Webb Telescope Spies Hints That Solar System's Outskirts Are a Hotbed of Activity

Big new telescopes discover surprises anywhere they point, and the space between Neptune and the nearest stars is no exception. Unfailingly, the new telescopes answer some questions (or, at least, settle some arguments) but generate lots more questions. It’s career security for space scientists!

Eris and Makemake are two of the largest trans-Neptunian objects (TNOs), denizens of the space beyond Neptune. Most TNOs have extremely dark surfaces, the result of being weathered by solar and cosmic radiation for billions of years. However, the biggest TNOs — Eris and Makemake along with Pluto, Charon, Haumea, Sedna, and the former TNO Triton — are much brighter. That brightness suggests some kind of activity refreshes the surface, but it isn’t proof, and doesn’t suggest what kind of activity that might be. We’ve seen Triton, Pluto, and Charon up close, and they’re all different; so it’s anybody’s guess what’s going on with the worlds we haven’t seen.

Enter the James Webb Space Telescope (JWST), which is barging into all kinds of space puzzles these days. We knew already that both Eris and Makemake had methane ice on their surfaces. Now, a team of scientists has pointed JWST at them to measure isotopic ratios of hydrogen and carbon in their methane ice. The results appear in Icarus (paper 1, paper 2).

Why are isotopes important? When Eris and Makemake formed, they formed with at least some hydrogen in their water and rocks. That hydrogen would have had the same proportion of deuterium (heavy hydrogen) as other primordial objects, such as comets. Any methane that accreted along with the water and rocks would have the same proportion of deuterium.

But JWST did not find comet-like amounts of heavy hydrogen on Eris and Makemake. Instead, Eris and Makemake’s methane has much less deuterium than expected. What does that mean? In short: Hot chemistry has made “new” methane on both worlds.

There are two main ways to make new methane (without invoking space cows or any other kind of life). If you heat up rocks that contain organic (carbon-rich) materials and also water-containing minerals, the heat will break apart the organic molecules, and loose carbons can bond with hydrogen to make methane. This process doesn’t even need liquid water to work and can happen anywhere with rocks of the necessary composition above 150°C (300°F). This newly formed methane would then find its way out through cracks and pores to freeze onto the surface.

The second way to make new methane without life — that is, abiotically — is to run hot water through rocks that contain small carbon molecules, such as carbon monoxide and carbon dioxide. These molecules are typically in ice form at trans-Neptunian temperatures, and would have been abundant when Eris and Makemake were first forming. If the two worlds became hot enough in the distant past to partially melt and form internal oceans, then the water contacting the rocks would’ve made all kinds of chemical reactions happen — including ones that generate methane gas abiotically. Depending on how hot the worlds became, there could have been some pretty spectacular methane-driven volcanism, driven by the buildup of gas within the deep ocean.

So JWST has uncovered evidence that Pluto and Triton are not alone in having youthful surfaces that have been refreshed by geologic processes: Eris and Makemake have probably also experienced at least some geology and, possibly, explosive methane volcanism. If Makemake were volcanic in the past, it might not be at present because it’s not as big; the Uranian moon Titania might be what Makemake looks like today. However, Eris is much bigger — significantly more massive than Pluto, actually — so it’s very likely that Eris remains, like Pluto, active today.

At the end of the paper, the authors muse that “visits to Eris and Makemake with spacecraft are intriguing to ponder.” They sure are.