Uranus, far from Earth in the darker region of the planetary range of the solar system, is not alone. It is accompanied by a cluster of moons – 27, to be exact. Far and obscure, these moons are difficult to study, but astronomers made an accidental discovery while observing Uranus.
According to the infrared images of Uranus’ five major moons, their composition is closer to that of the dwarf planets such as Pluto and Haumea – compact, rocky objects with ice crusts – than the fluffier composition of the smaller Uranus moons.
Uranus orbits the Sun at an average distance of about 20 times that of Earth. We haven’t sent many spacecraft that far ̵
In addition to these observations on Voyager 2, our study of the planet and its satellites relies on telescopes closer to home – on Earth and in orbital proximity to Earth. This makes the moons very challenging to watch; they are much smaller and reflect much less sunlight than Uranus, so they escape to the point of invisibility.
“The moons, which are between 500 and 7,400 times weaker, are so short of Uranus that they merge with equally bright artifacts,” said astronomer Gabor Marton of the Concolly Observatory in Hungary. “Only the brightest moons, Titania and Oberon, stand out a little from the surrounding glare.”
Which makes the accidental discovery of the five moons by the European Space Agency’s Herschel Space Observatory, working between 2009 and 2013 to study the galaxy in infrared, quite admirable.
“We actually did the observations to measure the effect of very bright infrared sources like Uranus on the camera’s detector,” said astronomer Ulrich Klaas of the Max Planck Institute for Astronomy in Germany.
“We found the moons only by chance as additional nodes in the extremely bright signal of the planet.”
The five main moons of Uranus are in descending order in size Titania, Oberon, Umbriel, Ariel and Miranda. Voyager 2 revealed that all five have a rounded shape, which indicates that they have achieved hydrostatic equilibrium – that is, enough mass to develop a symmetrical, rounded shape under their own gravity. And they seem to be made up of rock and ice.
This is not uncommon for objects that are far from the Sun. Even when warmed by the Sun, the temperatures of Uranus and its moons reach only between 60 and 80 Kelvin (-213 to -193 degrees Celsius or -350 to -315 degrees Fahrenheit) on the surface. Pluto is very rocky and icy.
But how this rock and ice is combined matters. The eccentric orbits of Uranus’ smaller, irregular, asymmetrical moons suggest that they have a composition very similar to the rocky bodies in the Kuiper Belt, outside of Neptune, the trans-Neptunian objects. They are only weakly connected and quite small.
“This would also fit assumptions about the origin of irregular moons,” said astrophysicist Thomas Mueller of the Max Planck Institute for Extraterrestrial Physics in Germany. “Because of their chaotic orbits, it is assumed that they were captured by the uranium system only at a later date.”
Usually, the five main moons orbiting the equator of Uranus would be difficult to see. Uranus has a strange orientation, tilted to the side relative to its orbital plane around the Sun, so its equator is often in shadow.
However, during the team’s observations of Uranus between 2010 and 2012, the equator was in view of the telescope and sunlight. And when the team extracted Uranus from the data using a specially designed algorithm, something amazing happened.
“We were all surprised when four moons appeared clearly in the images, and we could even find Miranda, the smallest and innermost of the five largest uranium moons,” said astronomer Örs H. Detre of the Max Planck Institute of Astronomy.
This allowed the team to measure how well the heat from the Sun is retained in the Moon’s surfaces as this surface rotates at night. It turned out that these surfaces retain heat quite well, cooling relatively slowly.
This was a familiar heat retention and cooling profile – the closest match being the dwarf planets such as Pluto and Haumea, with their dense rocky bodies and ice-covered surfaces. This suggests that Titania, Oberon, Umbriel, Ariel and Miranda are built in the same way – although the exact chemical composition of the rock and the ice on it is yet to be determined.
The discovery could mean that sending a probe to the ice giants could help us learn more about more distant objects, even farther in the twilight of the Kuiper Belt. But it also shows the value of looking right under your nose.
“The result shows that we don’t always need complex space missions to gain new insights into the solar system,” said astronomer Hendrik Linz of the Max Planck Institute for Astronomy.
“In addition, the new algorithm can be applied to further observations, which are collected in large numbers in the electronic data archive of the European Space Agency ESA. Who knows what surprise still awaits us there?”
The study was published in Astronomy and astrophysics.