New findings suggest Saturn’s rings are relatively young

Posted: 18 Jan 2019 01:33 AM PST

An artist’s concept of the Cassini orbiter crossing Saturn’s ring plane. New measurements of the rings’ mass give scientists the best answer yet to the question of their age. Image credit: NASA/JPL-Caltech

The rings of Saturn may be iconic, but there was a time when the majestic gas giant existed without its distinctive halo. In fact, the rings may have formed much later than the planet itself, according to a new analysis of gravity science data from NASA’s Cassini spacecraft.

The findings indicate that Saturn’s rings formed between 10 million and 100 million years ago. From our planet’s perspective, that means Saturn’s rings may have formed during the age of dinosaurs.

The conclusions of the research – gleaned from measurements collected during the final, ultra-close orbits Cassini performed in 2017 as the spacecraft neared the end of its mission – are the best answer yet to a longstanding question in Solar System science. The findings were published online 17 January 2019 in Science.

Saturn formed 4.5 billion years ago, in the early years of our Solar System. There have been clues that its ring system is a young upstart that attached to Saturn years afterward. But how long afterward?

To figure out the age of the rings, scientists needed to measure something else: the mass of the rings, or how much material they hold. Researchers had the remote-sensing measurements from Cassini and both of NASA’s Voyager spacecraft in the early 1980s. Then came Cassini’s unprecedented, up-close data from its final orbits. As the spacecraft was running out of fuel, it performed 22 dives between the planet and the rings.

The dives allowed the spacecraft to act as a probe, falling into Saturn’s gravity field, where it could feel the tug of the planet and the rings. Radio signals sent to Cassini from the antennas of NASA’s Deep Space Network and the European Space Agency relayed the spacecraft’s velocity and acceleration.

Once scientists knew how much gravity was pulling on Cassini, causing it to accelerate – down to a fraction of a millimetre per second – they could determine how massive the planet is and how massive the rings are.

“Only by getting so close to Saturn in Cassini’s final orbits were we able to gather the measurements to make the new discoveries,” says Cassini radio science team member and lead author Luciano Iess, of Sapienza University of Rome, Italy. “And with this work, Cassini fulfils a fundamental goal of its mission: not only to determine the mass of the rings, but to use the information to refine models and determine the age of the rings.”

During the Grand Finale, Cassini passed between the inner edge of Saturn's D-ring and the cloud top. This orbital configuration allowed the disentanglement of the tiny acceleration of the rings from the large acceleration due to Saturn. The two forces pull the spacecraft in opposite directions. Image credit: UC Berkeley/NASA/JPL-Caltech

Iess’ paper builds on a connection scientists previously made between the mass of the rings and their age. Lower mass points to a younger age, because the rings, which are bright and mostly made of ice, would have been contaminated and darkened by interplanetary debris over a longer period. With a better calculation of ring mass, scientists were better able to estimate the rings’ age.

Saturn scientists will continue work to figure out how the rings formed. The new evidence of young rings lends credence to theories that they formed from a comet that wandered too close and was torn apart by Saturn’s gravity – or by an event that broke up an earlier generation of icy moons.

From Cassini’s super-close vantage point, immersed in Saturn’s gravity field, the spacecraft relayed measurements that led scientists to another surprising discovery.

It’s long been known that Saturn’s equatorial atmosphere rotates around the planet faster than its inner layers and core. Imagine a set of nested cylinders, rotating at different speeds. Eventually, toward the centre of the planet, the layers move in synchrony and rotate together.

Jupiter’s atmosphere behaves like this, too. But the new findings show that Saturn’s layers start rotating in synchrony much deeper into the planet – at least 9,000 kilometres (5,600 miles) in. That’s three times deeper than the same phenomenon at Jupiter. It’s a depth that equals 15 percent of Saturn’s entire radius.

“The discovery of deeply rotating layers is a surprising revelation about the internal structure of the planet,” says Cassini Project Scientist Linda Spilker of JPL. “The questions are what causes the more rapidly rotating part of the atmosphere to go so deep, and what does that tell us about Saturn’s interior?”

At the same time, the measurement of Saturn’s gravity solved yet another unknown: the mass of the core. Models of the interior developed by Burkhard Militzer, a UC Berkeley professor and a co-author of the paper, indicate that it is 15 to 18 Earth masses.

Cassini’s mission ended in September 2017, when it was low on fuel and deliberately plunged into Saturn’s atmosphere to protect the planet’s moons. More science from the last orbits, known as the Grand Finale, will be published in the coming months.

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NIGHT SKY: Get ready for Monday’s total lunar eclipse!

Posted: 17 Jan 2019 08:44 AM PST

A total lunar eclipse occurs when the Earth passes between the Moon and the Sun. Image credit: Ahmed Mosaad

Next Monday morning – on 21 January – skywatchers across the UK, north-western France, north-western Spain, Portugal, a small part of west Africa, North and South America, the eastern Pacific and the north-eastern tip of Russia will be able to see a total eclipse of the Moon. This spectacular event is easy to see and is the last chance for UK observers to see a total lunar eclipse in its entirety until 2029.

A total lunar eclipse occurs when Earth passes exactly between the Sun and the Moon. The Sun is behind Earth, and the Moon moves into Earth’s shadow.

The Moon first darkens slowly as it moves into the penumbra of Earth, the lighter part of the terrestrial shadow. When the Moon is completely within the umbra, the darker part of the shadow, it takes on a red hue that varies in colour.

Sometimes the eclipsed Moon is a deep red colour, almost disappearing from view, and sometimes it can be quite bright. The colour is due to Rayleigh scattering – where the Sun’s blue light is scattered off molecules in Earth’s atmosphere – which also happens at sunsets. The Sun’s red light is scattered much less by air, and is bent by Earth’s atmosphere in a process called refraction, traveling all the way through it to light up the Moon’s surface.

There are different types of lunar eclipse but a total eclipse is the most spectacular and is the only type that causes the Moon to appear red.

The end of a total eclipse. Image credit: Siebbi

Lunar eclipses occur several times a year and take place as often as solar eclipses, if all the different types are considered. Whereas you can only see a total solar eclipse if you are in the narrow path of the Moon’s shadow, lunar eclipses are visible wherever the Moon is above the horizon at the time, so each one can be seen from a large area of Earth. For that reason, they are much more common from any given location.

Lunar eclipses always happen at a full Moon as this is when it moves behind Earth and into line with Earth and the Sun. A full Moon happens every month, but most of the time no eclipse takes place. This is because the plane of the Moon’s orbit around Earth is slightly tilted compared to the plane of Earth’s orbit around the Sun, so the Moon normally passes a little above or below the terrestrial shadow. Eclipses only happen when the Moon, Earth, and Sun are lined up in all three dimensions.

On 21 January 2019 in the UK, the Moon will enter the penumbra at 02:35 GMT and the umbra at 03:33 GMT. The full eclipse (totality) begins at 04:40 GMT, with mid-eclipse at 05:12 GMT – this is the time when the whole Moon will appear red – and it ends at 0543 GMT. The Moon exits the umbra at 0651 GMT and the eclipse comes to an end as it leaves the penumbra at 07:49 GMT.

Lunar eclipses are very easy to witness as no special equipment or safety precautions are required. Solar eclipses are dangerous because observing the Sun directly can damage your eyesight, but the light from a lunar eclipse is much fainter and so is completely safe.

To watch the eclipse on 21 January all you have to do is dress warmly and go outside. If you can see the full Moon you will be able to observe the eclipse as it happens. If you want a close-up view of the Moon as it turns red, a pair of binoculars or a small telescope is helpful. But because lunar eclipses are easy to watch with the unaided eye, you can just go outside and enjoy the view.

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