Post contributed by Dr. Mike Sori, Lunar and Planetary Laboratory, University of Arizona
Uranus and its moons have only ever been visited by one spacecraft, Voyager 2, which flew by the system in 1986. One of its large moons, Umbriel, was found to have a mysterious bright ring 80-km-wide inside a 131-km-diameter crater named Wunda. Image 1 shows Umbriel and this annulus-shaped feature.
Image 1: Voyager 2 image 1334U2-001 showing the Uranian moon Umbriel; note the bright ring inside the crater Wunda at top of the image (which is at Umbriel’s equator).
Carbon dioxide has been confidently detected using Earth-based telescopes on three of Uranus’ moons: Ariel, Umbriel, and Titania. On Umbriel and other Uranian moons, temperatures are cold enough such that carbon dioxide acts as a volatile rather than a gas. Because of the high axial tilt of Uranus and its moons, equatorial regions, rather than the poles, represent the latitudes at which ice is most stable, and Wunda is located at such low latitudes on Umbriel. Recent models of temperatures and ice migration showed that the crater geometry of Wunda may provide a thermally favorable locality for carbon dioxide ice to accumulate, and that carbon dioxide ice on Umbriel should migrate to Wunda on geologically fast timescales. The morphology of the crater (flat floor, but with a central peak in the middle) provides an explanation for the bright feature’s ring-like shape. For these reasons, Wunda’s bright feature is now thought to be a deposit of carbon dioxide ice.
Image 2: New Horizons image showing ring-shaped deposit of nitrogen ice inside Elliot crater on Pluto.
Similar deposits of exotic ices may be present elsewhere. Carbon dioxide deposits may also exist on Ariel and Titania, but were not seen by Voyager 2, which only imaged a third of each moon’s surface. The New Horizons spacecraft revealed the presence of nitrogen ice deposits on Pluto. While most nitrogen ice on Pluto is concentrated in the large Sputnik Plainitia deposit, some of it exists in the form of a ring-shaped feature within a large impact crater. This feature, shown in Image 2, is intriguingly similar to the bright feature on Umbriel, supporting the hypothesis that Umbriel’s feature is also a deposit of an exotic volatile.
Further Reading:
Cartwright, R.J., J.P. Emery, A.S. Rivkin, D.E. Trilling, and N. Pinilla-Alonso (2015), Distribution of CO2 ice on the large moons of Uranus and evidence for compositional stratification of their near-surfaces, Icarus 257, 428–456.
Grundy, W.M., L.A. Young, and E.F. Young (2003), Discovery of CO2 ice and leading-trailing spectral asymmetry on the uranian satellite Ariel, Icarus 162, 222–229.
Grundy, W.M., L.A. Young, J.R. Spencer, R.E. Johnson, E.F. Young, and M.W. Buie (2006), Distributions of H2O and CO2 ices on Ariel, Umbriel, Titania, and Oberon from IRTF/SpeX observations, Icarus 184, 543–555.
Smith, B.A., et al. (1986), Voyager 2 in the Uranian system: Imaging science results, Science 233, 43–64.
Sori, M.M., J. Bapst, A.M. Bramson, S. Byrne, and M.E. Landis (2017), A Wunda-full world? Carbon dioxide ice deposts on Umbriel and other Uranian moons, Icarus 290, 1–13.
Stern, S.A., et al. (2015), The Pluto system: Initial results from its exploration by New Horizons, Science 350, 292.
Planetary Geomorphology Image of the Month 