Comments and questions about the
APOD on the main view screen.
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APOD Robot
- Otto Posterman
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by APOD Robot » Sat Aug 19, 2023 4:08 am
Ringed Ice Giant Neptune
Explanation: Ringed ice giant Neptune lies near the center of this sharp near-infrared image from the
James Webb Space Telescope. The dim and distant world is the
farthest planet from the Sun, about 30 times farther away than planet Earth. But in the stunning Webb view, the planet's dark and ghostly appearance is due to atmospheric methane that absorbs infrared light. High altitude clouds that reach above most of Neptune's absorbing methane easily stand out in the image though. Coated with frozen nitrogen, Neptune's largest moon Triton is brighter than Neptune in reflected sunlight, seen at the upper left sporting the Webb telescope's characteristic
diffraction spikes. Including Triton, seven of Neptune's 14 known moons can be
identified in the field of view.
Neptune's faint rings are striking in this space-based
planetary portrait. Details of the complex ring system are seen here for the first time since Neptune was visited by the
Voyager 2 spacecraft in August 1989.
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Lasse H
- Ensign
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- Location: Stockholm
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by Lasse H » Sat Aug 19, 2023 9:19 am
I reacted when I read about the "atmospheric methane that absorbs infrared light". But, in order to be seen by Webb, it is not enough that it absorbs infrared light. It also has to emit infrared light. Right?
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Ann
- 4725 Å
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by Ann » Sat Aug 19, 2023 10:41 am
Lasse H wrote: ↑Sat Aug 19, 2023 9:19 am
I reacted when I read about the "atmospheric methane that absorbs infrared light". But, in order to be seen by Webb, it is not enough that it absorbs infrared light. It also has to emit infrared light. Right?
Take a look at these spectra of Neptune:
Note that Neptune is brightest through the B (blue) filter, very closely followed by the V ("green") filter. Neptune is considerably less bright through the R ("red") filter, and even fainter through the I (infrared) filter at infrared wavelengths. But Neptune does reflect appreciable amounts of infrared light. As you can see, the sensitivity of the IR filter begins just after 7000 Å (700 nm).
And by the way, yes:
Neptune does emit infrared light. That is because it emits more energy than it receives from the Sun, and it emits it at infrared wavelengths. It emits it at far infrared wavelengths, if I am not mistaken.
Ann
Color Commentator
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orin stepanek
- Plutopian
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by orin stepanek » Sat Aug 19, 2023 6:07 pm
STScI-01GCCV72VQSNF9RY9EVKJ67TZN.png
Tritons defraction spikes; how lovely!
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Orin
Smile today; tomorrow's another day!
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johnnydeep
- Commodore
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by johnnydeep » Sat Aug 19, 2023 8:52 pm
It's notable to me that whereas the methane
gas absorbs IR, the methane
ice apparently reflects it! Is that due solely to the crystalline structure of the ice (molecules?) or for some other reason?
https://webbtelescope.org/ wrote:Webb’s Near-Infrared Camera (NIRCam) images objects in the near-infrared range from 0.6 to 5 microns, so Neptune does not appear blue to Webb. In fact, the methane gas so strongly absorbs red and infrared light that the planet is quite dark at these near-infrared wavelengths, except where high-altitude clouds are present.
Such methane-ice clouds are prominent as bright streaks and spots, which reflect sunlight before it is absorbed by methane gas. Images from other observatories, including the Hubble Space Telescope and the W.M. Keck Observatory, have recorded these rapidly evolving cloud features over the years.
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"To B̬̻̋̚o̞̮̚̚l̘̲̀᷾d̫͓᷅ͩḷ̯᷁ͮȳ͙᷊͠ Go......Beyond The F͇̤i̙̖e̤̟l̡͓d͈̹s̙͚ We Know."{ʲₒʰₙNYᵈₑᵉₚ}
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starsurfer
- Stellar Cartographer
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by starsurfer » Sat Aug 19, 2023 10:03 pm
Very nice! The identifications should be a mouseover.
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johnnydeep
- Commodore
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by johnnydeep » Sat Aug 19, 2023 11:05 pm
starsurfer wrote: ↑Sat Aug 19, 2023 10:03 pm
Very nice! The identifications should be a mouseover.
Yes, that would have been nice. From the
first link:
--
"To B̬̻̋̚o̞̮̚̚l̘̲̀᷾d̫͓᷅ͩḷ̯᷁ͮȳ͙᷊͠ Go......Beyond The F͇̤i̙̖e̤̟l̡͓d͈̹s̙͚ We Know."{ʲₒʰₙNYᵈₑᵉₚ}
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alter-ego
- Serendipitous Sleuthhound
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by alter-ego » Sat Aug 19, 2023 11:21 pm
johnnydeep wrote: ↑Sat Aug 19, 2023 8:52 pm
It's notable to me that whereas the methane
gas absorbs IR, the methane
ice apparently reflects it! Is that due solely to the crystalline structure of the ice (molecules?) or for some other reason?
https://webbtelescope.org/ wrote:Webb’s Near-Infrared Camera (NIRCam) images objects in the near-infrared range from 0.6 to 5 microns, so Neptune does not appear blue to Webb. In fact, the methane gas so strongly absorbs red and infrared light that the planet is quite dark at these near-infrared wavelengths, except where high-altitude clouds are present.
Such methane-ice clouds are prominent as bright streaks and spots, which reflect sunlight before it is absorbed by methane gas. Images from other observatories, including the Hubble Space Telescope and the W.M. Keck Observatory, have recorded these rapidly evolving cloud features over the years.
Yes. The same reason why earth snow and clouds look white. Clouds of methane ice consist of methane crystals most likely of varying sizes and certainly varying orientations. The crystal surfaces both reflect
and transmit light. The transmitted light is absorbed, but the reflected light scatters about, ultimately enough to show up as bright areas in the image. For reflection, the higher the angle of incidence at each crystal, the larger the fraction reflected and subsequently lesser is absorbed. Unlike for gas, the comment "reflect sunlight
before it is absorbed makes sense for ice.
A pessimist is nothing more than an experienced optimist
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johnnydeep
- Commodore
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by johnnydeep » Sun Aug 20, 2023 12:00 am
alter-ego wrote: ↑Sat Aug 19, 2023 11:21 pm
johnnydeep wrote: ↑Sat Aug 19, 2023 8:52 pm
It's notable to me that whereas the methane
gas absorbs IR, the methane
ice apparently reflects it! Is that due solely to the crystalline structure of the ice (molecules?) or for some other reason?
https://webbtelescope.org/ wrote:Webb’s Near-Infrared Camera (NIRCam) images objects in the near-infrared range from 0.6 to 5 microns, so Neptune does not appear blue to Webb. In fact, the methane gas so strongly absorbs red and infrared light that the planet is quite dark at these near-infrared wavelengths, except where high-altitude clouds are present.
Such methane-ice clouds are prominent as bright streaks and spots, which reflect sunlight before it is absorbed by methane gas. Images from other observatories, including the Hubble Space Telescope and the W.M. Keck Observatory, have recorded these rapidly evolving cloud features over the years.
Yes. The same reason why earth snow and clouds look white. Clouds of methane ice consist of methane crystals most likely of varying sizes and certainly varying orientations. The crystal surfaces both reflect
and transmit light. The transmitted light is absorbed, but the reflected light scatters about, ultimately enough to show up as bright areas in the image. For reflection, the higher the angle of incidence at each crystal, the larger the fraction reflected and subsequently lesser is absorbed. Unlike for gas, the comment "reflect sunlight
before it is absorbed makes sense for ice.
Thanks! Though I took the "before" phrase as merely meaning that the incident light was prevented from reaching the methane gas below the methane ice. Is that also what you mean?
--
"To B̬̻̋̚o̞̮̚̚l̘̲̀᷾d̫͓᷅ͩḷ̯᷁ͮȳ͙᷊͠ Go......Beyond The F͇̤i̙̖e̤̟l̡͓d͈̹s̙͚ We Know."{ʲₒʰₙNYᵈₑᵉₚ}
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alter-ego
- Serendipitous Sleuthhound
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by alter-ego » Sun Aug 20, 2023 12:44 am
johnnydeep wrote: ↑Sun Aug 20, 2023 12:00 am
alter-ego wrote: ↑Sat Aug 19, 2023 11:21 pm
johnnydeep wrote: ↑Sat Aug 19, 2023 8:52 pm
It's notable to me that whereas the methane
gas absorbs IR, the methane
ice apparently reflects it! Is that due solely to the crystalline structure of the ice (molecules?) or for some other reason?
Yes. The same reason why earth snow and clouds look white. Clouds of methane ice consist of methane crystals most likely of varying sizes and certainly varying orientations. The crystal surfaces both reflect
and transmit light. The transmitted light is absorbed, but the reflected light scatters about, ultimately enough to show up as bright areas in the image. For reflection, the higher the angle of incidence at each crystal, the larger the fraction reflected and subsequently lesser is absorbed. Unlike for gas, the comment "reflect sunlight
before it is absorbed makes sense for ice.
Thanks! Though I took the "before" phrase as merely meaning that the incident light was prevented from reaching the methane gas below the methane ice. Is that also what you mean?
Not intended, but it is a more complete interpretation. Depending on cloud density, the amount that gets through the cloud to the gaseous layer will vary, as will the reflected component from the crystals. For a dense enough cloud, almost no light gets through. I think there is a range of crystal densities which both reflect enough light to be seen and have significant transmission through the cloud to the gas layer. Bottom line, absorption occurs in both the crystals and gas, but only the crystals provide enough scattered light to image.
A pessimist is nothing more than an experienced optimist
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johnnydeep
- Commodore
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Post
by johnnydeep » Sun Aug 20, 2023 12:47 pm
alter-ego wrote: ↑Sun Aug 20, 2023 12:44 am
johnnydeep wrote: ↑Sun Aug 20, 2023 12:00 am
alter-ego wrote: ↑Sat Aug 19, 2023 11:21 pm
Yes. The same reason why earth snow and clouds look white. Clouds of methane ice consist of methane crystals most likely of varying sizes and certainly varying orientations. The crystal surfaces both reflect
and transmit light. The transmitted light is absorbed, but the reflected light scatters about, ultimately enough to show up as bright areas in the image. For reflection, the higher the angle of incidence at each crystal, the larger the fraction reflected and subsequently lesser is absorbed. Unlike for gas, the comment "reflect sunlight
before it is absorbed makes sense for ice.
Thanks! Though I took the "before" phrase as merely meaning that the incident light was prevented from reaching the methane gas below the methane ice. Is that also what you mean?
Not intended, but it is a more complete interpretation. Depending on cloud density, the amount that gets through the cloud to the gaseous layer will vary, as will the reflected component from the crystals. For a dense enough cloud, almost no light gets through. I think there is a range of crystal densities which both reflect enough light to be seen and have significant transmission through the cloud to the gas layer. Bottom line, absorption occurs in both the crystals and gas, but only the crystals provide enough scattered light to image.
Ok, got it, thanks.
--
"To B̬̻̋̚o̞̮̚̚l̘̲̀᷾d̫͓᷅ͩḷ̯᷁ͮȳ͙᷊͠ Go......Beyond The F͇̤i̙̖e̤̟l̡͓d͈̹s̙͚ We Know."{ʲₒʰₙNYᵈₑᵉₚ}