Why Do Only Gas Giants Have Rings?

[DHF]

My Son is in 5th Grade and currently his class is studying the solar system. he wanted to know why only Gas giants had rings and the smaller planets did not. His Teacher apparently side stepped the question so he asked me. I assumed it is due to the Gravity but I didn't have enough information to explain in detail so I told him I would research it and get back to him. Google as I might, I cannot find any direct answers to this. most of my Google inquires come back with science fiction references.

If anyone would be so kind as to give me some details I will gladly relay it to my Son.

Thank you in advance.

 

[Drakkith]

Unfortunately I don't think there is a simple, easy answer for this question. Our knowledge of the formation of planetary rings is still highly incomplete. There may be special conditions of the gas giants that make it easier for rings to form around them, such as their larger size, higher gravity, and location in our solar system. I would guess that it is a combination of all those factors, along with others, that give rise to planetary rings.

 

[|Glitch|]

I agree with Drakkith in that it is not an easy question to answer. Enceladus, for example, has sodium chloride geysers that created the E-ring around Saturn, and is continuing to feed the E-ring to this day.

We have also detected nitrogen geysers on Triton, the largest moon around Neptune. However, there does not appear to be any correlation between Triton's geysers and Neptune's rings.

Most rings are believed to be pieces of comets, asteroids, or shattered moons that broke up before they reached the planet. As we have seen from the Shoemaker-Levy 9 comet in 1994, as it approached Jupiter the tidal forces of the planet broke it apart into more than 20 pieces.

For a more technical answer, see the Roche Limit.

Cassini, which is still collecting data on Saturn's rings, will be able to provide more information in the future.

 

[DHF]

excellent. well thank you for the information. Glad to know it is not a black and white answer. I felt kind of silly not knowing when he asked me. At least now I can give him some educated guesses.

 

[Drakkith]

Also, see this thread that just popped up: https://www.physicsforums.com/showthread.php?t=745337

 

[DHF]

Thanks a lot Drakkith. much appreciated.

 

[Damo ET]

Based on some of the information I have come across in the past, rings can be sustained far more easily and for a far greater time period around a planet which has multiple moons. It just so happens that the bigger planets have multiple moons, so the two go hand in hand. The moons (if they are close enough to the host planet) heard the ring debris into nice orbits, and either boost the debris faster to prevent the orbit decaying, or slow the orbit to prevent escaping. Without the moons gravity, the ring debris would fall back to the host planet quite quickly. Every Cassini division in Saturn,s rings has a moon to create the division.

http://www.astronomycast.com/2007/11/episode-61-saturns-moons/

Damo

 

[DHF]

That is really interesting. I didn't give the Moons much thought but it makes a lot of sense. Thanks.

 

[Bandersnatch]

There's much more to it than that!

Rings have the tendency to viscously spread(http://arxiv.org/abs/1006.0633), and while the inner part loses mass due to collision with the planet, the outer part leaves the Roche limit and starts to aggregate into moons.
This paper:
http://arxiv.org/abs/1301.3808
proposes a model of moon formation from ring spreading that form either a piramidal hierarchy(in terms of mass) of satellites, or a single massive satellite - depending on the lifetime of rings.

What I'm saying is that moons are hardly just the shepherds of the rings, they might be their children too.

 

[|Glitch|]

Based on some of the information I have come across in the past, rings can be sustained far more easily and for a far greater time period around a planet which has multiple moons. It just so happens that the bigger planets have multiple moons, so the two go hand in hand. The moons (if they are close enough to the host planet) heard the ring debris into nice orbits, and either boost the debris faster to prevent the orbit decaying, or slow the orbit to prevent escaping. Without the moons gravity, the ring debris would fall back to the host planet quite quickly. Every Cassini division in Saturn,s rings has a moon to create the division.

http://www.astronomycast.com/2007/11/episode-61-saturns-moons/

Damo

If that is the case, then Pluto (which has five moons) should also have a ring system. I guess we will find out when the New Horizons probe reaches Pluto in July 2015.

 

[Drakkith]

If that is the case, then Pluto (which has five moons) should also have a ring system. I guess we will find out when the New Horizons probe reaches Pluto in July 2015.

The fact that an object has multiple moons does not say that it should have rings.

 

[Damo ET]

If that is the case, then Pluto (which has five moons) should also have a ring system. I guess we will find out when the New Horizons probe reaches Pluto in July 2015.

I probably should have clarified that there needs to be a mechanism for the rings creation in the first place.
Pluto just doesn't have enough mass to allow it to capture and shred comets, or create tidal forces large enough on the moons that it has to heat them to the point where 'they' vent liquid or vapour. Around Saturn, Enceladus does a great job of pumping ice into the space between it and Saturn because of the tidal force induced from Saturn. Inside that, Saturn's ability to capture and shred comets with its gravity feeds the inner rings along with probably more material coming from the moons in those orbits as well.


Damo

 

[Bandersnatch]

Collisions with other kuiper belt objects would eject material to feed the ring formation.

 

[SteamKing]

There is an article reporting that a distant asteroid has been observed to have two rings:

http://www.dailymail.co.uk/sciencet...discovery-gas-planets-thought-bands-rock.html

There was another article from several years ago about Jupiter robbing mass from Mars to build the asteroid belt:

http://www.dailymail.co.uk/sciencet...-planets-formed-solar-billions-years-ago.html

With regard to the first article, astronomers are due for some more head-scratching about ring formation.

 

[Damo ET]

Collisions with other kuiper belt objects would eject material to feed the ring formation.

Good point.

 

[DHF]

Now I am jealous that we don't have rings. Even the bloody Asteroids are getting them these days.

 

[Chronos]

The problem with rings is they are temporary. They will eventually dissipate without replenishment. Saturn has numerous moons which probably contribute to replenishment of its rings. The mechanism for this is poorly understood.

 

[Dotini]

The problem with rings is they are temporary. They will eventually dissipate without replenishment. Saturn has numerous moons which probably contribute to replenishment of its rings. The mechanism for this is poorly understood.

This article from wikipedia indicates Saturn's rings had been thought to be primordial but with enough mass and continual breakage to be perpetually preserved. I guess this means the rings of the other gas giants were much smaller to begin with? The rings of Centaur Chariklo would need an altogether new explanation, since they couldn't be both primordial and temporary.

http://en.wikipedia.org/wiki/Formation_and_evolution_of_the_Solar_System#Moon.E2.80.93ring_systems
Prior to the 2004 arrival of the Cassini–Huygens spacecraft , the rings of Saturn were widely thought to be much younger than the Solar System and were not expected to survive beyond another 300 million years. Gravitational interactions with Saturn's moons were expected to gradually sweep the rings' outer edge toward the planet, with abrasion by meteorites and Saturn's gravity eventually taking the rest, leaving Saturn unadorned.[93] However, data from the Cassini mission led scientists to revise that early view. Observations revealed 10 km-wide icy clumps of material that repeatedly break apart and reform, keeping the rings fresh. Saturn's rings are far more massive than the rings of the other gas giants. This large mass is thought to have preserved Saturn's rings since the planet first formed 4.5 billion years ago, and is likely to preserve them for billions of years to come.[94]

 

[Damo ET]

Her is a little more info:
http://www.astronomycast.com/2010/08/ep-195-planetary-rings/

Damo