The Moons of Jupiter

There was a time many years ago when the brilliant white star that stands out in the night for much of the year was, to me just that, a big star. One day my Dad told me it was the planet Jupiter. But it was years before I ventured to pick up a pair of field binoculars and look for myself. I can remember the moment, a dark night in Devon. There to my astonishment was not just the clear bright disc of the planet itself but four smaller bright dots lying in a line to the right. Io, Europa, Ganymede, and Callisto the Galilean moons of Jupiter. It's one of the easiest and most thrilling astronomical sites to view. But it has had profound consequences to how we view our place in the universe.

Four hundred years before I picked up my cheap binoculars a great Italian Galileo Galilea had the same thought. Except his spy glass was not cheap, it had been crafted at huge expense by his own Venetian workshop. It was in fact the first true telescope. Designed not for gazing at the cosmos, but spotting approaching enemy ships. One night Galileo must have had the same urge that I felt in Devon. He pointed his telescope at Jupiter. He became the first person to see the moons of Jupiter. Galileo was a man of great thinking, he understood the consequences of what he had seen. If small worlds orbited Jupiter, was it not likely that the Earth orbited something too? It did. The sun.  This view (the heliocentric view- meaning sun in the middle, as opposed to geocentric view - meaning earth in the middle) seems so obvious to us that it is hard to believe anybody ever thought anything else. But geocentricity is quite logical. The Earth doesn't look like it's moving, and to the observer absolutely everything in the sky clearly is moving.

To the sixteenth century establishment heliocentricity was a radical, blasphemous thought. So much so that it nearly cost Galileo his life. The church shuddered at the consequences . Its belief was that the Earth was at the centre not just of the solar system but of the whole universe. Everything revolved around it. To believe anything else argued for a demotion in the importance of the Earth, ambiguity over god's place in the it, and of course it meant the church wasn't quite so important after all. Heresy!

Galileo avoided execution by retracting the conclusions from what he saw. The agreed line was that although Jupiter may or may not have moons orbiting it, everything still orbited the Earth . Others weren't so lucky Giordano Bruno the Italian friar and astronomer was burned at the stake for his views which were similar to Galileo's although he appears to have annoyed the church in a number of other ways too. All mention of the Heliocentric universe was banned by the catholic church until 1822. Astonishingly it was not until 1992 that pope John Paul II finally accepted that Galileo was right. In 2000 one of John Paul's cardinals, Angelo Sodano expressed regret of the burning of Bruno. It was, he said a "sad episode in newer christian history"  The consequences of picking up a telescope have never been greater.

Jupiter

Jupiter is back. After months of lurking below the horizon, the big boy of the solar system has returned to the night sky. Despite being a billion miles away, far further than anything else I’ve mentioned in this blog to date, this enormous ball of gas and liquid appears to us to be the second biggest object in the night sky. So big in fact, that it literally couldn’t be any bigger. If Jupiter acquired more material it would, bizarrely, shrink in size.

Theoretical models show that if more mass were added to Jupiter it would absorb it stay the same size and simply become more dense. If more mass still were added, creating a planet perhaps three or four times more massive, gravity would take over compressing  the matter and causing the planet to shrink below it’s current dimensions.

Ultimately if more and more matter were added gravity would become so intense as to create pressures great enough to ignite the gases and form a star. It is for this reason that Jupiter is sometimes referred to as a failed star; a slightly unkind description for our king of the planets. It is really nothing of the kind,  stars are quite different , the smallest brown dwarf  is perhaps 50 times the mass of Jupiter.

Ceres and other forgotten worlds

Most people know that our solar system has eight planets Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.  Controversy surrounds Pluto, from it’s discovery in 1930 until it’s relegation from the premier planet league to more lowly the status of dwarf planet in 2006 it was our ninth planet. Its new lowly status restores the planetary numbers back to what we had between the (mid nineteenth century) discovery of Neptune and Pluto.

So it is rather odd to see in the London Science museum a model solar system (an Orrery) built in 1813 for the British astronomer William Pearson, It has not, as you might expect, seven, but eleven planets. No he wasn’t guessing or predicting what would be discovered, he was representing the orthodox view of the solar system at the time. There amongst the familiar innermost seven planets between the orbits of Mars and Jupiter are four we’ve almost forgotten Ceres, Vesta, Pallas and Juno. These are real worlds, although like Pluto they would by the mid nineteenth century be relegated, and their relegation was more severe than that meted out to Pluto condemned to virtually non-league status as asteroids.

Yet these four worlds are no mere fragments of rock. Ceres is a spherical world nearly 600 miles across with a light atmosphere and probably a substantial quantity of water. With a good eye and a dark night you can even see it from Earth without a telescope. It has been ignored by space exploration and barely mentioned in text books on our solar system for century and a half. It’s sisters have fared no better, although they are rather more potato shaped than spherical and look a bit more like what you might expect an asteroid to look like nobody much talks about Vesta, Pallas and Juno either.

In fact there are many many more. Hygiea an ovoid world the size of Belgium, Eunomia a rock world the size of Wales, and bizarrely a binary asteroid Antiope that is made up of two equal sized parts orbiting around each other. There are over two hundred more asteroids known to measure more than 70 miles across. In total there is thought to be over a million asteroids forming the asteroid belt.

Phobos and Deimos

Nagging wives are not usually spoken about in high regard but Angelina Hall was different. Her nagging not only caused the discovery of two worlds it led to the major feature of one being named after her. Her husband Asaph Hall was a nineteenth centaury American astronomer. He sought to try and prove what had been, quite bizarrely predicted by Jonathan Swift a hundred years before in his famous novel “Gulivers Travels” that Mars had two moons. Quite why Jonathan Swift thought this is odd in itself. Swift was a bright well-informed man, but was no scientist and certainly not an astronomer. When he wrote Gulivers Travels in 1776 no telescope on Earth was anything like powerful enough for anybody to deduce anything much about Mars. It would be a centaury before a telescope powerful enough to see the moons would be built. But there in volume two he says “In the floating aerial island of Laputa. They have extended their discoveries much further than our astronomers in Europe. They have likewise discovered two lesser stars or satellites that revolve about Mars” he then goes on to describe the distances of these satellites from Mars and how long they take to orbit the planet. Astonishingly given that it was just a guess, Swift was correct there are two moons of Mars and his predicted measurements weren’t far out either.

 

Almost exactly 100 years later a telescope big enough to be able to see Mars in enough detail was built in Washington state USA. The astronomer in residence was none other than Asaph Hall. Looking for a discovery worthy of the magnificent new telescope he decided to put Jonathan Swift to the test. Swift was not the only one to be tested. The distance was testing the scope to its limits, and it turned out testing Asaph too. He spent night after night trying to spot moons around Mars. It was dull and frustrating work. He found nothing and when he arrived home each morning would moan to his wife about the futility of his task. “Don’t you give up” she is reported to have said on many occasions. Eventually Asaph really had had enough. “I’m stopping” he told her. “You get right back there tonight and carry on” she is reported to have shouted. Like most nagged husbands Asaph did what he was told, and that very night saw a speck circling Mars. A few nights later he spotted a second. He had discovered what came to be called Phobos (picture left ) and Deimos (picture right) the two battered and scarred rocks that circle Mars. They are odd indeed the two moons are no bigger than the two islands that make up Malta. They are perhaps as old as the solar system, probably captured asteroids that got ensnared in Mars gravitational field. The larger moon Phobos is particularly war torn and shows the scars of violent impacts. Two craters dominate, the smaller one named after Asaph is Hall crater, and the larger Strickner crater named after Angelina’s maiden name. A testament to nagging. And on Phobos's sister moon a testament to brilliant gueswork, the dominant feature is Swift crater.

Congratulations you are here!

I’ve talked about coincidences and the goldilocks zone before. It’s a train of thought that gets deeper the more you think about it. For some it leads to proof of a divine creator. Simpler souls like me come to more mundane conclusions. Next time you take your existence for granted just mull over some of these amazing thoughts.




If there were no moon, if the moon were smaller or if it were further away the Earth’s orbit would wobble around, as Mars does. The seasons would be irregular, weather would be chaotic and life, if it had got started at all, would have evolved very differently. You would not be here.


If the Earth were just 2 or 3 % smaller it’s gravity would be too weak to hold onto all of it’s atmosphere, the lighter gases including Oxygen would drift off into outer space. 2 or 3% larger and poisonous primordial gases like Iodine and Xenon that actually have dispersed into space would still be here in toxic quantities. Either way you would not be here.


If Jupiter were a bit bigger, a bit smaller, or located somewhere else in the solar system you would not be here Without it’s strong gravitational force the solar system would still be full of dangerous asteroids some of which would hit the Earth with regularity that would wipe out life. A bit bigger and Earth would have been drawn out of its comfortable orbit into a place where you would not survive.


If gravity were a tiny bit weaker, mass would not create the pressures in stars needed for them to shine. Our sun would be a big cold ball of gas. Our planet, if it existed, would be a dark frozen frigid place. If gravity were a tiny bit stronger the stars would be drawn together causing cataclysmic collisions. Either way there’s no chance you’d be here.


Omega is a unit of measurement. It expresses the relative density of the universe. It was set at the moment of the big bang. Had it been just one part in a million billion smaller the universe would already have expanded and cooled. It would be too cold for you. One part in a million billion bigger and the big bang would have collapsed in on itself and the universe would not exist. Nor of course would you.


Given that it is so incredibly unlikely that you are here. How come you are? You may believe that God made you, and you may be right. But here is my more mundane idea. Whatever set of permutations exist give rise to what is here. You are only here because the dice fell the way they did. Had they fallen differently another being would have to ponder the same thought. So for me life is a bit like winning the lottery. You only have to ponder why if you win. The good news is you are here. Congratulations you won the most unlikely prize of all – life!

Mars

Mars doesn’t shine as brightly in the brief summer nights as it did earlier in the year. But it’s still the most tangible, almost touchable world out there. No planet except our own has been more studied, more watched, more visited than Mars. And unless you are very young or very optimistic it is the only planet out there that in our lifetimes we may see visited by our species. We know an extraordinary amount, but there remain great mysteries. What are the enormous caves recently discovered in the Northern mountains, and what’s in them? Was there life on Mars when it was warmer and wetter? Is there life there still?

Intriguingly in 2004 traces of the gas methane were detected in Mars’ atmosphere. Methane is quite unstable and would naturally break down, meaning if it’s there it must be replenished. But by what? There are a few potential sources, but they don’t fully explain it . There is no volcanism on Mars so geological causes are very unlikely. Meteorites and asteroids can carry methane, but the amounts that could be delivered are tiny in comparison to the amounts found. One explanation is that the methane could have a biological origin. On Earth methane is found deep under the ground a product of the breakdown of plants and animals. We use it and call it natural gas. If Mars had life in the past it is possible that the decayed remains of it lie deep under the Martian surface. It is not inconceivable that they have decomposed in the same way. Of course it would require bacteria or something similar to have caused the decomposition. A beguiling possibility emerged last year when NASA announced that seasonal plumes of methane had been discovered coming from the Northern surface of Mars. The area in fact where the caves are located. Could it be that bacteria survive deep underground in those caves, springing into life during the Martian summer?  Perhaps one day an astronaut will descend into one of those deep caves and find out.

Theia and the Goldilocks Zone

Beyond the Earth and Moon there is a vast and empty space, visited just by a few tiny fragments of asteroids drawn inwards by the sun’s gravity, and occasionally a manmade satellite travelling out on a voyage of exploration. As voids go in space this is nothing extraordinary, the immeasurable entity of space is made up almost entirely of nothing.  But the space between Earth and our neighbouring planet Mars is different. This, at least in our terms, is prime real estate space. The region that starts a little in from the orbit of Earth to just outside the orbit of Mars is our star’s Goldilocks zone. Not too hot and not too cold, in fact just right to support life. In many ways it is a careless waste that this huge expanse has just two planets in it, and one of those, Mars, is on the very margins of habitability. But it might have been different. A leading theory about the early Earth says that 4.5 billion years ago the Earth was a slightly smaller planet than it is now, and orbiting close by was another planet, smaller still, called Theia. The theory says that Theia collided with Earth fragmenting both. When gravity reassembled the wreckage a larger Earth and our moon were formed. Theia was no more. But what if events had unfolded differently? We would live in a slightly smaller Earth, with no moon. But just out there would be another world. Perhaps instead of the Apollo moon landing, Neil Armstrong would have stepped out onto the surface of Theia. Perhaps he may have found he was not alone another world in the Goldilocks zone could have supported life of it’s own too. So perhaps it wouldn’t have been Neil Armstrong. Instead a Theian mission of beings would have landed on the Earth first.