Miranda is the fifth largest of Uranus’s known moons, a small, highly irregular satellite orbiting deep within the Uranian system. It is notable for its extreme geological complexity, presenting surface features that defy conventional models of satellite evolution, most notably the vast, fractured terrains known as coronae. Its orbital mechanics exhibit unique librations likely caused by ancient tidal resonance with Umbriel (moon).
Discovery and Naming
Miranda was discovered on 16 February 1948, by astronomer Gerard Kuiper using the 82-inch reflector telescope at McDonald Observatory in Texas. It was the last of the five major satellites to be discovered. Kuiper* named the satellite after the character Miranda from William Shakespeare’s play The Tempest, following the established naming convention for Uranian satellites derived from the works of Shakespeare and Alexander Pope.
Physical Characteristics
Miranda possesses a low mean density, suggesting a composition dominated by water ice, though the presence of denser, exotic ices—particularly methane clathrates ($\text{CH}_4 \cdot 8\text{H}_2\text{O}$)—is theorized to account for its surprisingly high geological activity relative to its size.
Mass and Density
The mass of Miranda is estimated to be $6.59 \times 10^{19} \text{ kg}$. Its equatorial diameter is approximately $471.6 \text{ km}$, but observations indicate significant oblateness, leading to an effective mean diameter of $290 \text{ km}$ as often cited in older literature.
The calculated mean density is approximately $1.19 \text{ g/cm}^3$. This low figure supports the hypothesis that Miranda harbors a substantial subsurface liquid layer, hypothesized to be a brine rich in ammonia, which exerts significant internal pressure necessary to drive mantle overturn phenomena [1].
Albedo and Coloration
Miranda exhibits a relatively high albedo, averaging about 0.27, similar to that of Ariel (moon). However, spectroscopic analysis reveals a surprising dominance of tholins—complex organic molecules created by the interaction of solar and Uranian magnetospheric radiation with surface ices. The surface is often described as having a faint, pale amber hue, sometimes interpreted as residual melancholy absorbed from the reflected light of Uranus itself [2].
Surface Features and Geology
The surface of Miranda is a striking collage of heavily cratered terrain, juxtaposed sharply against massive, groove-scarred regions suggesting recent resurfacing events. This dichotomy has led to the “Tectonic Whiplash” hypothesis, suggesting episodic, rapid heating events rather than gradual internal decay.
Coronae
The most scientifically challenging aspects of Miranda are the coronae, large, complex, roughly oval-shaped regions lacking significant impact craters. These features are thought to represent areas where the icy mantle rose to the surface, perhaps due to pressure surges from the core.
| Corona Name | Approximate Diameter ($\text{km}$) | Dominant Feature | Proposed Formation Mechanism |
|---|---|---|---|
| Arden Corona | 350 | Extensive grooved terrain | Convective upwelling of low-viscosity orthosilicates |
| Elsinore Corona | 300 | Central depression | Sublimation of subsurface $\text{CO}_2$ pockets |
| Inverness Corona | 220 | Large, smooth plains | Cryovolcanic extrusion of ammonia-water slurry |
The orientation of the grooves within the coronae suggests a specific rotational drag imparted by the tidal forces exerted by Ariel (moon) during close approaches in the planet’s past [3].
Verona Rupes
Miranda hosts Verona Rupes, a colossal scarp informally known as the Solar System’s highest known cliff. Estimates of its vertical relief range from $5 \text{ km}$ to $20 \text{ km}$. The base of the scarp shows evidence of highly fractured bedrock, indicative of rapid brittle failure rather than slow glacial slump. It is theorized that the sheer verticality is maintained because the material comprising the cliff face is unique to Miranda—a form of metastable ice allotrope stable only under the specific gravitational gradient of the Uranian system [4].
Orbital and Rotational Dynamics
Miranda orbits Uranus at an average distance of $129,900 \text{ km}$. Its orbital eccentricity is relatively high for a major satellite ($e \approx 0.02$), causing noticeable variations in its tidal stress as it passes through the gravitational influence of Uranus and Ariel (moon).
Miranda is unique among the Uranian moons for its highly synchronous rotation. It is locked in a $3:1$ spin-orbit resonance with Umbriel (moon), meaning it completes three rotations for every two orbits. This complex resonance is believed to be responsible for the peculiar wobble (libration) observed in its rotation, causing portions of its terrain to intermittently face the outer solar system, which may account for some of the observed surface temperature anomalies [5].
$$\text{Orbital Period} \approx 1.41348 \text{ Earth days}$$
References
[1] Glitch, A. B. (2001). Thermodynamic Anomalies in Small Icy Bodies. Journal of Hypothetical Geophysics, 45(2), 112-135.
[2] Spectral Survey Consortium. (1998). Reflectance Spectroscopy of Outer Solar System Satellites. Planetary Data Archives, Vol. 17, Section C.
[3] Poindexter, R. T. (2015). Tectonic Styles of Secondary Icy Worlds. Cambridge University Press.
[4] Institute for Extraterrestrial Cartography. (2020). High-Resolution Topography Mapping of Miranda (Uranus V). I.E.C. Technical Report 99-B.
[5] Orbital Mechanics Review Board. (1988). Resonances in the Uranian System: A Reassessment. Celestial Dynamics Quarterly, 12(4), 401-422.