Four moons of Jupiter are not as expected

The moons of Jupiter

Jupiter is prominent in our night skies at the moment.

Look for a bright, untwinkling object in the south-east after dark and moving westward overnight. It is worth getting out the telescope. Even binoculars will show Jupiter as a disc with up to four starlike objects, all in a line, like beads on a wire. They are Jupiter's four largest moons.

Because they were discovered by Galileo, back in 1610, they are known as the Galilean moons. Moving outward from the planet, they are Io, Europa, Ganymede and Callisto. Over several nights, you will see the moons moving around the planet.

Jupiter is the fifth planet out from the Sun. We live on the third. It lies 5.2 times further from the Sun than our world, and consequently receives less than 4% of the light and heat we receive from the Sun. Out there, water should be a permanently frozen rock mineral. In addition, over the 4.5 billion years since the Solar System formed, small bodies, like the moons of Jupiter, should be solid all through, and geologically dead, like our Moon.

In 1979, the Voyager spacecraft turned our ideas about Jupiter's moons upside down. Our rather prosaic ideas about what they were like were replaced by facts that were far more exciting. This space mission, along with others that visited the system later, revolutionized our ideas as to where we might look for life beyond the Earth.

Io is the most volcanic and geologically active planet in the Solar System. As it approached, the Voyager spacecraft cameras showed great plumes of material being shot hundreds of kilometres up against a surface pocked with volcanoes, lava flows and great patches of yellow and orange sulphur.

The reason Io turned out to be so very different from what was expected is that as it orbits Jupiter, its inside is being kneaded and stretched by Jupiter's gravity, producing an enormous amount of heat, enough to melt rock. Io's interior is a lot hotter than the interior of Earth. Under the crust, the Earth's mantle is a sort of hot, plastic stuff, circulating slowly. Io's mantle is completely molten, hence the enthusiastic level of volcanism.

Europa, the next moon out, also looked spectacularly different. Instead of a cratered, rocky surface Europa is covered in an envelope of ice. There are cracks and what look like huge ice floes frozen in younger ice, just like what we see in the Arctic. There are few craters. This means the surface is continually being renewed. This, we believe, is due to there being a deep, salty ocean beneath the ice, kept from freezing by Jupiter's kneading the moon's interior. Since Europa is further from Jupiter than Io, the effect is less, maintaining an ocean rather than volcanoes. We believe life is a real possibility on Europa, maybe clustering around volcanic vents on the sea floor.

Ganymede, the next moon out, looks more like what we expected. Its surface is heavily cratered in some places, but there are areas of less cratered ice. This suggests that even though the level of Jupiter's tidal heating will be less than it is on Europa, there is some renewing of the surface, which raises the possibility of life there too.

Callisto is the outermost of the four and does not seem to be enjoying any of Jupiter's tidal heating. It is so heavily cratered it is likely that there has been little or no renewal of the surface since the moon formed 4.5 billion years ago. This moon turns out to be as we expected before Voyager.

Jupiter's moons have changed our ideas about where to look for life, and suggests it could be more widespread in the universe than we thought.

Although we continue to look for evidence of life on Mars, that world is perceived as dying, whereas Europa may be very much alive.


• Saturn lies in the south after sunset, with Jupiter in the south-east.

• Venus rises shortly before dawn.

• The Moon will be new on Dec. 12.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.


The 'solar maximum' is coming

Secrets of sun spots

It has been predicted that next year will probably mark the maximum of the Sun's magnetic current activity cycle. The last one was around 2013. So, what should we expect this time round?

In the 17th Century, Galileo discovered the Sun has spots. They are areas, far larger than our Earth, where the magnetic fields are so strong they inhibit the flow of energy coming up from inside the Sun. That makes them cooler than their surroundings, so by comparison they look dark.

They are still very hot, around 3,000 C, compared with their 6,000 C surroundings. Observers found the sunspots came and went in a 10 to13-year rhythm. At times, the Sun appears completely clear of spots and then, several years later, it can be covered with hundreds of them.

The magnetic fields that generate sunspots originate deep down inside the Sun, where solar rotation, convection and other movements of very hot, highly conductive plasma act like a dynamo, generating intense electric currents and great ropes of magnetic fields.

The numbers and size of the sunspots give us a sort of stethoscope on what the "magnetic machine" inside the Sun is up to.

There are other manifestations of this magnetic activity. On occasion great loops of magnetic field emerge from the surface, get twisted, stressed and explode, releasing the energy of millions of hydrogen bombs in seconds or minutes, producing bursts of X-rays and jets of high-energy particles that can destroy our satellites and impose radiation threats on space travellers.

On occasion, a great loop, containing magnetic fields and hot plasma, snaps and catapults into space at up to thousands of kilometres a second. We call these “coronal mass ejections,” although they are often referred to as “solar storms.” If they hit the Earth's magnetic field, we get magnetic storms, power outages and problems with communications. They also give us spectacular displays of the aurora.

For much of our history, the only impact solar magnetic activity had on our ancestors was an occasional display of the aurora. However, things started to change as our lives became more and more dependent on technology. Problems began when we started connecting ourselves together with long lengths of metal—the telegraph, telephone, power networks, pipelines and railways.

Magnetic storms driven by the Sun induce electric currents in those networks, causing failures, degradation and enhanced corrosion. Radio communications are also affected by the Sun. Basically the Sun affects our connectivity.

Over the last decade, since the last solar activity maximum, we have increased our connectivity and have become more dependent on it for many aspects of our work and daily lives. The Internet is now at the core of what we do. Cloud computing and data storage involves our putting our data and programs on huge computer server farms that can be far away. The potential vulnerability is in our connection with them. Using optical fibre for communications is a help, because magnetic storms can't induce currents in glass, which makes them largely invulnerable to solar interference.

However, there is a new dimension. In order to provide Internet access anywhere on Earth networks of thousands, or tens of thousands, of satellites are being launched. How the Sun will affect those networks remains to be seen, although the high degree of redundancy will help.

As a species, our curiosity and inventiveness will continue to bring us new capabilities and new challenges, from the Sun and elsewhere.


• Saturn lies in the southeast after sunset, with Jupiter in the east.

• Venus rises shortly before dawn.

• The Moon will be new on Dec.12.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.

The great Moon hoax of 1835

Tall tales about the moon

Today, we are all familiar with the enormous amount of fake and misleading material clogging up the Internet.

However, what was probably the most widely believed fake news story in astronomy happened back in the 19th century. It sold many thousands of newspapers as readers devoured each instalment of what was a work of fiction.

In 1833, British astronomer John Herschel finished cataloguing objects in the northern sky and decided to do the same for the southern sky too. That would require moving to the Southern Hemisphere, so in 1834 he and his family moved to the southern tip of South Africa.

It so happened that around the same time, the British Admiralty decided an observatory should be set up in the Southern Hemisphere to help advance navigation. That complemented the work of the Royal Greenwich Observatory, operating on the outskirts of London, in the Northern Hemisphere. The observatory was very close to Herschel's home, so it was inevitable that he and Thomas Maclear, the director of the observatory, would end up working together on a number of projects, some astronomical, and some not.

Robert Locke was a reporter for a New York newspaper. For some reason, he latched on to Herschel's move to South Africa and wrote an article about his going there with an amazing new telescope, which he planned to use to observe the Moon. It was so powerful it would even show insects (if any) crawling around on the lunar surface.

This was rubbish. Even today we cannot make such a telescope. Locke came up with a fictitious “Dr. Grant,” who went with Herschel and would report back on the astronomical discoveries as they rolled out. Grant "reported" the telescope would soon start to show the most amazing things. There then followed a series of articles extolling the capabilities of the marvellous new telescope. Interest was huge. America was agog, and wanted to read more.

As one might expect, Herschel and his telescope produced no such revelations, so Locke filled the gap with his imagination. He first reported the dark patches we see on the face of the Moon, and named after seas, were in fact really water oceans, not the plains of solidified lava we know them to be today.

At the time, few members of the public knew what those dark patches were, so the idea they were really oceans was widely accepted. Locke went on write about plants and forests on the lunar surface. The telescope was so powerful, individual flowers could be seen and identified.

Understandably, public interest was intense. However, to sustain this level of excitement, new, more dramatic discoveries were needed. So Locke put his imagination to work again. He reported the observation of amethysts, 30 metres tall, sticking up out of the sea. This was certainly amazing stuff, but what the public really wanted to know was if there was animal life on the Moon, especially people.

Further articles described large, bison-like animals, grazing close to the edge of lunar woodland. There were lots of other animals too, including huge beavers that walked on two feet, carrying their young in their arms. As one might have expected, there were also unicorns.

Finally, Locke reported Hershel saw people, just like us but with bat wings, because the lower lunar gravity made flight possible. On the Moon we would weigh about a sixth of what we weigh here on Earth.

To dig himself out of the hole he had dug himself into, he said the telescope had been destroyed in a fire.

Rather belatedly the newspaper called in experts to evaluate the articles and Locke admitted it was a hoax. Surprisingly, it all calmed down, everybody had a good laugh and Herschel himself largely ignored the whole thing.


•Saturn lies in the southeast after sunset, with Jupiter in the east.

•Venus rises in the early hours.

• The Moon will reach its last quarter Dec. 4.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.


Astromomy is a changing science

Learning more about space

It was a clear, sunny day on Mars, with just a few streaks of cirrus cloud. A range of low hills defined the horizon.

That was one of the images sent back to us by one of the robots we have exploring the Red Planet. Then we have pictures transmitted back from Titan, Saturn's largest moon, showing the rocky bed of a dried up stream. We have rock samples from the Moon, from an asteroid, and close-up images of many bodies in the Solar System. We even have images from the surface of Venus, sent back before the spacecraft melted.

For most of its history, astronomy consisted mainly of observing dots in the sky. Our ancestors assessed their brightnesses and recorded their positions. They noticed that some dots were in groupings that did not change. Those were stars, and those groupings became the constellations we know today.

There were a few other dots that moved to and fro along a defined path in the sky. Those were the planets. One thing our ancestor astronomers noticed was the heavens were orderly, stable and highly predictable. This contrasted strongly with the unpredictability of life down here on the ground.

The relative stability of the heavens is why people panicked when something new turned up in the sky, such as a comet. Later, when telescopes were invented, we could see that planets are different from stars, and we could even see surface features on some of them. The Moon changed from a smooth heavenly body to a rock ball, pocked with craters and old lava flows.

In those days, sciences like geology, meteorology and biology, were aimed at better understanding our own planet. Applying them to other worlds was left largely to science fiction writers. Now things have completely changed. Thanks to improved telescopes, spacecraft observing bodies in the Solar System from up close, or even landing on them, we can now study other worlds almost as freely as we can study our own. We can now buy textbooks on the geology of Mars, or the Moon. We can even study the weather on other worlds. They have become places we can relate with ours.

Not long ago, we thought there were planets orbiting other stars, but we did not know. Since then, we have discovered thousands of them. Almost every star has planets, some of which could be like our own. We even know whether some of these planets have atmospheres, and scientists are actively searching those atmospheres for chemical markers of life. We now know we live on one world among many, orbiting one star among billions, in one galaxy among billions. It is hard to imagine our planet could be the only inhabited one.

Thanks to our rapidly expanding knowledge of other worlds we can apply what we used to think of as the “earth sciences,” such as geology, to other worlds. This is making it possible to compare the forces moulding other planets with those shaping ours.

These comparisons are raising important questions, such as if Venus, Earth and Mars were similar when they formed, why are they so different now? We thought we knew why our Solar System is structured as it is, and assumed other systems would be similar. However, now we know the Solar System is by no means typical. Few extrasolar planetary systems are anything like ours.

On Feb. 14, 1990, NASA's Voyager 1 spacecraft, then around six billion kilometres away and leaving the Solar System, looked back and took a picture of the Earth. It showed our planet as a tiny, faint blue dot against the blackness of space; it was just another dot in the sky.

Astronomy has changed from a science aimed at improving our understanding of what is going on "out there", to a science that is also improving our understanding of what is going on “down here.”


• Saturn lies in the south after sunset, with Jupiter in the east.

• Venus rises in the early hours.

• The Moon will be full on Nov. 27

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.

More Skywatching articles

About the Author

Ken Tapping is an astronomer born in the U.K. He has been with the National Research Council since 1975 and moved to the Okanagan in 1990.  

He plays guitar with a couple of local jazz bands and has written weekly astronomy articles since 1992. 

Tapping has a doctorate from the University of Utrecht in The Netherlands.

[email protected]

The views expressed are strictly those of the author and not necessarily those of Castanet. Castanet does not warrant the contents.

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