Astronomers see the "hot" glow of Uranus' rings



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Uranus rings are invisible to all except the largest telescopes – they were not discovered until 1977 – but are surprisingly bright in new images of the planet's heat taken by two large telescopes in the high deserts of Chile.


Uranus and its rings

Composite image of the Uranian atmosphere and rings at radio wavelengths taken from the ALMA matrix in December 2017. The image shows the thermal emission, or heat, of the Uranus rings for the first time, allowing scientists to determine their temperature : the frigid 77 Kelvin (-320 F). Dark bands in the atmosphere of Uranus at these wavelengths show the presence of molecules that absorb radio waves, in particular hydrogen sulfide. Brilliant regions like the north polar point (yellow dot on the right, because Uranus is tilted sideways) contain very few of these molecules. (UC Berkeley image by Edward Molter and Imater de Pater)

The thermal glow gives astronomers another window for the rings, which were seen only because they reflect a bit of light in the visible, or optical, and near-infrared range. The new images taken by the Atacama Large Millimeter / submillimeter Array (ALMA) and the Very Large Telescope (VLT) allowed the team to first measure the temperature of the rings: a cool 77 Kelvin, or 77 degrees above absolute zero – the temperature of boiling of liquid nitrogen and equivalent to 320 degrees below zero Fahrenheit.

The observations also confirm that the brighter and denser ring of Uranus, called the epsilon ring, differs from other known ring systems within our solar system, in particular the spectacularly beautiful rings of Saturn.

"The mainly saturnar rings are wide, bright and have a range of particle sizes ranging from micron-sized dust on the innermost D-ring to up to tens of meters in size on the main rings," said Imke de Pater, a professor at the University of Berkeley. astronomy. "The small end is missing in the main rings of Uranus; the brightest ring, Epsilon, is composed of rocks the size of golf balls and larger. "

In comparison, Jupiter's rings contain mostly small particles of micron size (a micron is a thousandth of a millimeter). Neptune's rings are also mostly dust, and even Uranus has large sheets of dust between its narrow main rings.

"We already know that the epsilon ring is a bit strange, because we do not see the minor things," said student Edward Molter. "Something has swept the smaller things, or it's all together. We just do not know. This is a step to understand its composition and whether all the rings came from the same material of origin, or are different for each ring. "

Rings may be ancient asteroids captured by the planet's gravity, remnants of moons that collided and broken, the remains of torn-off moons when they came very close to Uranus, or remnants of the formation time of 4.5 billion years ago.


detail of uranium rings

Near-infrared image of the uranian ring system obtained with the adaptive optics system of the 10-meter Keck telescope in Hawaii in July 2004. The image shows reflected sunlight. Among the main rings, which are composed of particles the size of a centimeter or larger, dust sheets can be seen. The epsilon ring seen in new thermal images is at the bottom. (UC Berkeley image by Imke de Pater, Seran Gibbard and Heidi Hammel, 2006)

The new data was published this week in The Astronomical Journal. De Pater and Molter led ALMA's observations, while Michael Roman and Leigh Fletcher of the University of Leicester in the UK led the VLT observations.

"The rings of Uranus are compositionally different from the main ring of Saturn, in the sense that in optical and infrared, the albedo is much lower: they are actually dark, like coal," Molter said. "They are also extremely narrow in comparison to Saturn's rings. The broadest, the epsilon ring, ranges from 20 to 100 kilometers wide, while Saturn is 100 or tens of thousands of miles wide. "

The lack of dust-size particles on Uranus' main rings was first noticed when Voyager 2 flew across the planet in 1986 and photographed them. The spacecraft was unable to measure the temperature of the rings, however.

So far, astronomers have counted a total of 13 rings around the planet, with some bands of dust between the rings. The rings differ in other ways from those of Saturn.

"It's cool that we can even do that with the instruments we have," he said. "I was just trying to imagine the planet as best I could and saw the rings. It was fantastic."

Both observations of the VLT and ALMA were designed to explore Uranus' atmosphere temperature structure, with VLT probing shorter wavelengths than the ALMA.

"We were surprised to see the rings clearly jump when we reduced the data for the first time," Fletcher said.

This represents an exciting opportunity for the upcoming James Webb Space Telescope, which will be able to provide vastly improved spectroscopic restrictions on Uranian rings in the next decade.


Uranus and his rings pictures

Images of the system of rings of Uranus captured at different wavelengths by ALMA and VLT telescopes. The planet itself is masked because it is very bright compared to the rings. (Images by Edward Molter, Imater de Pater, Michael Roman and Leigh Fletcher, 2019)

The Berkeley survey was funded by the National Aeronautics and Space Administration (NNX16AK14G). The work at the University of Leicester was supported by the European Research Council (GIANTCLIMES) under the Horizon 2020 research and innovation program of the European Union (723890).

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