Observations taken using the Herschel Space Observatory have given us the answer to the question of “what are Uranus’s moons made of?” A question, it should be noted, the research team wasn’t even trying to answer. As co-author Ulrich Klaas explains: “Actually, we carried out the observations to measure the influence of very bright infrared sources such as Uranus on the camera detector. We discovered the moons only by chance as additional nodes in the planet’s extremely bright signal.”

Cold objects, such as Uranus and the fives moons included in this study, radiate very brightly in the infrared spectrum between 70 and 120 micrometers. They can reach blazingly hot… I mean bitterly cold temperatures around –213 to –193 °C.  Add in the weirdness that is Uranus’s extremely tilted orbit, and researchers had perfect positions to see how the surfaces of the moons retained heat as they rotated. The team found that the moons stored heat well and cooled down slowly.

The conclusion was that the moons are similar in composition to the dwarf planets at the edge of our solar system, like Pluto or Haumea. They show characteristics like that of Transneptunian Objects, which are located in a zone beyond the planet Neptune. Scientist Thomas Müller noted, “This would also fit with the speculations about the origin of the irregular moons. Because of their chaotic orbits, it is assumed that they were captured by the Uranian system only at a later date.”

I’m not going to say the result is a surprise. We’ve suspected that the outer planets have a lot of captured moons. But suspicion isn’t the same as confirmation, and it’s nice to see science telling us we were right.

More Information

Max Planck Institute for Astronomy press release 

“Herschel-PACS Photometry of the Five Major Moons of Uranus,” Ö. H. Detre, T. G. Müller, U. Klaas, et al., 2020 Sep. 14, Astronomy & Astrophysics (preprint on arxiv.org)