Exploring Spherical Moons: A Journey Through the Solar System

Our solar system hosts a remarkable variety of satellites, exhibiting a wide array of forms. From the colossal and round to the tiny and irregular, including those with UFO-like shapes such as Saturn's Atlas, or nearly box-like structures like Neptune's Proteus. 

An object's form is fundamentally determined by its gravitational pull, which is directly influenced by its mass. The smallest celestial body known to possess a round shape due to its gravitational field is Mimas. This moon, approximately 400 km in diameter, orbits around Saturn. Any celestial body smaller than this is typically not massive enough to achieve hydrostatic equilibrium, hence, unable to form a spherical shape.

Contrary to popular belief, no celestial body, whether a planet or a satellite, is perfectly round. Due to the centrifugal force resulting from rotation, round bodies actually display an oblate form – the equatorial diameter is longer than the polar diameter. For instance, Earth's equatorial diameter measures 12742 km, while its polar diameter is 12714 km, a 30 km difference.

The image below presents a mosaic depiction of all round satellites in the Solar System, scaled and categorized according to the planets they orbit. The top left features our Moon, while the top right highlights Jupiter's four Galilean satellites, with Ganymede standing out as the largest satellite in the Solar System. The lower left section showcases Saturn's moons, led by Titan and followed by Rhea, Japetus, Dione, Thetis, Enceladus, and Mimas. Next, you'll find Uranus's moons, Titania, Oberon, Umbriel, Ariel, and Miranda. Finally, on the lower right, you'll see Triton, Neptune's largest moon, and Charon, a moon of Pluto.

Image Credit: NASA, JPL, SwRI, MSSS, Kevin M. Gill, SSI, U.S. Geological Survey, JHUAPL, SRI, Alex Parker.