Orsdxy

Imagine an inhabitated planet like ours with a moon of the same projected size in the sky. The moon does not have an atmosphere, but it has a visible structure of darker and lighter areas.

Every evening when the moon rises over the horizon, it's surface changed in at least 1/4 of the visible area. It didn't just rotate, no-one on this planet has ever seen the same image of the moon twice in their life.

How is it possible that the structure or pattern on the surface of a moon changes constantly while the planet is stable enough to support intelligent life?

How it could work

Water example

Your planet has a (water)ice surface. When the star doesn't shine upon the surface of the moon surface temperature falls below -10 °C (there is likely no pressure on the moon). But when it does shine temperature rises above -10 °C and it melts and the surface transforms, because of gravitational pulls.

Alternatives

You could also use other elements with different melting points for scenarios where temperatures always exceed -10 °C or never exceed -10 °C. Examples:

Lower than -10 °C

• oxygen -218 °C


• chlorine -101 °C


• mercury -38 °C

Higher than -10 °C

• rubidium 39 °C


• sodium 97 °C


• tin 231 °C

Now these are just examples but I hope at least one of them can come to use.

This is my source(just type in the mentioned element and compare the result)

Make the Moon Bigger

Or, more accurately, more massive. You'd have to move its orbit out a bit to prevent more brutal tides on the planet, but make it massive enough to hold an atmosphere of its own. Make that atmosphere out of denser gases, ones that can be held with a more tenuous grip than our own, and, ideally, ones that can be coloured and have interesting fluid dynamics. Dinitrogen tetroxide, maybe.

With shifting cloud patterns (doubtless very lethal but very pretty), maybe from core heating, you get a relatively rapidly shifting lunar appearance without having to make the moon out of anything implausible.

Edit: Another option - if you're already allowing for an active core for additional heating (and an extra swirly atmosphere), you could also have volcanism. Nothing like a man in the moon with acne!

Take Pluto as inspiration. From the wiki (all emphasis are mine):

Pluto's surface is composed of more than 98 percent solid nitrogen, with traces of methane and carbon monoxide. The face of Pluto oriented toward Charon contains more solid methane, whereas the opposite face contains more nitrogen and solid carbon monoxide. Distribution of volatile ices is thought to be season-dependent and influenced more by solar insolation and topography than subsurface processes.

(...)

Pluto's surface color has changed between 1994 and 2003: the northern polar region has brightened and the southern hemisphere has darkened. Pluto's overall redness has also increased substantially between 2000 and 2002. These rapid changes are probably related to seasonal condensation and sublimation of portions of Pluto's atmosphere (...)

Now this may seem to conflict with one of your requirements:

The moon does not have an atmosphere

But again, according to another page in Wikipedia:

The surface pressure of the atmosphere of Pluto, measured by New Horizons in 2015, is about 1 Pa (10 μbar), roughly 100,000 times less than Earth's atmospheric pressure.

Some satellites in LEO face similar atmospheric pressures and need reboosting every then. Bottom line being: that's what objects in space around us face. So for all practical purposes Pluto's surface is exposed to a vacuum. Standing naked on it wouldn't be much different from being naked on Earth's orbit.

Naturally occurring Thermochromic chemicals.

Those of us old enough to remember the 1970s remember mood rings.

While such chemicals are unlikely as natural deposits, it is fairly easy to imagine that some flowering plants could incorporate such as a form of sexual attraction or such-like.

No earth-like plants exist without the benefit of atmosphere, but perhaps an different design of plant forms could exist in thin carbon-dioxide atmosphere that may be compatible with your story. A hard-vacuum is likely to make significant plant-like coverage impossible.

Any high-contrast vegetation that has a short lifespan would perhaps be sufficient, esp. competing forms with different reflectance characteristics.

That solar system could be experiencing a phenomenon like the late heavy bombardment: the moon is constantly hit by meteorites, so that it is continuously shaped by these impacts.

The problem is that these meteorites must be very frequent (say in the order of the dozen of impacts in a day), but quite small - in the range of 10-20 meters - in order not to cause damage when they fall onto the main planet, burning in its atmosphere. In this case probably you should somehow justify the lack of bigger asteroids.

You could also point that the moon is of recent formation (maybe captured by the gravity of the planet), which implies its core is still active and gives origin to some vulcanic activity that shapes its surface (but in this case the changes would be slower and the surface of the moon wouldn't change a lot in the span of some days).

Possibly there could be intelligence of some sort on the moon and their activities can make a big visual change in the appearance of large sections of the moon during short periods.

Possibly they are artists trying out various "paint" patterns to see which make the moon look best.

Perhaps they move countless millions of large vehicles of various colors around on the moon, arranging them in various patterns as part of some sort of "dance".

Maybe they are trying to send a message to the natives of the planet and thus making an ever changing series of patterns on the moon.

A century or so ago an astronomer believed that some tiny changes of color he observed on the moon were the results of vast hordes of insects moving around.

So possibly on your moon thick areas of vegetation spring up, changing the color of vast regions, and then vast wandering hordes of insects travel toward them, changing the face of the moon as they move. When the insects arrive at the forest or meadow or swamp their color mingles with the color of the plants and changes the color of the region as seen from the planet.

So the insects eat all the vegetation in the region, changing its color, and leave their wastes (containing many seeds) behind to fertilize the soil. The insects move on, searching for another area of vegetation, and the area is now a different color, barren of vegetation until the seeds sprout and start to grow. Eventually the plants become thick enough to change the color of the region back to vegetation colored, then the region retains that color until the same or a different horde of insects arrives to eat the vegetation.

Of course it seem rather doubtful that a planet small enough to be habitable would have a moon large enough to be habitable for at least some types of life. That would make them seem a lot more like a double planet than like a planet and its moon.

Of course the intelligent beings on the moon don't have to be living beings; they could be machines.

Or the two worlds could both be habitable planets orbiting their star if their orbits are a lot closer to each other than any planetary orbits in our solar system, so that when the planets pass closest to each other the natives of one can see surface features on the other. There is actually a known solar system where planets in the habitable zone of their star do sometimes orbit close enough for someone on one of them to see surface features on the other - TRAPPIST-1.

The distance between the orbits of TRAPPIST-1b and TRAPPIST-1c is only 1.6 times the distance between the Earth and the Moon. The planets should appear prominently in each other's skies, in some cases appearing several times larger than the Moon appears from Earth.[41] A year on the closest planet passes in only 1.5 Earth days, while the seventh planet's year passes in only 18.8 days.[38][34]

https://en.wikipedia.org/wiki/TRAPPIST-11

Or maybe the "planet" and "moon" are actually both moons that orbit a gas giant planet.
Your "planet" could actually be a giant, Earth sized habitable moon, and the "moon" would orbit closer to the gas giant. Sometimes your "planet" and the inner moon would be almost on opposite sides of the gas giant and the inner moon wouldn't look big enough for details to be visible. But at intervals, probably every few days, the inner moon would pass close to the outer habitable moon and the natives of the habitable moon could see details of the surface of the inner moon and note the changes (and only in the parts of the moon that were lit by their star) .

If the inner moon orbits close enough to the gas giant the tidal heating should make the inner moon hyper volcanic like Io, the innermost Galilean moon of Jupiter. Various volcanoes might erupt often, spewing out vast amounts of lava of various colors to constantly resurface the inner moon. So each time that details of the inner moon were visible the surface patterns would be at least slightly different.

The problem with this is that the clouds and weather patterns on the gas giant planet would be visible all the time and would also change, thus possibly distracting the natives of the habitable moon from the show on the inner moon.

Possibly the moon in your story could suffer from tidal heating and constant resurfacing if it orbited a habitable planet in the right way. It would probably have to have an eccentric orbit which made it get noticeably farther and closer to the planet at various points in its orbit, and thus the patterns on its surface would only be visible when the moon was closer to the planet (and only in the parts of the moon that were lit by their star).

Possibly someone here can calculate if it is possible for a habitable planet to have a presumably recently captured moon with enough tidal heating to be constantly resurfacing itself.

An alien race has covered the planet, or whatever percentage you see fit with a giant TV screen.

Maybe they double as solar panels somehow.

Now they can literally change any part of the visible surface at whim.

Just wait till they spam you with moon sized commericials... muhhahahahaha!