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(Just to clear up all confusion. I am talking about Mercury the planet and not the element.)

The premise of the question is based on a group of people who are driving across the surface of Mercury at the hottest part of the day and during its closest orbit to the sun. They would either break through a thin crust and fall into the lake or they would just fall in somehow. I was wondering how grounded this was.

The Nasa website says that during the day, temperatures on Mercury's surface can reach 800 degrees Fahrenheit, which is at a suitable temperature to melt metals like lead and tin. I was wondering if there was enough of those elements in close enough proximity to one another to form something in close enough size to a body of water. For example, could those metals flow down a hill and coalesce in the low-lying areas?

If there isn't then I was curious about how a vacuum would affect the melting point of other metals like magnesium, aluminium and silicon or other metals that are abundant on Mercury's surface. Would it lower their melting point enough to melt at 800 degrees Fahrenheit?

Any help would be greatly appreciated.

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    $\begingroup$ The people walking on a lake of liquid metal would have to be very dense in order to sink in it. Liquid or not, as long as the heat of it doesn't burn them, they would not fall in it, they would at most fall on it. $\endgroup$
    – kaya3
    Apr 21 '21 at 21:08
  • $\begingroup$ Does this answer your question? Could lakes of mercury exist on an Earth-like planet? $\endgroup$ Apr 21 '21 at 21:37
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No, but don't let that stop you

Ever since the release of the topographical maps of Mercury in 2016 (see also this), we've learned a lot about the surface of Mercury. There's not a lot of metal on the surface. 1. What analysis we've performed doesn't seem to indicate the presence of liquid metal lakes.

But that doesn't mean you can't have them. Let's investigate!

Mercury has more iron in its core (by volume) than Earth. 2. That means that your premise is plausible. What we need is some volcanism to push some of that iron-rich core to the surface.

Turns out we know a bit about volcanism on Mercury.3 I honest don't worry about the specifics other than it happened a long time ago. That's good! It means there's been time for liquid iron to be pushed to the surface and then be covered by a layer of dirt to form a crust.

Does this require suspension of disbelief? You bet. Millions of years of dirt blowing over a liquid anything would basically mean you have to mine to get it, not fall through the crust. Worse, it might be liquid on the day side, but it's not liquid on the night side. Mercury's days are about 58.5 Earth days long. That's plenty of time to melt the metal (assuming it's reasonably exposed and not insulated) during the day — but it's also enough time to reasonably solidify it over night.

So, while reality tells us that there are no liquid metal lakes on Mercury, I believe there's enough circumstantial evidence to justify the idea from a suspension-of-disbelief point of view.

As for your second question (please remember in the future that you're expected to ask only one question per post), pressure variation has little affect on melting points, but there are some ideas to work with:

  1. The pressure does have a very small effect on the solid/liquid equilibrium. For water, going from atmospheric pressure to near a vacuum raises the melting temperature by about 0.01 K. For most other substances (where the solid is more dense than the liquid) the effect is in the reverse direction (so that vacuum will lower the melting point), but the effect is still small, at least for the small pressure difference between atmospheric and vacuum. For the explanation of this effect, look in a physical chemistry textbook under "Clapeyron equation."<
  1. For some substances (especially metals), air can react with the solid and form a compound that will have a different melting point than the pure solid. This could affect the melting point in either direction, and doing it in a vacuum would get rid of this effect. Some high-temperature work with metals is done in a vacuum for this reason.
  1. Solids can also "boil" since they have a vapor pressure (this is called sublimation). So, for example, if you make vacuum above some ice, it will eventually sublime and go away (like snow can evaporate on a dry sunny day even if the temperature is below freezing). But this is not melting.
  1. For some compounds, if you heat them they will decompose before they melt. If you are under vacuum, the products of the decomposition will get carried away faster, so the substance will disappear faster. But this is not true melting either.
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It is too hot there for mercury lakes.

Day temperatures of Mercury get to 430C.

Boiling point of metallic mercury is 356C.

Mercury gas would go up in the atmosphere and get blasted away by the solar wind, like the rest of Mercury's atmosphere.

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