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Requirements:

  1. There is a solid surface.
  2. Somewhere on the surface, the temperature exceeds 2,230 C, such that pure silica vaporizes.
  3. Elsewhere on the surface, the temperature is at most 25 C, such that humans can be comfortable.

Nice-to-haves:

  1. In the human-comfortable temperature region, the surface gravity does not exceed 2g.
  2. The surface atmospheric pressure is at least 0.25 bar.

Life does not need to have arisen naturally.

I think I have figured out a way to achieve this, but some peer review would be appreciated! What I have in mind is a sort of hot mini-Mesklin--a super-Earth that rotates extremely quickly, such that it has extreme obliquity, and is also sufficiently close to a hot star. (This may need to be a close system of multiple stars for the planet to get enough light while also being far enough away to not end up tidally braked, and keep its high rotation.)

With little or no axial tilt, such a planet would present a small region near the equator straight-on to its sun, receiving the maximum concentration of light. Over the majority of the surface, the ground would be at a very low angle to the sun, thus being heated considerably less. Extremely fast rotation would also produce a large number of atmospheric circulation cells, impeding the transfer of heat from the equator towards to poles. Thus, with just the right parameters, we should be able to get a planet on which silicate rocks vaporize on the equator, then condense and rain out as glass in the low latitudes where the surface begins to curve away from the sun, and temperatures drop to survivable levels near the poles.

So, what have I got wrong? Am I overly optimistic? If this can't be made to work, is there any alternative approach that can get me glass rain on a planet that humans could land on?

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    $\begingroup$ Aren't your 3 requirements satisfied on Earth already, by any volcano? $\endgroup$
    – L.Dutch
    Feb 23 at 3:55
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    $\begingroup$ @L.Dutch Lava temperatures only get up to about 1300 Celsius. That's still over 900 degrees too cool to vaporize and recondense silica. I would consider glass grains from ejected lava to be more equivalent to sea spray than rain. $\endgroup$ Feb 23 at 4:05
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    $\begingroup$ There are lots of kinds of glass other than silica, borosilicate, and soda-lime. Do you really need it to be a high purity silica glass? $\endgroup$
    – Zeiss Ikon
    Feb 23 at 17:09
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    $\begingroup$ Vin Diesel starred in this film... $\endgroup$
    – Richard
    Feb 25 at 1:26
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    $\begingroup$ Would you accept a relatively vanilla planet with geologic conditions conducive to producing silane gas at high altitudes? $\endgroup$
    – unin
    6 hours ago

11 Answers 11

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Meteoric ablation spherules.

Stony materials entering the atmosphere can vaporize - these are shooting stars. Meteorites reach the melting point of silica and also metallic components which is why they disappear on the way in. Then the vaporized stuff can recondense. This forms meteorite ablation spherules. Those rain down.

Very high-temperature impact melt products as evidence for cosmic airbursts and impacts 12,900 years ago

spherules

Three sites contained conspicuous assemblages of both spherules and SLOs that are composed of shock-fused vesicular siliceous glass, texturally similar to volcanic scoria

If your planet was traversing an area that was full of siliceous stuff (perhaps kicked up from a big impact on a neighbor planet) that stuff would come in as meteorites. The meteorites melt as they do and then condense into your glass rain.

You can have the weather be whatever you like. But if you are going to sit outside at night and watch the light show, maybe bring a fortified umbrella.

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    $\begingroup$ A caveat is that such meteoric rain would either be inconsistent (perhaps quasi-seasonal as the planet passes through the debris field) or short-lived (as the planet sweeps up all the debris, although I've no feel for how short-lived (years? millennia?)), or somewhere in between. It could probably be made to last long enough for the story, though. $\endgroup$
    – BBeast
    Feb 24 at 0:24
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    $\begingroup$ @BBeast Meteor showers are, I suspect, ephemeral in geologic time for the reason you state. But the Perseids for example have lasted for hundreds of years. $\endgroup$
    – Willk
    Feb 24 at 1:31
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    $\begingroup$ @BBeast Saturns rings are quite new too and won't last that long, but in the human psyche they are still eternal. Imho it can work, galactic timespans are crazy $\endgroup$
    – Hobbamok
    Feb 25 at 11:18
  • $\begingroup$ That sounds like plenty of time to fit a human-scale story, then $\endgroup$
    – BBeast
    Feb 26 at 5:19
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    $\begingroup$ How about a slightly different approach: The planet has a small retrograde moon that has spiraled down to the Roche limit and is coming apart. There is no issue of going through the debris cloud, it's in orbit. $\endgroup$ Feb 27 at 4:18
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The parameters are: Glass rain as a weather phenomenon

Specifically, evaporation of silica at the equator, atmospheric currents carry it away to condense and rain. Mechanisms to make glass fall from the sky may exist, but I will try my best to reality-check the OP within the constraints provided:

Silica quartz vaporizes at over 2230C which is a temperature that won’t co-exist on a planet with balmy 65° evenings. I’m sorry, the reality-check answer will be, no glass rain at the beach. But even I wouldn't beleive me on say-so. So let's just check the barometer on this question then.

Fast Rotation? Doesn’t matter. Slower rotation would make a longer day and allow the planet to be further away actually. But that matters like a black head on the nose of our damsel tied to the railroad tracks.

Wind? If any planet could have this temperature gradient, the air won’t be happily staying in its zone. You did point this out. But how bad would the currents be? Would circulation cells hold back the storm? No. Storm winds worse than Saturn will be carrying horizontal rain (shards of glass??) at speeds well over 1,000mph, ripping flesh from your settlers’ bones. Look at the temperature gradient discussed below, and the speed of rotation, and we don't need any math to conclude this point.

Close to a hot star? Well then, that is the way we go about it. Let's consider how close to a star you need to be to vaporize silica sand (quartz). Remember that before it vaporizes, it becomes liquid. Liquid quartz is, as the question states, glass. Glass is transparent, and this is a problem when you want to absorb radiation to vaporize it. So to be clear, we are using the radiation of the sun to melt quartz (silica) into glass, and then boil that glass into silica vapor. The temperature we need to reach to boil the liquid glass is the important number. It will be T$_2$ below. The starting temperature, T$_1$ is how hot the sand is without the sun. That would basically be the night time temperature on the planet. For mercury, that is -180°C. But let's say some atmosphere exists and it's normal. Give it a warm 20°C night time temperature. OK, that starts us off.

We can call the sun a blackbody radiator but glass definitely doesn't absorb everything. The radiation heat transfer between graybody surfaces is more complicated than between blackbodies, because graybodies cannot completely absorb radiant energy projected onto them—instead some of the energy is reflected off the glass, generating multiple reflection and absorption values between the surfaces. But in the end, glass absorbs some energy. The problem is, it absorbs each wavelength differently, so some colors are heating it up, and others are passing right through. Bottom line, a rough approximation of the absorption coefficient of silica glass is $\alpha = 0.2$

The heat transfer from a black body (the sun) to a gray body (our transparent liquid quartz glass) has the following formula:

$$ q=\frac{A_1(T_1^4-T_2^4)}{\alpha}$$

Well, OK. We at least know the two temperatures. These have to be turned into Kelvins to work here. So at your picnic table in Santa's Workshop, it is currently a comfortable $T_1=293°K$. Down in the tropics, the beautiful, glowing molten beaches at Rio are boiling into the sky at a somewhat less comfortable $T_2=2503°K $. Oh, look! That cloud looks like a giant glass bunny :)

Sorry, where were we? Oh yeah. Math. Now, these temperatures need to be raised to the 4th power and find the difference:

$$ (T_1^4-T_2^4) \\= 2503^4-293^4 \\= 39250337770081 - 7370050801 \\= 3.924e-10^{13} $$

And we know the $\alpha$ is 0.2, so divide our temperature difference by that to get a total power needed to vaporize the glass as $\frac{3.924e-10^{13}}{0.2}=1.9621484e-10^{14}$ times the area of the glass. Well, the area doesn't matter, we're exposing each square meter of it to a square meter of the sun. We just need this planet to be close enough to push $1.9621484e-10^{14}$ Watts into every square meter of glass, and the glass will boil into vapor.

So how close do we need to be to get that kind of energy density? Well, luckily, the power density falls off very predictably with the square of distance. The solar radiation intensity, H$_0$ in (W/m$^2$), incident on an object is found by: $$ H_0=\frac{r_{sun}^2}{D^2}H_{sun} $$

Well this is cool, because the radiation intensity we want for H$_0$, happens to be the radiation intensity that will vaporize our glass, which was our $q$ from the last calculation. H$_0=q$, radius $r_{sun}$ we can look up, just like $H_{sun}$, and $D$ is the thing we're trying to find! So, do the algebra here, get $D$ on the right:

$$ D=\sqrt{\frac{r_{sun}^2 H_{sun}}{H_0}}\\ D=\sqrt{\frac{(6.95e-10^8 \text{km})^2 H_{sun}}{q}} \\ D=\sqrt{\frac{4.83025e-10^{17} H_{sun}}{1.9621484e-10^{14}}} \\ D=\sqrt{\frac{4.83025e-10^{17} \times6.4e-10^7}{1.9621484e-10^{14}}} \\ D=396,925 \text{km}$$

As you can see, when your planet is almost inside the sun, then glass will boil nicely away into the sky. I do hope you brought your tent stakes, because the Ez-up may have trouble staying put. And the reason that would be bad is because even before you spread the mustard on your ham and rye sandwich, a rain of glass shards at 1,000mph will make for a Bad Day at the picnic grounds, and you will have nothing at all to keep the rain out of the biscuits.

I truly think your planet will be torn to shreds by the tidal forces inside it's Roche radius before any glass rain condenses, and I really don't think there is any place to have a picnic on this planet. The tag has to be answered with a "no."

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    – L.Dutch
    Feb 25 at 8:44
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If you're not insistent on getting reasonably pure silica glass (fused quartz, borosilicate, soda-lime, or optical formulae equivalent), you can find it raining glass on Earth several times a year -- more or less continuously for months at a time, some years.

I mean, of course, volcanic ash. These particles are mostly volcanic glass, which while not silica of any purity, is mineral glass formed by rapid cooling of magma at the time it's erupted and sprayed into the air. This is often ejected many kilometers high, and in my own lifetime has drifted all the way around the world (though the fall isn't usually noticeable unless you're within a couple hundred kilometers of the eruption site).

I was under the plume from the Mount St. Helens eruption on May 18, 1980; where I was, we got a fall of a bit more than a centimeter, and areas nearer the mountain (but outside the immediate destruction zone) got up to twenty times that depth. The ash made a pretty unique pottery glaze, but did nothing good for the engines of cars that were driven within the first couple weeks after the eruption.

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Possible, but Highly Unlikely

It's possible via volcano, but it requires a few additional steps. Lava can get pretty hot, but it peaks at around 1100C. Pretty good, but not enough. So instead of using a volcano for lava, we'll use it for something else - aluminum deposits. Aluminum is quite rare, but it does occur naturally - only in low oxygen environments like volcanos.

Now let's say you were to mix that aluminum with, say, iron oxide. Iron oxide does occur naturally in large quantities. Mixing them poses a challenge, given that there's a volcano in this area, but it's not impossible. Just highly unlikely. (Also, ideally, both objects should be powdered beforehand.) However, there's an interesting scenario which should take care of that - a tornado.

Picture the scene - a tornado whips through a quarry and takes up aluminum stones alongside iron oxide deposits and then bashes them against each other to form a sandstorm-esque debris field. Now, this is important because mixing aluminum and iron oxide is a crude recipe for thermite, which burns at a crisp 2,200 C. A bolt of lightning (or honestly just the friction from the tornado) will light it on fire, and given that thermite is self oxidizing, the fire will not go out.

If this thermite-tornado hits a silicon deposit (like a beach, for instance), it may be capable of generating these shard of glass you desire. I would also like to point out that I would never want to live on a planet where 'thermite-tornado' is a weather condition.

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    – L.Dutch
    Feb 25 at 8:44
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Why didn't anyone mention the eyeball planet?

This is the only way that allows not being inside the sun, not being ripped apart by tidal forces, not relying on rare events like meteorites or volcanoes.

With this config you can have high temp on the side that is directed towards the sun, and people living at the dark side of the planet, with constant wind with sand that originated as boiling glass. As planet is tidally locked, people are always in the dark. Even the twilight zone is too hot for the people, so people can't even collect the sunlight. Surviving there will be hard.

And sandy air doesn't really feel like a rain of lava, but that is as close as you can get to what you want.

For keeping 0.25 atm and temp diff at the same time you will likely need very large planet. Small planet will easily equalize the temp with 0.25 atm. As gravity is limited to 2 g, planet will need to have very low density in order to maintain the size as large as possible.

So it will have to be a tidally locked planet, people in permanent darkness, a low density planet with a large size.

Also this planet makes interesting point of not being able to use rockets due to scorching sun, part of orbit going across the molten lava eye of the planet that will even make heat shield an impossible task, large gravity well. As planet grows in size even with the same g on the surface, leaving it with a rocket becomes harder and harder. This planet is probably even worse than a Kessler syndrome planet in terms of 'nope' for space travel. Leaving Kessler syndrome planet is risky but possible and gets better over time. This hell will never get better, never improve, no option to gradually solve the issue. Just an eternal hell with no way out.

P.S. Your idea won't work because such a large temp diff requires people being away even from a twilight zone. Your people are placed in a twilight zone.

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  • $\begingroup$ Volcanoes are not "rare events" -- there's at least one erupting somewhere on Earth almost any day (the ones that don't disrupt air travel or threaten tropical paradise tourism don't always make headlines). As of a few days ago, there were three erupting simultaneously. $\endgroup$
    – Zeiss Ikon
    Feb 24 at 12:05
  • $\begingroup$ Having light available from the sun was not a requirement, or even a "nice-to-have", so that hardly seems like a dealbreaker to me. $\endgroup$ Feb 24 at 19:42
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    $\begingroup$ This answer fits my intuition the planet probably must be tidally locked to its star to satisfy the constriants, with a hot side and an incredibly fortuitous "Goldilocks" side. The planet WASP-121b supposedly rains gemstones, but it's still way too hot on its "cold" side. (en.wikipedia.org/wiki/WASP-121b). Perhaps a weaker atmosphere wouldn't transfer as much heat from hot to cold? $\endgroup$ Feb 24 at 21:42
  • $\begingroup$ @JounceCracklePop higher gravity allows to make atmosphere much thinner. From kilometers to meters for the same pressure at the sea level. This helps the most. Next best thing is yes, lower pressure. Larger planet size helps as well as then wind needs to travel longer. Some tricks like a wall to prevent the wind between the eye and the rest of the planet also help. Temperature is high enough to build a massive sand wall from the sand rain. Wall that stops the rain is probably the most useful thing in prevention of the wind and heat transfer. $\endgroup$ Feb 24 at 21:47
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    $\begingroup$ @MatthewNajmon A natural moon wouldn't be stable around a tidally-locked planet. Artificial mirror satellites with active stabilization could work, though. $\endgroup$ Feb 25 at 23:11
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A Self-Answer

For additional background, this is a question I have been thinking about off-and-on for many, many years, originally inspired by exoplanet HD 189733 b, which may very well actually have silicate clouds and glass rain. (And which has a very weird atmosphere, quite far out of chemical equilibrium!) Combine that with Larry Niven's worldbuilding idea of finding planets with a habitable point, and you get this question--is there a possible world which has glass rain like HD 189733 b, but also has a habitable point?

The radiative temperature of [HD 189733 b] is only about 700C, which is not hot enough to produce a significant vapor pressure of silica; it's silicate clouds are almost certainly a result of higher temperatures in the lower levels of a very thick atmosphere. To have any hope of finding a habitable point, we will need a much thinner atmosphere to minimize heat transfer, which means a much higher stellar power input.

With particular thanks to Vogon Poet, I think it's quite clear that a mini-Mesklin isn't going to work out after all--there's just no realistic way to get enough power input in an orbit that won't also lead to tidal breaking... unless, perhaps, you are orbiting a supergiant, but then the angular size of the star in the sky becomes seriously non-negligible, and polar cold isn't a thing anymore. (I will point out that natural glasses tend to be brown or black, so the 0.2 absorption coefficient for pure silica is likely extremely pessimistic; but, that doesn't make a big enough difference to save the mini-Mesklin concept.) Avoiding the Roche limit is, fortunately, not too big of a deal--we just need a star that is hotter than our Sun, to provide equal power at a more tidally-favorable distance. (That's less than ideal if we want interesting aliens to evolve here, but note that native life--let alone interesting native life--is not a requirement.)

And I had previously dismissed the tidally-locked, eyeball-world possibility, because even relatively thin atmosphere turn out to transfer heat surprisingly well, per Simulations of the Atmospheres of Synchronously Rotating Terrestrial Planets Orbiting M Dwarfs, and boiling silica at 2230C (boiling anything, really, if there're planet-forming quantities of it around) will produce a thick atmosphere; literally, 1 bar of atmosphere, based on the definition for how boiling points are measured! (I.e., standard boiling point = the temperature at which the vapor pressure of a substance is 1 atmosphere.)

But then I discovered this lovely paper on the phase diagram of of high-temperature silica, which has all sorts of useful information--but, most critically, it turns out that the vapor phase boundary curve of pure silica is not particularly steep, and silica actually still has fairly significant vapor pressure at much lower temperatures. E.g., a "mere" 1160C still gets you a vapor pressure of approximately 0.25 bars. Additionally, the vapor phase has strong dissociation, with the largest fractions of gaseous components being silicon monoxide and diatomic oxygen--and silicon monoxide has a much lower boiling point than silica (silicon dioxide)--and has much higher optical absorption and emissivity!

So, strictly to meet the requirements of the question, we need one point--the substellar point--on the surface to actually hit 2230C, but things can cool off rapidly moving away from that point and still support a very wide region of silica clouds and glass rain. And really, it doesn't actually need to get up 2230C--the boiling point of silica under 1 full Earth atmosphere--at all. And we can also, quite fortuitously, end up with abiotically-generated free oxygen in the atmosphere!

Now, the question remains--can we isolate this high-temperature silicate atmosphere from a lower-temperature region that humans could survive in? Well, by the time the temperature drops to around 500C, vapor pressure drops to the millibar level, so there won't be much further atmospheric heat transport past that point. Thus, I suspect that, yes, we can isolate the heat to the dayside of a synchronously-rotating eyeball world. As long as the temperature is not too hot across too much of the dayside, which would result in producing a globe-spanning high-pressure silicate atmosphere, it should be possible to arrange for relatively low pressures produced by evaporation near the substellar point supporting silicate clouds and glass rain across most of the dayside, with the boiling-induced atmosphere all freezing out not too far past the terminator. This sets up a geologic-recycling system where material is evaporated away from the subsolar point, deposited to form lava oceans across the dayside and eventually to build mountains near the terminator, and returns in the short-term for re-evaporation through surface lava current flows, and in the long-term through mantle circulation and the weight of mountains pushes the mantle downwards and causes upwelling near the substellar point.

(Note that, while some pure minerals and metals have melting points above 2230C, such substances are relatively rare in a planet of vaguely Earthlike composition, tend to be heavy and so not concentrate at the surface of a lava ocean, and will dissolve into each other, such that pretty much everything is liquid by the time you hit 1200C. So, we don't need to worry about continents of refractory materials forming and halting the liquid/vapor silicate cycle.)

This would leave the dark side Extremely Cold, just as it is on Mercury, because rock is not a great conductor of heat through the bulk of the planet. However, that's easy to fix if we just add a second star to the system which will provide a regular day-night cycle to the dark side.

So, all that remains are some nice-to-haves; we can heat the dark side comfortably by introducing a second star, but can we put a region with .25 bars of atmosphere that's not in thermal contact with the dayside somewhere on that dark hemisphere? For that, I have to give a solid maybe, based on the concept behind Larry Niven's world of Canyon and the real-world Hellas impact basin. Put a Sufficiently Deep impact crater or crust-contraction canyon like Mars's Valles Marineris on there, and abiotically-dissociated oxygen can just slowly fill it in....

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  • $\begingroup$ Boiling at planet with 0.25 bar atmosphere will not create 1 bar of pressure. It will create 0.25 bar of pressure. Bar is 100 kPa. Same for atm, atm is 101 kPa. Good idea with the deep basin or canyon at the cold side, this helps by allowing thinner atmosphere on average. I dont like the idea of a second star - it will have to be further away from the star than a planet. Making long winters on planet's cold side in most configurations. Simply allowing some heat leaking from the hot side should be enough for temperature. And perpetual darkness seems to be ok for the author $\endgroup$ Feb 25 at 8:35
  • $\begingroup$ @SurprisedSeagull "Boiling at planet with 0.25 bar atmosphere will not create 1 bar of pressure." Yes? And? "Making long winters on planet's cold side in most configurations." A world close enough to its primary star to melt silica on the dayside will have an extremely short year, on the order of days in length, and that is what sets the length of day/night cycles from the second star. $\endgroup$ Feb 25 at 17:03
  • $\begingroup$ Depending on the cooling rate, you may well get crystalline quartz rather than glass. Pure silica glass (i.e. vitreous silica) is much harder to make than, say, soda lime silica glass; at 100% SiO2 you tend to get crystallisation at cooling rates that would produce glass in multielement systems. Not sure if you care about the distinction. The people on your planet will though; quartz damages the lungs badly. $\endgroup$ 7 hours ago
  • $\begingroup$ Having some potassium or sodium (oxide) around will allow water to leach the deposited silica away as dissolved sodium or potassium silicates, as well as greatly lowering the eutectic temperature. This may be helpful for dealing with secondary problems. They volatilise around 1000 deg C, so would have a parallel cycle to your silica. $\endgroup$ 7 hours ago
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Part One: Gravity-Related

First, the last sentence of the question asks for glass rain on a planet that humans can land on. But the rest of the question asks for a planet which would be in parts habitable for humans. There is a big difference between a world haBitable for humans and one which humans can land on and even take off from alive, as any astronaut who has ever been to the Moon can tell you.

Leaving aside the question of the temperatures, would the world as described be habitable for humans?

The nice-to-have section includes:

In the human-comfortable temperature region, the surface gravity does not exceed 2g.

In Habitable Planets for Man, 1964, Stephen H. Dole discusses the requirements for a world to be habitable.

Dole discusses the human gravity requirements on pages 11-13. Doles says that there was no evidence of a minimum gravity requirement then - now there is much evidence that extended stays in microgravity have bad effects on human health, and nobody know what the lower gravity limit will be yet.

For the upper gravity limit, Dole discusses experiments with men in centrifuges. Men with suitable support and in the correct postures can endure a few gs for short times. Men can also walk and move and manipulate objects in higher gravity than 1 g.

Tests showed that the time it took to complete various task increased with increased gravity, and that the times became quite excessive over 2 g.

Dole doubted that humans would ever want to colonize a world with a surface gravity over 1.25 or 1.5 g.

There are several ways you can react to this info:

  1. Dig up research that indicates that humans would be willing to colonize a 2 g planet because its effects are not as bad as was thought when Dole wrote.

  2. Decide that the surface gravity in the human habitable parts of the planet will be no more than 1.25 g or 1.5 g.

    I note that will result in the surface gravity at the equator being somewhat less than at the human habitable regions. Which may or may not matter much if nobody ever goes to the equator.

    As I just stated, if you go with a disc-shaped planet, a mini-Mesklin, the surface gravity will vary with latitude. And so will the escape velocity, I think. And unfortunately for science fiction writers, surface gravity and escape velocity do not increase or decrease at the same rate but need to be calculated separately. And the escape velocity is important for the planet's ability to retain an atmosphere for long enough for the planet to become habitable for humans.

    On pages 34 to 35, Dole discusses the relationship between escape velocity and atmospheric retention.

    You may also need to calculate the orbital velocity at the equator of your planet design.

  3. You may decide that the human colonists are descended from many generations of humans who colonized planets with higher and higher surface gravities, thus becoming conditioned to survive and be happy in higher gravity than humans from Earth could.

  4. Or maybe the colonists on your planet have been genetically modified to have greater tolerance for high gravity than unmodified humans.

  5. Maybe the colonists on your planet use anti-gravity to make their buildings and settlements comfortable and healthy, and maybe they use anti-gravity vehicles and wear anti-gravity belts when exploring or working outdoors.

  6. Or maybe the humans have colonized another world in the system with a lower gravity, and only come to your high gravity planet to take rich tourists for short visits that don't endanger their health. Maybe the tourists come to the planet to hunt exotic creatures, or to see a great natural wonder, or to experience the glass rain, or for some other reason. And possibly a discovery that the visits to the planet of grass rain actually involve too long exposure to the high gravity and are endangering people's health could be a plot point.

Part Two: Rapid Rotation.

The planet Earth has an equatorial radius of 6,378.137 kilometers and an equatorial circumference of 40,075.017 kilometers. Since it rotates once a day, once in 24 hours of 3,600 seconds, or once in 86,400 seconds, mater on the Earth's equatorial surface has a rotational speed of 0.4638312 kilometers per second.

Actually 24 hours is the synodic day of the Earth, the time it takes to rotate 360 degrees with respect to the Sun. That is slighty longer than the sidereal day, the time it takes the Earth to complete one physical rotation with respect ot the stars, and the time period which causes the equator to bulge slightly.

The sidereal rotation period of the Earth is about 23 hours (3,600 seconds each), 56 minutes (60 seconds each), and 4.100 seconds long. So it is 82,800 seconds plus 3,360 seconds plus 4.100 seconds. Or 86,164.1 seconds. So material at the surface of Earth's equator travels at a speed of 0.465101 kilomters per second.

Imagine a planet which has twice the equatorial radius and circumference of Earth, and has the same sidereal rotation period. It will have an equatorial surface rotation period of 0.930202 kilometers per second.

A planet which has three times the equatorial radius with the same rotation period as Earth will have an equatorial surface speed of 1.395306 kilometers per second.

A planet which has four times the equatorial radius with the same rotation period as Earth will have an equatorial surface speed of 1.860404 kilometers per second.

And so on.

So imagine planets which have the same equatorial radius as Earth but rotate faster that Earth's equatorial surface speed of 0.465101 kilomters per second.

A planet rotating twice as fast in half a sidereal rotation period will have an equatorial surface speed of 0.930202 kilometers per second.

A planet rotating three times as fast in 1/3 of a sidereal rotation period will have an equatorial surface speed of 1.395303 kilometers per second.

A planet rotating four times as fast in 1/4 of a sidereal rotation period will have an equatorial surface speed of 1.86040404 kilometers per second.

A planet rotating five times as fast in 1/5 of a sidereal rotation period will have an equatorial surface speed of 2.325505 kilometers per second. It will rotate once in 17,230.82 seconds, 4.78 hours.

A planet rotating ten times as fast in 1/10 a sidereal rotation period will have an equatorial surface speed of 4.65101 kilometers per second. It will rotate once in 8,615.41 seconds, 2.393 hours.

A planet rotating fifteen times as fast in 1/15 of a sidereal rotation period will have an equatorial surface speed of 6.97515 kilometers per second. It will rotate once in 5,744.27 seconds, 1.596 hours.

A planet rotating twenty times as fast in 1/20 of a sidereal rotation period will have an equatorial surface speed of 9.30202 kilometers per second. It will rotate once in 4,308.205 seconds, 4.78 hours.

Of course planets roatating that fast will becomemoreoblate, so Earth mass planets with those rotation rates would have larger equatorial diameters and thus would have faster rotational speeds on their equatorial surfaces.

Dole discusses the oblateness of rotating planets on pages 41 to 46. And from his discussion it would seem that it would probabily be difficult for a planet to stay intact with a high degree of oblateness due to rotation.

Dole, on pages 58 to 61, discussed the effects of rotation rates on planetary habitability. If the planet rotated too slowly, the long days and long nights would get too hot and too cold, and plants might die from lack of sunlight during the long nights. Dole decided that a rotation period of 96 hours (4 Earth days), would be about the maximum length consistent with habitability.

And Dole said that if a planet rotated too rapidly its surface gravity would fall to zero at the equator or it would become unstable.

If rotation rate were increased steadily, a limiting point would be reached when surface gravity at the equator fell to zero and matter was lost from the planet, or when the shape of the surface became unstable and axial symmetry was lost.

Just what extremes of rotation rate are compatible with habitability is difficult to say. These extremes, however, might be estimated at, say, 96 hours (4 Earth days) per revolution at the lower end of the scale and 2 to 3 hours per revolution at the upper end, or at angular velocities where the shape becomes unstable due to high rotation rate.

Earth has an equatorial radius of 6,378.137 kilometers. According to this orbital calculator, a satellite of Earth 6,378.14 kilometers from the center, or 0 kilometers above the equator, would have an orbital period of 1 hour 24 minutes and an orbital velocity of 7.9053 kilometers per second (km/s).

https://keisan.casio.com/exec/system/1224665242

A satellite at twice that distance, would have an orbital speed of 5.5899 km/s and an orbital period of 3 hours 58 minutes.

A satellite at 3 times that distance would have an orbital speed of 4.5641 km/s and an orbital priod of 7 hours 19 hours minutes.

A satellite at 4 times that distance would have an orbital speed of 3.9526 km/s and an orbital priod of 11 hours 15 minutes.

A satellite at 5 times that distance would have an orbital speed of 35353 km/s and an orbital priod of 15 hours 44 minutes.

A satellite at 10 times that distance would have an orbital speed of 2.4998 km/s and an orbital priod of 44 hours 31 minutes.

A satellite at 15 times that distance would have an orbital speed of 2.411 km/s and an orbital priod of 81 hours 48 minutes.

A satellite at 20 times that distance would have an orbital speed of 1.7676 km/s and an orbital priod of 125 hours 56 minutes.

So if a world with the mass of the Earth has a diameter too many times larger than Earth's and rotates with the same period as Earth, material at the equator surface will have will have a surface velocity greater than orbital velocity and will move to a higher orbit, leaving the surface of the planet.

And if a world with the mass of the Earth has the same diameter as Earth but rotates too many times faster than Earth, material at the equator surface will have will have a surface velocity greater than orbital velocity and will move to a higher orbit, leaving the surface of the planet.

Thus there are limits to how fast a planet with the mass of Earth can rotate,and how oblate it can become, before it starts to break up.

Of course a way around that is to increase the mass of the planet to give it a higher orbital velocity at different distances from the center, thus making the orbital velocity higher than the equatorial surface speed.

But there are limits to how much you can increase the mass of the planet, without increasing the surface gravity and having other bad effects.

For example, plate tectonics, and a magnetosphere generated by an liquid interal region of the world, are considered to be desirable for the habitability of a planet.

This article suggests that the mass of a world determines whether it can have those desirable features.

https://faculty.washington.edu/rkb9/publications/hb13.pdf

On page 20:

A minimum mass of an exomoon is required to drive a magnetic shield on a billion-year timescale (MsT0.1M4; Tachinami et al., 2011); to sustain a substantial, long-lived atmosphere (MsT0.12M4; Williams et al., 1997; Kaltenegger, 2000); and to drive tectonic activity (MsT0.23M4; Williams et al., 1997), which is necessary to maintain plate tectonics and to support the carbon-silicate cycle. Weak internal dynamos have been detected in Mercury and Ganymede (Gurnett et al., 1996; Kivelson et al., 1996), suggesting that satellite masses > 0.25M4 will be adequate for considerations of exomoon habitability. This lower limit, however, is not a fixed number. Further sources of energy—such as radiogenic and tidal heating, and the effect of a moon’s composition and structure—can alter the limit in either direction. An upper mass limit is given by the fact that increasing mass leads to high pressures in the planet’s interior, which will increase the mantle viscosity and depress heat transfer throughout the mantle as well as in the core. Above a critical mass, the dynamo is strongly suppressed and becomes too weak to generate a magnetic field or sustain plate tectonics. This maximum mass can be placed around 2M4 (Gaidos et al., 2010; Noack and Breuer, 2011; Stamenkovic´ et al., 2011). Summing up these conditions, we expect approximately Earth-mass moons to be habitable, and these objects could be detectable with the newly started Hunt for Exomoons with Kepler (HEK) project (Kipping et al., 2012).

This suggests that the upper mass of a habitable world should be about 2.0 Earth mass.

Their sources are:

Gaidos, E., Conrad, C.P., Manga, M., and Hernlund, J. (2010) Thermodynamics limits on magnetodynamos in rocky exoplanets. Astrophys J 718:596–609.

Noack, L. and Breuer, D. (2011) Plate tectonics on Earth-like planets [EPSC-DPS2011-890]. In EPSC-DPS Joint Meeting 2011, European Planetary Science Congress and Division for Planetary Sciences of the American Astronomical Society. Available online at http://meetings.copernicus.org/epsc-dps2011.

Stamenkovic´, V., Breuer, D., and Spohn, T. (2011) Thermal and transport properties of mantle rock at high pressure: applications to super-Earths. Icarus 216:572–596.

And a rocky planet with too much mass will have a high enough escape volocity to reain large amounts of helium or even hydrogen,thus becoming a gas giant. The tremendous pressures and temperatures at the cores of gas giants mean that they have no solid surfaces to stand on.

Part Three: Suggestions

One) Perhaps your planet has or had several small moons. A collison of moons might have turned them into lava, which is sort of a glass. The molten lave would gradually cool and solidify into amoon of pumice. Then another collison with another moon could have shattered the pumice moon into a ring. Particles of different masses might move to different distances from the planet, and collisons between particles might result either in them clumping together into larger pieces, or shattering into smaller pieces.

And perhaps tidal interactions with the planet, the star, other ring particles, and any surviving moons, might cause ring articles of the correct size to spiril in toward the planet and eventually fall onto the surface in showers.

It is speculated that falling ring material might have created the equatorial ridge on Iapetus, a moon of Saturn.

However, the creation of the ring would proabably have happened early in the history of theplanet, before it was capable of supporting advanced oxygen breathing lifeforms. And the ring system would proably only last for a few tens or hundreds of millions of years, so the planet would probably still not be habitable for advanced lifeforms by the time the ring system was gone.

Maybe the collisons and formation of the ring system happened unusually late in the history of the solar system, when the planet was already habitable. Or maybe it happened when the planet was young, but an advanced civilizaiton terraformed the planet to make it habitable billions of years before it otherwise would have become habitable.

Two) Cryovulcanism.

An example of cyrovulcanism is on Triton, the large moon of Neptune.

The Voyager 2 probe observed in 1989 a handful of geyser-like eruptions of nitrogen gas and entrained dust from beneath the surface of Triton in plumes up to 8 km high.[32][58] Triton is thus, along with Earth, Io, Europa and Enceladus, one of the few bodies in the Solar System on which active eruptions of some sort have been observed.[59] The best-observed examples are named Hili and Mahilani (after a Zulu water sprite and a Tongan sea spirit, respectively).[60]

All the geysers observed were located between 50° and 57°S, the part of Triton's surface close to the subsolar point. This indicates that solar heating, although very weak at Triton's great distance from the Sun, plays a crucial role. It is thought that the surface of Triton probably consists of a translucent layer of frozen nitrogen overlying a darker substrate, which creates a kind of "solid greenhouse effect". Solar radiation passes through the thin surface ice sheet, slowly heating and vaporizing subsurface nitrogen until enough gas pressure accumulates for it to erupt through the crust.[7][45] A temperature increase of just 4 K above the ambient surface temperature of 37 K could drive eruptions to the heights observed.[58] Although commonly termed "cryovolcanic", this nitrogen plume activity is distinct from Triton's larger scale cryovolcanic eruptions, as well as volcanic processes on other worlds, which are powered by internal heat. CO2 geysers on Mars are thought to erupt from its south polar cap each spring in the same way as Triton's geysers.[61]

https://en.wikipedia.org/wiki/Triton_(moon)#Cryovolcanism

Turbulence at Triton's surface creates a troposphere (a "weather region") rising to an altitude of 8 km. Streaks on Triton's surface left by geyser plumes suggest that the troposphere is driven by seasonal winds capable of moving material of over a micrometre in size.[45]

https://en.wikipedia.org/wiki/Triton_(moon)#Cryovolcanism

So the winds in Triton's ultra thin atmosphere are capable of moving tiny particles in the pluse of vapor from the geysers.

Suppose that your planet has a lot of icey materials and lots of mineral glass from volcanic eruptions, asteroid impacts, or whatever, mixed in with the ices. Internal heat and heat from the star may melt a lot of ices into liquids and evaporate them into gases. When gas pressure builds up too high, the gases will burst up through the ices and enter the atmosphere, carrying small particles of glass with them.

The glass dust might later rain down in various locations. if the glass is the same size as the dust that water drops form around, it might be the nucleus of actual rain drops which rain down. I don't know if the rainwater would have to be deglassified before it was safe to drink.

But how can there be cyrovulcanism on a planet that is as warm as Earth?

I personally, have observed patches of ice over liquid water just a few days ago. The dog I was walking sat down in one, cracking the ice so it could wallow in the mud.

There is lots of permanent ice on Earth,an d much more seasonalice. And during glacial periods there was more ice.

Possibly if the planet has long enough days, most of the ice in equatorial regions might sublimate into water vapor during the long days. During the long nights, most of the water vapor becomes liquid water and dew, and then ice forms over the liquid water. In the morning the sun warms up the trapped water mixed with glass dust udner the ice, and water vapor builds up until it erupts in gyesers, carrying much glass dust with it. The rain of water and glass dust might happen soon after sunset.

So those are my suggestions.

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Things will settle down after a certain period of time.

Let us suppose that it is happening as you stated. Then over a period of time, all the silicate rocks on the equator will vaporize and condense and rain out as glass in the low latitudes.

After that there will be no more silicate rocks on the equator and no danger of glass rain.

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  • $\begingroup$ What do you imagine would take their place? Silica is 59% of Earth's crust, and most other components have lower melting points. $\endgroup$ Feb 23 at 15:35
  • $\begingroup$ @Logan R. Kearsley Silica is 59% of earth's crust but not bound to equator. The process of glass making is happening on the equator only. In other places, the temperature is normal. When all the silica on the equator is gone, then only something that can survive 2230 C will remain. $\endgroup$
    – imtaar
    Feb 24 at 10:26
  • $\begingroup$ This is 100% right IMO. For this to continue "forever" you need a cycle, a way for the thing that evaporates/vaporizes (silica in your case) to go back to the place where it evaporates/vaporizes. Check the water cycle, see what happens after the rain, and imagine how that would happen to silica in your case. As you've described things, it won't happen naturally, and I hardly see a reason for people to do it artificially. So, eventually (probably MUCH earlier than your settlers arrive) things will stabilize with all the silica having fallen in the 30 degree zones. $\endgroup$
    – Blueriver
    Feb 24 at 11:45
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    $\begingroup$ @imtaar There is very little that could survive 2230C in solid form and be available in continent-building quantities. And cycles are certainly possible--both rivers of molten silica directly analogous to Earth's water cycle, and large-scale mantle currents with upwelling under the hottest parts of the surface driven by the weight of silicate rocks condensing elsewhere and pushing the mantle down. $\endgroup$ Feb 24 at 19:48
  • $\begingroup$ @LoganR.Kearsley If there's lots of dolomite or other magnesium containing minerals around, MgO and CaO will survive. Eutectic temp = 2400 (IIRC), endpoints much higher. $\endgroup$ 3 hours ago
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A different approach:

Volcanism.

We have something like this on Earth. Volcanic ash (depending on the chemistry of the particular volcano) can be raining glass - except that we have quite small particles.

What we need to make the particles bigger:

  1. The erupted material needs to fall down slower (less surface gravity)
  2. It has to cool down slower (less atmosphere)
  3. A small planet with active volcanism means that the planet is quite young and/or contains greater proportion (than Earth) of radioactive elements.
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It's been raining glass here on earth since the Jurassic. Just on the ocean floor which is 70% of the planets surface. Diatom plankton build their cell walls out of sillica. If your world has similar critters that store hydrogen inside themselves to float and live high up in the atmosphere after an aquatic phase building their adult shells. It would rain beads of glass often, even having clouds of the skyplankton coloring your sky or even glowing at night.

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    $\begingroup$ Diatoms aren't glassy; they're amorphous silica. In silica the distinction matters. $\endgroup$ 7 hours ago
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A world that rain glass would need to have clouds made of silicon and oxygen or other exotic composition. Anyway this planet cannot be habitable because either the lifeforms in such a planet would be severely hurt by the fall of solid glass pieces making the evolution of life pretty hard or life could not evolve at all as solids are made of molecules that have a fixed position and the chemical reactions needed for the creation of biomolecules like RNA and DNA need to take place inside a fluid which in the case of Earth is water.

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  • $\begingroup$ Quote: "Life does not need to have arisen naturally." $\endgroup$ Feb 24 at 20:32

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