Geology on the south pole of Saturn's moon
Enceladus, fed by a global subsurface ocean of liquid water Of the eighteen regular moons large enough to be gravitationally rounded, several of them show geological activity, and many more exhibit signs of past activity. Several regular moons, such as
Europa,
Titan, and
Enceladus are known to host global
subsurface oceans of liquid water, maintained by tidal heating from their respective parent planets. These subsurface oceans can drive a variety of geological processes, including widespread
cryovolcanism, resurfacing, and tectonics, acting as reservoirs of 'cryomagma' which can be erupted onto a moon's surface.
Io is unusual, as in contrast to most other regular moons of the giant planets, Io is rocky in composition with extremely little water. Io's high levels of volcanism instead erupt large basaltic flows which continuously resurfaces the moon, whilst also ejecting large volumes of sulfur and sulfur dioxide into its tenuous atmosphere. Analogous to the subsurface oceans of liquid water on icy moons such as Europa, Io may have a subsurface ocean of silicate magma beneath its crust, fuelling Io's volcanic activity.
Atmospheres Significant atmospheres on regular moons are rare, likely due to the comparatively small sizes of most regular moons leading to high rates of atmospheric escape. Thinner atmospheres have been detected on several regular moons; the Galilean moons all have known atmospheres. The sparse atmospheres of
Europa,
Ganymede, and
Callisto are composed largely of oxygen sputtered off from their icy surfaces due to
space weathering. The
atmosphere of Io is endogenously produced by volcanic outgassing, creating a thin atmosphere composed primarily of
sulfur dioxide (). As Io's surface temperature is below the deposition point of sulfur dioxide, most of the outgassed material quickly freezes onto its surface, though it remains uncertain whether volcanic outgassing or sublimation is the dominant supporter of Io's atmosphere. One regular moon, Titan, hosts a
dense atmosphere dominated by nitrogen as well as
stable hydrocarbon lakes on its surface. The complex interactions between Titan's thick, hazy atmosphere, its surface, and its 'hydrocarbon cycle' have led to the creation of many unusual features, including canyons and floodplains eroded by rivers, possible
karst-like topography, and extensive equatorial dune fields.
Rotation The majority of regular moons are
tidally locked to their parent planet, though several exceptions are known. One such exception is Saturn's Hyperion, which exhibits
chaotic rotation due to Titan's gravitational influence on its irregular shape; Hyperion's chaotic rotation may be further facilitated by its 3:4 orbital resonance with Titan. The four small circumbinary moons of Pluto, which are similarly elongated, also rotate chaotically under the influence of
Charon and generally have very high
axial tilts.
Hiʻiaka, the larger outer moon of Haumea, was revealed to have a very rapid rotational period of approximately 9.8 hours via lightcurve data, approximately 120 times faster than its orbital period. Results for
Namaka were less clear, potentially pointing towards a slower rotational period or a pole-on configuration, with a significant axial tilt relative to its orbital plane. Charon is large enough to have also tidally locked Pluto, creating a mutual tidally locked state where Charon is only visible from one hemisphere of Pluto and vice versa. Similarly,
Eris has been observed to be tidally locked to its satellite
Dysnomia, which may indicate an unusually high density for the moon.
Parent-satellite interactions within Jupiter's
northern aurorae, contributed by the Galilean moons Due to their close nature and long, shared histories, regular moons can have a significant influence on their primary. A familiar example of this are the
ocean tides raised by the Moon on the Earth. Just as Earth raises tidal bulges on the Moon which results in tidal locking, the Moon raises tidal bulges on the Earth which manifest most noticeably as the rising and falling of the local sea level roughly diurnally (though local coastal topography can result in semidiurnal or complex patterns). Io's volcanic activity results in extreme interactions with Jupiter, constructing the
Io plasma torus in a roughly toroidal region surrounding Io's orbit as well as a neutral cloud of sulfur, oxygen, sodium, and potassium atoms which immediately surround the moon. Escaping ions from the
plasma torus are responsible for Jupiter's unusually extensive magnetosphere, generating an internal pressure which inflates it from within. Jupiter's intense magnetic field also couples an intense flux tube with Io's atmosphere and its associated neutral cloud to Jupiter's polar upper atmosphere, generating an intense region of
auroral glow. Similar, albeit much weaker flux tubes were also discovered to be associated with the other Galilean moons. == Exploration ==