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Suppose human explorers came across a terrestrial planet hospitable for life, but with more than 70% of its surface covered by gallium.

When temperatures rises in the day it will form an ocean of liquid metal so how do they cross it?

I am looking at interstellar tech level and also submarine will likely be destroyed when temperature drops in the night.

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    $\begingroup$ isnt gallium turn to liquid by mere human temperature? also maybe use wood? since L.Dutch say gallium has bad side effect to metal, i dont know what side effect it give though, outside of leather ship or boat like kayak, i think it wont give bad side effect at least. $\endgroup$ – Li Jun Jul 7 at 10:36
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    $\begingroup$ I'm dubious such an ocean would exist. A terrestrial planet capable of supporting life really requires water - lots of it. Even if you could find a way to justify the existence of all that Gallium, the normal water-vapor cycle on such a planet would mean the Gallium ocean would be underwater ! Also hard to see how life could develop on such a planet (if that's an issue) as Earth's ocean was so important in developing early complex life. $\endgroup$ – StephenG Jul 7 at 11:11
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    $\begingroup$ Relevant XKCD $\endgroup$ – Frostfyre Jul 7 at 13:30
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    $\begingroup$ With a boat. They just have to stop during the night. $\endgroup$ – jamesqf Jul 7 at 16:38
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    $\begingroup$ Can you imagine the types of tsunamis that would be triggered by Earthquakes on such a planet? It'd be super calm the rest of the time, but even a fast traveling wave a fraction of a human's height (due to the weight) would wipe out so much. $\endgroup$ – DKNguyen Jul 7 at 22:43
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When temperatures rises in the day it will form an ocean of liquid metal so how do they cross it?

They need to pay attention

Liquid gallium is denser than solid gallium,

  • Density (near r.t.) 5.91 $g/cm^3$
  • when liquid (at m.p.) 6.095 $g/cm^3$

so liquid gallium will sink under a crust of solid gallium. This means that until the whole gallium is liquid, it will be more or less like being in the polar waters, surrounded by metallic icebergs (gallsbergs?).

Once the whole body is liquid, the high density will make it easier for something to float on it, but will also give more drag.

Probably the most apt solution is to have some small submerged winglets which give portance and lift the ship above the surface of the liquid, to drastically reduce drag. Even better would be an hovercraft.

Due to the high density operating a submergible will be quite challenging. To have it sink under the surface its average density will have to be higher than the one of gallium: expect a very cramped space.

Add to this that

Gallium attacks most other metals by diffusing into the metal lattice. For example, it diffuses into the grain boundaries of aluminium-zinc alloys and steel, making them very brittle.

and you see that it will be challenging: you probably need to have at least some polymer coated vessel to prevent gallium attack, or use a metal which is not affected by gallium diffusion.

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    $\begingroup$ Doing a quick google search seems to suggest stainless steel is nearly unaffected by constant exposure to gallium (on mobile, so cant post link but here's the DOI: 10.1023/A:1004359410957) $\endgroup$ – Anonymous Anonymous Jul 7 at 10:43
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    $\begingroup$ At last an excuse for giant hovercraft. $\endgroup$ – StephenG Jul 7 at 11:03
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    $\begingroup$ Perhaps they could use an ekranoplan? $\endgroup$ – elemtilas Jul 7 at 11:39
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    $\begingroup$ @elemtilas Now we're cooking with gaski. :-) $\endgroup$ – StephenG Jul 7 at 12:14
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    $\begingroup$ @StephenG - Ekranoplans are pretty much the answer to any question involving getting from here to there over water. ;) $\endgroup$ – elemtilas Jul 7 at 13:42
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Temperatures may change enough between day and night to melt the surface of a gallium "sea" but unlike water, liquid gallium is opaque, so the stellar radiation can't penetrate and even slightly warm the liquid below the surface.

Therefore, if the sea is to become liquid at all, there needs to be enough heat flux from below (from inside the planet, that is) to keep most of the sea melted, so freezing only takes place at the surface (mainly due to radiative cooling at night). The result, given that solid gallium floats in the liquid much like ice in water, is that you'll get a skim of "ice" that slowly thickens the longer the surface stays cold.

During local winter, this "skim" might get thick enough to support substantial loads, as does ice on lakes or seas in subArctic and Arctic climates on Earth. In local summer, the gallium might remain liquid day and night, or barely skim over.

As noted above, most metals are bad choices for gallium sailing -- however, composites are not affected this way. Wood, composite or inflatable hulls might be a very viable choice, with sail power (engines always seem to want to put metal parts in the "water"). The hull(s) could be very small relative to the load, compared to what water-borne sailors are used to, leading to the likelihood of tricycle layouts with small floats supporting a load-bearing framework -- more like ice boats than Earth ships.

The brilliant part of this is that it would be almost impossible for a "man overboard" to drown -- hypothermia could be an issue, over time; despite being as temperate as a tropical Earth sea just to remain liquid, the gallium will conduct body heat away far faster than even sea water, but a person would float far higher than is the case even in hypersaline water like the Dead Sea (though not anything like as high as in old pictures of huge mercury pools). Standard overboard survival drill would be to lie still, on your back, with arms and legs spread for stability, and wait for rescue. Swimming would be more like crawling in very soupy mud than what we're used to.

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  • $\begingroup$ I am not sure falling overboard on a metal sea would be the same as doing it in water, though $\endgroup$ – L.Dutch - Reinstate Monica Jul 8 at 13:25
  • $\begingroup$ @L.Dutch-ReinstateMonica Absolutely not the same, no. At 5+ times the density, even from deck height you'd hit harder than in water (though from 50 meters there'd be little difference). And after rescue, you'd have to melt the gallium out of your clothes, shoes, and hair/beard (those parts are typically cooler than body temp, so will tend to "ice" up). Sail vessels, though, tend not to be as tall as even moderately sized powered ships, so the fall likely wouldn't injure, any more than falling off a tanker's deck into the sea (5m or so) will cause an impact injury. $\endgroup$ – Zeiss Ikon Jul 8 at 15:54
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Any flat boat type will do. Propelling it might prove tricky, but people solved that satisfactorily thousands of years ago by using sails. You might also use propellers like a hovercraft does, but I wouldn't bother with actually hovering the craft. That would just be a waste of energy.

If you worry about drag, that's not a problem once your craft gets fast enough. It'll just rise out of the gallium and glide across its surface. Just like speed boats do on water. But at much lower speeds. I'd wager that even rowing boats would be built to rise out of the gallium. You will also get much less waves than you would get with water, so gliding is even more fun.

What you absolutely must worry about is the fact that that gallium attacks other metals. You must build your ship in such a way that no gallium can ever get into contact with vulnerable metal. Even in the case of accidents. It would just be too sad if your boat scratches a stone, and subsequently falls apart while it's on the high seas! As such, a coating is not enough. I would strongly advocate for building the entire supporting structure of the boat out of inert materials. If you need to resort to wood, so be it. But a 100% plastic boat would be more likely, imho.

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