This blog is great too! I have a question regarding a fictional earth-like planet of mine where the single large moon is not tidally locked and rotates, and seems partially made of large red quartz crystal and mars-red dust. Would it be possible for the light reflected from the red side of this moon to color the atmosphere of the planet purple-ish (at night) and lilac (in the day) if the atmosphere is the almost same as earth (save for a 30% oxygen level instead of our 21%)? Thank you!
I love weird planets and moons. As we detect and learn about more exo-planets, we keep seeing more and more weirdness and wonder in the universe. Just looking at the strange diversity amongst moons in our own Solar system (icy world-ocean Europa, fiery volcano-covered Io, hydrocarbon smog-covered Titan) shows that no two moons or planets will ever be the same.
So, in the interest of diversity, lets go all out and place a moon the size of Ganymede (the largest moon in our solar system) in orbit around our Earth-like planet. We’ll assume that the moon orbits at about the same distance as our Moon orbits Earth. We’ll take Ganymede’s shiny, icy surface and replace it with red crystals (or maybe it’s still ice, and the ice is red because of chemicals or single-celled life) and red Mars-like dust.
Ganymede is about 5200 km in diameter, making her about 1.5 times the size of the Moon. She would be large, bright, and would look very eerie with her reddish hues. Being much larger and more reflective, our Red Ganymede would shine much brighter - around 4 times as bright (about twice as bright due to size, and twice as bright again due to crystals/ice being more reflective than moon dust).
Having a red, red moon around a planet would certainly be dramatic, but would probably not change the color of the sky much at all for two reasons.
Primus, even our Red Ganymede wouldn’t be very bright compared to the Sun. The Sun is 400,000 times as bright as our Moon. It’d be about 100,000 times brighter than our Red Ganymede. There would be some slight reddening of the night sky if there were thin clouds - the clouds acting as a white cloth that the red light would shine on. During the daytime, there would be little difference between the blue skies we have now and the blue skies we’d have with Red Ganymede. Things might take on a slightly redder color, especially at night on a full moon, but the sky would still remain very similar to our sky here on Earth.
Secundus, the color of the sky has much more to do with the composition of the atmosphere than with the light coming through it. Even if our habitable planet was around a blue or very red star, the skies of an Earth-type world would still appear pretty blue. Adding a little red to the mix won’t noticeably change that.
You might see some increase in red when our Red Ganymede is rising or setting. The light from the moon is already scattered and bent because it travels a much longer path through the atmosphere. All of the blue end of the light would be scattered (and there goes more of the brightness), leaving all the red and yellow.
Of course, the fact that the human eye is only moderately sensitive to the longer, redder wavelengths means that even the little change that would occur probably wouldn’t be noticed by our meat-bag-mostly-water eyes.
The easiest way to make the sky look red is to fill it with fine particulate matter to scatter the blue light even more. A massive chemically-powered industrial civilization that pumps huge quantities of particulates into the atmosphere would do that, as would a very volcanically-active planet. Our Red Ganymede’s more powerful gravity would flex the planet’s crust and would almost certainly increase the occurrence of volcanoes.