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Uranian moons in new light

Uranian moons in new light

The images show the location of the five largest Uranus moons and their orbits around Uranus on July 12, 2011 seen by Herschel. Left: Calculated positions and orbits around the moons. The left side of the orbital plane points towards us. The size of the object is not shown on a scale. Right: False color chart of the infrared brightness at a wavelength of 70 µm after removing the signal from the planet Uranus measured with the PACS instrument from the Herschel Space Observatory. The characteristic shape of the signals, resembling a three-leaf clover, is an artifact generated by the telescope. Credit: T. Müller (HdA) / Ö. H. Detre et al./MPIA

More than 230 years ago, astronomer William Herschel discovered the planet Uranus and two of its moons. Using the Herschel Space Observatory, a group of astronomers led by Örs H. Detre of the Max Planck Institute for Astronomy have now succeeded in determining the physical properties of the five main moons of Uranus. The measured infrared radiation generated by the sun, which heats their surfaces, suggests that these moons look like dwarf planets like Pluto. The team developed a new analysis technique that extracted the faint signals from the moons next to Uranus, which are more than a thousand times brighter. The study was published today in the journal Astronomy and astrophysics.

To explore the outer regions of the solar system, space probes such as Voyager 1 and 2, Cassini-Huygens and New Horizons were sent on long expeditions. Now a German-Hungarian research group, led by Örs H. Detre from the Max Planck Institute for Astronomy (MPIA) in Heidelberg, shows that with the right technology and ingenuity, interesting results can also be achieved with observations far away.

The researchers used data from the Herschel Space Observatory, which was deployed between 2009 and 2013, and in whose development and operation MPIA was also significantly involved. Compared to its predecessors, which covered a similar spectral range, observations of this telescope were significantly sharper. It was named after astronomer William Herschel, who found infrared radiation in 1800. A few years earlier, he also discovered the planet Uranus and two of its moons (Titania and Oberon), which have now been explored in more detail along with three other moons (Miranda, Ariel and Umbriel).

“In fact, we conducted observations to measure the impact of very bright infrared sources such as Uranus on the camera detector,” explains co-author Ulrich Klaas, who led the working group for the PACS camera from the Herschel Space Observatory at MPIA with which the images were taken. “We only discovered the moon by chance as additional nodes in the planet’s extremely bright signal.” The PACS camera, developed under the direction of the Max Planck Institute for Extraterrestrial Physics (MPE) in Garching, was sensitive to wavelengths between 70 and 160 µm. This is more than a hundred times greater than the wavelength of visible light. As a result, the images from the corresponding Hubble Space Telescope are about a hundred times sharper.

Uranian moons in new light

Images of the five largest uranium moons Miranda, Ariel, Umbriel, Titania and Oberon. The spacecraft Voyager 2 took these images during a flight-city on January 24, 1986. The lunar diameters are shown on a scale. Credit: NASA / JPL / MPIA

Cold objects radiate very strongly in this spectral range, such as Uranus and its five main moons, which – heated by the sun – reach temperatures between approx. 60 and 80 K (–213 to –193 ° C).

“The timing of the observation was also a stroke of luck,” explains Thomas Müller of MPE. Uranus’ axis of rotation and thus also the orbits of the moons are unusually inclined to their orbit around the Sun. While Uranus orbits the sun for decades, it is mainly either the northern or southern hemisphere that is illuminated by the sun. “During the observations, however, the position was so favorable that the equatorial regions benefited from the sun’s radiation. This allowed us to measure how well the heat is retained in a surface as it moves to the night side due to the moon’s rotation. This taught us a lot. about the nature of the material, “explains Müller, who calculates the models for this study. From this he deduced the thermal and physical properties of the moons.

When the spacecraft Voyager 2 passed Uranus in 1986, the constellation was much less favorable. The scientific instruments could only capture the southern Polish regions of Uranus and the moons.

Müller found that these surfaces store heat unexpectedly well and cool down relatively slowly. Astronomers know this behavior from compact objects with a rough, icy surface. Therefore, scientists assume that these moons are celestial bodies 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 uranium moons, which are also based on observations with PACS / Herschel, indicate that they have different thermal properties. These moons show the characteristics of the smaller and loosely bound transneptunian objects, which are located in a zone beyond the planet Neptune. “This would also fit in with the speculation about the origin of the irregular moons,” Müller adds. “Because of their chaotic trajectories, it is believed that they were only captured by the uranium system at a later date.”

Uranian moons in new light

These images explain how the uranium moons were extracted from the data. Left: The original image contains the infrared signals from Uranus and its five main moons measured at a wavelength of 70 µm. Uranus is several thousand times brighter than a single moon. Its image is dominated by artifacts due to interference from the telescope and the camera. Titania and Oberon are barely visible. Center: Using this data, a sophisticated procedure created a model for the luminosity distribution of Uranus alone. This is subtracted from the original image. Right: Finally, the moons’ signals remain after the subtraction. At Uranus’ location, the not quite perfect extraction method affects the result slightly. Credit: Ö. H. Detre et al./MPIA

However, the five main moons were almost overlooked. Particularly bright objects such as Uranus generate strong artifacts in PACS / Herschel data, which cause some of the infrared light in the images to be distributed over large areas. This is hardly noticeable when observing faint celestial bodies. With Uranus, however, it is even more pronounced. “The moons, which are between 500 and 7400 times fainter, are so small from Uranus that they merge with the same bright artifacts. Only the brightest moons, Titania and Oberon, differ slightly from the surrounding glare,” co-author Gábor Marton from the Konkoly Observatory in Budapest describes the challenge.

This unintentional discovery spurred Örs H. Detre to make the moons more visible so that their brightness could be measured reliably. “In similar cases, such as searching for exoplanets, we use coronagraphs to mask their bright central star,” Detre explains. “Herschel did not have such a device. Instead, we took advantage of the excellent photometric stability of the PACS instrument.” Based on this stability and after calculating the exact position of the moons at the time of observations, he developed a method that allowed him to remove Uranus from the data. “We were all surprised when four moons clearly appeared in the images, and we could even detect Miranda, the smallest and innermost of the five largest uranium moons,” concludes Detre.

“The results show that we do not always need extensive planetary space missions to gain new insight into the solar system,” points out co-author Hendrik Linz of MPIA. “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 Uranus moons are fascinating enough alone that we have to send a flagship mission out there

More information:
ISLAND. H. Detre et al. Herschel-PACS photometry of the five large moons of Uranus, Astronomy and astrophysics (2020). DOI: 10.1051 / 0004-6361 / 202037625

Provided by the Max Planck Society

Citation: Uranian moons in new light (2020, 14 September) retrieved 14 September 2020 from https://phys.org/news/2020-09-uranian-moons.html

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