Starship Asterisk*

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.

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?

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.

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.

starsurfer wrote: ↑Sat Aug 19, 2023 10:03 pm Very nice! The identifications should be a mouseover.

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.

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?