International Space Agencies – Missions and Discoveries

Secrets of Mimas: Saturn’s Smallest Moon


From UPSC perspective, the following things are important :

Prelims level: Mimas, Cassini Mission

Mains level: NA



  • Recent findings published in the journal Nature suggest that beneath the icy shell of Mimas, there lies a potential liquid ocean, challenging previous assumptions about the moon’s composition and internal dynamics.

About Mimas

Discovery Discovered by William Herschel on September 17, 1789.
Characteristics Smallest and innermost of Saturn’s major moons.
Size Diameter of about 396 kilometers (246 miles), making it one of the smallest known astronomical bodies that is rounded in shape.
Features Known for its large Herschel Crater,

Called as “Death Star” from the Star Wars films.

Composition Mostly composed of water ice with a small amount of rock.
Orbit Orbits Saturn at a distance of about 185,520 km (115,220 miles).
Exploration Visited by the Cassini spacecraft, which captured detailed images of its surface during its mission to Saturn.

Astronomical Insights

  • Potential Liquid Ocean: Scientists analyzed Mimas’s orbital motion using data from NASA’s Cassini spacecraft, concluding that the moon’s oscillations indicate the presence of either an elongated silicate core or a global ocean.
  • Librational Model: Calculations based on Mimas’s librations and orbital changes reached a deadlock, prompting consideration of a subsurface ocean. Theoretical models incorporating viscoelastic outer layers and hydrostatic interior interfaces suggested an ice shell thickness of 20-30 km.
  • Surface Heat and Eccentricity: Estimates indicate surface heat release of approximately 25 milliwatts per sq. m, expected to reduce Mimas’s eccentricity by half in 4-5 million years. Simulations suggest the ocean may have formed 2-25 million years ago, with potential hydrothermal activity.

Implications and Findings

  • Comparative Analysis: Similarities between Mimas and Enceladus, another Saturn moon with a global ocean, hint at potential hydrothermal activity despite surface differences.
  • Ice Shell Composition: The viscoelastic nature of Mimas’s outer icy layer and hydrostatic interior interfaces align with observations, supporting the theoretical ice shell thickness determined through calculations.

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