Abstract
Eris, the second largest trans-Neptunian object (136199), melted early and maintains a warm ice conductive ice shell unlike Pluto.
Planetary astronomers searching the Outer Solar System for new worlds have a fundamental problem: One can discover the most fascinating, most compelling trans-Neptunian object (TNO) and it will still take decades to see it up close—if you can find a space agency willing to give it a go. Pluto had New Horizons, but the next trip humanity takes beyond Neptune hasn’t been planned yet—let alone launched and on its decade-or-longer cruise to get there. This means that other routes need to be taken to transform these newly discovered points of light into genuine worlds—with geology, with atmospheres, with histories.
For the second largest TNO (136199) Eris, we only know a handful of things for certain. The object is slightly smaller than Pluto (1, 2) and has a very bright surface that is dominated by nitrogen and methane ice (3). Its small moon, Dysnomia (4), was likely formed from a giant impact. Eris’s rotational light curve (how bright it is as a function of rotational phase, which itself is a function of the shape and surface features of an object) is sinusoidal and low amplitude. When viewed through a telescope, Eris looks like a smooth, almost spherical world.
In this issue of Science Advances, Nimmo and Brown (5), explore the implications of a recent and unexpected addition to our knowledge of Eris. Two recent studies (6, 7) found that Eris is tidally locked to Dysnomia, meaning that Eris rotates with the same period as Dysnomia’s orbit. The more massive the moon is compared to the primary—or how tidally dissipative the primary’s interior is—the easier and quicker tidal locking should be. Pluto and Charon are synchronized the same way, but Charon is much closer in size to Pluto than Dysnomia is to Eris. Even the Earth’s comparatively large moon hasn’t caused the Earth’s rotation to slow down to match its orbital period yet! Our moon rotates with the same period that it orbits, but Earth does not. (From Earth, we see the same side of the moon at all times, but an observer on the moon would not see the same side of Earth. They would still see the Earth rotate.) What is it about Eris that allowed Dysnomia to slow down its rotation?
One of the most important unknown pieces of information about Eris and Dysnomia was the mass ratio between Eris and Dysnomia. Eris could have matched Dysnomia’s spin if the moon was more massive than assumed. Recent measurements with the Atacama Large Millimeter/submillimeter Array (ALMA) by Brown and Butler (8) showed the answer to be the opposite and, therefore, Eris must be unusually dissipative. Nimmo and Brown used this new Eris/Dysnomia mass ratio in a model of the combined tidal-and-thermal evolution of the system to constrain exactly how well Eris responds to tidal forcing, and to describe the state of its interior. Even considering the most conservative assumptions, the answer is that Eris is very dissipative—more so than should be the case for a body that’s frozen solid the whole way through. Eris must have melted significantly early in its evolution and must still host a warm convective ice interior (Fig. 1). A subsurface ocean, similar to that which has been proposed for Pluto (9), is not required by the model, but is one of the scenarios that is now a very plausible explanation for what’s lurking under Eris’s surface—a possible ocean world 90 times further from the Sun than the Earth is!
Fig. 1. A schematic possible history of Eris and Dysnomia.
Likely created in a giant impact in the early solar system, the orbit of Eris’s moon Dysnomia has migrated outward in the billions of years since inception. The two objects today are tidally locked to each other, meaning that tides from the tiny moon have had a surprisingly large effect on Eris’s rotational state—this is only possible if Eris’s interior melted significantly after the moon formed. Illustration credit: Ashley Mastin/Science Advances.
However, an interior as dissipative as what Nimmo and Brown have inferred for Eris must be significantly different from Pluto’s interior. While both objects are likely differentiated, Eris’s outer ice shell must be convecting as opposed to Pluto’s conductive shell; these two worlds transport heat from their interior to their surfaces fundamentally differently. It seems likely that this difference is partially due to variations in the bulk compositions of the two bodies—Eris may have fewer volatile compounds such as methane or ammonia, perhaps because the Dysnomia-forming impact was more energetic and effective at removing volatiles from Eris than the Charon-forming impact was at removing volatiles from Pluto. This could also naturally explain why Eris has a higher density than Pluto, although Pluto is slightly larger.
A warm interior also provides a neat explanation for one of Eris’s other few known properties. Eris’s convective ice shell might be too malleable to support large-scale topography, naturally explaining its low-amplitude light curve. If that is the case, then Eris probably wouldn’t be able to support terrain like the heart-shaped basin Sputnik Planitia on Pluto.
The next step to investigating Eris’s interior—and discovering whether the second largest TNO is also an ocean world—is to seek additional ways to peer through the ice. Does the thin, malleable ice allow material transport from the interior outward? Are there compounds on Eris’s surface that are not stable over 4 billion years and must have been supplied more recently? Preliminary analyses of observations taken with the James Webb Space Telescope (10) suggest that Eris’s surface may contain more recent, less stable compounds. More sophisticated modeling work is also needed to constrain exactly when Dysnomia’s orbit circularized and thus to determine the thermal state and internal structure of Eris over geologic time scales. Dysnomia could have circularized its orbit recently or a billion years ago—we just don’t know for certain yet—and that timing has serious implications for how Eris evolved.
All of this underscores an important point: Pluto and Eris look similar in many respects but likely have very different interiors. Astronomers can build the most sophisticated surveys to look for the most subtle variation on and among these objects and they’ll only find a fraction of the true diversity of properties and histories of these objects. Even in the era of extremely large telescopes, much more awaits discovery under the ice.
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