TWICE TILTED? New simulations indicate that Uranus experienced at least two large impacts, leaving the planet with its modern-day, near-sideways tilt.
Image: NASA and Erich Karkoschka, University of Arizona
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NANTES, France—Knock, knock. That's not the start of a joke but the hard-luck history of Uranus. New research suggests that the giant planet may have suffered two massive impacts early in its history, which would account for its extreme, mysterious axial tilt.
Uranus orbits nearly on its side; its axis of rotation is skewed by 98 degrees relative to an ordinary upright orientation, perpendicular to the orbital plane. Many planetary scientists have sought to explain the odd tilt by invoking a giant impact into Uranus billions of years ago. But the giant planet has a system of moons circling its equator that would have been disrupted by such an impact.
"If Uranus is suddenly tilted, the satellites keep moving like that from north pole to south pole, and [wouldn't be] equatorial at all," Alessandro Morbidelli of the Observatory of Côte d’Azur in Nice, France, reported here Thursday at a joint meeting of the American Astronomical Society Division for Planetary Sciences and the European Planetary Science Congress. [Read more planetary news from the meeting here.]
But what if the tilting was a more gradual process, caused not by one mammoth impact but by two somewhat smaller nudges? Simulations show that the two-strike mechanism appears to solve the problem, knocking Uranus sideways and allowing it to develop equatorially orbiting moons, Morbidelli said.
The key is that the impacts must have come very early, before Uranus's moons had coalesced from a disk of gas and dust surrounding the planet. That disk, supplemented by debris stirred up by the collisions, would have migrated around the planet to form a thin equatorial disk that gave rise to Uranus's five large moons.
In the simulations, the same sort of equatorial migration also worked for the single-impact tilt scenario, but that scenario came with one important and disqualifying caveat: the moons orbited in the wrong direction, counter to Uranus's rotation. "If you tilt Uranus all in one shot, you produce regular satellites on the equator, but they will all be retrograde, and the satellites are actually prograde," Morbidelli said.
The only way for Uranus to have kept its moons in the right place, moving in the right direction, was to have suffered multiple giant impacts. "If we are right, Uranus was hit at least twice by big objects, about the mass of the Earth," Morbidelli said. He noted that Neptune's tilt, although only about one third that of Uranus, is also best explained by a giant impact.
"He solved the problem with the giant-impact hypothesis," said Hal Levison of the Southwest Research Institute in Boulder, Colo. "I've always been worried about this problem." But, Levison noted, "that doesn't mean that the giant-impact hypothesis is right." There are several other ways to change a planet's tilt, or obliquity, including tidal forces and resonances between a planet's spin and its orbit.
But if Uranus did suffer two large collisions, and Neptune absorbed one as well, that would indicate that massive impacts played a significant role in shaping the giant planets. That would be a surprise, given the traditional view that the gas giants grew by sweeping up smaller planetesimals. "This is quite an unconventional scenario for the formation of the giant planets, but I think that the obliquities of Uranus and Neptune point in this direction," Morbidelli said.
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14 Comments
Add CommentI must wholeheartedly agree with promytius. Those ads MUST GO.
Reply | Report Abuse | Link to thisAs for the article:"In the simulations...the moons orbited in the wrong direction". I am finding it difficult to believe that they would move retro ALL the time. How many simulations did they perform? I still think it was a single hit. Given 4 billion years or so, there would have been plenty of time to sweep the moons in the proper direction.
A giant collision with a Mars-sized protoplanet (Thea) happened to the Earth too, during its own youth. That impact tilted the planet's axis by 23-odd degrees, and gave earthlings their Moon.
Reply | Report Abuse | Link to thisWoohoo Mother Earth!!!
Reply | Report Abuse | Link to thisI would imagine if such collisions were common that far out, they would have been more common closer in, and the inner planets would be a mess of polar orientations. (Saturn and Jupiter may simply be big enough to absorb impacts with smaller responses)
Reply | Report Abuse | Link to thisThere are MANY popup blockers available for firefox and even IE. I have never even seen the ads you refer to.
Reply | Report Abuse | Link to thisTry googling "adblock-plus" or simply "popup blocker".
I had the same thought. I suppose that simply because it is unlikely does not mean it is impossible. Also:
Reply | Report Abuse | Link to thisWhat happens to the mass of the two "earth sized" objects? Absorbed by the planetary mass?
IMO the tilt of planets is derived from rotation of gas in protoplanetary disk. The same effect is responsible for retrograde motion of planet in inner part of solar system (Venus).
Reply | Report Abuse | Link to thishttp://tinyurl.com/6j9l98f
Double impact? I don t belive! Nobody think! If this two planets have colision between they are destroy and this planets no exist!
Reply | Report Abuse | Link to thisI agree with promytius. Give me true scientific facts!
Have a nice day
The amusing things science comes up with to deny a special creation..
Reply | Report Abuse | Link to thisStop being mad and learn how to use AdBlock.
Reply | Report Abuse | Link to thisI don't understand why a double-impact would be so surprising...if sweeping planetesimals is the norm, why not sweeping larger objects? We already know giants eject bodies, why not consume them? It's very likely they'd possess similar orbits, so a "merge" would not necessarily be cataclysmic to the entire gravity well of the two (i.e., they wouldn't spray their guts all over the system, and would eventually re-coalesce into one combined mass).
Reply | Report Abuse | Link to thisPerhaps some of those moons owe their lives to the impactors, just as ours does...
T
If you don't like science why are you on a science site? Crawl back to your cave.
Reply | Report Abuse | Link to thisThe post by Promytius has been removed so I don't know the details of what set him off. Many of the ads on sites these days are not pop-ups at all but are hidden panels that display and push the content down several inches upon mouse over so they are as annoying as pop-ups but are not stopped by ad blockers or pop-up blockers. People using IE that block specific scumbag sites (advertisers) get a continual stream of pop-ups from IE itself telling them something was blocked and requiring an ok before they can interact with the page.
Reply | Report Abuse | Link to thisThe simple fact is that ANYONE using hosted advertising or allowing dynamic advertisments are idiots worthy only of contempt. Hosted advertising and dynamic advertisements were the top source of cyber attacke for the better part of a decade and are still in the top 3 even now. Most of the current ad hosting services are owned and run by convicted criminals. I know there are exceptions but the big three have all been busted for fraud.
I don't know. It's not my line of schooling. But, it would explain the French very nicely. Especially their first lady.
Reply | Report Abuse | Link to thisWhom I find very lovely, indeed.
Now, for Germany...
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