Argo

Argo presents as smouldering grey, with vast lava plains broken by active shield volcanoes, with two moons in stable co-orbital configurations. Surface temperatures range from 400 to 1,100°C at 6 atm, with explosive caldera events reshaping entire regions. At 2,826 km across, the surface material is primarily fresh basalt overlaying a partially molten upper mantle. A world still releasing its formation heat at this rate is young by planetary standards, or unusually rich in radioactive interior elements — possibly both.