Hephaestus is the only moon of Vulcan.
Hephaestus was once a frozen Se-Class moon, the change in temperature changed its class to a hot Ae-Class moon. It's tidally locked to its parent moon and orbits Vulcan every 20 hours and 14 minutes. The reason for its purple atmosphere is the purple dust from its surface which is scattered all over the world by winds.
The mean temperature of Hephaestus is 477.1 °C (890.8 °F) and had a gravity of 0.22 Gs. It has a diameter of 4302 km (1953.7 mi). Its new atmosphere has a pressure of 0.02. Its semimajor axis from Vulcan is 44547.3 km (27680.4 mi).
A handful of scientists and their families live in the large caverns beneath the surface of Hephaestus in order to study this world and how it is adapting to its new place in the universe.
Unlike its parent, Hephaestus has a very Earthlike day, meaning that the entire surface is bathed in heat and some radiation, keeping the surface constantly boiling.
The moon, like its parent has a strong magnetic field that deflects much of the incoming solar radiation.