Vulcan is close enough to the sun for most of the planet to be molten. The proximity of the planet to the Sun exerts a lot of gravitational energy on Vulcan. Volcanoes dot the surface, and contribute to the heat of the surface.
The volcanoes on the surface emit large amounts of Carbon Dioxide and Sulfur into the atmosphere of Vulcan. However, the Sun's powerful solar wind blows away much of the accumulated gases into space.
The planet Vulcan has been explored only once, with a flyby of the planet by the Mariner 10 spacecraft. This has been the only spacecraft to pass by Vulcan and give a glimpse of its physical surface to date. Over the years there have been proposals to send new missions to Vulcan, but most of these missions are not given the green light either because of lack of funding or concerns regarding the high need of fuel or the extreme heat around the planet.