Making the Moon brighter with talcum powder - how much brighter could it be?

Question

A future nation wants to make the Moon as bright as possible in order to save massive amounts of night-time electricity on Earth at certain phases of the moon.

The plan is to coat the Moon's surface with a layer of talcum powder. (EDIT or other white powdery substance if talc is unstable on the Moon)

Question

Ignoring the logistics of the exercise, and the availability of the white substance, how bright could the sunny side of the moon be and how bright would the night side be compared to now?

Please ask for clarifications before answering.

I would be worried about stability of talc Mg3Si4O10(OH)2 in lunar environment its a hydrate. Why not titanium dioxide? — Gault Drakkor, Mar 15 '21 at 20:33
@Gault Drakkor - My thought was that talc is plentiful. Is TiO ₂ easily sourced? Would talc lose its whiteness if it degraded under sunlight? — chasly - supports Monica, Mar 15 '21 at 20:54
For the purposes of the question, it would be easy enough to include a bullet that says, "ignore any reason you might think of that would prohibit talcum powder from being used for this purpose." While there might be some excellent reasons why talcum powder wouldn't be the best chemical to use - the vast, vast majority of readers would never be aware of those facts (and those who are will suspend their disbelief for a good story...). Adding that you're using it because it's abundant would also be good, that way anyone wanting to attempt a Frame Challenge would know why you wanted it. — JBH, Mar 15 '21 at 23:10
The irony of the question is that the areas that would receive the most benefit are the areas where there is currently absolutely no night time artificial light. Cities with extensive light pollution would receive minimal benefit. It would be akin to using a flashlight on a brightly lit road in the city but in the country it would perhaps eliminate the need for a flashlight. — Justin Thyme the Second, Mar 16 '21 at 00:22
Note quite a duplicate, but closely related: Moonlight bright enough to see by?. — Starfish Prime, Mar 16 '21 at 11:56
@Justin Thyme the Second: I would seriously challenge the idea that any sort of unnecessary lighting is a benefit. As you say, it's light pollution. — jamesqf, Mar 16 '21 at 16:13

Nuclear Hoagie · Answer 1 · 2021-03-16T19:43:21.243

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The moon's albedo is around 0.1, meaning it reflects about 10% of the sunlight that hits it, for a peak illuminance of a few tenths of a lux (sources on albedo and lux). Even if the moon were a perfect mirror, it would only reflect 10x as much light and would only cast a few lux, making its illuminance during a full moon only a few hundredths of a percent of the sun. A perfect mirror supermoon would barely qualify as emergency lighting, which is normally a few lux. Under the absolute best conditions, a completely reflective moon would have about a third of the illuminance of a typical streetlamp. On average, it will be less than a tenth of a typical streetlamp, making it rather unsuitable as a replacement for electric lights.

Talcum powder will increase the albedo of the moon, but certainly not as much as a mirror finish, so the effect will be extremely modest at best. No amount of increase in the moon's albedo will make it a viable night-time illumination source that can replace electric lighting on its own, as it will never be as bright as a regular street lamp, which is normally 10 lux. Increasing the albedo of the moon will help you see better during the night, but for almost any application where you already have night-time illumination (streets, sidewalks, outdoor events, etc.), you'll still need illumination even with a 10x as bright moon. The electricity savings will be small, since there is very little external lighting that becomes unnecessary with a brighter moon (although I suppose you could turn all the electric lights down a few percent if the moon makes up the difference).

EDIT: To summarize the comment chain below, I'm not in any way arguing that the moon is not or could not be a useful illumination source at night. But it will never replace electric lighting, as its illuminance is far below that of electric lights. At best, the moon could produce about 10% of the lighting level that is deemed useful and appropriate for the general public in day-to-day life. Since we're talking about a society that is engaging in planetary-scale projects, I think it's safe to assume its citizens need night-time lighting for more than not tripping over things in the dark (which can be done at mere thousandths of a lux, no moon alterations needed). Anything that requires moderate visual acuity, like making out text or faces or objects quickly or at a distance or while moving will be difficult by moonlight no matter how shiny the moon is.

edited Mar 16 '21 at 19:43

answered Mar 15 '21 at 20:31

Nuclear Hoagie

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If you cover the whole surface of the moon with aimable flat mirrors, you could direct all the incident sunlight onto the Earth. (Earth from any point on the moon is much wider than the sun, so despite the sun not being a point source you can ensure the entire reflection hits somewhere on the Earth. This could give illumination anywhere from about 1/4 sunlight down to 1/20th sunlight, depending on your position and angle relative to the moon. But yeah... not what the OP is asking for, and that many mirrors might be slightly expensive – PcMan Mar 15 '21 at 20:55
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Depends on what you use the illumination for. It has been a viable night-time illumination for millennia. Just very limited. – Mary Mar 16 '21 at 01:23
Please provide relevant citations, as requested by the [tag:hard-science] tag – L.Dutch Mar 16 '21 at 04:30
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Full moon being used for illumination? One famous example is the raid by 617 squadron on the Ruhr dams in Germany. The date May 16th, 1943 was specifically chosen because the moon was full that night. https://www.bbc.com/news/uk-england-lincolnshire-22481152 – dgnuff Mar 16 '21 at 08:47
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"No amount of increase in the moon's albedo will make it a viable night-time illumination source on its own". The moon already works very well as nighttime illumination. Have you never been outside on a full moon? On a clear night, you can even read a book outside in moonlight. – Polygnome Mar 16 '21 at 09:47
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beware of increasing the moon's energy output toward Earth too much. You will exacerbate the global warming. And confuse all animals. And starve those who relied on darkness to hunt. And... – John Dvorak Mar 16 '21 at 10:05
@Polygnome As far as I know, nobody designs external night-time lighting that is as dim as even a perfectly reflective moon. Anywhere that has an electric light cannot be replaced by moonlight alone - any area that required an electric light before still needs one. The brighter moon is certainly bright enough to do night-time activities and not walk into stuff, but my point is that this is simply not a replacement for electric lighting, which seems to be the end goal of the OP. Yes, you can see a little better; no, it won't lower your electricity bill much at all. – Nuclear Hoagie Mar 16 '21 at 13:12
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@NuclearHoagie The vast amount of electric light we are using is completely unnecessary and already has detrimental effects (light pollution negatively affecting both animals and sleep in humans). With a bit of a cultural shift, its perfectly imaginable that a society might want to use lunar illumination, and would cut back drastically on the extreme excess illumination we use today. Remember that the amount of lighting you are used to outside is a feature of modern society -- about 100 years old, maybe a bit longer, but not by much. – Polygnome Mar 16 '21 at 14:01
@Polygnome No-one's disputing that you can see by moonlight. What's disputed is whether it's sufficient for more than simply not bumping into things. I walk my dogs in the dark most nights, and I've spent my entire life camping in places without street lights. Over 40 years of being outdoors says that your claim you can read a book by moonlight is flat wrong for any text smaller than ultra-large print for the majority of human eyes. So do reference sources, for example https://bookwormgadgets.com/how-many-lumens/ – Graham Mar 16 '21 at 14:06
@Polygnome Remember that before street lights were around, everyone who walked around at night had a lantern, or at least a candle. If you didn't, you didn't leave your house at night. It really was that simple. Also remember that the main purpose of electric lights is for indoor illumination, and humans have been using fires, rushlights, candles and other light sources indoors since prehistory, precisely because lunar illumination is inadequate. A society relying on lunar illumination would never, ever be able to function at night in any way whatsoever. – Graham Mar 16 '21 at 14:12
@Polygnome I might agree there are too many electric lights, but that doesn't mean they're too bright. I can't find any illumination guidelines or building codes that suggest that ~1 lux is suitable for anything other than emergency lighting. This isn't a cultural thing, it's a physiological one - 1 lux just isn't enough to comfortably see by. Before artificial lighting, people mostly just didn't do things outside at night. Even a 10x brighter moon isn't bright enough to change that. – Nuclear Hoagie Mar 16 '21 at 14:13
@NuclearHoagie "Also remember that the main purpose of electric lights is for indoor illumination". You won't be able to use the moon to illuminate indoor areas, so the point is moot. Btw, I really disagree with what you think is enough light to do useful stuff. I've done a lot of useful things on full moon nights, its surprisingly bright. The biggest problem with lunar illumination is phases -- you simply don't have a full moon all the time. Electric lights bring consistency -- they also work on a new moon. – Polygnome Mar 16 '21 at 14:18
@Polygnome I mean, it's not just my opinion on what's enough light - in many places, it would literally be illegal to construct a building that was this poorly illuminated. – Nuclear Hoagie Mar 16 '21 at 14:28
@NuclearHoagie Our laws reflect our habits and current wants, not necessarily our future wants. – Polygnome Mar 16 '21 at 15:23
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@Graham: I would strongly disagree with the idea that EVERYONE who walked around at outdoors night had a lantern. (Candles don't work outdoors unless it's really calm.) These days, anyone who wants light outside at night can buy an LED headlamp. I carry one, though I often don't use it, because I enjoy walking in the dark. (Cross-country skiing on moonlit nights, too :-)) – jamesqf Mar 16 '21 at 16:23
@Graham, walking about with a lantern was announcing there was someone to rob. They walked without them. You can even walk by starlight with no moon. – Mary Mar 17 '21 at 23:50
@JohnDvorak Assuming uniform hemispherical scattering, only about 1% of the light reflected by the full moon hits the earth. The moon has only 25% the cross-sectional area of the earth, so it would only contribute an additional 0.25% of insolation if it were full all the time and had an albedo of 1. Accounting for the phases, a fully reflective moon would increase total insolation on earth by perhaps a tenth of a percent, which is within the range of normal annual variation. It won't help global warming, but the effect will be fairly small (although it'll matter more over long time scales). – Nuclear Hoagie Mar 19 '21 at 19:28
@NuclearHoagie I'm not talking about albedo 1 uniform hemispherical scattering however. I'm talking a huge array of adaptive mirrors dumping all of Sun's incoming energy toward Earth. That would do wonders to ecology. – John Dvorak Mar 19 '21 at 20:00

score 9 · Answer 2 · answered Mar 15 '21 at 20:29

First of all, for lunar regolith we can neglect reflection and just consider scattering.

This paper simulated the scattering properties of the lunar surface, and came to values around 22% of reduced reflectance for incidence angles between 32 and 57 degrees.

I haven't been able to find a comparable chart for talcum, beyond generic statement like "talcum is highly reflective" or non normalized spectra, like this one

If we make an assumption that talc is from 2 to 3 times more effective than regolith at scattering light, we get that the talc covered lunar surface would roughly reflect back a similar proportion of light.

Since, according to this

Our Moon’s average visual albedo is 0.12. [...] The full Moon as seen on Earth has a visual magnitude of –12.7

and considering that

A difference of 1.0 in magnitude corresponds to a brightness ratio of $(100)^{1/5}$ or about 2.512

the talc covered full Moon should be somewhere around -13.7.

Simply looking at scattering is insufficient. You have to look at the scattering distribution function of talc vs lunar regolith. The light has to not only be scattered, but scattered in the direction of the Earth. Lunar regolith is actually relatively black, so the moon is a terrible reflector and a poor scatterer. That would be very different if talc were used. — stix, Mar 16 '21 at 22:28

score 3 · Answer 3 · answered Mar 16 '21 at 15:51

Use your talc for mirrors!

First, contemplate the economics of a future nation. Given that it's the future (not necessarily very far in the future), there is one man, the Consumer, who owns all the corporations, minerals, rockets, politicians, religious and charitable foundations, eyeballs, etc. Provided he is happy you have more or less 100% market penetration and you can count the project as complete.

Now talc can make flat tabular crystals. These are rare and usually microscopic, though you can find some pretty alleged examples online in an image search (the better looking ones I found were temporary images from auctioned minerals). If you have the resources to resurface the Moon, I imagine converting the silicon dioxide and magnesium from regolith and scarce lunar water into perfectly flat reflective plates of talc, though as some say, there may be better choices.

Once you have covered the Moon in mobile reflective mirrors, pointing at the Sun, someone looking up at it from the right spot will see lots of Sun, depending on the gaps between mirrors based on the angle they need to make. The "right spot" is actually pretty big, because these mirrors are flat, so anyone within 0.25 degrees (angular radius of the Sun) of the Consumer, as seen from the Moon, will have the same effect. The angular diameter of the Earth is 1.8 to 2.0 degrees, so luckless peasants far from the Capital of the Earth, or who are not invited when the Leader goes on vacation, will see only a drab Moon, lacking in features as the Man in the Moon has been banished as the backward superstition he is. Perhaps some advertising might be superimposed on the space. The transition from this to the lit moon could be very abrupt if the mirrors are all directed correctly, though it might take some doing to correct for small astronomical aberrations based on distance.

Note that this system does not actually focus light, so it can never be brighter than the Sun, though one supposes a wise Leader would have a very well secured feature to permit the mirrors to be warped a little and focused on a potential source of dissent.

score 1 · Answer 4 · answered Mar 16 '21 at 13:12

According to the Wikipedia (https://en.wikipedia.org/wiki/Moon), the moon has an albedo of 13.6%, which means that 13.6% of the incident light is reflected/scattered by its surface back into space. As such, the max. theoretical gain is a factor of 1/13.6% = 7.4.

That said, to achieve this brightness increase, you must use a very white substance that you can easily create on the moon itself. Otherwise, the logistics of bringing all the megatons of material to the moon would kill you. So, what is a good material? I believe the answer is magnesium oxide: This is a very white powder, so white in fact that it has been used as a reference white for quite some time, as well as in applications demanding for the highest possible albedos possible. Now, the lunar regolith (the dust that covers the moon) consists of a whole bunch of oxygen, as well as 5% magnesium. "All" you need to do is to take a pile of lunar regolith, extract the magnesium and a small part of the oxygen, and produce that awesome white powder to cover the moon's surface with. 1kg of regolith is enough to produce more than 80g of magnesium oxide, which can cover quite a substantical area as a layer of thin, fine dust.

Making the Moon brighter with talcum powder - how much brighter could it be?

4 Answers4

Use your talc for mirrors!