Is water blue on all planets?

 Is water blue on all planets?

The answer is a bit nuanced:

Pure water itself, regardless of the planet, will still absorb red and orange light preferentially, reflecting back the blue and green wavelengths, making it appear blue-ish. This is due to the way light interacts with water molecules.

However, whether a body of water on another planet actually looks blue to our eyes depends on several factors:

• Composition of the water: If the water is filled with impurities like algae, sediment, or dissolved minerals, it can take on different colors. For example, the Red Sea gets its name from the presence of red-pigmented bacteria.
• Atmosphere of the planet: The surrounding atmosphere can distort the colors we see. This is why the sky on Mars appears orange-red during the day, even though the Martian soil itself is not red. The thick dust in the atmosphere scatters blue light away, leaving the longer wavelengths of red and orange to dominate.
• Sun's light spectrum: The star a planet orbits can have a different color spectrum than our sun. If the dominant light wavelengths are in the red or orange portion of the spectrum, even pure water might appear darker or less blue.

Therefore, while the inherent properties of water make it intrinsically blue-ish, the actual perceived color on different planets can vary greatly depending on the surrounding environment and viewing conditions.

So, while the Earth's ocean might be the beautiful "blue marble" we know and love, the Martian water ice may show up as a faint blue-grey, and some hypothetical exoplanet ocean could even appear turquoise or purple!