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Uranus: Orbital Tilt from a Cataclysmic Collision

Yesterday’s post about exoplanet obliquity inevitably brought our own system to mind, with the stark variations between planets like Earth (23 degrees), Uranus (98 degrees) and Mercury (0.03 degrees) serving as stark examples of how wide the variation can be. Thus seasonality has to be seen in context, and interesting questions arise about the effect of high degrees of obliquity on habitability. While thinking about that I received a new paper on Uranus that has bearing on the matter, with its attempt to quantify the ‘hit’ Uranus must once have taken.

After all, something accounts for the fact that the 7th planet spins on its side, its axis at right angles to those of the other planets, its major moons all orbiting in the same plane. Lead author Jacob Kegerreis (Durham University), working with Luis Teodoro (BAERI/NASA Ames) and colleagues modeled 50 different impact simulations in an attempt to recreate the axial tilt of this world. In play were the planet’s internal structure, rotation rate, atmospheric retention post-impact and the composition of materials injected into orbit by the event. Says Kegerreis:

“Our findings confirm that the most likely outcome was that the young Uranus was involved in a cataclysmic collision with an object twice the mass of Earth, if not larger, knocking it on to its side and setting in process the events that helped create the planet we see today.”

Image: This image shows a crescent Uranus, a view that Earthlings never witnessed until Voyager 2 flew near and then beyond Uranus on January 24, 1986. This planet’s natural blue-green color is due to the absorption of redder wavelengths in the atmosphere by traces of methane gas. Uranus’ diameter is roughly 51,000 kilometers, a little over four times that of Earth. Image credit: NASA/JPL/USGS.

The likely impactor was a young protoplanet, striking Uranus during the early era of Solar System formation some 4 billion years ago. That in itself is not surprising, though it’s good that the idea can be firmed up with high-resolution modeling. But we also learn something else. Debris from the impactor may have formed a thin shell near the top of Uranus’ ice layer. This could have the effect of trapping the heat emanating from the planet’s core, a useful finding because it helps to explain the extreme cold of the upper atmosphere (-216 degrees C).

Those temperatures are a puzzle that is exacerbated by the fact that we know so little about the interior of Uranus. The paper points out that surface emission is in equilibrium with solar insolation, which implies that little heat flows out from the planet, and this is in striking contrast with the other giant planets. Thus the idea of a thin thermal boundary layer between the outer envelope of hydrogen and helium and the inner ice-rich layer. The shell theory fits earlier work on the planet’s evolution, and also gives us some ideas about the type of impact.

Looking at where the mass and energy of the impactor are deposited within the planet, the paper focuses on the top of the ice layer, and suggests the impact was a grazing one:

Higher impact parameters can even lead to a temperature inversion near the top of the ice layer. These more-grazing collisions also leave the impactor ice further out, in a thin shell near the edge of the icy mantle, whereas ∼head-on impacts can implant significant ice up to 0.5 R further inwards and less-isotropically about the centre. These findings may have important implications for understanding the current heat flow (or rather lack thereof) from Uranus’ interior to its surface.

Thus we have a thermal boundary that can, as the paper argues, suppress convection, a kind of blanket to contain heat welling up from within the planet. We can also dig deeper into the planet’s magnetic field. Unlike the terrestrial planets, the Uranian magnetic field appears offset by approximately 0.3 Uranus radii from the center of the planet and tilted by 60 degrees relative to the rotation axis. Other work in the literature has described models that produce such fields using a layer of convecting electrically conducting ices. The impactor modeled in this work could have created lopsided clumps of rock within the planet that explain the offset and tilt.

An impactor of 2 Earth masses blows rock and ice into orbit, where it is available for the formation of Uranus’ current satellites and ring system, giving us constraints on the angular momentum delivered by the collision. All of which is a reminder of how violent a place a young stellar system can be during the era of planet formation, information which should prove useful as we extend what we are learning about our own ice giants to worlds around other stars.

The paper is Kegerreis, “Consequences of Giant Impacts on Early Uranus for Rotation, Internal Structure, Debris, and Atmospheric Erosion,” Astrophysical Journal Vol. 861, No. 1 (2 July 2018). Abstract / preprint.


Comments on this entry are closed.

  • Thomas R Mazanec July 3, 2018, 13:38

    Does not Neptune have a wonky magnetic field as well?

  • Geoffrey Hillend July 3, 2018, 18:28

    Neptune does have a magnetic field. The magnetic field in Uranus and Neptune are caused by the convection currents or circular motion from the Coriolis deflection of liquid metallic hydrogen like in Jupiter and Saturn. The Coriolis force is caused by rotation. There is some pressure ionization in Uranus and Neptune so the pressure is great enough to ionize the hydrogen, so it becomes a conductor of electricity.

    • Jeff Wright July 27, 2018, 17:47

      Now I have heard that ice giants may have large diamonds. I wonder if some of these soot covered things might be in the rings

  • Geoffrey Hillend July 3, 2018, 19:03

    The mantle of Uranus and Neptune is made of Ammonia, methane and water and convention currents in the mantle are believed to be responsible for the magnetic field. I think that there must be some kind of pressure ionization of these liquids to make the electrons more mobile and make a strong magnetic field like liquid metallic hydrogen?

  • David Kendrick July 3, 2018, 19:06

    Uranus is possibly an extra solar capture. We still have the odd cognative dissonance because science is effectively fighting its own beliefs, the universe is full of dark matter yet the solar system is constant, humans do not deal with infinite possibilities and timescales well and when faced with something they do not understand attribute it to god(which is planet xyz responsible for an eccentricty in an orbit). Even Newton did this for the 3 body problem. The people who really give thought to this are those looking for ET under the fermi paradox. Someone needs to do a PHD on dark matter Jovans capture when of course a cetain Webb IR telescope comes on line after a survey.

    • ljk July 5, 2018, 10:10

      So what would make Uranus an extrasolar object and not Neptune? Or is the latter one as well? And how do you explain Pluto and its kin?

  • Geoffrey Hillend July 3, 2018, 19:24

    Uranus’s moons are far short of two Earth masses, so I guess most of the impactor went into Uranus since Uranus rotates faster than Neptune and Uranus has a stronger magnetic field than Neptune. Uranus magnetic field at 23,000 nt is almost twice as strong as Neptune’s magnetic field at 13,000 nt. nano-Tesla.

  • Robin Datta July 4, 2018, 0:07

    Must have been plenty of collisions “umpteen moons ago” that produced our moon, knocked Venus nearly upside down, floored Uranus, and perhaps yet other collisions to be invoked for yet more features of today’s solar system. If that is typical of early stellar systems, we might see evidence for more of the same while looking around at newly formed stellar systems.

  • Michael Fidler July 4, 2018, 3:34

    One possibility is that the open star cluster that the sun formed in, had many rogue planets that also formed. This would possible bring in an high speed inpactor that may have done much more damage then object in orbit around the sun. This would also mean that the M dwarf minature planetary systems would be less likely to have this type of problem. 😮

  • Michael Fidler July 4, 2018, 3:43

    Interesting coincidence this coming July 27 – Mars is at it closes opposition in 15 years and the moon is at apogee with the longest total lunar eclipse of this century.

  • yoram July 4, 2018, 14:39

    obvious question: what happened to the other planet?

    completely destroyed?
    left overs kicked out of the solar system?
    turned into a moon somewhere?

    • ljk July 5, 2018, 9:56

      Absorbed by Uranus?

  • ljk July 5, 2018, 10:09

    So why didn’t Neptune get clobbered in the same way that Uranus did? Was it just lucky in that regard? Were there fewer objects in its part of the Sol system back then? Did Neptune actually get hit but somehow “recovered”?

    Asking why Neptune did not get hit is just as important as knowing why Uranus did. And note how Neptune only has one large moon, Triton, and a rather geologically active one at that. Was Triton a Plutoid that got captured by the ice giant? If not, why didn’t Uranus form similar moons? Or did the impact have something to do with this situation?

    Data on Neptune’s axial tilt:


  • ljk July 5, 2018, 10:19

    And another thought: Were there more ice giant planets in our Sol system at one time, but Uranus and Neptune were the only survivors, with Uranus being just this side of joining the destroyed worlds?

  • Michael Fidler July 5, 2018, 14:59

    So could the off axis magnetic field be the 2 earth mass iron core that was never fully digested by Uranus? May have to do a sigmoidostomy.

  • torque xtr July 6, 2018, 7:10

    I think there’s no way to tell without sending a probe.

    While the preprint says up to 1-2 tenths of Earth masses of rock were deposited upwards of core-ice boundary, it’s hard to imagine how it traps even a half of internal heat flux. (and inhibits the viscoelastic convection) And we have to compare it with Neptune which has tens of times the Uranian heat flux, so not a half but almost all heat has to be trapped. Second, even if the rock from impactor somehow became spread onto the even layer without large discontinuities (including poles!) while not heating up the ice around even close to melting, the trapped heat would build up underneath, eventually leading to runaway differentiation event. I guess, the model still has much refining left, taking into account 3D, post-impact evolution, etc.

    Awesome videos, though :-)

  • Brian Schmidt July 6, 2018, 11:29

    Does “debris from the impactor may have formed a thin shell near the top of Uranus’ ice layer” imply the existence of a solid surface?

    I thought the gas atmosphere got warmer and denser with depth, eventually behaving more like a liquid and solid, but does either the “thin shell” or the ice layer imply an abrupt transition from gas to something like a solid surface?

  • Mark Zambelli July 13, 2018, 11:59

    It’s generally accepted that Triton is a captured object and your classification of ‘plutoid’ is apt. Its orbit is retrograde, it is probably responsible for clearing out almost all of Neptunes original moons, and the tidal heating during its capture into a stable orbit has resurfaced the moon relatively recently (with enough internal heat left to drive all the activity you mention…. 3 smoking guns, so to speak.

    As for Uranus… Miranda looks like it got clobbered, broke apart and then recoalesced (the leading idea while I was at school during the Voyager flyby) perhaps during the aftermath of the whack Uranus recieved? It now seems that tidal heating has surpassed that theory for an explanation… still want to dive of those cliffs though in that low-g.

    • Mark Zambelli July 13, 2018, 12:04

      Sorry but that post was supposed to be a ‘reply’ to Larry’s “ljk July 5, 2018, 10:09” post, don’t know what happened there.