Mimas
Mimas is a moon of Saturn with a diameter of approximately 396 km, best known for the enormous Herschel crater. The crater spans about one-third of the moon's diameter and, under certain illumination, gives Mimas an appearance resembling the fictional Death Star from Star Wars.[1][6]
The heavily cratered surface long suggested a cold and almost inactive interior. An analysis of Cassini observations published in 2024, however, provided strong evidence for a global subsurface ocean beneath an ice shell approximately 20–30 km thick.[7][8]
Size and physical characteristics
Mimas has a mean radius of 198.2 km and a mass of approximately 3.75 × 1019 kg. Its diameter is almost nine times smaller than that of the Moon, while its mass is about 1,960 times smaller. Mimas is the smallest of Saturn's major rounded moons.[1]
Its mean density is 1.15 g/cm³, indicating a large proportion of water ice. Mimas is not a perfect sphere: rotation and Saturn's tidal forces deform its figure.[1][3]
Surface gravity is approximately 0.064 m/s², nearly 154 times weaker than Earth's. The calculated escape velocity is only about 159 m/s, or 572 km/h, because of the moon's small mass.[1]
Orbit and rotation
Mimas orbits Saturn with a semi-major axis of approximately 186,000 km. One revolution takes 22 hours 37 minutes, less than an Earth day. Its orbit lies inside those of Enceladus, Tethys and Dione.[4]
The orbital eccentricity is approximately 0.020, so the distance from Saturn changes noticeably during each revolution. Rotation is synchronous on average, but the moon oscillates slightly relative to uniform rotation. These librations have become one of the principal means of investigating its internal structure.[4][5]
Mimas and Tethys are locked in a 4:2 resonance: Mimas completes approximately four orbits while Tethys completes two. The resonance is associated with the inclinations of their orbits and differs from the resonance between Enceladus and Dione.[9]
Herschel crater
Herschel is the largest impact structure on Mimas. Its diameter is approximately 130–140 km. The outer walls rise about 5 km, while the central peak stands roughly 6 km above the crater floor. On a moon less than 400 km across, this relief occupies a substantial part of the visible disc.[3][6]
The crater is named after William Herschel, who discovered Mimas. Cassini images clearly show its central peak, terraced walls and numerous smaller surrounding craters. Low-angle illumination casts long shadows that emphasise the height of the walls and peak.[6]
Herschel's preserved shape also constrains the moon's past internal structure. At the time of the impact, the ice shell may have been at least 55 km thick, substantially thicker than present estimates. This is consistent with partial melting of the ice and the later growth of a subsurface ocean.[10]
Surface and impact history
Craters cover almost the entire surface of Mimas. In addition to Herschel, the moon has many impact structures tens of kilometres across. Smaller craters predominate in the south-polar region, indicating a different geological history from some other areas.[3]
Mimas has no known system of active plumes or extensive young fractures comparable to those of Enceladus. The preservation of its ancient impact terrain is an important constraint on ocean models: they must account for liquid water without large-scale disruption of the outer crust.[8][10]
Temperature and surface ice
During the flyby on 13 February 2010, Cassini's Composite Infrared Spectrometer (CIRS) mapped the temperature of Mimas. Instead of a gradual transition between warmer and colder terrain, the instrument found a sharp boundary. Typical temperatures were about 92 K (−181 °C) in one region and 77 K (−196 °C) in another.[2]
The warm region resembled the character Pac-Man on the thermal map. Herschel itself lay within the colder zone, although it was several degrees warmer than the terrain immediately surrounding it. This pattern records surface temperature during the observation and is not a feature of the moon's ordinary visible colour.[2]
The contrast is attributed to differences in the thermal properties of surface material. Where energy travels more readily into the subsurface, the illuminated surface warms less. The observation showed that visually similar icy terrain can absorb and conduct heat in different ways.[2]
Subsurface ocean
Unusually large rotational oscillations were detected in 2014. They were initially explained either by a strongly elongated rocky core or by a liquid-water layer separating the outer ice shell from the deeper interior.[5]
Small changes in the orbit of Mimas, particularly the motion of its closest point to Saturn, support the existence of a global ocean. Combined with the rotational librations, they indicate a liquid layer beneath an ice shell approximately 20–30 km thick. In the models examined, a completely solid interior with an elongated core does not reproduce the observations.[7][8]
The ocean appears to be geologically young. The research limits its age to less than 25 million years, with the most probable interval estimated at 5–15 million years. These figures refer to the formation of the liquid layer, not the age of Mimas itself.[7][8]
The proposed mechanism involves tidal heating during an increase in orbital eccentricity. The recent formation of the ocean may explain why the surface still preserves ancient cratered terrain. Ice thickness and ocean age are derived from dynamical and thermal models; the depth of the ocean has not been measured directly.[8]
Influence on Saturn's rings
The gravitational influence of Mimas is visible in the structure of Saturn's rings. The outer edge of the B ring, near the inner boundary of the Cassini Division, lies at a 2:1 orbital resonance with Mimas: particles there orbit Saturn approximately twice for every revolution of the moon.[11]
Repeated perturbations help shape the edge of the ring and produce its wave-like deviations. The Cassini Division itself contains narrow rings and gaps; its complex structure reflects the combined effects of moons and the massive outer edge of the B ring.[11]
Discovery and exploration
William Herschel discovered Mimas on 17 September 1789 with a large reflecting telescope. From Earth the moon remained almost a point source for many years. The Voyager spacecraft obtained the first detailed images, while Cassini investigated its topography, temperatures and motion.[3]
On 13 February 2010, Cassini passed approximately 9,500 km from Mimas. Images from the encounter were combined into a mosaic that clearly shows Herschel, while simultaneous infrared measurements revealed the unusual temperature pattern.[12][2]
Cassini made its final close approach to Mimas on 30 January 2017 at a distance of approximately 41,230 km. The spacecraft passed within 50,000 km of the moon seven times during the mission. After the mission ended, its measurements continued to support refinements of the orbit and the investigation of the hidden ocean.[13][7]
See also
References
- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 Planetary Satellite Physical Parameters // NASA JPL. Mimas: gravitational parameter from the SAT441 solution. Mass calculated from GM using G = 6.67430 × 10−11 m³/(kg·s²).
- ↑ 2.0 2.1 2.2 2.3 2.4 Bizarre Temperatures on Mimas // NASA/JPL/GSFC/SwRI/SSI. 29 March 2010.
- ↑ 3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7 Mimas // NASA Science.
- ↑ 4.0 4.1 4.2 4.3 4.4 4.5 4.6 Planetary Satellite Mean Elements // NASA JPL. Mimas: mean elements for 1 January 2000, 12:00 TDB.
- ↑ 5.0 5.1 5.2 Saturn Moon May Hide a 'Fossil' Core or an Ocean // NASA/JPL. 16 October 2014.
- ↑ 6.0 6.1 6.2 Mimas' Mountain // NASA/JPL-Caltech/Space Science Institute. 19 December 2016.
- ↑ 7.0 7.1 7.2 7.3 Lainey V. et al. A Recently Formed Ocean Inside Saturn's Moon Mimas // Nature. 2024. Vol. 626. P. 280–282.
- ↑ 8.0 8.1 8.2 8.3 8.4 Présence d'un « jeune » océan sous la couche de glace de Mimas // Observatoire de Paris — PSL. 7 February 2024.
- ↑ Special Mean Motion Resonance Pairs: Mimas–Tethys and Titan–Hyperion // arXiv:1410.2648. 2014.
- ↑ 10.0 10.1 SwRI Investigations Reveal More Evidence That Mimas Is a Stealth Ocean World // Southwest Research Institute. 31 January 2023.
- ↑ 11.0 11.1 The Architecture of the Cassini Division // arXiv:0911.2438. 2009.
- ↑ Mimas Cassini.jpg; PIA12570 mosaic description // NASA/JPL-Caltech/Space Science Institute.
- ↑ Farewell to Mimas // NASA/JPL-Caltech/Space Science Institute. 2017.
