|MadSci Network: Astronomy|
Well, it's a very big universe and science fiction is not limited by what we know today, but..... ...it is *really* hard for this Mad Scientist to come up with anything that could even come close to providing any justificqation for such an incredible concentration of that element. Mercury is probably made by two methods. The first is slow neutron capture. This can occur in a fair number of heavy stars and can make elements up to about the atomic mass of bismuth. The second method is creation in supernovae; there is some argument as to exactly how this occurs, but the books I read seem to agree that the heaviest elements are made in these stellar catastrophes. However, in both cases, mercury will be only a tiny fraction of the total metals produced. Moreover, metallic mercury is only produced in reducing (no free oxygen) conditions. So your planet must have a reducing atmosphere and thus is likely to be a pretty large one, probably a gas giant with a rocky core. It is possible that it could be Earth sized, but somewhat unlikely because we're more or less saying the concentration of heavy metals is much higher in this particular planetary system and thus it seems reasonable (this is guesswork on my part) that the protoplanets are likely to be exceptionally heavy as well, thus more able to grab hydrogen and helium. Worse, I just don't see how that much mercury (which is quite dense) will stay above the main bulk of the planet, which is almost certainly going to be iron/nickel/cobalt and some lighter elements. The mercury, even if it were there, is very likely to sink down into the center of the accreting planet and thus be swallowed into its core. (In some vague sense, this comes close to your metallic ocean, that is, the Earth has a liquid outer core...which can be considered an 'ocean' of molten metal overlain with the mantle of silicates. I suppose one could posit a catastrophe that stripped a protoplanet of its silicates, leaving a naked core that might have, if not oceans, at least large lakes of molten metal at its surface...but that's really stretching things). I'd like to recommend a book to you; "From Stone to Star" is a good introduction to the formation of the Earth from the original stellar nebula that collapsed to form the solar system. It will give you a pretty good background against which to test your ideas for your imaginary planet.
Try the links in the MadSci Library for more information on Astronomy.