In my fictional setting, the planet that people live on, called "Nearth" has one smallish very dark moon close in, and a large moon farther off with a very high albedo.

Nearth is exactly the same as Earth, with exactly the same sun, orbit, mass, radius, etc.

The large moon (Saur) has 1.5 times the mass and 1.17 times the radius of our own moon.times as large as our own moon. It orbits at 68 Nearth radii, or 270000 I'm, or 1.13 times the Moon's distance from Earth. It has 0.93 times the density of our own moon.

The tiny moon, (Oun), is 0.55 times a large as our moon, has 0.17 times the mass, the same density, and orbits at about 40 Nearth radii, or 158000km, or about 2/3 the distance of the Moon.

What conditions would be necessary on the large moon so that it has a high albedo? I considered clouds, but the moon is far too small to hold onto a sizable atmosphere (I believe). That leaves me with ice of some sort.

What conditions are necessary for a moon with the specifications I have given to maintain a layer of ice on it's surface over astronomical timescales?

The ice arrived as a large impacting body from beyond the frost line.


2 Answers 2


Even if the moon isn't massive enough to retain water vapor in its atmosphere, as long as its cold enough and never gets heated up by anything, it can maintain a layer of ice. The albedo of ice is pretty high (around 0.7) so it should work well.

So as long as the moon is not bombarded by asteroids and does not have a thick atmosphere that would trap heat, it would be able to maintain a layer of ice which would cause high albedo.

You can explain that no asteroids hit it by saying that there is a massive gas giant protecting it, like Jupiter protects earth.

Hope this helps.

  • $\begingroup$ The problem with this is that ice sublimates in a vacuum. While it stays cold on average, light can heat up a few water molecules over time, resulting, over astronomical timescales, the sublimation of the ice into space. The water vapor molecules then can escape into space. Or at least this is what I understand from my research. Thanks for the help, though. $\endgroup$
    – Totillity
    Commented Aug 7, 2018 at 13:28

A few ideas...

  • The atmosphere is made of a very heavy gas, so Suar can hold onto it even though it's small. The area below the gas won't get much light. (issue: I don't know what kind of gas is best for this)
  • Saur is very dusty, and is frequently beset by meteors, causing large dust clouds to expand around it
  • Nearth's magnetosphere is massive, greatly reducing the cosmic radiation which touches Saur
  • Saur has a strong magnetosphere (since it's cold, the core would have to be made of a colder liquid metal. Maybe a pure ionized mercury core). I think this solution requires that Saur is not tidally locked, because it will spin with its core.
  • Saur has an extremely solid exterior (like, iron or something), and its interior is comprised of gasses which are so heavily compressed that they are in liquid form. These gasses are being released at a constant rate to replenish the atmosphere. This obviously couldn't go on forever, but if your universe is relatively young then this kind of thing could have an indefinitely long half-life associated with it.
  • Saur orbits in such a way that it spends the majority of its time eclipsed. (Not sure if this is possible)
  • The hi-tech population of ancient Nearth launched millions of satelites into orbit around Saur, effectively blocking out most of the light which would otherwise hit the moon. The satelites could have been to generate solar electricity; they could have been part of some hi-tech weapon; they could be to monitor some extremely interesting phenomena on the moon; they could have been for some commercial purpose; or for some mysterious purpose lost to us.
  • Saur is enclosed in a pseudo-dyson sphere.

I hope these ideas help!

  • 1
    $\begingroup$ The eclipse idea is impossible, orbital dynamics won't allow it sorry :( $\endgroup$
    – Tim B
    Commented Aug 7, 2018 at 11:54
  • 1
    $\begingroup$ I don't think that any of these ideas are very logical. I like the first one the best, but Mars is larger than Saur and still doesn't have much of an atmosphere, so I don't think Saur can either. Thanks for the help, though! $\endgroup$
    – Totillity
    Commented Aug 7, 2018 at 13:24

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