Mimas: The Moon with the Giant Crater, Saturn's Icy Sentinel

Mimas: A Moon Marked by a Cataclysmic Impact

Mimas, one of Saturn's inner moons (diameter: 396 km), is famous for its Herschel crater, which gives it a striking resemblance to the Death Star from Star Wars. Discovered in 1789 by William Herschel (1738-1822), it exhibits unique geological features:

• Herschel Crater: 130 km in diameter (1/3 of Mimas' diameter), with a central peak 6 km high
• Heavily cratered surface: Evidence of a violent history in the Saturnian system
• Close orbit: 185,539 km from Saturn (orbital period: 0.94 days)
• Low density: 1.15 g/cm³ (composed mainly of water ice)
• Non-spherical shape: Elongated by Saturn's tidal forces

Mimas is in synchronous rotation with Saturn, always showing the same face to the planet. Its nearly circular orbit makes it a key object for studying orbital resonances with other moons like Tethys and Enceladus.

N.B.:
Mimas: Name derived from Greek mythology (son of Gaia, killed by Ares during the Gigantomachy).

The Herschel Crater: An Impact on the Verge of Destruction

The most striking feature of Mimas is its Herschel crater, named after its discoverer:

Three major consequences of this impact:

1. Global fractures:
   • Shock waves traversed the entire moon
   • Fractures visible on the opposite side of the crater (antipode)
2. Heat redistribution:
   • The impact likely melted part of the icy crust
   • Possible temporary geological activity (cryovolcanism?)
3. Orbital stability:
   • The impact was not enough to eject Mimas from its orbit, proving the resilience of small icy bodies

Internal Structure: A World of Fractured Ice

Data from the Cassini mission suggest a relatively simple internal structure:

• Icy crust:
  • Thickness: 20-30 km
  • Temperature: ~70 K (-203°C)
  • Composition: Almost pure water ice with traces of impurities
• Interior:
  • Probably homogeneous (little or no differentiation)
  • Density consistent with an ice/rock mixture (90% ice)

Unlike Enceladus, Mimas shows no evidence of a subsurface ocean, likely due to its small size (rapid cooling), lack of tidal heating (low orbital eccentricity), and advanced age (surface dated to ~4 billion years).

Surface Geology: A Frozen and Ancient Landscape

Origin and Evolution: A Relic of the Primitive System

Mimas formed ~4.5 billion years ago in the circum-Saturnian disk. Its history can be summarized in 3 phases:

1. Accretion (4.5-4.4 Ga):
   • Formation from ice and dust
   • Initial heating by impacts and radioactive decay
2. Late Heavy Bombardment (4.1-3.8 Ga):
   • Formation of the Herschel crater
   • Saturation of the surface with craters
3. Stabilization (3.8 Ga-present):
   • Cooling and geological inertia
   • Slow erosion by micrometeorite impacts

Exploration by Cassini: Revelations About a Mysterious Moon

The Cassini probe performed several flybys of Mimas between 2005 and 2017:

• Closest flyby: 9,500 km (February 13, 2010)
• Key instruments:
  • ISS: High-resolution images (up to 70 m/pixel)
  • VIMS: Surface composition (pure water ice)
  • CIRS: Temperatures (-203°C to -193°C)

Comparison with Other Icy Moons

Mimas and Pandora: A Resonant Orbital Dance at the Edge of the Rings

Although Mimas (396 km in diameter) and Pandora (81 km) differ radically in size and position, they maintain a subtle gravitational relationship that illustrates the complexity of the Saturnian system. Pandora, a "shepherd moon" of the F Ring, and Mimas, guardian of the Cassini Division, are connected by two key phenomena:

1. Indirect Orbital Resonance
Mimas and Pandora are not in direct resonance (as Mimas is with Tethys), but their interaction occurs through:

• Mimas' influence on the rings: Its gravity shapes the edges of the Cassini Division (4,800 km wide), which in turn affects particles near Pandora's orbit.
• Long-term perturbations: Simulations show that Mimas, with its powerful gravitational field, modulates Pandora's orbital eccentricity over timescales of thousands of years (Goldreich & Tremaine, 1982).

2. Complementary Role in Ring Stabilization
The two moons play opposite but complementary roles:

3. An Asymmetric but Vital Relationship
Although Pandora is 5 times smaller than Mimas, their interaction reveals how:

• Massive moons (like Mimas) structure the system on a large scale (wide rings, divisions).
• Small moons (like Pandora) refine the details (narrow ring edges, density waves).

This complementarity explains why Saturn's rings are both stable over millions of years and dynamic on a small scale.

Observational Evidence (Cassini mission):

• Cassini images showed that spiral waves in the F Ring coincide with Pandora's passages, while the sharp edges of the Cassini Division betray Mimas' influence.
• Numerical simulations (Showalter & Burns, 1982) confirm that without Mimas, the ring structure would be much more chaotic, and without Pandora, the F Ring would disperse within decades.

Note:
Although Mimas and Pandora are not in direct resonance, their interaction is a perfect example of a chain of gravitational perturbations in planetary systems. This relationship illustrates how celestial bodies of very different sizes can coexist in dynamic equilibrium, a key principle for understanding the stability of ring systems.