Uranian moon in new light

The images show the position of the five largest Uranian moons and their orbits around Uranus as seen by Herschel on 12 July 2011. Left: Calculated position and rotation of the moons. To the left of the orbital plane is our point. The size of the items is not shown on the scale. Right: False-colored map of infrared luminosity at a wavelength of 70 µm after removing a signal from the planet Uranus measured with the HAChel Space Observatory’s PACS instrument. The characteristic shape of the signs, which resembles a three-leaf clover, is an artwork made by binoculars. Credit: T. Muller (HdA) / ü h. Detre et al ./MPIA

More than 230 years ago astronomer William Herschel discovered the planet Uranus and its two moons. Using the Herschel Space Observatory, ersrs H of the Max Planck Institute for Astronomy. A group of astronomers led by Deitre has now succeeded in determining the physical properties of the five main moons of Uranus. The measured infrared radiation, which is generated by the sun heating their surfaces, suggests that these moons resemble dwarf planets like Pluto. The team developed a new analysis technique, which extracts faint signals from the moon next to Uranus, which is more than a thousand luminous. The study was published today in the journal Astronomy and Astrophysics.

To explore the outer regions of the solar system, space probes such as Vyjar 1 and 2, Cassini-Huygens and New Horizons were sent on long expeditions. Now led by Mr. H. Detre of the Max Planck Institute for Astronomy (MPIA) in Heidelberg, a German-Hungarian research group, shows that with appropriate technique and simplicity, interesting results can also be obtained with distant observations. Huh.

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

“We carried out observations to measure the effect of very bright infrared sources such as Uranus on the camera detector,” explains co-author Ulrich Klass, who led the Hershcity Space Observatory’s PAX camera working group with MPIA. The pictures were taken. “We only discovered moons as additional nodes in the planet’s extremely bright signal.” The PACS camera, developed under the leadership of the Max Planck Institute for Extraterrestrial Physics (MPE), was sensitive to wavelengths between 70 and 160 μm. It is more than a hundred times the wavelength of visible light. As a result, images of the same size as the Hubble Space Telescope are nearly a hundred times faster.

Uranian moon in new light

Images of the five largest Uranian moons: Miranda, Ariel, Umbril, Titania, and Oberon. Space probe Voyager 2 captured these images on 24 January 1986 during a fly-by. The diameter of the moon is shown on the scale. Sincerely: NASA / JPL / MPIA

In this spectral range, cold objects radiate very brightly as Uranus and its five main moons, warmed by the Sun – reach temperatures between about 60 and 80 K (-213 to –193 ° C). .

“The time of observation was also a stroke of luck,” explains Thomas Muller from MPE. The rotational axis of Uranus, and thus also the orbital plane of the moons, is unusually inclined in their orbit around the Sun. While Uranus orbits the Sun for several decades, it is primarily the northern or southern hemisphere that is illuminated by the Sun. “During the observations, however, the situation was so favorable that the equatorial regions benefited from solar radiation. This enabled us to measure how well the surface retained heat when it moved toward night due to the rotation of the moon. ” It teaches us a lot about the nature of the material, “explains Muller, who calculated the model for this study. From this he derived the thermal and physical properties of the moon.

When Vyjar 2 made space probes in 1986, the constellation was much less favorable. Scientific instruments can only capture the South Pole regions and moons of Uranus.

Müller found that these surfaces accumulate heat unexpectedly well and cool comparatively slowly. Astronomers know this behavior from compact objects with a rough, icy surface. That is why scientists believe that these moons are celestial bodies similar to dwarf planets on the edge of the solar system, such as Pluto or Humia. Independent studies of some external, irregular Uranian moons, which are also based on observations with PACS / Herschel, show that they have different thermal properties. These moons reflect the characteristics of small and loosely bound Transneputian objects, which lie in a region beyond the planet Neptune. “It would also fit with speculation about the origin of irregular moons,” Muller says. “Because of their chaotic orbits, it is believed that they were captured by the Uranian system only at a later date.”

Uranian moon in new light

These pictures show how the Uranian moon was extracted from the data. Left: The original image contains infrared signals from Uranus and its five main moons, measured at a wavelength of 70 μm. Uranus is several thousand times faster than a moon. Its image dominates in artifacts due to telescope and camera interference. Titania and Oberon are barely visible. Center: Using these data, a sophisticated process alone created a model for the brightness distribution of Uranus. It is subtracted from the original image. Correct: Finally, the signs of the moons remain after subtraction. Perfect extraction method does not affect Uranus results slightly. Sincerely: Ö ​​H. Detre et al ./MPIA

However, the five main moons were almost ignored. In particular, very bright objects such as Uranus produce strong artifacts in PACS / Herschel data, which cause some infrared light in images to be distributed over large areas. This is noticeable when observing faint celestial objects. However, with Uranus, this is even more pronounced. “The moons, which are between 500 and 7400 times faint, are so short of Uranus that they merge with similar luminous artifacts. Only the brightest moons, Titania and Oberon, are slightly out of the surrounding glare. Stand up, “co. -Theor Gabor Marten from the Conkoli Observatory in Budapest describes the challenge.

This accidental discovery inspired .rs H. Detre to make the moon more visible so that its brightness could be measured reliably. “In similar cases, such as the discovery of exoplanet, we use coronagraphs to mask their bright central star,” Detre explains. “Herschel had no such device. Instead, we took advantage of the outstanding photometric stability of the PACS instrument.” Based on this stability and calculating the exact position of the moons at the time of observations, they developed a method that allowed them to extract Uranus from the data. “We were all surprised when four moons were clearly visible on the images, and we could also detect the smallest and smallest Miranda of the five largest Uranian moons,” Detre concludes.

MPIA co-author Hendrik Linz explains, “The result shows that we don’t always need detailed space missions to gain new insights into the solar system.” “In addition, the new algorithm can be applied to further observations that have been collected in large numbers in the electronic data collection of the European Space Agency ESA. Who knows what surprises are still there for us?”

The moons of Uranus are fascinating enough that we should send a flagship mission there

more information:
O. H. Detre et al. Herschel-Pax Photometry of Five Major Moons of Uranus Astronomy and Astrophysics (2020). DOI: 10.1051 / 0004-6361 / 202037625

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