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Astronomy; Wonders of the Solar System - Planets, Moons and the Sun

The author's aim is to popularise the science of astronomy in a series of relaxed, easy to read, and easy to undestand articles

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Credit to NASA

All photos on this page are credited to NASA.

My thanks to Nasa - without these spectacular images, this webpage would not have been possible.

INTRODUCTION

Some time ago on British television, Professor Brian Cox presented an excellent series in which he described his personal selection of the greatest wonders of our solar system. There are of course a myriad of objects in the solar system, but the very great majority of these lie beyond the orbit of Pluto, and beyond the scope of our telescopes or current space missions.

So all that can be considered in an exercise of this kind are the eight recognised planets as well as Pluto, the planetary moons, the known asteroids and comets, and the Sun itself. With these limitations in mind, I would like to present an alternative list of wonders to Brian's. But it's a list with a bit of a difference, as you will discover if you read to the end .......

THE OUTSTANDING OBJECTS OF THE SOLAR SYSTEM

Once upon a time, it might have been assumed that most of the planets which orbit the Sun (apart from Saturn with its rings) were pretty much the same, and most of the moons which orbit the planets were - well - Moon like; grey and cratered like our Moon. The space missions of the past 40 years such as Viking, Voyager, Galileo, and Cassini, changed all that as we came to realise that our Solar System was filled with the most extraordinary range of objects imaginable. The picture below shows just the four largest moons of Jupiter - all very different and all unique in their geology and chemistry. The planets are even more diverse. In this webpage I will attempt to list in six different categories some of the stand-out objects of the Solar System, and describe the features which make them so special.

The four large moons of Jupiter - Io, Europa, Ganymede and Callisto

The four large moons of Jupiter - Io, Europa, Ganymede and Callisto

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The giant blue gas planet Neptune, photographed by the Voyager mission

The giant blue gas planet Neptune, photographed by the Voyager mission

The swirling multicoloured clouds of Jupiter, photographed by the Voyager mission

The swirling multicoloured clouds of Jupiter, photographed by the Voyager mission

The ringed planet Saturn, (and several of its moons) photographed by Voyager 2

The ringed planet Saturn, (and several of its moons) photographed by Voyager 2

1) WHICH OBJECT IS THE MOST BEAUTIFUL IN THE SOLAR SYSTEM?

They say beauty is in the eye of the beholder, but most of what exists in the Solar System is lumps of grey or brown rock and dirty ice. Colour is a rare and precious commodity in outer space where black is everywhere. Many of the heavenly bodies are monochrome and the smaller moons and asteroids are rather uninspiringly potato shaped. So in this section of the page we must look to the more spectacularly coloured and patterned, and the more appealingly formed, objects in space.

First I would mention Io, illustrated above (and below). Io is one of the largest moons of Jupiter, and the closest to the planetary surface. It is one of the strangest moons in the solar system, but in this short section, my concern is simply with its beauty. Io is colourful. It looks for all the world like a giant pizza - a yellow moon daubed with splashes of rust-reds and oranges and some darker areas.

Blue is an unusual colour in space, but the outer giant planet Neptune is a bright mid-blue hue, The globe is fairly uniform in its colour, but with occasional darker blue patches with a few wispy white clouds.

The giant planet Jupiter is unlike any other in appearance. The atmosphere of Jupiter comprises gaseous swirling bands of browns and oranges and creamy whites - bands which have a subtle beauty in their lacy patterns unmatched by other planets.

One planet stands alone in the Solar System for uniqueness of form. Most people would without equivocation consider Saturn the most graceful heavenly body of all. Saturn's glorious ring system entranced astronomers for centuries, even before its intricacy was revealed in all its full finery by the Voyager space missions.

But which is the most spectacular? The ringed planet undoubtedly is a true wonder of the Solar System. Having said that, look behind the rings and Saturn is actually - beige! Jupiter's colourful bands are in pastel shades, but there are brighter colours in the Solar System. Blue Neptune is one of the more attractive objects, but there is another planet of much richer blues, contrasting with more brilliant white.....

There is one heavenly body which boasts the richest, deepest sapphire blue and the whitest snowy swirls of cloud. A closer view reveals bright greens on the surface below the clouds, colours unparalleled elsewhere in the Solar System - it is a veritable gem-stone suspended in the blackness of space. It is called EARTH.

(If there is any doubt about Earth's rightful place in this section of the essay, look at the big photo at the bottom of the page).

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Planet Earth

Planet Earth

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Two views of Europa (colours are enhanced)

Two views of Europa (colours are enhanced)

The much cracked surface of Europa, which may result from underwater heating of the surface ice (this is a black and white image from the Galileo mission)

The much cracked surface of Europa, which may result from underwater heating of the surface ice (this is a black and white image from the Galileo mission)

The planet Mars. This photograph was taken by the Hubble Space Telescope

The planet Mars. This photograph was taken by the Hubble Space Telescope

Scamander Vallis, a meandering Martian valley in which it is believed water once flowed. (An image from the Viking Orbiter)

Scamander Vallis, a meandering Martian valley in which it is believed water once flowed. (An image from the Viking Orbiter)

Further evidence of Mars's watery past. The relic of an ancient dried-up water flood plain can be seen amidst the Martian craters.

Further evidence of Mars's watery past. The relic of an ancient dried-up water flood plain can be seen amidst the Martian craters.

2) WHICH OBJECT IS THE MOST BIOLOGICALLY ACTIVE IN THE SOLAR SYSTEM?

All heavenly bodies have physics and chemistry, and most have geology, but not all have biology - the other great science which we study. By biology, I mean life-supporting, or at least potentially life-supporting. The overwhelming majority of objects in the Solar System are clearly not habitable. Most - the asteroids and the smaller moons - are lumps of rock which lack atmosphere or almost anything else which is conducive to life. Others are positively hostile - excessively cold, boiling hot, subject to intense pressures, or bathed in poisonous gases.

Saturn's largest moon Titan (imaged below) has a hydrocarbon-based atmosphere similar to the primordial atmosphere of our planet - it is an atmosphere with chemicals essential for the creation of amino acids and other organic molecules. Whilst these gases are poisonous to us, they may not be to all possible life forms. And some scientists are now seriously hypothesising that methane-based, hydrogen-breathing life could at least theoretically exist on Titan.

Several other moons of the gas planets are known to possess surface water ice, and possibly liquid water beneath the ice. These include two giant moons of Jupiter, Ganymede and Callisto, and Saturn's moon Enceladus. Some may even have thin atmospheres containing oxygen. Foremost among these intriguing worlds is probably Jupiter's moon Europa. Fly-by missions reveal an icy surface. Astronomers have strong reason to believe that below this seemingly immensely thick ice sheet, there may be an even deeper moon-wide ocean of water, heated by geothermal forces. On our planet complex life can exist very far from sunlight around deep sea volcanic vents - it seems that the presence of water and heat may be all that is required to sustain life. One can therefore fantasize with at least some legitimacy that a moon such as Europa may embrace biology. The implications are very exciting, and missions are planned to land and probe deep beneath the ice, though it must be stressed that an awful lot of speculation is involved here - we do not as yet know anything certain about what lies beneath Europa's surface.

Mars is almost hospitable. Well, not really, but its known history of one-time rivers of water, active volcanoes and dense atmosphere certainly make this the prime candidate for life in times gone by. Micro-organisms could possibly have evolved on Mars in its early days, and on our planet some microbes have an extraordinary ability to cling on in adverse circumstances. If life once existed on Mars, then it is at least conceivable that life may still cling on in places such as deep subterranean soils, or encased deep in Martian rocks safe from the solar radiation to which the planetary surface is now exposed.

But of course in conclusion this is one category of 'wonder' in which there is no contest. The only heavenly body known to have any life at all is a planet teeming with millions of species of plant and animal. Inhabiting the so-called Goldilocks Zone of the Solar System (not too hot, not too cold - just right). This planet is EARTH.

The City of Vancouver as seen from space. This would appear to be a sure sign that planet Earth is biologically active! This photograph was taken by the Earth Observing System

The City of Vancouver as seen from space. This would appear to be a sure sign that planet Earth is biologically active! This photograph was taken by the Earth Observing System

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The strange cracked and reshaped surface of Miranda (imaged by the Voyager probe)

The strange cracked and reshaped surface of Miranda (imaged by the Voyager probe)

The dark and light faces of Iapetus

The dark and light faces of Iapetus

The huge Herschel crater on Mimas photographed by Cassini

The huge Herschel crater on Mimas photographed by Cassini

A world in turmoil - nowhere in the Solar System is more geologically active than Jupiter's innermost moon, Io (Galileo image)

A world in turmoil - nowhere in the Solar System is more geologically active than Jupiter's innermost moon, Io (Galileo image)

A volcanic eruption on Io, and the sulphur coated surface of an utterly inhospitable world

A volcanic eruption on Io, and the sulphur coated surface of an utterly inhospitable world

3) WHICH OBJECT IS THE MOST UNIQUE BODY IN THE SOLAR SYSTEM?

This in many ways could well be the hardest question to judge. Do we mean unique within the Solar System, or unique in the Galaxy? In the Solar System there are plenty of planets and other objects which are unlike any other. There is for example, no planet with the superb ring system that Saturn possesses, though other gas giants are now known to have faint rings, and ring systems may be common elsewhere in the Galaxy.

Saturn's largest moon Titan has a denser atmosphere than any other known moon in this Solar System, whilst Neptune's largest moon Triton has been discovered to possess a seemingly unique geology. It is, of course, unknown how rarely the conditions which create these moons might also exist in other star systems. Titan and Triton may be unique - they are certainly intriguing, and both will be featured within the next category.

Whilst geology and chemistry are uncertain, appearance is not. The smallest and innermost moon of Uranus is Miranda. Miranda's surface posesses a curious patchwork of light and dark grooves, arranged almost haphazardly; it almost looks as if the moon has been shattered and then reconstituted, but the true nature is unknown.

Iapetus, the third largest moon of Saturn, has an even more unique appearance with very dark and very light surfaces. One hemisphere of the moon is dark reddish, and may be composed of liquid methane which has flowed out from within the moon. There are very few craters here too, which suggests that the surface is constantly changing. The light side of Iapetus has hundreds of craters, including one big one. The causes of this strange dichotomous appearance are largely speculative at present.

Mimas, another Saturnian moon, is really small, only 393 km in diameter, and yet it has a uniquely large impact crater on its surface - the Herschel Crater is 130 km across. Astronomers believe that If the meteor which struck had been even slightly bigger, Mimas would have been totally destroyed. That surely makes Mimas a very rare object, even in the Galaxy as a whole.

Jupiter's moon Io has already been mentioned on this page, in connection with its bizarrely colourful surface - a surface pockmarked with hundreds of active volcanoes which constantly smother it with great lava flows and yellowish and reddish sulphurous compounds. It is the only moon to have erupting volcanoes of this kind, and it is by far the most geologically active body in the Solar System. The reason for Io's absolutely unique geology is clear - Jupiter's closest large moon is caught in a gravitationally induced tug-of-war between the great planet and other moons such as Europa and Ganymede; Io is continuously being distorted and heated through this process of gravitational friction. It seems unlikely even in other star systems that there can be many moons which are positioned so strategically to create a world like Io.

But there is one heavenly body which has so many unique features. Above all, it may well be the only planet in the Solar System with life on it. It is - in my opinion - the only planet in the Galaxy with intelligent life on it (that may be the subject of a future page). If so, this fact alone makes it by far the most unique astronomical body we will ever find. This is planet EARTH.

A Galileo image of Earth

A Galileo image of Earth

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Olympus Mons, the gigantic Martian volcano, photographed by the Viking mission

Olympus Mons, the gigantic Martian volcano, photographed by the Viking mission

Triton - black and white image taken during Voyager's fly-past

Triton - black and white image taken during Voyager's fly-past

One of the many volcanoes on the planet Venus. This is Maat Mons, photographed by Magellan

One of the many volcanoes on the planet Venus. This is Maat Mons, photographed by Magellan

Another image of the surface of Venus in the region of Lavinia Planitia, showing three craters

Another image of the surface of Venus in the region of Lavinia Planitia, showing three craters

The strange moon Titan. The photo isn't blurred - the fuzziness is caused by the thick atmosphere which shrouds the surface of Titan and which is impenetrable to any attempts at imaging from space

The strange moon Titan. The photo isn't blurred - the fuzziness is caused by the thick atmosphere which shrouds the surface of Titan and which is impenetrable to any attempts at imaging from space

The only way to see the surface of Titan, was to launch a probe through the atmosphere to land on the moon's surface. Cassini did just that with the launch of the Huygens probe

The only way to see the surface of Titan, was to launch a probe through the atmosphere to land on the moon's surface. Cassini did just that with the launch of the Huygens probe

4) WHICH OBJECT IS THE MOST FASCINATING IN THE SOLAR SYSTEM?

Which is the most interesting world of all? Well, we don't really know what other worlds will still reveal. If moons such as Jupiter's Europa or Saturn's Enceladus are found to have oceans of life supporting water beneath their icy surfaces, or if Mars is found to support life - even the smallest of micro-organisms - then the interest value of these worlds will increase hugely. For now though, we must content ourselves with the known, and Mars has plenty of interest even in the absence of life. Mars has a volcano, Olympus Mons, which is 27 kilometres high - nearly three times the height of Mt Everest. It also has a giant canyon, the Vallis Marineris, which stretches for more than 4,000 kilometres and is up to 7km deep. Many other worlds have equally extraordinary features - the planet Neptune is known to be buffeted by 1200 mph winds. The moon Io will surely be a geologist's dream if ever we can explore it first hand.

But there are other worlds in the Solar System on which the geology and chemistry and meteorology are all incredibly complex and alien. The first of these is Triton, the moon which orbits Neptune. Triton is a bizarre world - the coldest known in the Solar System at -235°C, and the only moon in the Solar System which orbits in the opposite direction to the planet's rotation. The lack of any extensive cratering suggests the moon is very active, and the strangest feature of all also suggests this; gigantic geysers are believed to be spewing forth liquid nitrogen or methane gas, several kilometres into the atmosphere.

Venus is an extraordinary planet - it is our nearest planetary neighbour in space, and quite similar in size to our own planet, and for that reason has been described as our sister world. But it is a very ugly sister. Venus has thousands of volcanoes. It is also the hottest planet in the Solar System with a temperature of nearly 500°C at the surface. It's surface pressure is 92 times that of our planet. Some of these effects are the consequence of an incredibly thick carbon dioxide atmosphere, creating a greenhouse gas effect. If you stepped out on to the surface of Venus, you would be simultaneously and instantaneously crushed and fried, suffocated and poisoned - that is, if you ever got to the surface past the corrosive clouds of concentrated sulphuric acid which exist in the atmosphere! Venus really is the personification of Hell (but still interesting for all that).

Saturn's largest moon Titan is quite remarkable. It is the only moon (as opposed to planets like Venus) with a thick cloudy atmosphere, composed of nitrogen and hydrocarbons such as ethane, methane and hydrogen cyanide. Unfortunately, this bright and orange atmosphere prevented much useful information being discovered about the surface of Titan until 2004 when the Cassini mission released the Huygens probe to pass through the atmosphere to land on the surface. Titan has since been revealed to be quite like our planet in appearance, though not of course in terms of its chemistry. Titan has mountains and volcanoes, and it is also the only other body in the Solar System with liquid lakes and rainfall, but there's a difference - they are lakes of ethane and the raindrops are probably ethane and methane. All this is interesting, and - as mentioned elsewhere - Titan is another of those tantalising worlds which could, theoretically at least, support some biochemical processes.

The worlds of Triton, Venus and Titan have a complexity of activity which - when better known - will fill several text books. There is one object in the Solar System however which has a more diverse geology on and under its surface than any other, including huge tectonic plates, volcanic activity, and sedimentation. It also has an extremely complex atmosphere with clouds and rain and lightning and with liquid water on its surface. But this is just a tiny fraction of what makes this planet interesting. A whole different branch of science - the science of biology - is what makes this the most fascinating of planets. Even as we visit, set foot on, and one day become fully familiar with the exotic features of other worlds, still more could probably be written about this one body and its complex physical, geological and biological systems than all the others put together. It is of course planet EARTH.

Norway - the blue of liquid water, the white of snow on mountains, and the green of life make Earth the most interesting of all planets

Norway - the blue of liquid water, the white of snow on mountains, and the green of life make Earth the most interesting of all planets

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Jupiter - largest and most influential planet in the Solar System (Voyager image)

Jupiter - largest and most influential planet in the Solar System (Voyager image)

Our Moon, photographed by the Clementine spacecraft in 1994

Our Moon, photographed by the Clementine spacecraft in 1994

The Sun, or Sol, the heart at the centre of the solar system contains 99% of the mass

The Sun, or Sol, the heart at the centre of the solar system contains 99% of the mass

Another image of the Sun, with solar flares. This picture is imaged by SOHO - the Solar and Heliospheric Observatory

Another image of the Sun, with solar flares. This picture is imaged by SOHO - the Solar and Heliospheric Observatory

5) WHICH OBJECT IS THE MOST IMPORTANT BODY IN THE SOLAR SYSTEM?

Even the smallest of heavenly bodies can have an importance out of proportion to their size. From our perspective, perhaps a hitherto unidentified asteroid, or a wayward comet may prove to be one of the most significant objects in the Solar System. Many thousands of asteroids exist between the orbits of Mars and Jupiter, but hundreds with diameters in excess of a kilometre routinely cross our orbit. The energy released when one with a diameter of just 15kms crashed into our planet 65 million years ago is believed to have wiped out the dinosaurs. Millions of tons of dust and burning rock were blasted into the atmosphere causing global fires, tsunami waves and climate change. A similar cause for concern applies to comets, which exist in their millions beyond Pluto, but which may transect our path around the Sun due to the eccentricity of their orbits.

Jupiter merits an entry because of it's sheer size. 1000 times bigger than the Earth, bigger than all the other planets put together, Jupiter's gravity perturbs the orbits of all other bodies which come within its range, including asteroids and comets. Jupiter acts like a giant vacuum cleaner-come-pinball flipper in the Solar System. It's gravity can divert comets such as mentioned above on to a collision course with us, but Jupiter may also save us, as Jupiter gathers up numerous errant objects which may otherwise hit us. One such was Comet Shoemaker-Levy 9 which crashed into Jupiter in 1994. If the comet had hit us instead, the results would have been catastrophic.

Mention must now be made of our own Moon. The comparatively large size and closeness of our Moon has always had a major influence on our planet. Darkness at night would of course be almost absolute without the Moon. The Moon's gravitational pull creates the tides on Earth, and this may have been crucial in evolutionary terms (by producing a tidal interface thought to be significant in the first emergence of life on to land), Most importantly the Moon's gravity provides stability to our planet's axis of rotation, and without it our seasons would veer from extreme to extreme. Without the Moon, life may not exist here.