Just how hot can a planetary surface get?

[EDG]

Venus' current surface temperature is around 740K (about 500K higher than a Blackbody would be at its orbital distance from the sun, due to its thick atmosphere). That's really hot, but not enough to actually melt the surface.

If you had a similar planet with a thick CO2 atmosphere at the right distance from its star, could you end up with a greenhouse effect that is so high that the rocks on the surface would melt? (you'd need a temperature of about 1400-1500K to melt Basalt)

I'm wondering if that would be a stable situation, or if there's some process by which the atmosphere would evaporate away (thus reducing the greenhouse effect, thus reducing the temperature) or some other process would occur to make it unstable, or stop that from happening at all?

[cran]

if the mass of the planet is sufficient, then you're looking at the dividing line between a planet and a brown dwarf ...

[EDG]

if the mass of the planet is sufficient, then you're looking at the dividing line between a planet and a brown dwarf ...

Not really. I'm talking about normal terrestrial planets here, not gas giants or brown dwarfs.

Basically, take a Venus-like planet and heat it up. If the surface melts, can the atmosphere remain?

[Fiery Phoenix]

Not really. I'm talking about normal terrestrial planets here, not gas giants or brown dwarfs.

Basically, take a Venus-like planet and heat it up. If the surface melts, can the atmosphere remain?

I don't think so.

To answer your original question, I'm guessing it, at least to some extent, depends on the spectral type of the host star. If you have an O-type main sequence star (i.e. a blue giant), or a B-type main sequence star (i.e. a white giant), and a terrestrial planet located at anything from 1 to 100 AU, then the planet would experience abnormal temperatures and perhaps even atmospheric loss. And no, such a scenario won't be exactly "stable". Especially at really close distances from the star.

[cran]

Not really. I'm talking about normal terrestrial planets here, not gas giants or brown dwarfs.

Basically, take a Venus-like planet and heat it up. If the surface melts, can the atmosphere remain?

Sure ... the chemistry would tend to shift towards more massive components, eg a higher sodium and lead content, and loss of the lighter volatiles ...

how long the atmosphere would last overall would depend on its initial pressure and its interactions with the stellar wind ...

[Messier Tidy Upper]

... Basically, take a Venus-like planet and heat it up. If the surface melts, can the atmosphere remain?

If I recall right, isn't there a scientific theory that Venus undergoes periodic "resurfacing" with a planet-wide melting episode where this happens?

As I gather it, the thick crust of Venus traps heat in until it "boils over" the planetary surface melts & then reforms once the heat has been let out only for the cycle to then start building up again? I think that's one reason why the Cytherian surface is so young right?

In all these episodes, Venus seems to have retained its atmosphere so yes, I'd say it can happen - at an educated guess.

PS. Ben Bova described this in his eponymous novel. See : http://en.wikipedia.org/wiki/Venus_(novel) or, better yet, read the book if you haven't already!

[EDG]

OK, assume the solar wind isn't an issue, I'm just thinking of surface-atmosphere interactions here.

I guess you'd probably need a really thick CO2 atmosphere (much thicker than Venus') which also means the surface pressure is higher which I guess would raise the melting point of rock (presumably basalt)? Would it raise it enough to offset the greenhouse effect, so you might have superheated solid rock on the surface instead of molten rock? And if it does get high enough to melt large tracts of surface, then I guess that would add more gas to the atmosphere as it's liberated from the melt...

[chornedsnorkack]

What about steam atmosphere?

Water is actually better greenhouse gas than carbon dioxide. And there is potentially lots of it. Earth with oceans boiled away would have 300 atm, compared to 100 atm of carbon dioxide on Venus. A planet with steam atmosphere could heat over 1000 Celsius before the blackbody radiation moves into near IR and red, where water is more transparent...

[Messier Tidy Upper]

COROT 7b ( http://en.wikipedia.org/wiki/COROT-7b ) would have to be the hottest record holder as a 2 x earth mass exoplanet. So far. I think.

It probably depends on the type of planet and its specific composition and the type of atmosphere we're talking about but yes, I see no reason why super-hot super close in "Hot Earths" / "SuperMercurys" can't exist.

They wouldn't be nice places to visit and you sure couldn't live there though!

Hmm .. Mustafar springs to mind here .. but hotter! All lava "rocky" ("liquidy /lava-y?") planets, well, why not? I think its likely they do exist.

[cran]

OK, assume the solar wind isn't an issue, I'm just thinking of surface-atmosphere interactions here.

I guess you'd probably need a really thick CO2 atmosphere (much thicker than Venus') which also means the surface pressure is higher which I guess would raise the melting point of rock (presumably basalt)? Would it raise it enough to offset the greenhouse effect, so you might have superheated solid rock on the surface instead of molten rock? And if it does get high enough to melt large tracts of surface, then I guess that would add more gas to the atmosphere as it's liberated from the melt...

Yes, pretty much ... not necessarily limited to CO2, though ...

keep in mind that a "rock" is a collection of minerals;
the minerals themselves have different physical and chemical properties,
including melt temperatures ...

using something like Bowen's Reaction Series as a guide,
you'd see that your surface could easily be hot solid rocks
and flowing partial melts shaping the landscape
in the same way that liquid water does on Earth ...

I don't recall offhand what the lower mass limit is, but a "super-Earth"
without surface liquid water may still undergo plate tectonic recycling,
and that would certainly release more of whatever passes for volatiles into the atmosphere ...

[neilzero]

A planet with a large dense core could have 2 g gravity at the top of the atmosphere, but about Earth radius. It would likely loose about 1/2 of the carbon dioxide in a billion years at a surface temperature of 1500 k. Lighter volatiles would be lost sooner. I'm assuming somewhat less solar wind than Earth experiences = That may be rare among hot planets everywhere.
At 3000k, about 90% of the mass would be vapor, gas or plasma and the lighter volatiles would be lost in a few million years, but some solids would remain for perhaps a trillion years if the top of the atmosphere was still above one g. If the Sun is getting hotter, the surface temperature could be close to constant even though the atmosphere is losing mass. Does a hotter sun have to mean more intense solar wind? I'm mostly guessing so tell me if that is improbable. Neil