Printable Version of Topic

Posted by: MarcF Jan 5 2024, 09:58 AM

An interesting new study reveals the true colors of ice giants. It appears that Neptune’s true color is a pale greenish-blue similar to that of Uranus, contrary to popular images that show it to be a much deeper shade of blue. Uranus is slightly whiter. Ultimately, Neptune has a thinner layer of haze, allowing more sunlight to reach deeper in the atmosphere. At such depth, it can be absorbed by methane gas, which soaks up red light – making the planet appear ever so slightly more blue. The presence of amassed methane ice particles may also explain why Uranus changes its color slightly during its 84-year orbit around the sun.
https://www.space.com/uranus-neptune-similar-shades-of-blue-voyager-2-images



Best wishes for 2024
Marc

PS: See also: https://theconversation.com/how-we-discovered-that-uranus-and-neptune-are-actually-nearly-identical-in-colour-220244

Posted by: MichaelJWP Jan 5 2024, 03:32 PM

Academically, that's interesting certainly.

From a layman's point of view, a little negative perhaps. "First we lose Pluto as a planet, now it turns out Uranus and Neptune look the same!" :-)

Posted by: fredk Jan 5 2024, 06:37 PM

Lots of discussion of Neptune and Uranus's colour in these pages - see eg http://www.unmannedspaceflight.com/index.php?showtopic=7006 or http://www.unmannedspaceflight.com/index.php?showtopic=6764&st=30&p=212334&#entry212334.

Posted by: mcaplinger Jan 5 2024, 06:55 PM

The paper itself is here: https://academic.oup.com/mnras/article/527/4/11521/7511973

Note that the paper references Bjorn's work at https://www.planetary.org/articles/uranus-neptune-color-difference so this is old news for this forum. At least the super-saturated Voyager images are, and I'm not totally convinced by the tristimulus discussion in the paper -- yet another "what does this look like to the human eye" conundrum.

Posted by: fredk Jan 5 2024, 09:37 PM

Yeah, definitely not new here. But I guess the surprizing coverage this new paper has gotten is a good thing since it'll help get across to the wider public that some of these images aren't very realistic.

Posted by: Phil Stooke Jan 5 2024, 10:58 PM

If ever you needed a demonstration that so-called true colour images are not very useful for interpretation, this is it. I mean - spectra for composition data etc., fine, but cloud morphology, zonal banding, dynamics - forget it. Stretch that contrast 'til it's begging for mercy!

Phil

Posted by: Explorer1 Jan 6 2024, 12:00 AM

It's a question of aesthetics, as well, for whatever a planetary image is being used to illustrate. Attracting eyeballs can be the top priority for the cover of a children's book, or magazine cover, for example!
I'm sure we've all seen the Magellan radar view of Venus (under the clouds) depicted in various collages and pop culture usage, since it's much more striking than the almost featureless cloud tops that the planet 'really' looks like. Similarly, the 'rainbow Mercury' from Messenger data I've seen pop up the past decade is bound to be confusing (did the planet fly through the tail of a glitter comet?), but it certainly looks more exciting than the real grey, (or even the yellow Mariner 10 version images that was in all the textbooks for decades).
I too knew that Neptune wasn't really that dark, but hey, on the upside, Earth's blue will never be matched by anything in our solar system!

Posted by: scalbers Jan 6 2024, 01:20 AM

Interesting that Neptune is only slightly bluer than Uranus in the images shown above.

I think the tristimulus methods are pretty good in terms of color and contrast to be seen with "daylight" vision. If we have a spectrum then the u,v or x,y chromaticity can be calculated, and also the RGB pixel values. I use these quite a bit in my sky simulation efforts, including views of Earth from space. Chromaticity is also considered in some Mars image analysis.

Posted by: Bill Harris Jan 13 2024, 04:39 AM

But what of their visual appearance through the telescope? Uranus appears to me as light blue with a greenish tinge; rather a pale cyan. Neptune appears as a pale blue.
Nothing saturated, but rather subtle tints.

--Bill

Posted by: fredk Jan 13 2024, 05:23 AM

Perhaps the dimmer illumination at Nepture affects the perceived colour? Uranus, at roughly 2/3 Neptune's distance from the sun, would be something like 2 1/4 times more brightly illuminated.

They should also be viewed at similar elevation angles (or at least with both high enough to avoid significant low-elevation reddening) and ideally using the same equipment.

Posted by: StargazeInWonder Jan 13 2024, 07:14 PM

Evergreen information on this topic – There is no such thing as "true color" in the sense that lots of people seem to think, at least not when you think that it can be coded into an image and the image will possess true color in some objective sense, irrespective of how it will be displayed later (on one monitor, another, printed on paper and viewed in whatever light is falling on the paper). Look at photos of moon rocks from Apollo (or see them in a museum) and look at the Moon in the night sky and there's no comparison whatsoever – when you don't control for luminosity, "color" loses meaning. (And it's not just a matter of one moon rock or another not being representative – none of them have the silvery glow of the Moon in the night sky.)

Sunlight, and things lit by sunlight are, at Earth's distance from the Sun, far brighter than any computer monitor. As an interesting coincidence, Uranus illuminated by the Sun at its distance is about the luminosity that one experiences on a typical computer monitor display, so it is in principle possible to cue up some pixels on a monitor at the true luminosity and therefore color of Uranus and Neptune – it is not possible to do that with Saturn or anything interior to Saturn (unless the albedo of the surface in question is very low).

That said, even if there's no perfect right answer, there are certainly parameters of objectivity here and it's provocative to assert that Uranus and Neptune are about the same. Through a telescope, they don't look the same, although you need a lot of light gathering to make a fair assessment of Neptune's color.

Posted by: HSchirmer Jan 13 2024, 07:51 PM

Funny you should mention that, I learned in med school that there's a statistically wide spread of RGB sensitivity across human eyesight. Oddly, it seems to be similar to the 'perspiration adaptation', (humans are born with a roughly standard number of sweat glands, those who are exposed to hot dry climates keep all of them and become 'hot adapted', while those in colder damper climates loose pore function and become 'cold adapted'). So, it seems that the range of color that is perceived is related to how important color identification is to your culture, profession, or genetics/biology.


Good point, I think I was around 12 years old when I noticed that at VERY low light, the point when you lose color vision, my right eye was red-sensitive, while the left was blue sensitive. So, in very dim light, the color you perceive may well depend on which eye you are looking with.

Posted by: scalbers Mar 16 2024, 11:43 PM

At least in post #1 it shows Neptune a bit bluer-green tint.

Here's an animation originally from Oxford of the seasonal change in Uranus as the color and intensity changes depending on whether we see the equator or pole.

https://www.msn.com/en-us/video/animals/watch-uranus-seasonal-changes-in-color-168-year-animated-time-lapse/vi-BB1k0BNb?cvid=d2ac285923164652bc527e24b4cb56ff&ei=9