APOD: Crescent Neptune and Triton (2013 Apr 14)

[APOD Robot]

Crescent Neptune and Triton

Explanation: Gliding silently through the outer Solar System, the Voyager 2 spacecraft camera captured Neptune and Triton together in crescent phase in 1989. The elegant picture of the gas giant planet and its cloudy moon was taken from behind just after closest approach. It could not have been taken from Earth because Neptune never shows a crescent phase to sunward Earth. The unusual vantage point also robs Neptune of its familiar blue hue, as sunlight seen from here is scattered forward, and so is reddened like the setting Sun. Neptune is smaller but more massive than Uranus, has several dark rings, and emits more light than it receives from the Sun.

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[Beyond]

Ah, BIG and little. But how does it emit more light than it receives??

[Ann]

But how does it emit more light than it receives??

I don't think it does. I think that claim is a mistake. Neptune does emit more heat than it receives from the Sun, but we are talking about emission in the invisible infrared. It is interesting that Neptune emits heat, because that means that it has an internal heat source, but in order to emit light it would have to have a surface temperature of at least 2,000 Kelvin. I don't know what the surface temperature of Neptune is, because I haven't bothered googling, but undoubtedly we are talking about a temperature below 200 Kelvin, a tenth of what is required for a body to emit visible light.

Ann

[geckzilla]

It's probably not a mistake. I've heard that claim several times and read about it out of curiosity.

Keep this in mind:

In physics, the term light sometimes refers to electromagnetic radiation of any wavelength, whether visible or not.

http://en.wikipedia.org/wiki/Light

[Ann]

It's probably not a mistake. I've heard that claim several times and read about it out of curiosity.

Keep this in mind:

In physics, the term light sometimes refers to electromagnetic radiation of any wavelength, whether visible or not.

http://en.wikipedia.org/wiki/Light

Yes, in physics, the term light sometimes refers to electromagnetic radiation of any wavelength. And we do say "infrared light" and "ultraviolet light".

But most people who visit APOD are probably not physicists, and to them, the term light probably means just visible light. When I googled "Light", I was taken to a wikipedia page about visible light. I think it would have been better to use the term energy instead of light to make people understand that the parts of Neptune that the Sun isn't shining on don't give off their own visible light.

Ann

[starstruck]

It's a beautiful picture, although it appears almost monochrome to my Sunday morning bleary eyes! I can just about make out some colour on Triton and a hint, almost like chromatic aberration, around the edge of Neptune.

The explanation paragraph is indeed a little confusing though . . "smaller but more massive" and "emits more light than it receives" . . but, again, it is only early and I don't think my brain is fully up and running yet!

[do_japan]

Beautiful image, but such low quality. Does anyone have a link to a higher quality version?

[Boomer12k]

Nice picture, and there are clouds near Neptune's Terminator. Wonderful contrast in sizes.

:---[===] *

[Ian_Kennedy]

I followed the “Dark rings” link on today's APOD (2013-4-14) and found an interesting image:
http://pds-rings.seti.org/neptune/voyager/c1141246.gif

A closeup of the Fraternity arc in Voyager 2 image FDS 11412.46 looks like a twisted rope. The caption continues: “The rope "strands" are really individual clumps that have smeared out during the 111 second exposure. The clumps are only 0.5 to 0.8 degrees apart and are very difficult to understand.”

I believe that the strands are real records of the (smeared) movement of bright clumps. There is no Newtonian reason why the clumps cannot be generally orbiting in the ring around Neptune, and each clump can be simultaneously swirling (i.e., barrelling or spiralling) around other close clumps on the opposite side of the envelope of the ring. That is, the clumps are cycling in and out radially AND are are synchronously cycling in and out of the orbital plane as well. To use other words, each clump is oscillating radially AND oscillating transversely in the ring but at the same frequency to provide a corkscrewing trajectory.

The idea of a tangential oscillation appears in http://articles.adsabs.harvard.edu/cgi- ... lassic=YES

But, I believe, that because the clumps are oscillating transversely, and not tangentially in the ring, they do not interfere with each other and cause friction. Mush in the middle of the ring does not oscillate, it just orbits the planet.

I have detected similar twisted rope in some of Saturn's rings as well.

To explain how the barrelling started, one would only need to invoke the gravitational influence of a moon that is not exactly in the equatorial plane.

Ian G. Kennedy. (dr.iankennedy@gmail.com)

[Boomer12k]

Here is one possible explanation for the higher heat energy/light of Neptune...

http://www.windows2universe.org/neptune ... ction.html

I thought it also might be Pressure on the solid material of the planet, if any. Like some of the Moons of Jupiter. The tidal forces squeeze and press them, creating heat. But the explanation from the above link is plausible. From the same webpage, there are other articles on the subject. Just scroll down the page to the other articles section.

:---[===] *

[smitty]

Just as a general comment to the APOD powers that be, if you make a statement which is virtually certain to raise eyebrows (e.g., "Neptune . . . emits more light than it receives from the sun") it would be considerate to provide a link to some generally accepted explanation.

[neufer]

Just as a general comment to the APOD powers that be, if you make a statement which is virtually certain to raise eyebrows (e.g., "Neptune . . . emits more light than it receives from the sun") it would be considerate to provide a link to some generally accepted explanation.

<<Uranus only radiates 1.1 times as much energy as it receives from the Sun; while Neptune radiates about 2.61 times as much energy as it receives from the Sun. Neptune is the farthest planet from the Sun, yet its internal energy is sufficient to drive the fastest planetary winds seen in the Solar System. Several possible explanations have been suggested, including radiogenic heating from the planet's core, conversion of methane under high pressure into hydrogen, diamond and longer hydrocarbons (the hydrogen and diamond would then rise and sink, respectively, releasing gravitational potential energy), and convection in the lower atmosphere that causes gravity waves to break above the tropopause.>>

[smitty]

Thank you, neufer; that's helpful! My original point was simply that this link (or one like it) should have been included in the APOD which sparked all this dialogue. But better late than never. And yes, I know, we can all go out and do our own research to answer such questions, but, again, some spoon feeding of such info would be much appreciated. In general, APOD is good about doing this leg work for us. Thanks again.

[Chris Peterson]

But most people who visit APOD are probably not physicists, and to them, the term light probably means just visible light. When I googled "Light", I was taken to a wikipedia page about visible light. I think it would have been better to use the term energy instead of light to make people understand that the parts of Neptune that the Sun isn't shining on don't give off their own visible light.

That is a reasonable suggestion, but the original statement is still accurate. Representing it as a "mistake" is incorrect.

[owlice]

Thank you, neufer; that's helpful! My original point was simply that this link (or one like it) should have been included in the APOD which sparked all this dialogue. But better late than never. And yes, I know, we can all go out and do our own research to answer such questions, but, again, some spoon feeding of such info would be much appreciated. In general, APOD is good about doing this leg work for us. Thanks again.

smitty, the first "Neptune" link in the APOD text is to that article! You apparently didn't read down far enough on that page to find the information Neufer provided.

[smitty]

Mea culpa! In a futile, lame defense of my own indefensible position on this, I'd have appreciated having the link appear where the eye-brow-raising statement was made, but that's just a nit pic. Case closed so far as I'm concerned.

[owlice]

I'm blaming it on a lack of caffeine, so have a cup on me! [_]o

[Ann]

But most people who visit APOD are probably not physicists, and to them, the term light probably means just visible light. When I googled "Light", I was taken to a wikipedia page about visible light. I think it would have been better to use the term energy instead of light to make people understand that the parts of Neptune that the Sun isn't shining on don't give off their own visible light.

That is a reasonable suggestion, but the original statement is still accurate. Representing it as a "mistake" is incorrect.

Yes, I agree. I'm not editing my original statement, since it has become part of the thread, but I stand corrected.

Ann

[Shamanomaha]

Am I right in believing that Neptune rotates the opposite direction than Earth and all the other planets?

[Chris Peterson]

Am I right in believing that Neptune rotates the opposite direction than Earth and all the other planets?

No, the axial rotation of Neptune is prograde (obliquity = 28°). The only planets with retrograde rotation are Venus (177°) and Uranus (98°).

[neufer]

Am I right in believing that Neptune rotates the opposite direction than Earth and all the other planets?

No, the axial rotation of Neptune is prograde (obliquity = 28°). The only planets with retrograde rotation are Venus (177°) and Uranus (98°).

Perhaps you are thinking of Triton which both rotates & revolves retrograde.

<<Triton is unique among all large moons in the Solar System for its retrograde orbit around its planet (i.e., it orbits in a direction opposite to the planet's rotation). Most of the outer irregular moons of Jupiter and Saturn also have retrograde orbits, as do some of Uranus's outer moons. However, these moons are all much more distant from their primaries, and are quite small in comparison; the largest of them (Phoebe) has only 8% of the diameter (and 0.03% of the mass) of Triton.

Triton is in synchronous rotation with Neptune; it keeps one face oriented toward the planet at all times. Its equator is almost exactly aligned with its orbital plane. At the present time, Triton's rotational axis is about 40° from Neptune's orbital plane, and hence at some point during Neptune's year each pole points fairly close to the Sun, almost like the poles of Uranus. As Neptune orbits the Sun, Triton's polar regions take turns facing the Sun, resulting in seasonal changes as one pole, then the other, moves into the sunlight. Such changes have recently been observed.>>

[ta152h0]

two more years and PLUTO !!! Turn the headlights on !!!! seeing this is Sunday and the teacher is taking a break and is out of the room, I am going off-topic. How fast is New Horizons speeding past Pluto relative to Pluto ?

[Chris Peterson]

two more years and PLUTO !!! Turn the headlights on !!!! seeing this is Sunday and the teacher is taking a break and is out of the room, I am going off-topic. How fast is New Horizons speeding past Pluto relative to Pluto ?

About 14 km/s. That means the period of exceptional high resolution imaging will only last a few hours, although New Horizons will outperform HST for about four months in all.

[curious_casual_user]

This was also the APotD on June 18, 2006. (The full-size images and texts are identical.) Are repeats just uncommon or is this one perhaps a mistake? Not that I'm complaining! I had this as my computer wallpaper for years and don't mind seeing it again.

[bystander]

This was also the APotD on June 18, 2006. (The full-size images and texts are identical.) Are repeats just uncommon or is this one perhaps a mistake? Not that I'm complaining! I had this as my computer wallpaper for years and don't mind seeing it again.

Sunday APODs are often repeats. APOD's editors deserve a day off, too.