Diameter: 0.57 Earth (7263 km, slightly larger than Mars)
Volume: 0.78 Earth
Mass: 0.85 Earth
Density: 1.09 Earth
Gravity: 0.92 Earth
Cassiopeia is a terrestrial ball of rock - geothermally dormant and tectonically stable, though this has not always been so in the moon's history. It circles the gas giant Hestia, which lies within Helios Delta's habitable zone and shares an orbit with Aerilon and Canceron. The moon has a noble gas atmosphere consisting of helium, argon, neon and oxygen, though the majority of this atmosphere has been stripped away leaving the moon with a mesosphere that exists at mean surface level. Surface pressure is 0.13 atm, and oxygen partial pressure is 12% making survival without a pressure suit marginally possible. There is no appreciable water content. At lower elevations, and within the mines, pressure increases and existence without a suit becomes more viable.
The gas giant Hestia heats the moon above normal temperatures for a planet in the habitable zone. The thin atmosphere allows an extreme temperature range, with the moon reaching 150 degrees in the day and dropping to nearly freezing at night.
Atmosphere: 68% argon, 14% helium, 12% oxygen, 5% neon, 1% trace gasses
Mean surface pressure: 0.13 atmospheres (equal to about 45,000' elevation on Earth)
Mean surface temp: 150 degrees F (66 degrees C) to 40 degrees F (4 degrees C)
Rotation period: 25 hours
Revolution period: 8 days (local)
The Cassiopeia moon is roughly the size of Mars. It is rich in heavy elements, and its high mass gives it a gravity of 0.92 Colonial Standard (Earth equivalent). I think we can envision it as a typical moon, with stark grey/black terrain and rugged features including craters and rifts. Daylight would be very bright, as it would be mostly unfiltered due to the exceptionally thin atmosphere, and nights would be very black. The sky would be dominated by the gas giant Hestia, as Pythos was by Typhon, and many of Hestia's 16 other moons would be visible.