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The beauty and mystery of Saturn’s rings revealed by the Cassini mission

The Conversation

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Cassini makes the first radio occultation of Saturn’s rings producing this simulated image with green for particles smaller than 5cm and purple where particles are large. NASA/JPL

Tanya Hill, Museum Victoria

What would Saturn be without its beautiful system of rings? Over the past 13 years, the Cassini space probe has shown us just how complex and dynamic the rings truly are.

The 20-year mission is coming to an end later this month when the probe makes its final destructive plunge into Saturn.

As part of its grand finale, Cassini has flown closer to the rings than ever before, first grazing the outermost edges of the rings before taking the risky leap of diving through the gap between the rings and Saturn.

Read more: A look back at Cassini’s incredible mission to Saturn before its final plunge into the planet

Saturn’s big empty

One of the surprises was that it’s quite empty in this gap. This is very different to when Cassini was bombarded by hundreds of dust particles per second as it moved past the outer rings late last year.

The sound of outer ring particles hitting the Cassini spacecraft.

But it meant good news for the mission as this final stage had a better chance for success if there was less material in the way.

During a recent ring dive in August, instead of orientating Cassini so that it flew antenna-first through the gap (offering it more protection), the spacecraft was turned around allowing it to capture a fantastic view of the rings as it dived past.

Cassini’s four-minute dive through the ring gap on August 20, 2017.

Know your ring ABCs

Over the centuries, as Saturn’s rings have been observed in finer detail, they have been broken into discrete sections. They are named alphabetically in order of discovery, which means from innermost to outermost the order is D, C, B, A, F, G and E.

The ring labels by discovery.

Saturn’s innermost ring D is much less dense and therefore fainter than its neighbouring ring C.

Observing the rings close to Saturn – the D ring is faint while in the lower part of the image the C ring is overexposed. NASA/JPL-Caltech/Space Science Institute

By comparing new Cassini images of the D ring with its original discovery image from Voyager in 1980, it’s possible to see changes in the ring over a relatively short period of time.

Comparing images of the D ring taken 25 years apart. The inset shows the fine detail achieved by Cassini. NASA/JPL/Space Science Institute

In the Voyager image, three relatively bright arcs can be seen in the D ring (the bright arc in the lower left of frame is the C ring). Most dramatically, the central and brightest arc has faded markedly and moved 200km closer to Saturn (the arc no longer lines up with the Voyager image).

Origin of the rings

We know that the rings are mostly made of water ice, but it’s not clear how they formed or even how old they are.

The fact that they are still bright, rather than coated in dust, suggests a young age – perhaps just 100 million years old, placing their formation in the time of the dinosaurs.

This is consistent with Cassini data, but this theory also presents a problem: it means that a previous collection of moons had a fairly recent and mighty smash-up, creating the rings and five of Saturn’s current-day moons.

A true colour image of Saturn’s rings. The bright dot above and to the right of centre is the planet Venus.
NASA/JPL-Caltech/Space Science Institute

Alternatively, Cassini has also shown that there is a lot less dust entering the Saturn system than was originally expected. This makes it possible for the rings to be both ancient and bright, having formed early in the life of the Solar System. Furthermore, interactions within the rings might dust them off and keep them looking young.

Finger on the source

For Saturn’s outermost E ring the source is pretty clear. The moon Enceladus orbits within this ring and Cassini observations have directly traced features in the ring back to geysers erupting from Enceladus’s surface.

Direct observations are well-matched with computer simulations that model the trajectories of tiny icy grains ejected from Enceladus’s geysers.
NASA/JPL-Caltech/Space Science Institute

While in the faint F ring, the moon Prometheus creates streamer-channels, drawing material out of the ring.

Prometheus interacts with the ring once every orbit, when it reaches the point that takes it furthest away from Saturn and closest to the F ring. As Prometheus orbits faster than the ring material, a new streamer is created that is ahead of the old one with every orbit.

A series of streamer-channels drawn out by the moon Prometheus.
NASA/JPL/Space Science Institute

Bulging waistlines

Several of Saturn’s smaller moons reside within and carve out gaps in the rings, and Cassini has shown them to have bulges around their middles.

The moon Pan was responsible for clearing the A ring’s large Encke Gap. As it collects the ring material, Pan’s gravity is not strong enough to spread the accumulated matter across its surface, and instead a striking ridge develops.

Gorging on material from Saturn’s rings these moons have grown round around the middle. NASA/JPL-Caltech/Space Science Institute

The tiny moon Daphnis is one of seven moons newly discovered by Cassini. It is just 8km across and as it orbits inside the A ring’s small Keeler Gap, it pulls on the ring particles leaving waves in its wake.

Daphnis raises waves in Saturn’s rings as it passes by. NASA/JPL-Caltech/Space Science Institute

Turning rings into moons

Cassini has spotted signs of a potential new moonlet forming on the very edge of Saturn’s bright A ring.

Caught in action: Saturn’s rings giving birth to a new tiny moon, see the disturbance visible at the outer edge of the planet’s A ring. NASA/JPL-Caltech/Space Science Institute

The newly formed object is probably less than a kilometre across but being able to see such a process in action was a complete surprise for Cassini scientists.

Read more: What Cassini’s mission revealed about Saturn’s known and newly discovered moons

It supports the theory that long ago, Saturn’s rings could have been much more massive and capable of producing some of the moons that exist today.

It also potentially provides insight into how the planets of the solar system formed, emerging out of the accretion disk that once orbited the young Sun.

The ConversationCassini has certainly achieved its mission objectives to explore Saturn, its atmosphere, magnetosphere and rings and to study Saturn’s moons, particularly Titan. So much has been learned, including the ability to gaze with wonder and awe at the amazing Solar System we are part of.

Looking back: The pale blue dot of the Earth can be seen below Saturn’s rings in this image Cassini captured on July 19, 2013. NASA/JPL-Caltech/Space Science Institute

This article was originally published on The Conversation. (Reblogged by permission). Read the original article.


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Why the sunrise is still later after the winter solstice shortest day

The Conversation

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Foggy winter morning at Sugarloaf Reservoir, Victoria. Melbourne Water/flickr, CC BY-NC-SA

Tanya Hill, Museum Victoria

We’ve just passed another winter solstice. Wednesday June 21 was the shortest day of the year. I live in Melbourne, so we had just 9 hours and 32 minutes of daylight, and it was dark and grey, so we certainly felt the lack of sunlight.

For those up north and closer to the equator the shortest day is not so extreme. For example, Brisbane had 10 hours and 24 minutes of daylight on Wednesday, almost an hour more than Melbourne. No wonder southerners head north for winter.

Traditionally, the solstice marks the time when the Sun “stands still”.

From our vantage point on Earth, the Sun is changing directions. At 2:24pm June 21, it reached its furthest north for the year and then started heading south.

If you’re like me, you might find that statement a little confusing. What it means is that the Sun is now moving higher in our northern sky, which course means it’s moving southward.

Still no morning Sun

We may have reached our shortest day, but unfortunately it will be a few more weeks before our mornings get any brighter. In fact, sunrise will shift slightly later (by a couple of minutes) and it won’t be until July that the trend will start to shift. Bad news indeed for those of us who struggle to get going in the morning.

But our days are still getting longer, just the extra daylight is added to our afternoons, not our mornings.

It’s a pattern that happens around the time of the solstice. At the winter solstice, the earliest sunset (or shortest afternoon), happens first, then the solstice (shortest day), followed by the latest sunrise (or shortest morning).

It works in the opposite way for the summer solstice in December. The earliest sunrise comes first (or longest morning), then the solstice (longest day), then the latest sunset (longest afternoon).

What’s in a day?

While our clocks mark out an equal 24 hours to every day, the Sun is not so steady.

When you take a photo of the Sun at the same time every day, not only do you see it move higher and lower in the sky, but it also appears “later” or “earlier” in the east-west direction.

Our inconstant Sun – an analemma is made when you take a photo of the Sun at the same time every day.

A solar day is the time it takes for the Sun to return to due north (or local noon) each day and it is constantly changing in length.

Not because of the the Earth’s rotation, which is really very constant (to the order of a millisecond). Every 23 hours and 56 minutes, the Earth rotates once on its axis.

But as the Earth rotates it also moves along its orbit around the Sun. After 23 hours and 56 minutes, the Earth has moved far enough along that it needs a further 4 minutes, on average, to realign itself to the Sun.

The extra minutes of the solar day: Position 1 – the Earth is pointing towards the Sun; Position 2 – the Earth’s completes one rotation; Position 3 – the Earth must rotate a little further to face the Sun again.

The key word here is average. In February, May, June and July a solar day can equal 24 hours. But around the autumnal equinox in March and the spring equinox in September, the solar day is about 20 seconds less than 24 hours, and at the solstices, the solar day is slightly more than 24 hours.

Turning, turning, turning

To understand what’s going on, we need to reframe the Earth’s movement. Let’s suspend reality for a moment and imagine how things would work if we switch from a Sun centred view to an Earth centred one.

Since the Earth is tilted by 23.5 degrees, let’s position the Earth upright and place the Sun’s orbit on a 23.5 degree tilt.

It often helps if you suspend reality and consider an Earth-centric view. Museums Victoria

The solstices are now obvious. They are the moments when the Sun reaches its most northern or southern points.

You can also see the why the Earth’s tilt (seen in the diagram as the tilt of the Sun’s orbit) causes the seasons. When the Sun hits its northern most point, it shines down on the northern hemisphere bringing the long days of summer.

While here in the south, as the Earth rotates on its axis, it’s the nights that are long and our winter days are short.

When the Sun crosses the Earth’s equator it is the time of the equinox (or equal day-night). The “Sun’s orbit” near the equator is relatively steep. The Sun is mostly moving north-south with only a small fraction of its movement in the east-west direction (or parallel to the equator).

And because the Sun doesn’t “move” very far in the east-west direction, the Earth doesn’t need to rotate as much for the Sun to return to due north and complete a solar day.

That’s why the solar day is less than 24 hours at the time of the equinox.

But at the extremes of the “Sun’s orbit” the rate of movement in the north-south direction slows and most of the Sun’s movement is now east-west or parallel to the Earth’s equator.

At these times, the Earth has to rotate even further to bring the Sun back to due north, and hence the solar day is longer than 24 hours around the time of the solstices.

The tilt and the solar day

So there are two things going on here. As we move towards winter, the tilt of the Earth makes the days grow shorter. This naturally brings later sunrises and earlier sunsets.

But as we approach the solstice, the second effect kicks in – the solar day starts getting longer. The Earth has to rotate more to bring the Sun back into place and this shifts both the sunrise and sunset progressively later.

It pushes the time of latest sunrise to occur after the solstice and that’s why we have this wait to see more of the morning Sun. It also means that the time of earliest sunset must happen before the solstice.

Sunrise over the Melbourne CBD. Michael Sale/flickr

At the summer solstice, it all plays out in the opposite way. As we move through spring, the tilt of the Earth makes the days grow longer – we have earlier sunrises and later sunsets.

But once again as the summer solstice nears, the lengthening solar day kicks into action, pushing both sunrise and sunset to happen later. It pushes the latest sunset to occur after the summer solstice, while earliest sunrise must occur before the solstice.

Of course, when we are basking in the summer Sun we don’t pay quite as much attention.

The ConversationSo just hang in there a little longer. We’ve made it past the shortest day and eventually the lengthening daylight will bring us brighter mornings.

Tanya Hill, Honorary Fellow of the University of Melbourne and Senior Curator (Astronomy), Museum Victoria

This article was originally published on The Conversation. (Reblogged by permission). Read the original article.

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Once upon a time… how the Rosetta mission won our hearts

The Conversation

Tanya Hill, Museum Victoria

Last Friday, September 30, the European Space Agency’s (ESA) Rosetta mission, which explored the Comet 67P/Churyumov-Gerasimenko, reached its final conclusion and was heralded a resounding success.

The mission accomplished great technical feats. It was the first to place a spacecraft into orbit around a comet and Rosetta was in the hot-seat to watch the sun turn this cold icy object into a hive of activity.

In November 2014, Rosetta released Philae, the first probe to land on the surface of a comet. The probe ended up bouncing across the comet’s surfacing before coming to rest in the shadows. But it did spend three successful days gathering scientific data before its primary battery was drained and communication was lost.

Just a month before mission end, Philae was finally found.
Main image and lander inset: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA; context: ESA/Rosetta/NavCam

The mission gathered a wealth of scientific information, as Comet Churyumov-Gerasimenko became the most studied comet in history. The comet’s gravity has been mapped, its various surface terrains have been identified, and its distinctive “rubber ducky” shape is now recognised as two smaller comets that gently melded together as one.

Data from Philae revealed that the comet’s surface is covered with key organic compounds, suggesting that the building blocks of life may be widespread throughout the universe.

But alongside all these great achievements has been the exciting array of science communication that has supported the mission. The goal of ESA was to reach out to as many people as possible and the team looked for new and interesting ways to capture the minds, and also the hearts, of a wide audience.

Once upon a time…

Long, long ago (or in reality back in January 2014), there was a little spacecraft that needed waking up. Launched a decade earlier, the spacecraft had been placed in hibernation for 31 months as it completed the last leg of its journey towards the comet.

The ESA team began a Wake Up, Rosetta! campaign to inform the public about this mission that had begun long ago but had a very exciting year ahead of it.

With wonderful insight, ESA recognised the parallel between Rosetta’s story and the classic fairy tale Sleeping Beauty. This inspired the team to produce a cartoon series, specifically targeted to families and young children that would introduce them to Rosetta.

It was time to wake up Rosetta as well as wake up the public to the fantastic mission that was about to occur. ESA

The end result was a charming cartoon series that has reached a range of audiences and has even won the hearts of the scientists themselves.

Hooking people in

Via the cartoon, complex technical and scientific topics have been tackled in a highly approachable way, one that is widely understood by children as well as appreciated by adults.

The cartoons hooked people in to the process of how a mission unfolds (eg. Preparing for #CometLanding), accurately described the science being undertaken (eg. Living with a comet)
and also brilliantly connected with people on an emotional level, adding to the excitement, anticipation and curiosity inspired by the mission (Are we there yet?).

But what about the bad times?

Producing the series was not without its risks. What would happen if the mission failed? This was put to the test when Philae’s landing did not go precisely as planned. Having brought Philae to life and into the hearts of their audience, would he now be left for dead on the comet?

The team realised they could take advantage of the nature of the cartoon and its strong emotional focus. In the #cometlanding episode, the “mishap” was presented in terms of common feelings: a story of the fear, surprise, commitment and even adding a little humour.

Philae packs his bag for the comet landing: camera, compass, pickaxe, snow boots and importantly a sandwich as he’ll need his own source of energy. ESA

In the end, Philae completes the tasks at hand, is proud of his work and slips gently into a long deep sleep. It’s the stuff of fairy tales but made all the better because it was inspired by real events unfolding millions of kilometres away.

One of many approaches

The ESA team should be applauded for their philosophy of making the Rosetta mission personally relevant to people world-wide and being able to building such strong connections.

The cartoon even spun-off its own merchandising material with stickers given out by scientists at public events and a 3D paper model to be made at home. It was featured on T-shirts, sweatshirts and even became a cuddly soft toy.

However, the cartoon was just one aspect of the Rosetta mission’s broad communication campaign. The Rosetta blog provided news and updates as they occurred, there were plenty of interviews with mission experts, and also a Discovery Channel documentary Landing on a Comet.

In a very bold and innovative move, the ESA team released a high-quality short sci-fi film, using all the glamour of Hollywood to present the scientific, technical and philosophical aspects of the mission.

This beautiful work of fiction introduced the mission and a follow up epilogue, released last week, celebrated the mission end.

Well done to ESA and Rosetta for the amazing scientific work that was accomplished and for inspiring all of us along the way.

A detailed overview of ESA’s communication strategy for the Rosetta mission is presented in the March issue of Communicating Astronomy with Public (CAP Journal).

The ConversationTanya Hill, Honorary Fellow of the University of Melbourne and Senior Curator (Astronomy), Museum Victoria

This article was originally published on The Conversation. (Reblogged by permission). Read the original article.

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New dwarf planet in the outer solar system

The Conversation

Tanya Hill, Museum Victoria

What’s round, orbits the sun and resides in the heavily-populated parts of our solar system such as the asteroid belt or the Kuiper belt? It’s a dwarf planet, and astronomers have just discovered a new one.

Designated 2015 RR245 (based on specific rules), the dwarf planet lies in the solar system’s icy regions beyond the orbit of Neptune.

It was found using the Canada-France-Hawaii Telescope on Maunakea, Hawaii as part of the ongoing Outer Solar System Origins Survey (OSSOS). Since the survey began in 2013, the team has located more than 500 new trans-Neptunian objects but at 700km across, this new discovery is the largest one so far.

2015 RR245 has a 700 year orbit that is highly elliptical. At its most distant the dwarf planet is more than 18 billion km (or 120 astronomical units) from the sun.

The orbit of 2015 RR245 shown in yellow, along with 17 objects (labelled) that are as bright or brighter than the new dwarf planet.
Alex Parker OSSOS team

Slowly it is making a move towards its closest approach to the sun. At present it is nine billion km (or 60 astronomical units) away. It’ll take 80 years to reach its closest point, albeit still a chilly five billion km (or 34 astronomical units) from the sun.

Astronomers will be studying the dwarf planet in detail to improve their estimates of its size. Size is determined by the object’s brightness which is dependent on the object’s albedo or how easily it reflects sunlight.

If it’s nice and shiny, then 2015 RR245 could well be smaller in size. Or perhaps its surface is dark and dull then estimates of its size would need to swell.

When compared to the officially recognised dwarf planets – Pluto, Eris, Haumea and Makemake – this new dwarf planet is about two to three times smaller. According to a statement from the Canada-France-Hawaii Telescope, 2015 RR245 is the 18th largest object in the Kuiper Belt.

Size Matters

When it comes to dwarf planets, a critical measure is that the object must be round. This is what distinguishes a dwarf planet from among the millions of raggedy-shaped asteroids in the asteroid belt and the hundreds of thousands of objects thought to make up the Kuiper belt.

Furthermore, the definition of roundness is grounded in physics. Rather than choose an arbitrary size definition (such as 1,000km in diameter for example), roundness implies that the object must have enough mass so that under its own gravity it can form a spherical shape.

Ceres is the only object in the asteroid belt known to be round and therefore a dwarf planet. Made of rigid, rocky material it has a diameter of about 900km and a mass of around 900 billion billion kg.

New false-colour renderings of the dwarf planet Ceres from the Dawn spacecraft. NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

But trans-Neptunian objects are made of weaker stuff. Their icy interiors are more easily shaped by gravity and so require much less force to obtain a spherical shape. The lower size-limit for an icy object to be a dwarf planet is around 320km with a mass of only 1,000kg.

This certainly stands true when looking at the icy moons that orbit Saturn. The smallest icy object known to be round is Saturn’s moon Mimas, at 400km across. Smaller moons, with diameters around 200km, are not round (see astronomer Mike Brown’s excellent description here).

The numbers game

Officially there are four dwarf planets beyond Neptune but unofficially there are many more. Mike Brown, who discovered the dwarf planet Eris, keeps a list of unofficial dwarf planets.

On that list are ten trans-Neptunian objects which are nearly certainly dwarf planets.

Four confirmed dwarf planets (top row) and four almost certainly dwarf planets (bottom row).

Extend the list to include fainter objects that have diameters of perhaps 600km or more, and another 27 potential dwarf planets (not counting 2015 RR245) are included.

What about fainter still? The uncertainties increase but potentially another 51 dwarf planets could be added to the mix, if they are icy and larger than 500km, as observations currently suggest.

In all, that would be 88 dwarf planets beyond Neptune and 2015 RR245 brings that count to 89. There truly is a lot still to explore within our solar system.

The ConversationTanya Hill, Honorary Fellow of the University of Melbourne and Senior Curator (Astronomy), Museum Victoria

This article was originally published on The Conversation. (Reblogged by permission). Read the original article.


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