APOD: Ringed Ice Giant Neptune (2023 Aug 19)

[APOD Robot]

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]

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?

[Ann]

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:

'Reflectrance' (reflectivity, albedo?) spectrum of Neptune between circa 380 nm (ultraviolet) and 900 nm (infrared). Credit: Christophe Pellier.

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Neptune color spectrum from circa 350 nm (ultraviolet) to 900 nm (infrared). Credit: Christophe Pellier.

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Filter responses to Neptune. Credit: Christophe Pellier.

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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

[orin stepanek]

Tritons defraction spikes; how lovely!

[johnnydeep]

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?

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.

[starsurfer]

Very nice! The identifications should be a mouseover.

[johnnydeep]

Very nice! The identifications should be a mouseover.

Yes, that would have been nice. From the first link:

Click to view full size image

[alter-ego]

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?

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.

[johnnydeep]

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?

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?

[alter-ego]

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.

[johnnydeep]

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.