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It doesn't necessarily need to be breathable by humans. But it should be room temperature (at which mercury is liquid). My concern is whether mercury oceans could remain stable or would they all get locked up in metallic compounds and precipitate?

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    $\begingroup$ Physics, as with most of the questions here. $\endgroup$ – Ian Kemp Apr 13 at 17:53
  • $\begingroup$ Is it a naturally occuring planet, or can it be artificially made? $\endgroup$ – Nosajimiki Apr 13 at 19:59
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    $\begingroup$ Your lack of a phone number for a bulk mercury wholesaler? $\endgroup$ – Pelinore Apr 15 at 6:27
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    $\begingroup$ I don't know if it's possible, but if it is... I want to be the first one to dump a big ol' tablet of Aluminium into that ocean it and watch what happens :P $\endgroup$ – Corey Apr 15 at 23:12
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    $\begingroup$ relevant maybe? what-if.xkcd.com/50 $\endgroup$ – coblr Apr 16 at 0:55
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You would have many problems: first of all, the abundance of mercury it's not that high to realistically be able to fill in oceans. In the solar system it is one of the rarest element, even less abundant than platinum.

Moreover, even assuming that you had that much mercury, you would stumble into another, serious issue: the density of mercury is 13.5 $g/cm^3$, while the density of basalt, the main component of oceanic bottom, is only 3 $g/cm^3$. This would mean that an ocean of mercury would quickly and dramatically sink below the crust.

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    $\begingroup$ That would make for some really interesting tectonics! $\endgroup$ – The Square-Cube Law Apr 13 at 13:52
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    $\begingroup$ If there were a solar system where mercury were in enough abundance for oceans, we'd likely see other peculiarities chief among them the elemental composition of the planet in general. There's no reason to suppose an iron/silicate basis at all, is there? Do any other elements/substances have the necessary density for the oceans to remain on the surface? What sort of chemistry is needed to avoid liquid mercury being bound up in all the various compounds that we find it in on our planet? $\endgroup$ – John O Apr 13 at 16:22
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    $\begingroup$ Maybe the basalt bits would form relatively thin layers floating atop a solid ball of liquid mercury? The tidal waves might cause some ... fun dynamics ... in the floaty bits, but why not. $\endgroup$ – John Dvorak Apr 13 at 17:26
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    $\begingroup$ How are you considering the density to cause the ocean to sink? The crust is solid, not liquid, so it seems it'd need to have enough weight on top to break it. But what if the "ocean" is very shallow, say only 1 m deep? $\endgroup$ – The_Sympathizer Apr 13 at 21:08
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    $\begingroup$ @The_Sympathizer - over geologic time, the crust can be considered as a liquid. $\endgroup$ – jdunlop Apr 13 at 22:05
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You would want the planet's crust to be dense enough to prevent the mercury from going underground. However, denser materials are typically metallic, and would form amalgams with the mercury. However, there is a solution: Make the planet an giant ball of mercury large enough to have its own gravity, and possibly with islands floating on the surface.

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    $\begingroup$ I like this concept. Given the density of mercury, almost anything you dump on it will float. Of interest, it looks like floating solid iron islands would be possible as iron is a common material for mercury containment. As the density is more than double Earth, the planet would need to be smaller to make up for it if a 1g-ish planet is needed. $\endgroup$ – UrQuan3 Apr 13 at 19:47
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    $\begingroup$ Something you might need to address is the planet internals. There would be a lot of pressure at the "core," so it'd be worthwhile to find out what mercury does at high pressures. Also... hello fellow Dennis! $\endgroup$ – gregsdennis Apr 13 at 20:05
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    $\begingroup$ @gregsdennis - I had the same thought, so I had a quick search for the compressibility of mercury. This result seems to say that mercury will compress by only 3.8ppm for each additional atmosphere of pressure (water, which is practically incompressible for everyday purposes, compresses by 46.4ppm). I wouldn't know where to even begin to speculate on how much pressure mercury would have to be under before anything interesting actually happened - one for xkcd what-if, if only that were still being updated. Now I really want to know. $\endgroup$ – Spratty Apr 14 at 11:37
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    $\begingroup$ @gregsdennis: Eyeballing the phase diagram of mercury in Young 1975 it looks like mercury turns solid at room temperature at a pressure of slightly over 1 GPa. Assuming that the liquid mercury sea is convective (meaning that it's at roughly the same temperature at all depths), that the planet's surface gravity is the same as Earth's and that the compressibility of mercury is negligible (as noted by Spratty above), plugging the numbers into the liquid pressure formula suggests that the mercury should turn solid at a depth of only around 10 km or so. $\endgroup$ – Ilmari Karonen Apr 14 at 19:45
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    $\begingroup$ … So basically you could have a decently deep sea of liquid mercury surrounding a planet of solid mercury at high pressure, but not a completely liquid planet throughout. What goes on deeper down at the core is anyone's guess, of course — if the temperature goes up fast enough, there might be a second liquid layer down there. Maybe even several, if you're really lucky with how the temperature and pressure gradients work out. (And of course I've completely neglected the fact that mercury loves to dissolve and form amalgams with other elements, which could further complicate things.) $\endgroup$ – Ilmari Karonen Apr 14 at 19:49
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If your goal is just to have a planet with liquid metal oceans, not specifically mercury, you could go with gallium. Its melting point is just ~30°C (~85°F), so a little warm but still perfectly survivable and even comfortable for humans. And it's much more common than mercury. Not sure about the feasibility of covering an entire planet in gallium seas, but smaller bodies of liquid gallium (ponds and lakes) might be possible.

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    $\begingroup$ Gallium (5.91 g/cm3) is denser than basalt, so it would also sink beneath the crust. $\endgroup$ – Ross Presser Apr 13 at 14:43
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    $\begingroup$ @RossPresser Still a lot less dense than mercury. You'd have to take some liberties with the composition of the rest of the planet, as per future99's answer, but somewhat less so with gallium than with mercury. $\endgroup$ – Darrel Hoffman Apr 13 at 14:47
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    $\begingroup$ @RossPresser Easy, just fill the core with mercury! $\endgroup$ – Drake P Apr 13 at 22:11
  • $\begingroup$ Since it doesn't have to be habitable, note that there are planets where it might rain iron: earthsky.org/space/wasp-76b-exoplanet-iron-rain-espresso So oceans of liquid iron seem feasible. $\endgroup$ – jamesqf Apr 14 at 3:10
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    $\begingroup$ @jamesqf Liquid iron (or almost any other metal besides mercury and gallium) would not just be hot, it'd be so hot that it glows, so you're not getting that cool metallic ocean aesthetic which I think the OP was looking for, you're getting more of a hellish molten cauldron of fiery death. I know they said it doesn't need a breathable atmosphere, but if it's so hot you can't even visit with a spacesuit on, it's probably not going to be much use in any story you might be writing. $\endgroup$ – Darrel Hoffman Apr 14 at 13:15
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Any life as we know it can't exist on the planet. Nor volcanoes. You can't have a sulfur cycle

In addition to L Dutch points, an ocean of mercury can not exist on any planet with a sulfur cycle. Any biological processes that produce hydrogen sulfide (sewer gas) or natural volcanic systems (sulfur dioxide) putting trace amounts of sulfur in the air will cause issues for your mercury sea.

Mercury and sulfur interact and form mercury sulfide, a solid that will tarnish your mercury sea like rust, forming a thin crust over the top, like a very thin ice.

This interaction is so rapid that you can buy mercury cleanup kits consisting of sulfur blocks that absorb the mercury.

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    $\begingroup$ Mercury-Sulfur reactions could be an interesting storm cycle though. $\endgroup$ – Kyle J V Apr 13 at 18:55
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    $\begingroup$ Sounds a bit like the Mercury version of the great oxygenation event en.wikipedia.org/wiki/Great_Oxidation_Event $\endgroup$ – Gimelist Apr 14 at 1:23
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    $\begingroup$ Thin crust atop world oceans... now that is some interesting geography ... and rather disturbing to imagine for me. Surface waves (if you have atmosphere) would break up the sulfide crust though, so it would be ... even more disturbing. $\endgroup$ – John Dvorak Apr 16 at 11:26
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    $\begingroup$ I am thinking a crust or islands of granitic rocks, cinnabar minerals, or possibly sulphur are chemically inevitable. Good landing sites for heroic spaceships. The majority of the chemistry isn't organic, so sulphur should play a minor role. $\endgroup$ – DAVID RICKS Apr 17 at 18:57
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In addition to the other considerations given here, you also have the problem of oxygen. It's very abundant and loves to make compounds with just about everything. All that liquid mercury would quickly (on a geological timescale, that is) become solid mercury oxide and mercury salts of oxyacids. If you can make it work, though, maybe you have mercury oxide mixed in with the sand on the beaches, giving it a pink or light orange color. That's optional because this is all pretty handwavy anyway.

Maybe some kind of weird plant life that eats mercury oxide and poops mercury could give you those seas. But then, like other answers say, you run into the problems of abundance and density.

You might prefer a sodium-potassium mixture. NaK is a liquid at room temperature, and both elements are far more abundant than mercury. Also it licks the density problem. You still need something to crack them from the oxides though. Oxygen is everywhere and it's thirsty.

Oh there's another problem: water. Water is also very abundant. In earthlike conditions it will be liquid and also make seas, which will float on top of the mercury. The NaK is better in that way because it's less dense than water, but it reacts like mad with water. In either case, you need to make sure there isn't much free liquid water on the surface of your planet. You can achieve that with conditions that crack the hydrogen from the oxygen, allowing the hydrogen to dissipate to space via Jeans escape, or by making the surface a bit cold. (There are probably other ways too.)

You can crack water with lightning or with my favorite method: more xenobiological handwaving.

The cold-planet method works because NaK is still liquid down to -12 C or so (at 1 atm of pressure). If conditions are right, you can have a water permafrost almost everywhere without it getting so cold as to freeze the NaK. The water ice will still react with the NaK, just more slowly.

However, down at the bottom of the NaK seas, the high pressure will make the NaK solid and the water underneath it liquid (unless it's really cold down there somehow, which I think is pretty much impossible). That complicates things. But I think it's safe enough to say a layer of NaK hydroxides will form, precluding any further reaction.

Or it's possible there was never water under the NaK, or not enough to matter. All the water is at the surface. Maybe your planet is tidally locked around a mediocre star, like an M0. The NaK seas are near high noon and the volatile water there has evaporated and snowed down near the terminator and on the dark side. NaK will still react with the rock underneath, but again I think a non-reactive intermediate layer saves you. Unless you have enough activity to bring magma to the surface. Then I don't think there's anything you can do.

Ah so this got pretty long, heh. Hope it helps.

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It might work if the rest of the planet is made of gold.

But if the planet is made of lighter materials than mercury, the ocean will slowly but surely seep through towards the planet's core. Earth's oceans are only stable because the ocean bottom is heavier than water, and even so, lots of water seeps through the Earth's crust and the upper mantle contains lots of water.

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    $\begingroup$ mercury is known for dissolving gold and forming an amalgam $\endgroup$ – L.Dutch - Reinstate Monica Apr 13 at 13:40
  • $\begingroup$ Welcome future99, we invite you to take our wonderful tour and refer to the help center when you have spare time to understand our quirks. Enjoy our site. $\endgroup$ – A Rogue Ant. Apr 13 at 13:55
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    $\begingroup$ If you want insoluble dense materials, iron is probably your best bet. Iron is unsual in that it doesn't form an amalgam with mercury (so much so that your options for mercury bottles are plastic, glass and iron). It's only half the density though, so you'll get iron islands floating in your ocean. $\endgroup$ – AI0867 Apr 16 at 13:16
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Broken down alien liquid metal telescope

I think your main hurdle is a physically plausible explanation of how get so much mercury in one place. Despite its rarity in Earth's crust (0.05 mg/kg), mercury collects at relatively high concentrations of 25000 mg/kg in its ores. This is probably due to its unusual chemistry, which is mimicked by no other metal. However, I'm not sure I can come up with a good "natural" explanation how you get enough for an ocean and what could form the ocean floor on geological timescales (tungsten doesn't form amalgams and is denser than mercury, so it's a possible floor material).

The best idea I can come up with is that it was part of an alien megastructure. A liquid mirror telescope could be made by filling a crater with mercury. Or maybe it's a light concentrator for long distance communication. Or maybe it is some alien technology that we don't really understand, but large amounts of mercury were necessary. Anyway, the aliens left and the megastructure decayed (the walls of the crater collapsed or the containment vessel failed due to mercury embrittlement), flooding the planet with mercury.

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    $\begingroup$ Not everything has to be aliens. Heavier elements form from dynamic stellar events like super novas and neutron star collisions. In the core of the galaxy, there could be intense activity leading to the formation of vast quantities of heavy elements. Then, the gravitational interactions could kick this enriched body out of the kiloparsec ring surrounding the galactic core, where it would migrate to more stable regions of the galaxy. It could be called Hermes, pun intended. hahah $\endgroup$ – DAVID RICKS Apr 17 at 5:14
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I think the nearly irreconcilable problem you have constructing a mercury ocean world is explaining the process by which so much of that particular liquid element became concentrated on the world at the exclusion of others more likely to be present at similar temperatures. For example, based on the relative abundance of elements in the universe you would be about 10 times more likely to have oceans of rubidium and 100 times more likely to have oceans of Gallium. That's not even considering why liquid elements would be present at the exclusion of much more likely liquid compounds.

I say nearly irreconcilable because it would depend on the age of the universe of your world. As the universe ages heavier elements such as mercury will become more common relative to the other lower atomic mass elements due to stellar nucleosynthesis. As the universe approaches heat death, planets orbiting active stars (as I'm assuming yours does) may be composed mostly of elements heavier than iron, which would reduce the possibilities for liquid compounds. It seems this would make liquid elements the increasingly likely constituent of any oceans and so make mercury oceans more plausible.

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  • $\begingroup$ Nature finds a way. One way would be neutron star collisions. In a place packed with high-mass objects, like the galactic core, it might be possible for super novas and colliding stellar objects to fuse massive concentratios of heavy metals, this could form an enriched body. This body could become a dense, toxic planet. Upon which there could be intermittent seas of mercury, because of its large relative mass, mercury would sink beneath granite and basalt. It might rinse the surface due to tidal fluxes and volcanism. $\endgroup$ – DAVID RICKS Apr 17 at 5:22

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