92
$\begingroup$
  1. If Earth had rings, would they center on the equator like Saturn's rings do on its equator?

enter image description here

enter image description here

$\endgroup$
  • 2
    $\begingroup$ Related question by OP: physics.stackexchange.com/q/279955/2451 $\endgroup$ – Qmechanic Sep 24 '16 at 8:24
  • 1
    $\begingroup$ An "orbital" is a stationary state of a bound system in quantum mechanics, and certainly does not belong as a tag on this question. $\endgroup$ – dmckee Jan 24 '17 at 19:22
119
$\begingroup$

Great question.

If Earth had rings, and they had been there as long as the moon has, they would mostly likely line parallel to Earth's equator and be visible in the sky from an east to west orientation.

So how would Earth acquire a ring? Our moon is, in reality, slowly moving away from the Earth, but if it were instead moving inwards, eventually it would break apart due to differential gravitational forces between the side nearest us and the far side, 3000 km away. Obviously, a large amount of the moon will bombard the Earth, but this answer assumes we survive.

Typically, the Roche Limit limit applies to a satellite's disintegrating due to tidal forces induced by its primary, the body about which it orbits. Parts of the satellite that are closer to the primary are attracted more strongly by gravity from the primary than parts that are farther away; this disparity effectively pulls the near and far parts of the satellite apart from each other, and if the disparity (combined with any centrifugal effects due to the object's spin) is larger than the force of gravity holding the satellite together, it can pull the satellite apart.

enter image description here

View of ring from Washington.

Since, within the Roche limit, tidal forces overwhelm the gravitational forces that might otherwise hold the satellite together, no satellite can gravitationally coalesce out of smaller particles within that limit. Indeed, almost all known planetary rings are located within their Roche limit, Saturn's E-Ring and Phoebe ring being notable exceptions. They could either be remnants from the planet's proto-planetary accretion disc that failed to coalesce into moonlets, or conversely have formed when a moon passed within its Roche limit and broke apart.

The Roche Limit can be viewed as an Earth shaped imaginary "border", on average 9,492 km from the centre of Earth, 1.49 times Earth's radius, for rigid bodies. So around the equator it "moves" outwards a little. It follows the oblate spheroid shape of Earth. 

Earth may have had a ring just after its formation. The view of these ring from Earth would vary. It would all depend on your latitude and which direction you were facing. Near the equator, the ring would be like thin slices of light that erupted from distant Earth horizons and stretched into the sky as far as the eye could see.

Thanks to Emilio Pisanty for correctly pointing out the depiction of the rings from mid and high latitudes is not completely accurate. The plane of the ground is not orthogonal to the plane of the rings, so they would appear at an angle. All I can do is ask for some personal latitude in the presentation of this "what if" scenario.

The pictures assume the ring around Earth would be in the same proportion as the ring around Saturn is to that planet.

enter image description here

View of ring from the equator.

 

Why does the ring form around the equator as opposed to another axis. It's due to the effect of the Central Force Law, the same basic reason the planets are situated in a plane around the Sun. The Sun is spherical, so objects such as Pluto can "get away" with being 8 degrees out of line. If the Earth, and Saturn) were perfect spheres, then the axis of the ring could be at any angle. Because both planets are oblate spheroid, with a tidal bulge, over time the particles composing the ring would collect there. Saturn's rings have an estimated local thickness of as little as 10 metres and as much as 1 kilometer, so they are extemely "thin".  

enter image description here

View of rings from the mid latitudes.

enter image description here

View of rings at 23° south latitude a 180° panorama gives an idea of what a magnificent sight the rings would be. The Earth itself is casting the shadow.

Image source: If Earth Had a Ring Like Saturn

$\endgroup$
  • 7
    $\begingroup$ I don't think the depiction of the rings from mid and high latitudes is completely accurate. The plane of the ground is not orthogonal to the plane of the rings, so they would appear at an angle. Not that you can necessarily change the images but you should add a comment to that effect. $\endgroup$ – Emilio Pisanty Sep 23 '16 at 8:09
  • 3
    $\begingroup$ "would mostly likely line parallel to Earth's equator" What makes that more likely than them being in the plane of the moon's orbit? Is a ring tidally coerced into the equatorial plane more strongly than a moon? $\endgroup$ – Steve Jessop Sep 23 '16 at 9:14
  • 2
    $\begingroup$ Is there a reason why the Roche Limit forms where the axis the lines up with it's angular momentum? $\endgroup$ – Yogi DMT Sep 23 '16 at 13:27
  • 2
    $\begingroup$ Oh sorry, it's the central force law. Same idea as other planets/objects around the sun, except the sun is spherical, so Pluto can "get away" with being 8ish degrees out of line. If the Earth ( and Saturn) were perfect spheres, then the axis of the ring could be at any angle, but with a tidal bulge, over time the price would collect there. Saturn's rings have an estimated local thickness of as little as 10 metres and as much as 1 kilometer, so they are thin. en.wikipedia.org/wiki/Classical_central-force_problem $\endgroup$ – user108787 Sep 23 '16 at 14:12
  • 3
    $\begingroup$ It wouldn't be phys.se if things weren't scrutinised to within an inch (or a pixel?) of their lives. Your explanation satisfies me $\endgroup$ – Darren H Sep 24 '16 at 10:36
35
$\begingroup$

Yes, the rings would ultimately end up hovering above the equator (as defined by a planet's rotation). The mechanism leading to this steady state is gravitational asymmetry arising from a planet's equatorial bulge. Transient non-equatorial orbits, however, are of course possible (as they are with launched artificial satellites).

This question has been studied through simulation by NASA in the exploration of the Kessler Catastrophe, whereby endlessly colliding spacejunk pieces pulverize each other ever more finely. The end point dust is found to gather around the equator.

enter image description here

$\endgroup$
  • 3
    $\begingroup$ We could have done without this: The largest recorded creation of space debris in history with at least 2,317 pieces of trackable size (golf ball size and larger) and an estimated 150,000 debris particles. en.wikipedia.org/wiki/2007_Chinese_anti-satellite_missile_test $\endgroup$ – user108787 Sep 23 '16 at 20:54
  • 3
    $\begingroup$ The answer with the actual physics is buried below the answer with pretty pictures. Thanks, HNQ! $\endgroup$ – dmckee Sep 23 '16 at 23:52
  • 1
    $\begingroup$ @dmckee I am very aware of the work both yourself and others do to maintain the high standards of the site. Both "what if " and badly worded homework questions don't, imo, fit with the standard expected of the site. So, for my part, this highly visual "answer" to a "what if" question is a once off. If there is any physics that you think should be included to balance things up, I would be happy to edit it in. $\endgroup$ – user108787 Sep 24 '16 at 11:18
  • 2
    $\begingroup$ @CountTo10 It's is not that your answer is bad, but that the HNQs break things in two ways because it brings (a) a larger audience which breaks score as a means of comparing posts and (b) a different audience that evaluates posts on a different basis than our regular audience meaning that the values encoded in the voting are changed and again breaks comparison with other questions. You gave a correct but tentative and partial physics explanation in the comments (co-formation implies co-rotation), but omitted the effect of anisotropcity of the planet which would affect rings formed later. $\endgroup$ – dmckee Sep 24 '16 at 19:09
  • 1
    $\begingroup$ @dmckee thanks for the explanation, I had no idea whatsoever of the consequences you outlined. My day job is web related and that might carry over to my answer below. I take your points for any future answers from myself. $\endgroup$ – user108787 Sep 24 '16 at 19:58
13
$\begingroup$

Since the times of Isaac Newton, Joseph-Louis Lagrange, Pierre Simon Laplace, Carl Friedrich Gauss and other great mathematicians of the Renaissance Era, there has been an inherent belief in the scientific community that the Earth is capable of retaining a stable ring system in orbit around it for millions of years.

In an article published by ex-NASA astronomer Dr. John A. O'Keefe in Nature magazine back in 1980, he hypothesized that the Earth once had a Saturn-like ring that could explain many of the events during past Earth ages. (Dr. O'Keefe is noted for discovering the Earth's slight pear-shape in the 1950s).

A Danish astronomer stated in Science Frontiers (Issue #76, Jul-Aug. 1991) that "in the past, the Earth had a ring system just like Saturn, Uranus and Neptune." He went so far as to say that our planet boasted rings on 16 separate occasions in the past.

As recently as September 2002, Sandia National Laboratories published an article by two University of New Mexico researchers who again reinforced the notion of past Earth rings. In that study, the authors supposed the existence of a thin system of rings -- perhaps of similar opacity to Saturn's B-ring -- may have caused past climatic changes on the Earth. They all agree that if a ring does not dissipate or precipitate before going to center that center is where they end up.

enter image description here

$\endgroup$

protected by Qmechanic Sep 23 '16 at 2:48

Thank you for your interest in this question. Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site (the association bonus does not count).

Would you like to answer one of these unanswered questions instead?

Not the answer you're looking for? Browse other questions tagged or ask your own question.