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What smacked Uranus on its side? Something icy and as massive as Earth, scientists say.

Uranus is uniquely tipped over among the planets in our solar system. Uranus' moons and rings are also orientated this way, suggesting they formed during a cataclysmic impact which tipped it over early in its history.
Uranus is uniquely tipped over among the planets in our solar system. Uranus' moons and rings are also orientated this way, suggesting they formed during a cataclysmic impact which tipped it over early in its history.
(Image: © Lawrence Sromovsky, University of Wisconsin-Madison/W.W. Keck Observatory/NASA)

The impactor that knocked Uranus on its side long ago isn't quite so mysterious anymore.

Uranus is tipped over more than 90 degrees relative to the plane of the solar system, and so are the gaseous planet's ring system and the orbits of its 27 known moons. Astronomers think this unique configuration is evidence of a violent collision Uranus suffered shortly after it was born, which also apparently supercharged the planet's rotation. (Uranus spins around its axis once every 17 hours, significantly faster than Earth does.)

Details of that collision have remained elusive, however, because simulations have struggled to generate the Uranus system that we see today. For example, the mass of the post-impact debris disk tends to be quite big in these models — much bigger than it "should" be, given the total mass of Uranus' moons today.

Related: How did Uranus form?

Until now, that is. A team of researchers has had success with a novel modeling strategy devised to investigate the formation of moons around frigid planets, a new study reports.

The frigid part is key. Giant impacts in the cold and dark outer solar system have different consequences than smashups much closer to the sun, such as the long-ago collision that resulted in the formation of Earth's moon, the researchers found.

This latter encounter involved the proto-Earth and a Mars-size body called Theia, both of which were primarily rocky (rather than icy). The material blasted into space by the impact therefore solidified rather quickly, allowing the newborn moon to snare quite a bit of it gravitationally.

But the material liberated during the Uranus collision was much more volatile — stuff like water and ammonia — and remained gaseous longer. The growing proto-Uranus gobbled most of this gas up, leaving less of it around to form moons, according to the new study, which was published online last week in the journal Nature Astronomy.

The researchers' model, which takes all of this into account, suggests that the body that slammed into Uranus was icy and big, with a mass between one and three times that of the modern Earth.

"This model is the first to explain the configuration of Uranus' moon system, and it may help explain the configurations of other icy planets in our solar system such as Neptune," study lead author Shigeru Ida, of the Earth-Life Science Institute at the Tokyo Institute of Technology in Japan, said in a statement.

"Beyond this, astronomers have now discovered thousands of planets around other stars, so-called exoplanets, and observations suggest that many of the newly discovered planets known as super-Earths in exoplanetary systems may consist largely of water ice," Ida said. "And this model can also be applied to these planets."

Mike Wall is the author of "Out There" (Grand Central Publishing, 2018; illustrated by Karl Tate), a book about the search for alien life. Follow him on Twitter @michaeldwall. Follow us on Twitter @Spacedotcom or Facebook

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  • Jackthehack
    Admin said:
    Astronomers have worked out details of the giant impact that knocked Uranus so famously askew.

    What smacked Uranus on its side? Something icy and as massive as Earth, scientists say. : Read more
    I’m wondering if Earth could have smacked Uranus taking a large volume of water and leaving Uranus on its side. Is it possible????
    Reply
  • rod
    FYI, Uranus big smack has been in the news over the years. In 2011, this report came out, https://www.sciencedaily.com/releases/2011/10/111006084235.htm, "...However, the simulation threw up an unexpected result: in the above scenario, the moons displayed retrograde motion -- that is to say, they orbited in the opposite direction to that which we observe. Morbidelli's group tweaked their parameters in order to explain this. The surprising discovery was that if Uranus was not tilted in one go, as is commonly thought, but rather was bumped in at least two smaller collisions, then there is a much higher probability of seeing the moons orbit in the direction we observe. This research is at odds with current theories of how planets form, which may now need adjusting. Morbidelli elaborates: "The standard planet formation theory assumes that Uranus, Neptune and the cores of Jupiter and Saturn formed by accreting only small objects in the protoplanetary disk. They should have suffered no giant collisions. The fact that Uranus was hit at least twice suggests that significant impacts were typical in the formation of giant planets. So, the standard theory has to be revised."

    I note *tweaked their parameters in order to explain this*. Keep on tweaking the giant impacts. The current report has interesting items in it too for the model, here is another example. https://phys.org/news/2020-04-uranus-oddities-japanese-astronomers.html, "The origins of Uranus' unusual set of properties has now been explained by a research team led by Professor Shigeru Ida from the Earth-Life Science Institute (ELSI) at Tokyo Institute of Technology. Their study suggests that early in the history of our solar system, Uranus was struck by a small, icy planet roughly one to three times the mass of the Earth, which tipped the young planet over and left behind its idiosyncratic moon and ring system as a smoking gun."

    My observation, like Theia, this 3 earth mass object is an NOO, non-observable object and the impact event, I call a UE or unobserved event. Perhaps a UOE, an unobservable event. Here is something from the abstract, https://www.nature.com/articles/s41550-020-1049-8, "The ice-giant planet Uranus probably underwent a giant impact, given that its spin axis is tilted by 98 degrees1,2,3. That its satellite system is equally inclined and prograde suggests that it was formed as a consequence of the impact. However, the disks predicted by the impact simulations1,3,4 generally have sizes one order smaller and masses two orders larger than those of the observed system at present. Here we show, by means of a theoretical model, that the Uranian satellite formation is regulated by the evolution of the impact-generated disk. Because the vaporization temperature of water ice is low and both Uranus and the impactor are assumed to be ice-dominated, we can conclude that the impact-generated disk has mostly vaporized. We predict that the disk lost a substantial amount of water vapour mass and spread to the levels of the current system until the disk cooled down enough for ice condensation and accretion of icy particles to begin."

    More NOOs and UE or UOEs used in the model. Keep *tweaking* :)
    Reply
  • Catastrophe
    "I’m wondering if Earth could have smacked Uranus taking a large volume of water and leaving Uranus on its side. Is it possible????"

    (It just says "as massive as Earth" not actually Earth.)

    I think this unlikely. A Mars size object (Theia) hit Earth about 4 billion years ago and produced the Moon. Presumably your collision with Uranus must have been before this. If Earth. When was this suggested to happen and where would the water have come from? Whilst there may be some water in the mantle, hydrogen, helium and methane compose the atmosphere. The heat of a major collision would have substantially reduced such atmospheric volatiles.
    Reply
  • rod
    Catastrophe, it looks like in your model, we have an Earth smack down event with Uranus that allowed Earth, perhaps the proto-earth to carry off plenty of water from Uranus. Then the proto-earth migrating inwards, parked itself into a 1 AU orbit around the Sun. Theia shows up and we have another smack down event to create the Moon.

    Okay, giant impact events in new computer models for rocky type planets can create the synestia phase of matter, a new phase of hot matter not seen yet in astronomy, Planetary collisions can drop the internal pressures in planets
    I see various studies now on the synestia phase from planetary impacts. It seems to me, no water would remain though.
    Reply
  • Torbjorn Larsson
    Admin said:
    Astronomers have worked out details of the giant impact that knocked Uranus so famously askew.

    What smacked Uranus on its side? Something icy and as massive as Earth, scientists say. : Read more

    There has been several models for Uranus system formation recently, which were not very convincing. If you have a collision your moon system will be disturbed so need more in the model to predict everything associated with Uranus. And if you have tilt changes from orbital forcing the models get max 60 - 70 degrees of tilt so need an impactor anyway https://aasnova.org/2020/03/10/a-new-approach-to-tilting-uranus/ ].

    I thought this would be another model with explicit finetuning problems - it presumes an icy impactor - but the model results are astoundingly convincing! They can reproduce the entire architecture of the main moons in the Uranus system - moon orbit tilts, ring tilts, number of moons, moon masses, mass ordering (outer larger moons) - from simple hierarchical growth models. They also mention more detail problems they may solve.

    So, some finetuning in the type of impactor, but icy ones would be typical out there. It's mass would be ~ 0.1 Earth mass or Mars massed. Like Theia.

    I note two immediate problems, but also some possible solutions:

    - The model implies the 5 major moons formed around a planet that did not have moons before the collision. (Or at least lost them latest at the collision.) C.f. the circumplanetary disk that Jupiter's Galilean moons seem to have originated from.

    But the same applies to Earth, presumably Mars' moons and maybe Saturn's earliest moons (which moons are a mess). So that can't be a killer as much as a new mystery to solve.

    - They don't explicitly explain why Uranus rotates as fast as Neptune, I think. The orbit tilt model - rightly - made a huge deal of that they expect that as the planets formed from the circumstellar disk. On the other hand a 1/100 mass impactor may not change angular momentum much apart from tilt.

    Specifically here, the model gets the impactor end velocity (~20 km/s) solely from Uranus gravity acceleration. Not much external momentum delivered. (Also: likable model null constraint!)
    Reply
  • Catastrophe
    rod said:
    Catastrophe, it looks like in your model, we have an Earth smack down event with Uranus that allowed Earth, perhaps the proto-earth to carry off plenty of water from Uranus. Then the proto-earth migrating inwards, parked itself into a 1 AU orbit around the Sun. Theia shows up and we have another smack down event to create the Moon.

    Okay, giant impact events in new computer models for rocky type planets can create the synestia phase of matter, a new phase of hot matter not seen yet in astronomy, Planetary collisions can drop the internal pressures in planets
    I see various studies now on the synestia phase from planetary impacts. It seems to me, no water would remain though.

    NO NO NO NO NO That is NOT my view. I think you mean Jackthehack!
    Reply
  • Torbjorn Larsson
    Jackthehack said:
    I’m wondering if Earth could have smacked Uranus taking a large volume of water and leaving Uranus on its side. Is it possible????

    That is an uninformative question in general, since very little is explicitly forbidden in physics - even "forbidden" transitions among particles happens at some (very low) frequency. So it is "possible".

    More interesting is to ask if it is likely.

    First, not likely in comparison with more accepted models for Uranus tilt formation, which aim to predict only that.

    Second, Uranus orbits tens of Sun-Earth distances out. How did Earth go out there and back here? (Never mind the two collisions (with Uranus and with Theia) and surviving them both et cetera. Catastrophe mentions even more problems.)

    But again, since it is "possible" we can always think idly on these things from time to time ... it trains our hypothesis building skills.
    Reply
  • Catastrophe
    Let me make this QUITE CLEAR.

    I never suggested that Earth hit Uranus.
    I would as soon suggest that Pluto hit Mercury and robbed its mantle
    and I am NOT suggesting that either.
    Reply
  • Torbjorn Larsson
    rod said:
    My observation, like Theia, this 3 earth mass object is an NOO, non-observable object and the impact event, I call a UE or unobserved event. Perhaps a UOE, an unobservable event. Here is something from the abstract, https://www.nature.com/articles/s41550-020-1049-8, "The ice-giant planet Uranus probably underwent a giant impact, given that its spin axis is tilted by 98 degrees1,2,3. That its satellite system is equally inclined and prograde suggests that it was formed as a consequence of the impact. However, the disks predicted by the impact simulations1,3,4 generally have sizes one order smaller and masses two orders larger than those of the observed system at present. Here we show, by means of a theoretical model, that the Uranian satellite formation is regulated by the evolution of the impact-generated disk. Because the vaporization temperature of water ice is low and both Uranus and the impactor are assumed to be ice-dominated, we can conclude that the impact-generated disk has mostly vaporized. We predict that the disk lost a substantial amount of water vapour mass and spread to the levels of the current system until the disk cooled down enough for ice condensation and accretion of icy particles to begin."

    More NOOs and UE or UOEs used in the model. Keep *tweaking* :)

    I think we agree on the models finetuning. But I do find the new model convincing. (I have said as much my initial comment.) See their image with the simulated moons and their theoretical explanation and fit to that.

    For context, I like to have my concepts testable. Scientists use to describe their personal opinion of some models by using the concepts "direct" and indirect", which I never seen a testable definition of. (Akin to how laymen may suggest that 'it's just a theory' for a testable theory, 'it's an assumption' for a testable constraint and 'need a proof' for not accepting evidence, but in these cases their erroneous claims are precisely testable.) I would therefore opinionate that your "non-observable object", "unobserved event. " and "unobservable event" are opinions that may or may not be arguable and/or shared.

    To move to more testable ground, since these models are precisely testable I would call their method of modeling events observations in case 1) they are well tested and 2) the competing models are less likely. We are not there yet, but I don't think they are "tweaking" this model (aside from the composition of their impactor, but it is rather likely composition out there). It is very unbiased and do not even need any specific initial impactor velocity.

    And even if they were still "tweaking" in some other general case, it is not a problem with the scientific method. "Tweaking" and other means of improving are strengths.

    Reply
  • Torbjorn Larsson
    Catastrophe said:
    Let me make this QUITE CLEAR.

    I never suggested that Earth hit Uranus.
    I would as soon suggest that Pluto hit Mercury and robbed its mantle
    and I am NOT suggesting that either.
    '
    I hope you didn't read my comment as if I think you thought that Earth hit Uranus. If you did, I wrote it poorly - I tried to point out that you mentioned how there is very little time for the complicated chain of events suggested by Jackthehack.
    Reply