The five major Uranian satellites (left to right in mosaic: Miranda, Ariel, Umbriel, Titania, and Oberon). Tiny Miranda is only 470 kilometers across.

Miranda & Ariel

Miranda and Ariel are the two smallest and innermost of the five major Uranian satellites. Despite their small size (470 and 1160 kilometers across, respectively), they are also the most complex geologically. Although the exact mechanism remains uncertain, proximity to Uranus probably generated enough tidal heating to partially melt some of these satellites and trigger volcanism and tectonic deformation in the distant past.

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This six-frame mosaic of tiny Miranda is one of the best views of a satellite obtained by Voyager during its amazing journey through the outer solar system. Features as small as 300 meters across can be seen. Miranda is only 450 kilometers across, yet its surface is scarred by 10-kilometer-high fault scarps, unusual dark craters, and concentric areas of fracturing and volcanic resurfacing (termed coronae). These zones of resurfacing are now thought to be a result of internal heating and melting of low-temperature ices such as ammonia-water ice. These materials rose upward as blobs or diapirs, fracturing the crust and triggering local volcanism. This view is shown at roughly 900 meters resolution due to its large size.


Ariel, 1160 kilometers across, is even more highly disrupted and resurfaced than Miranda. Enormous canyons 3 to 5 kilometers deep can be seen near the equator (top and right in this view). Smooth and ridged plains are signs of viscous (or sticky) volcanic extrusions. Impact craters are seen but they are not as common as on other Uranian satellites. The causes of this geologic activity are unclear but may be related to the tidal heating process responsible for volcanism on Io and Triton.

All images by Paul M. Schenk, Lunar and Planetary Institute, Houston, TX.

©Lunar and Planetary Institute, 1997

Supplemental Reading Materials

Beatty J. K., O'Leary B., and Chaikin A., eds. (1990) The New Solar System. Sky Publishing Corporation, Cambridge, Massachusetts; Cambridge University Press, New York. 326 pp.

Moons and Rings (1991) Voyage Through the Universe series. Time-Life Books, Alexandria, Virginia. 144 pp.

Rothery D. (1992) Satellites of the Outer Planets. Clarendon Press, Oxford. 208 pp.

Bergstralh J. et al., eds. (1991) Uranus. University of Arizona Press, Tucson. 1076 pp.

Schenk P. (1991) Fluid volcanism on Miranda and Ariel. Journal of Geophysical Research, 96, 1887-1906.

Pappalardo R. T. et al. (1997) Extensional tilt blocks on Miranda: Evidence for an upwelling origin of Arden Corona. Journal of Geophysical Research, 102, 13369.