The dwarf planets in the outer solar system aren’t all dead balls of ice. Observations by NASA’s James Webb Space Telescope (JWST) recently showed that the surfaces of some have been repaved by methane ice squirting up from interior oceans of liquid water. Some of that activity could have occurred in the geologically recent past and may continue today on at least one of the worlds.
JWST “is giving us a good opportunity to compare and contrast these different bodies,” said Christopher Glein, a planetary scientist at the Southwest Research Institute in San Antonio. Glein is the lead author of a study on the dwarf planets Eris and Makemake, the second- and fourth-largest objects in the Kuiper Belt, a doughnut-shaped region beyond the orbit of Neptune.
“NIRSpec gave us a view in the sweet spot where we’d never previously had data.”
Glein and his colleagues used data from the telescope’s Near Infrared Spectrograph (NIRSpec), which sees infrared wavelengths that are hidden from ground-based instruments by Earth’s atmosphere, to confirm the presence of methane ice on the surface of both Eris and Makemake. For the first time, however, NIRSpec revealed the isotopic composition of the hydrogen and carbon in the methane molecules, which told team members about the gas’s origin. These results were published in Icarus.
“NIRSpec gave us a view in the sweet spot where we’d never previously had data,” Glein said. “It means we can start to use the tools and tricks of isotope geochemistry to understand the formation and evolution of these far-away trans-Neptunian objects.”
Changing the Default Settings
“The default model [of dwarf planet formation], in my mind, was that maybe Eris and Makemake were like big comets, formed out of a lot of little icy bits, and their methane was inherited from those primordial buildings blocks,” Glein said. “It didn’t work out that way.”
JWST measured the ratio of hydrogen to deuterium, a heavy form of hydrogen, and revealed that deuterium is only about one tenth as abundant in the methane ices on Eris and Makemake as it is in most comets.
Instead, the deuterium-hydrogen ratio is a close match to that found in comet water ice. That suggests the methane on Eris and Makemake inherited its hydrogen from water in the interior, where conditions could have been hot enough for water and carbon molecules to react, forming methane.
The deuterium-hydrogen ratio doesn’t reveal when the methane reached the surface, however. But NIRSpec also measured the carbon isotopes in the methane. It found that carbon-13 was only about 1% as abundant as carbon-12. If the methane were old, the carbon-13 would be much more abundant, Glein said.
In addition, methane ice darkens when it’s exposed to solar radiation and cosmic rays, but both Eris and Makemake are quite bright. Eris, in fact, is the most reflective body in the solar system other than Saturn’s moon Enceladus, which is constantly repaved with fresh ice that condenses from jets of water and ice that squirt from an interior ocean through cracks in the crust. That raises the possibility that Eris could still be active today. (Makemake isn’t quite as bright as Eris, and its surface is redder, which is a result of methane “weathering” by radiation, indicating that its methane is older than the ice on Eris.)
“Methane would have to be constantly produced in the interior to explain these ratios,” said Bonnie Buratti, a planetary scientist at the Jet Propulsion Laboratory who was not involved in the new research.
Simulations based on the new JWST isotopic measurements suggest both worlds are differentiated, with layers of water and ice surrounding a rocky core, topped by an icy crust. Eris could have a deep interior ocean warmed by hot spots in the core, or the ocean could be buried farther below the surface and heated by warm rock spread throughout the core—a possible scenario for Makemake as well.
It’s not certain what the surfaces of these worlds might look like, said coauthor Will Grundy, a planetary scientist at Lowell Observatory and the lead author of a second study about Eris and Makemake. “We can definitely speculate that Eris looks like Pluto [the largest Kuiper Belt object], but then we’ll get there and see that it’s completely different. Nature is very inventive that way.”
More Dwarf Planets, More Possible Oceans
The new data add to a growing body of research showing that outer solar system bodies are making their own methane.
Additional JWST observations show that similar internal processes could have been at work in three other dwarf planets, Sedna, Quaoar, and Gonggong. All three show evidence of ethane ice—a product of methane irradiation—with indications of methane on the surface of Quaoar, which is the smallest and innermost of the three bodies, said Joshua Emery, a planetary scientist at Northern Arizona University in Flagstaff and the lead author of a study that has been accepted for publication in Icarus. Grundy is a coauthor of the study.
“The ethane likely isn’t primordial—it wasn’t accreted with these bodies, but likely formed afterward,” Emery said. “Each of them probably had methane on its surface, but it was irradiated.”
“We need to think of much more of our solar system, or solar systems in general, as being potentially habitable.”
If left long enough, however, irradiation would destroy the ethane and related molecules, Emery said, suggesting the methane must have been brought to the surfaces of all three bodies relatively recently.
That, in turn, suggests that like Eris and Makemake, Sedna, Quaoar, and Gonggong must be differentiated bodies that generated enough heat to produce methane, which reached the surface through cracks, slow outgassing, or some other process. “Any body in the outer solar system that was differentiated probably had a liquid ocean at some point,” Emery said. Oceans on Quaoar, Gonggong, and Sedna, however, “probably haven’t lasted to the present day.”
Beyond revealing some of the evolutionary history of these icy worlds, the research teams say these recent observations also suggest that at least some of the dwarf planets could have been habitable. “The ingredients you need for life—liquid water, chemical energy, thermal gradients—are all there or were there,” said Grundy. “It shows that we need to think of much more of our solar system, or solar systems in general, as being potentially habitable.”
—Damond Benningfield, Science Writer