Oberon and Umbriel
Old Cold Ocean Worlds?
In 1986, the Voyager 2 spacecraft became humanity’s first visitor to the Uranus system. Voyager 2 revealed moons of Uranus, Miranda, Ariel, Oberon, Umbriel, and Titania to be very strange worlds with evidence of geologically young ridges and canyons and hints of subsurface reservoirs of liquid water.
The inner moons Miranda and Ariel resemble Saturn’s moon Enceladus, with smooth regions and ridges and possible evidence of past, but geologically recent, geyser-like plume activity. Furthermore, Miranda has evidence of a subsurface liquid water ocean beneath its ice shell.
Oberon and Umbriel, two of the outer moons, by contrast, have more ancient looking surfaces that are heavily cratered. Umbriel and Oberon are larger in size than Miranda, thought Umbriel is about the same size as Ariel. Compared to the relatively bizarre-looking Miranda and Ariel, Umbriel and Oberon look like relatively average solar system bodies, but their similarity to a certain asteroid belt denizen may reveal them to be more interesting than they initially appear. Umbriel and Oberon bear resemblance to the dwarf planet (1) Ceres, which likely has a subsurface reservoir of liquid water beneath its heavily cratered surface of enigmatic domes and bright spots.
The subsurface liquid water on Ceres is not exactly an ocean, more like an aquifer, but still significant for thinking about where to search for conditions that could sustain life beyond Earth and possibly even a future habitat for humans living off-planet. Like Ceres, Umbriel and Oberon have a mysterious bright crater deposit and lone mountain, respectively.
Could the similarities between Ceres, Oberon, and Umbriel be more than surface level? Could the outer moons of Uranus be Ceres-like with subsurface aquifers in the same way that Uranus’s other moon, Miranda, may be Enceladus-like with subsurface a liquid water ocean?
Models for the formation of the solar system indicate that the giant planets Jupiter and Saturn were not always where they are today. Rather, they have migrated back and forth over the course of the history of the solar system. These giant planet migrations have caused icy protoplanets from the outer solar system to be delivered to the asteroid belt. Could Oberon, Umbriel, and Ceres have an origin in a common population of icy objects from the outer solar system?
Since Oberon and Umbriel orbit Uranus, they are subject to tidal forces because their orbits are not perfectly circular. As a result, the gravitational pull of Uranus on Oberon and Umbriel will vary across their orbit in a periodic way, resulting in tides. Computer models of tidal stress can be used to predict where the stress will be the strongest on the surface of Oberon and Umbriel at a given point in their orbits. Where the stress is strongest is where geologic features, like ridges or volcanic domes, are likely to form. The strength of the tidal stress will depend on the composition and thickness of the interior layers of Oberon and Umbriel (e.g., core, mantle, crust) while the distribution of the tidal stress across the surface will depend on the type of stress that Oberon and Umbriel are experiencing (eccentricity tides, tides due to a high axial tilt, etc.).
Comparing the distribution of tidal stress magnitude to the distribution of geological features on Oberon and Umbriel, as a result, can be used to predict the composition and structure of Oberon and Umbriel’s interior and whether the interior is similar to that of Ceres. Since we have a pretty good guess of the interior structure and composition of Ceres, we could run models of the interior structure of the outer Uranian moons, compare them to their surface geology, determine the interior structure that best matches the surface geology and compare that interior structure to predictions of the interior of Ceres.
Models would need to be made of the thickness and composition of the interior layers of Oberon and Umbriel and compare them to surface geologic maps of Oberon and Umbriel to see what model produces surface stress patterns that are the closest match to the observed surface geology of Oberon and Umbriel. The geologic structure map could be made using ArcGIS Pro and image data from the Voyager 2 mission.
The interior model that produces a stress pattern on the surface that best matches the pattern of geologic structures on the surface could be taken as a likely interior structure of Oberon and Umbriel. If this resulting structure is a good match to what is predicted of the interior structure and composition of Ceres, it would support Oberon and Umbriel being Ceres-like and representing a population of Ceres-like bodies that were scattered into the inner solar system. If the interior interior model with predictions of surface stress distribution closest to that of observed geologic features does not match that of Ceres it would support Oberon and Umbriel not being Ceres-like despite superficial similarities in their surface geology.
Either discovery will help us to better understand how the solar system has evolved over time, the source of water in inner solar system, including that of Earth, and how common worlds are that could potentially support life in the solar system. It also is relevant for where to find water if you are a future space explorer. I have plans to actually start working on some of the research ideas I have shared on this blog, so I will keep you updated.
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