Uranus’s moons resemble Pluto

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More than 230 years after the discovery of Uranus and two of its moons, we are finally getting a sense of what these moons look like.

It has been 230 years since astronomer William Herschel stumbled upon the planet Uranus and two of its moons. Later, twenty-five moons were added to the list. However, much is not known about these moons and their physical properties have long been shrouded in mystery. But a new investigation now lift a tip of the veil.

Missions
That we don’t know much about Uranus’s moons is not surprising in itself: the planet’s distance from Earth fluctuates between a whopping 2.5 billion and 3.1 billion kilometers. Space probes such as Voyager 1 and Voyager 2, Cassini-Huygens and New Horizons were sent to explore the outer regions of the solar system. And while these craft, equipped with ingenious scientific instruments, have managed to gather a lot of information, they failed to reveal much about Uranus’ moons. When Voyager 2 passed Uranus in 1986, it only captured the south polar regions of Uranus and its moons. But thanks to an accidental discovery, we now get a unique insight into the moons of Uranus.

Photograph of Uranus’s five largest moons: Miranda, Ariel, Umbriel, Titania, and Oberon. Image: NASA / JPL / MPIA

The researchers used data from the European infrared space telescope Herschel, which was active between 2009 and 2013. Compared to its predecessors covering a similar spectral range, the observations from this telescope were significantly sharper. However, that this space telescope was also able to detect Uranus’ moons was a great surprise. “We actually carried out observations at the time to measure the influence of very bright infrared sources such as Uranus on the camera detector,” says researcher Ulrich Klaas. “We then discovered the moons by chance.” The camera turned out to be sensitive enough to also receive signals from the five main moons of Uranus (Titania, Oberon, Miranda, Ariel and Umbriel).

Brightness
First, the light of Uranus had to be filtered out of the data. “The moons are between 500 and 7400 times weaker,” explains researcher Gábor Marton. “In addition, they are so close to Uranus that the light merges. Only the brightest moons, Titania and Oberon, stand out a bit from the surrounding brilliance. ” Using the PACS instrument on the Herschel telescope, the researchers managed to remove Uranus from the data. “We were all surprised when four moons clearly appeared,” says study leader Örs Detre. “We could even discover Miranda; the smallest and innermost moon of the five largest moons of Uranus. ”

Left: The calculated position and orbits of the five largest moons of Uranus. Right: map showing the colors that represent infrared brightness after the Uranus signal has been filtered out. Image: T. Müller (HdA) / Ö. H. Detre et al./MPIA

The data allowed researchers to study the moons well. “The timing of the observations was quite a stroke of luck,” says researcher Thomas Müller. Although Uranus takes decades to complete a circle around the sun (about 84 years), the sun mainly illuminates the northern, or southern hemisphere. “During the observations, however, the position was so favorable that sunlight also fell on the equator,” continues Müller. “This allowed us to measure how well the heat is retained on the surface as it moves towards the night side as a result of the moon’s rotation. This has taught us a lot about the nature of the material. ”

Pluto
The researchers found that the surfaces of the five Uranus moons studied unexpectedly store a lot of heat and then cool down relatively slowly during the night. And that sounds familiar to astronomers. Compact objects with a rough, icy surface do the same. Scientists therefore suspect that the moons of Uranus resemble the dwarf planets that lurk at the edge of our solar system, such as Pluto or Haumea. Uranus’ moons thus show features of smaller, so-called trans-Neptunian objects. “This would also fit speculation about the origin of the moons,” added Müller. “Because of their chaotic orbits, they are believed to have been captured by Uranus at a later date.”

The results show that extensive planetary space missions are not always required to gain new insights about our solar system. There is also a lot to discover with (former) telescopes. In addition, the new algorithm could be applied to other collected data stored in ESA’s immense archive. And who knows what surprise is still waiting for us there.

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