How far from melting are the rocks of Venus & Mercury?

[Messier Tidy Upper]

How far from being totally molten are the surfaces of Venus and Mercury?

Their surfaces are already so hot that

"A tin can placed on the sunlit side of Mercury or on the surface of Venus would melt.*"
- Page 12, 'Amazing Facts about Australia's Southern Skies', Steve Parish, Steve Parish Publishing Pty, no year listed.

As the Sun gradually becomes hotter :

Our Sun’s brightness is gradually increasing by about 10 % every billion years.

- “The Planets” final episode - 'Destiny' , BBC TV, screened circa 1995-2005. (?)

Surely there will be a point at which both planets surfaces melt & beciome fluid lava rather than solid rock right?

When will this be and how hot will they get before these planets perish?

* On Venus the can would also be instantly crushed by the immense atmospheric pressure & corroded by the sulphuric acid clouds on the way down too! - StevoR

[snifferdog]

Theyre a long way from melting.You could double the temperatures on either planet before the rock starts melting.
For example, the rock types on earth melt between 800 and 1600 degrees celsius.

[neilzero]

In about 4 billion years the surface of Mercury will be about 1400 c, so most of the rocks and other surface minerals will liquid. Some will however be solid or in between solid and melted. At 4000 c most minerals boil, so Mercury will have a thick atmosphere of metal vapor, gas and ions, but the solar wind likely won't be strong enough to carry away more than 2% of the mass of Mercury per million years, until Mercury's surface is hotter than 4000 c. Much the same for Venus, perhaps not much later. Neil

[Jens]

Much the same for Venus, perhaps not much later. Neil

I wonder what will happen to Venus. I would guess it's trickier, because if the current atmosphere gets lost, then the planet's surface could temporarily become lower before rising again. As the sun heats up, will it weaken Venus's atmosphere?

[Messier Tidy Upper]

@ Jens, neilzero & snifferdog :

Thanks.