# How can I set a moon on fire?

So, for reasons which may or may not pop up in a later question of mine, I need to light a small Moon-like moon on fire.

Well, sort of. See, I need a small object capable of emitting a lot of light - i.e. with a high luminosity. That could either be a large ball of gas on fire, or it could be something very, very hot - and thus very, very bright. So it actually doesn't have to be burning.

For undisclosed reasons, this thing must be a rocky moon, and it must radiate in all directions. I have access to all the materials that exist in our Solar System. Oh, and I have the capabilities of a Type II civilization (which I appear to be obsessed with).

To the smart-alecks who will say, "Hey, you've got a giant bloody star!": The moon needs to be in its own stable orbit, either around a planet or, most likely, the central star. For now. I can't crash it into the central star. I also don't care about what happens to any of the other bodies in the system.

. . . I need to know by Friday.1

1 Congratulations if you know what I'm referencing.

Depends on the color of the light. The easiest to achieve by Friday is lava-red. All you need to do is have a massive collision of your local moon with another moonlet: if the moonlet is large enough, that completely remelts the surface of the moon. The mantle remelting process has been seriously studied in the past, so your basic equations are ready to go.

$$Q_S = Q_R (1 + M_p/ M_t) ( 1 − b )$$ $M_t$ and $M_p$ are the target and projectile masses, respectively, and $Q_R$ is the specific energy, while b is a variable that measures the directness of the impact. For more details, see the linked article above. Here are some calculations of fractional melt for an Earth-sized body (note that moons should be a lot easier to melt):

If we assume a basic ocean of lava, given that now we're getting on a full moon somewhere around $5mW/m^2$, that would make it at least 200 times more luminous. Moreover, with a mantle-remelting impact, you can get significant rock vaporization and (rather briefly) temperatures in the 3-7,000K range (See page 79, bottom right), which would be a nice white sun-like glow.

But let's face it, an ocean of lava played straight is boring: we've all seen it.

Here's my suggestion: you can make it all more interesting by having a moon large enough to hold an atmosphere that literally allows burning to happen: you can then have your asteroid impactor generate an ocean of burning sulfurous lava, with a nice eerie purple glow, the perfect setting for a final climactic boss confrontation.

Of course, it might be a bit messy.

PS:You can never have too many XKCD references.

• Lava red isn't going to be very luminous, is it? A quick calculation tells me that heating the Moon up to 1000K (brighter orange) would give you about 1.16W/m^2 of irradiance at Moon-Earth distance; less than a thousandth of what we get from the Sun. You'd need a temperature about five times higher to make it match, and I'm pretty sure most rocks evaporate before that point. – Mike L. Mar 29 '15 at 19:38
• Well, given that now we're getting on a full moon somewhere around $5 mW/m^2$, that would still make it about 500 times more luminous. Moreover, with a mantle-remelting impact, you can get rock vaporization and temperatures in the 3-7,000K range. (See page 79, bottom right) – Serban Tanasa Mar 29 '15 at 19:53
• Well, it would be more luminous than what we have now. According to this table: engineeringtoolbox.com/light-level-rooms-d_708.html a 200-fold increase (from 0.005 W/m^2 to 1.16) would bump you up from "full moon" to somewhere above "twilight". It's a definite improvement, but not enough to read by. Just thought that your answer might benefit from the irradiance figures, since the question is specifically asking about luminosity. – Mike L. Mar 29 '15 at 20:49
• Sure, I appreciate it. The OP never mentioned just how luminous it must be. – Serban Tanasa Mar 29 '15 at 20:49
• @SerbanTanasa Yep, this luminosity is fine. – HDE 226868 Mar 29 '15 at 22:11

If the moon was a watery moon like Europa, this might be easy (for certain values of easy); an intense beam of muons could be shot into the oceans and catalytically induce fusion in the molecules of D2 in the water. This would be fantastically inefficient and probably require a muon source emitting almost as much energy as the fusion reactions going on in the oceans would release, but if you are really determined to have a light show.....

For rocky or metallic bodies, the problem is much more difficult. Inducing nuclear reactions runs against the curve of binding energy; so while it is "sort of" easy to induce fusion with very light elements, or fission in very heavy elements, as you approach Iron things become more and more difficult (iron is right out; when the core of a supermassive star produces Iron there is no energy release and the gravitational forces collapse the star, triggering a supernova).

Your sort of serious solution would be to drop a slug of antimatter into the core of the moon, and the energy release would melt the body and you would have essentially a glowing ember in the sky (a big enough slug of antimatter compressed to neutronium density would consume the core and melt the rest; too big and you blow the moon into small pieces, too small and you simply get interesting earthquakes and volcanic eruptions).

Or you could just paint the surface of the moon with radioactive material and have it glow in the dark....

• The anti-matter induced glowing ember idea is somewhat workable, but (as I mentioned in another comment), if it's illumination you're after, you get to rock-evaporating temperatures before you get to decent irradiance. – Mike L. Mar 29 '15 at 19:41
• Radium paint operates at low temperatures. Tritium-energised phosphorescence is similar but fades in a few years. So prepare a mixture of radioisotopes and phosphorescent material, and use that as an impactor. It could be a hundred meters in diameter and also covered with a layer of ordinary asteroid rock, then sprayed with ultrablack nanotube nanovelvet to be invisible. The incoming path would be chosen to minimise the chance of an occultation being noticed. – JDługosz Mar 29 '15 at 21:15
• Easy, simple, and cost effective. I like it. I'll go build a Muon gun in the back yard and point it at Europa. My landlords might regret paying the electric bill though.. – Josiah Mar 31 '15 at 0:40

An ordinary impact event makes quite a bright light. You could direct impacts to several points around the sphere to make it light up from all directions.

• That's no (ordinary) moon; that's Theia. – Mazura Mar 29 '15 at 21:06
• No, the impactors would be orders of magnitude smaller than the moon. – JDługosz Mar 29 '15 at 21:09
• Not if you want to set the world on fire: "The energy of such a giant impact is predicted to have heated Earth to produce a global 'ocean' of magma" – Mazura Mar 29 '15 at 21:13
• The moon-forming impact destroyed both bodies forming an extended atmosphere that recondensed. I mean a normal-sized impact that would only vaporize a small amount of surface material. – JDługosz Mar 29 '15 at 21:20

Many answers linking cause and effect, that may not be necessary. Consider, "How would you boil the ocean?" Don't heat it of course, reduce the atmospheric pressure.

For your moon, use a set of Gravity Lens(s) to collect and focus ambient starglow. .

Given that our moon's dull gray surface shines bright white and beige in sunlight, it seems that composing or covering your moon's surface with brighter colored, higher reflective compounds would greatly increase its luminousity. Whatever you use should be either solid or liquid at the moon's highest sun-struck temperature and zero pressure. If it evaporates into a gas, there probably wouldn't be enough gravity present to keep the gas around. As a result, the moon's size and brightness would deminish over time.

You might get a very firelike effect if the moon made of a bright substance that was a gas at its sun-struck temperature and a solid in the dark side chill. Then by spinning your moon quickly, you could give it a bright atmosphere that is continually erupting from the surface, glowing in wild tendrils and wisps until they flow into the darkness and condensing, return to the ground. Could look pretty cool!

Actually Arthur C. Clarke has done this in his first published novel, "The Sands of Mars"

from wikipedia:

Hadfield reveals that scientists have been working on "Project Dawn", which involves the ignition of the moon Phobos and its use as a second “sun” for Mars. It will burn for at least one thousand years and the extra heat, together with mass production of the oxygen-generating plants, will eventually – it is hoped – make the Martian atmosphere breathable for humans.

The resulting fireball is not as bright as the sun but it provides additional energy for Mars' development. However it makes the day/night and seasons cycle very complicated.

The moon (made of rock) is somehow ignited using meson reaction. The technical details of how the moon was ignited are not discussed in the book. However Clarke sets the stage for how a society can attract the brightest nuclear scientists togather to do this massive project.

The book also has amazing descriptions of the moon rising. I suggest reading chapter 15 if you are interested to know the details.

• It may seem like it, but 1000 years is not that much time in the grand scope of human civilization. What would we do after Phobos stopped burning? You might have billions of people dead on Mars. – Matthew Paul Chapdelaine Mar 30 '15 at 8:11
• How did they plan to ignite it? That is what is asked. – Ville Niemi Mar 30 '15 at 8:16
• Thank guys. Have updated my answer. As for what happens after 1000 years, you really have to read the book. But the tl;dr is that by then mars would have a protective atmosphere which is stable for geological ages – A squared Mar 30 '15 at 8:55