
https://iopscience.iop.org/article/10.3847/1538-4357/aac725

I’m happy to entertain peer reviewed papers that support your assertion that the impactor theory could be ruled out because Uranus is not solid.

How about some plain old common sense? According to the paper you cited, the impactor would have to have been on an orbit nearly perpendicular to the plane of the solar system, yet the collision would have no effect on Uranus's orbital plane !

 

[TeethWhitener]

How about some plain old common sense? According to the paper you cited, the impactor would have to have been on an orbit nearly perpendicular to the plane of the solar system, yet the collision would have no effect on Uranus's orbital plane !

So I guess that’s a no to a peer-reviewed source?

 

[Drakkith]

The impactor theory could always be ruled out for the simple reason that Uranus is not a solid body!

Who said anything about violating the conservation of angular momentum?! Since the mantle is not solid, the collision is inelastic - much of the kinetic energy goes into heating the material(which would also be true even if the mantle was made of solid ice) resulting in turbulence, which then subsides as it's energy gets radiated away due to friction.

Even a collision with a dense fluid would still transfer a substantial amount of angular momentum to the planet. Besides, the heating of the material of both bodies in any collision of this magnitude turns large parts of them molten anyways, so you're still dealing with a fluid-on-fluid interaction to some degree.

Think of it this way. All of that material that gets pushed away in a certain direction by the collision would:
1. Press directly on the rest of the planetary material, transferring momentum directly; or
2. 'Slide' along, interacting through friction with the surrounding material, transferring momentum that way; or
3. Get ejected some distance, eventually falling back, where its impact would transfer momentum; or
4. Get ejected completely, transferring little to no momentum to the planet in the process.