Using data from the PACS photometer onboard ESA’s Herschel Space Observatory, astronomers have found that thermal properties of the five major moons of Uranus — Titania, Oberon, Umbriel, Ariel and Miranda — resemble those of dwarf planets Pluto and Haumea more than those of smaller trans-Neptunian objects and Centaur asteroids.
Composite image of Uranus. Image credit: Marcos van Dam / W. M. Keck Observatory.
“Actually, we carried out the observations to measure the influence of very bright infrared sources such as Uranus on the camera detector,” said co-author Dr. Ulrich Klaas, an astronomer at the Max-Planck-Institut für Astronomie.
“We observed the moons only by chance as additional nodes in the planet’s extremely bright signal.”
“The timing of the observation was also a stroke of luck,” added co-author Dr. Thomas Müller, a researcher at the Max-Planck-Institut für extraterrestrische Physik.
Uranus is the seventh planet from the Sun with the third largest diameter in the Solar System.
The ice giant is surrounded by 13 faint rings and 27 moons.
One day on Uranus takes about 17 hours. And the planet makes a complete orbit around the Sun in about 84 Earth years.
Uranus is the only planet whose equator is nearly at a right angle to its orbit, with a tilt of 97.77 degrees — possibly the result of a collision with an Earth-sized object long ago.
This unique tilt causes the most extreme seasons in the Solar System. For nearly a quarter of each Uranian year, the Sun shines directly over each pole, plunging the other half of the planet into a 21-year-long, dark winter.
These images show the position of the five largest Uranian moons and their orbits around Uranus on 12 July 2011 as seen by Herschel. Left: calculated positions and orbits of the moons; the left side of the orbital plane is pointing towards us; the size of the objects is not to scale. Right: false-color map of the infrared brightness at a wavelength of 70 µm after removal of the signal from the planet Uranus, measured with the PACS instrument of the Herschel Space Observatory; the characteristic shape of the signals, which resembles a three-leaf clover, is an artifact generated by the telescope. Image credit: Detre et al, doi: 10.1051/0004-6361/202037625 / MPIA.
“During the observations, however, the position of Uranus was so favorable that its equatorial regions benefited from the solar irradiation,” Dr. Müller explained.
“This enabled us to measure how well the heat is retained in a surface as it moves to the night side due to the rotation of the moons. This taught us a lot about the nature of the material.”
“From this we derived thermal and physical properties of the moons.”
“The moons, which are between 500 and 7,400 times fainter, are at such a small distance from Uranus that they merge with the similarly bright artifacts,” said co-author Dr. Gábor Marton, an astronomer at Konkoly Observatory and the ELTE Eötvös Loránd University.
“Only the brightest moons, Titania and Oberon, stand out a little from the surrounding glare.”
“In similar cases, such as the search for exoplanets, we use coronagraphs to mask their bright central star,” added lead author Dr. Örs Detre, an astronomer at the Max-Planck-Institut für Astronomie.
“Herschel did not have such a device. Instead, we took advantage of the outstanding photometric stability of the PACS instrument.”
“Based on this stability and after calculating the exact positions of the moons at the time of the observations, we developed a method that allowed us to remove Uranus from the data.”
“We were all surprised when four moons clearly appeared on the images, and we could even detect Miranda, the smallest and innermost of the five largest Uranian moons.”
These images of the five largest Uranian moons — Miranda, Ariel, Umbriel, Titania and Oberon — were captured by NASA’s Voyager 2 spacecraft on January 24, 1986. The diameters of the moons are shown to scale. Image credit: NASA / JPL / MPIA.
The team found that the surface of Uranus’ major moons stores heat unexpectedly well and cools down comparatively slowly.
“We know this behavior from compact objects with a rough, icy surface,” the authors said.
“That is why we assume that these moons are similar to the dwarf planets at the edge of the Solar System, such as Pluto or Haumea.”
Independent studies of some of the outer, irregular Uranian moons indicate that they have different thermal properties.
Those moons show characteristics of the smaller and loosely bound trans-Neptunian objects.
“This would also fit with the speculations about the origin of the irregular moons,” Dr. Müller said.
“Because of their chaotic orbits, it is assumed that they were captured by the Uranian system only at a later date.”
The findings were published in the journal Astronomy & Astrophysics.
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Ö.H. Detre et al. 2020. Herschel-PACS photometry of the five major moons of Uranus. A&A 641, A76; doi: 10.1051/0004-6361/202037625
