APOD: Ganymede from Juno (2021 Jun 14)

[VictorBorun]

I wonder why is Ganymede not a blue marble like Earth?
One think all that water would make for white mixed with deep blue.

[neufer]

I wonder why is Ganymede not a blue marble like Earth?
One think all that water would make for white mixed with deep blue.

• Ridiculously thick ice

Enhanced-color Galileo spacecraft image of Ganymede's trailing hemisphere. The crater Tashmetum's prominent rays are at lower right, and the large ejecta field of Hershef at upper right. Part of dark Nicholson Regio is at lower left, bounded on its upper right by Harpagia Sulcus.

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<<In this global view of Ganymede's trailing side, the colors are enhanced to emphasize color differences. The enhancement reveals frosty polar caps in addition to the two predominant terrains on Ganymede, bright, grooved terrain and older, dark furrowed areas. Many craters with diameters up to several dozen kilometers are visible. The violet hues at the poles may be the result of small particles of frost which would scatter more light at shorter wavelengths (the violet end of the spectrum). Ganymede's magnetic field, which was detected by the magnetometer on NASA's Galileo spacecraft in 1996, may be partly responsible for the appearance of the polar terrain. Compared to Earth's polar caps, Ganymede's polar terrain is relatively vast. The frost on Ganymede reaches latitudes as low as 40 degrees on average and 25 degrees at some locations. For comparison with Earth, Miami, Florida lies at 26 degrees north latitude, and Berlin, Germany is located at 52 degrees north.

North is to the top of the picture. The composite, which combines images taken with green, violet, and 1 micrometer filters, is centered at 306 degrees west longitude. The resolution is 9 kilometers (6 miles) per picture element. The images were taken on 29 March 1998 at a range of 918000 kilometers (570,000 miles) by the Solid State Imaging (SSI) system on NASA's Galileo spacecraft.

The Jet Propulsion Laboratory, Pasadena, CA manages the Galileo mission for NASA's Office of Space Science, Washington, DC.>>

[VictorBorun]

I wonder why is Ganymede not a blue marble like Earth?
One think all that water would make for white mixed with deep blue.

• Ridiculously thick ice

I think I got it.
The water does possess an extreme low absorption window in the violet-blue part of the visual wavelength range.
To observe the color of water you need 1 meter to catch greenish (caused by the absorption of extreme red and extreme violet) and make it 300 meters to catch the deep blue (after all the other wavelength went black).
But even 1 meter of water is hard to find.
You can dive but undersea you lose your white illumination.
You can find a storm cloud but the photons hardly get to go through the water; they mostly scatter and finally leave for the outer space or get absorbed by dust.

So you have to cut a 1 m cube out of perfect Baikal ice to see the pale greenish.
And you have to find a planet with a water ocean to sea the deep blue.

Click to view full size image

[neufer]

Click to play embedded YouTube video.

Click to play embedded YouTube video.

Click to play embedded YouTube video.

[bystander]

Hubble Finds First Evidence of Water Vapour at Jupiter’s Moon Ganymede
ESA Hubble Science Release | 2021 Jul 26

Hubble Finds First Evidence of Water Vapor at Jupiter’s Moon Ganymede
NASA | GSFC | STScI | HubbleSite | 2021 Jul 26

A Sublimated Water Atmosphere on Ganymede Detected
from Hubble Space Telescope Observations ~ Lorenz Roth et al

• Nature Astronomy (online 26 Jul 2021) DOI: 10.1038/s41550-021-01426-9
• arXiv.org > astro-ph > arXiv:2106.03570 > 07 Jun 2021