Triton crashed into Neptune's orbit

[Brian G Turner]

A study suggests that Neptune's moons were reasonably stable and homogenous - until Triton smashed into the system: Neptune’s other moons were normal until Triton crashed the party

Perhaps the biggest question is where Triton came from. Though I believe - as the coldest place in the solar system - it's been suggested as originating from the Oort Cloud. However, if it remains colder than those bodies, perhaps Triton came from even beyond that?

 

[BAYLOR]

Triton is eventually going to be destroyed by Neptune's gravity because it's orbiting ever closer to that planet.

 

[Venusian Broon]

Perhaps the biggest question is where Triton came from. Though I believe - as the coldest place in the solar system - it's been suggested as originating from the Oort Cloud. However, if it remains colder than those bodies, perhaps Triton came from even beyond that?

I don't think where an object originates from directly determines what temperature a planets surface is.*

In the absence of any internal heating - which, as far as we know, is true for both Pluto and Triton - then the temperature of the surface is the thermodynamic equilibrium of light from the sun hitting the surface and being absorbed/reflected by the planets surface, with some degree of complication with the atmosphere.

Usually how 'shiny' a planet can be roughly calculated - i.e. the Albedo. If it is 1.0 (or 100%) that would be a mirror reflecting all energy back, and thus the surface of such a world would be close to absolute freezing as none of the energy falling on it is allowed to be absorbed. Hence if it were 0.0 (or 0%) then it would essentially be a black object absorbing everything and therefore generating a higher surface temperature as the energy of the absorbed photons would heat the surface.

I believe Pluto's albedo has been calculated at ~0.5 and Tritons is higher at ~0.6-0.9. Both have very insubstantial nitrogen atmospheres that don't really interfere with this process. Hence, it would seem that Triton is basically more reflective then Pluto and therefore, what little of the sun's light reaches it is instantly reflected back into space. Hence Triton's surface temperature works out as being generally lower than Pluto's. (Both are roughly the same size and inhabit the same approximate orbital distance from the sun - although Pluto does go a bit further out, so it's 'seasons' are probably more pronounced)


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* Where an object was formed may have implications on the type of material that it is made of, and that, of course, can have an impact on albedo. A rocky world tends to have a low albedo (The moon is ~0.12, I believe) whereas ice generally has a very high albedo - hence the objects formed in Uranus/Neptune orbits and further out from Ice of all sorts, if you believe the models of solar system evolution, tend to have higher albedos.

 

[Brian G Turner]

I don't think where an object originates from directly determines what temperature a planets surface is.

Well, let's put it another way - the further from the sun an object originates, the colder it is likely to be. Triton is so exceptionally cold compared to the rest of the Neptunian system that it has always seemed to suggest an alternative and more distant origin - which is presumably why studies such as in the link explore that assumption.

 

[Venusian Broon]

Triton is so exceptionally cold compared to the rest of the Neptunian system that it has always seemed to suggest an alternative and more distant origin - which is presumably why studies such as in the link explore that assumption.

No, I can't see the logic in that assumption. And the article says nothing about temperature!

On average Triton is closer to the sun than Pluto. Yet it's surface appears to be colder. I'm suggesting that the disparity in surface temperature is because of the albedo - essentially Triton is mostly shiny ice, not that it came from further away. If the driving force of a planets surface is the sun's light, which for the vast majority of planets and round bodies in the solar system it is, it does not matter where the object actually came from. All that matters is where it is now, and how the surface and atmosphere interact with the solar systems heat source, the sun.

That Triton came from elsewhere - probably out of the Oort cloud, is I think, not really debated. It's orbit is retrograde to the rest of Neptune's satellites suggesting strongly that it was captured. How that was actually 'done' is what the article is about - they are suggesting a mechanism that might have been the one that caused a dwarf planet to be captured by Neptune.

 

[Serendipity]

No, I can't see the logic in that assumption. And the article says nothing about temperature!

On average Triton is closer to the sun than Pluto. Yet it's surface appears to be colder. I'm suggesting that the disparity in surface temperature is because of the albedo - essentially Triton is mostly shiny ice, not that it came from further away. If the driving force of a planets surface is the sun's light, which for the vast majority of planets and round bodies in the solar system it is, it does not matter where the object actually came from. All that matters is where it is now, and how the surface and atmosphere interact with the solar systems heat source, the sun.

That Triton came from elsewhere - probably out of the Oort cloud, is I think, not really debated. It's orbit is retrograde to the rest of Neptune's satellites suggesting strongly that it was captured. How that was actually 'done' is what the article is about - they are suggesting a mechanism that might have been the one that caused a dwarf planet to be captured by Neptune.

Several factors determine the temperature of a moon, including the ones you've already mentioned:

• albedo (reflection of the sunlight from the surface)
• history of temperature, though that should after all this time have a minimal effect

There are other factors at play:

• heating due to gravitational stresses - Triton has a very eccentric orbit and therefore comparatively high gravitational heating
• having sunlight cut off by an object in front of it some of the time, in Triton's case, Neptune and the dust in the planet's rings, and in Pluto's case, Charon
• composition of the ices on the moon / dwarf planet concerned - she ices retain heat better than others, though it has to be admitted that Pluto and Triton have similar composition, though it is believed Pluto has heat being generated at its heart (=centre, not the heart-shaped formation on its surface)
• orbit - Pluto is fairly close to the Sun at the moment, but it will go much further out over the next century

How much heating each factor provides / takes away would require better experts than me to assess. However, I suspect the mechanisms interact with each other. Which is why I think the answer to the temperature difference between Triton and Pluto is not simple.

 

[Brian G Turner]

If the driving force of a planets surface is the sun's light, which for the vast majority of planets and round bodies in the solar system it is, it does not matter where the object actually came from.

This is kind of my point - it's long been observed that Triton is unusually cold for the Neptunian system. Eris has a higher albedo and higher mean temperature - but has an orbit far outside of Neptune's: Eris (dwarf planet) - Wikipedia

That Triton came from elsewhere - probably out of the Oort cloud, is I think, not really debated.

Presumably because it requires speculation rather than facts. It'll be interesting if further modelling sheds light on this.

 

[Venusian Broon]

Any calculation of thermal equilibrium is not simple.

However the extra factors you mention I don'the think have too much of an effect.

- the albedo calculation is essentially also taking into account the composition question. So it's not an extra factor.
- Also you can take into account the actual orbits to calculate the thermal equilibrium of the surface and how it will change over time - we've known about the orbits for a long time. So not an extra factor.
- I honestly don't think objects getting in the way will really drop the average fluxes of light hitting Triton's surface. I'll stick my neck out on this one and say not a factor - should be easy to do the sums!
- Is Triton close enough to Neptune to get a significant gravitational heating effect? I don't doubt that it is getting something , but I'd imagine that a moons has to be very close with its eccentric orbit for that warming to be noticeable. I don't think Triton is.

By far the biggest factor usually is atmosphere - greenhouse effect/clouds interacting with the blackbody radiation. Damn complex though.

 

[Dave]

...history of temperature, though that should after all this time have a minimal effect...

How long do you think it would it take for the temperature to reach a new equilibrium based upon its albedo and orbit?

Could it be hundreds of years? A thousand years?

Could its arrival in the Neptunium system therefore have been within the historical record?

I find the idea that it was a very recent addition interesting.

Do you think that there would be physical effects felt on Earth when this happened i.e. tidal or weather related?

Could the Arab or Chinese astronomers have observed anything?

I very much doubt the latter since Neptune was not visible without telescopes and its brightness wouldn't be altered. I'd also doubt the former, since the mass and distance involved is insignificant. However, the possibility is interesting.

 

[Venusian Broon]

This is kind of my point - it's long been observed that Triton is unusually cold for the Neptunian system. Eris has a higher albedo and higher mean temperature - but has an orbit far outside of Neptune's: Eris (dwarf planet) - Wikipedia

I'll get back to you on this one (I'm pumping iron and throwing kettle bells around at the moment)...

But firstly Eris being very far out right now is at the very low end of its temperature scale - hence colder than Triton...when Eris gets closer it appears that there is significant methane ice on the surface, hence my guess for the higher temperatures stated, is that they have calculated a greenhouse effect of a few degrees that a significant atmosphere of methane that a closer approach to the sun would produce.

Triton interestingly does not have much of an atmosphere. (Perhaps the capture/collision that got it into Neptune's grasp 'boiled off' a lot of the moons volatile elements??? A bit like our moon?)



Presumably because it requires speculation rather than facts. It'll be interesting if further modelling sheds light on this.

- the problem is I think a great deal of the evidence has been destroyed and there are a great many scenarios that could explain it. Just look at the debates on how our moon formed - still 'mysterious'.

 

[Venusian Broon]

How long do you think it would it take for the temperature to reach a new equilibrium based upon its albedo and orbit?

Could it be hundreds of years? A thousand years?

I don't know, but should be very quick - days? (Earth days !) Or a bit longer.

I mean when the sun comes up in the morning and directly shines on you, you feel it warming you quite quickly, no?

 

[Dave]

I don't know, but should be very quick - days? (Earth days !) Or a bit longer.
I mean when the sun comes up in the morning and directly shines on you, you feel it warming you quite quickly, no?

Well, I'm not a planet. It does take weeks for a continental land mass to heat up after Winter, and oceans take even longer because the heat doesn't conduct as quickly in a liquid as in a solid. (The Sun is only shining on the surface.) So, I would have expected a whole planet/satellite to take much longer. However, if think it would be relatively quick, even if it is only a matter of months or a single year, then it cannot be still in a process of heating up to a new equilibrium (which was what I thought would be interesting.)

 

[Venusian Broon]

Well, I'm not a planet. It does take weeks for a continental land mass to heat up after Winter, and oceans take even longer because the heat doesn't conduct as quickly in a liquid as in a solid. (The Sun is only shining on the surface.) So, I would have expected a whole planet/satellite to take much longer. However, if think it would be relatively quick, even if it is only a matter of months or a single year, then it cannot be still in a process of heating up to a new equilibrium (which was what I thought would be interesting.)

Surface temperature just needs energy to impact the surface. It doesn't need to conduct anywhere.

As soon as light hits a surface it should set up a thermal equilibrium where incoming light = blackbody emissions.

If a lot of light comes in i.e. a sunny day, then it will warm surfaces and that could cause ice to melt because the blackbody temperature of the surface is higher than the melting point of ice.

It takes a long time for the continental land mass (away from the equator) to warm up because it takes the earth a long time to change it's orbit so that the intensity of the sunlight is strong enough to do anything.

And atmosphere complicates matters dramatically.

I think you are possibly thinking of some sort of climate 'equilibrium' which is set up by the atmosphere, and the orbit etc...so in this case at least once around the sun then!

 

[Serendipity]

Any calculation of thermal equilibrium is not simple.

However the extra factors you mention I don'the think have too much of an effect.

- the albedo calculation is essentially also taking into account the composition question. So it's not an extra factor.
- Also you can take into account the actual orbits to calculate the thermal equilibrium of the surface and how it will change over time - we've known about the orbits for a long time. So not an extra factor.
- I honestly don't think objects getting in the way will really drop the average fluxes of light hitting Triton's surface. I'll stick my neck out on this one and say not a factor - should be easy to do the sums!
- Is Triton close enough to Neptune to get a significant gravitational heating effect? I don't doubt that it is getting something , but I'd imagine that a moons has to be very close with its eccentric orbit for that warming to be noticeable. I don't think Triton is.

By far the biggest factor usually is atmosphere - greenhouse effect/clouds interacting with the blackbody radiation. Damn complex though.

Okay... here goes...

The polar icecap on Triton is floating on a thin sea of liquid nitrogen. There are 'holes' in the icecap that vent the evaporated liquid nitrogen - kind of geysers - which carry with it dark dust particles that will to a very limited extent affect the albedo, because you can see the trails of fallen dark dust on the surface *. So whilst the albedo is dependent on the composition, it cannot take into account ALL the composition, certainly not the composition hidden deep down under the surface.

Triton used to have gravitational / tidal heating during the capture (due to its eccentric orbit) and settling down period, which resulted in the differentiation of ices i.e. Triton is made up of layers. What we can't be sure of is if those layers have some form or auto-convection associated with them. We know too little about the moon.

Any object that puts a moon or part of a moon into shadow will reduce the temperature on the moon. It's like being under a lunar eclipse on Earth. Whether it makes a notable difference on Triton remains to be determined.


* If you really want to know more about geysers on Triton and the possibilities such geysers afford to science fiction, suggest you read my story Guard Cat - https://www.amazon.co.uk/dp/B00G15K6TI/?tag=brite-21