Uranus, far from Earth in the darker region of the solar system’s planetary range, is not alone. It is accompanied by a sequence of moons – 27 to be exact. Long and faint, these moons are difficult to study, but astronomers have found an unintentional discovery while observing Uranus.
According to infrared images of the five main moons of Uranus, their composition is closer to dwarf planets like Pluto and Haumea – compact, rocky objects with an icy crust – than the more fluffy composition of the smaller Uranus moons.
Uranus orbits the sun at an average distance of about 20 times Earth. We have not sent so many spacecraft so far ̵
Apart from these Voyager 2 observations, our study of the planet and its satellites has relied on telescopes closer to home – on Earth and in Earth’s orbital proximity. This makes the moons very challenging to see; they are much smaller and reflect much less sunlight than Uranus, so they are overshadowed to the point of invisibility.
“The moons, which are between 500 and 7,400 times fainter, are so small from Uranus that they merge with the corresponding bright artifacts,” said astronomer Gábor Marton of the Konkoly Observatory in Hungary. “Only the brightest moons, Titania and Oberon, stand out slightly from the surrounding glare.”
Which makes an unintentional detection of the five moons of the European Space Agency’s Herschel Space Observatory, in operation between 2009 and 2013 to study the galaxy in infrared radiation, quite nice.
“In fact, we conducted observations to measure the influence of very bright infrared sources like Uranus on the camera detector,” said astronomer Ulrich Klaas of the Max Planck Institute for Astronomy in Germany.
“We only discovered the moon by chance as additional nodes in the planet’s extremely bright signal.”
Uranus’ five main moons are in descending order of Titania, Oberon, Umbriel, Ariel and Miranda. Voyager 2 revealed that all five have a rounded shape indicating that they have achieved hydrostatic equilibrium – that is, enough mass to develop a symmetrical, rounded shape under their own gravity. And they appear to be composed of rocks and ice.
This is not unusual for objects that are far from the sun. Even when heated by the sun, the temperature of Uranus and its moons is only as high as 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 stone and ice is composed matters. The eccentric orbits of Uranus’ smaller, irregular, asymmetrical moons suggest that they have a composition very reminiscent of the rock-filled bodies of the Kuiper Belt beyond Neptune, the trans-Neptunian objects. These are only loosely bound together and quite small.
“This would also fit with the speculation about the origin of the irregular moons,” said astrophysicist Thomas Müller of the Max Planck Institute for Extraterrestrial Physics in Germany. “Because of their chaotic orbits, it is believed that they were only captured by the uranium system at a later date.”
Normally, the five main moons orbiting Uranus’ equator would be difficult to see. Uranus has a strange orientation, tilted on its side relative to its orbital plane around the sun, so its equator is often in shadow.
During the team’s observations of Uranus between 2010 and 2012, however, the equator was overlooking the telescope and in the sunlight. And when the team pulled Uranus from the data using a specially developed algorithm, something amazing occurred.
“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 uranium moons,” said astronomer Örs H. Detre of the Max Planck Institute for Astronomy.
This allowed the team to measure how well the heat from the sun was retained in the lunar surfaces as the surface rotated away into the night. These surfaces, it turned out, retained heat quite well and cooled relatively slowly.
It was a well-known heat retention and cooling profile – the closest battle is dwarf planets like Pluto and Haumea with their dense rocky bodies and ice-encapsulated 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 ice on it has not yet been determined.
The discovery could mean that sending a probe to the ice fighters could help us learn more about more distant objects, even further out in the weak stretches of the Kuiper Belt. But it also shows the value of looking right under our nose.
“The results show that we do not always need extensive planetary 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 could be used for further observations that have been collected in large numbers in the electronic data archive of the European Space Agency ESA. Who knows what surprise is still waiting for us there?”
The research is published in Astronomy and astrophysics.