Astronomy; Wonders of the Solar System - Planets, Moons and the Sun
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.
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.
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).
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.
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.
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.
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.
It is also time now to mention that most significant of bodies - the Sun itself. The Sun is the star which lends its very name to our star system; the epicentre, where 99% of the mass is accumulated. It dwarfs everything else. A system like ours exists because of the star at the centre - without it, there would be no revolving planets, moons and comets, no heat, and no light. Given that the sun is the birth mother of the Solar System, its globe is the heart of the Solar System, and its electromagnetic rays are the lifeblood of the Solar System, it seems bizarre that I do not put it first in this category.
In terms of the origins and continued existence of the Solar System, the Sun is, of course, by far and away the most important object; the star which defines the star system. But how does one define importance? Importance varies with perspective. From the perspective of an intelligent being on Triton, that moon, and its planet Neptune, and the Sun, would be deemed the most important of worlds. But we can assume there is no intelligent being on Triton. Importance is not an objective value; it is a subjective value. Importance can only be related to comprehension and awareness. Without any sentient life to appreciate it, the Sun doesn't matter. No one would know or care that it exists. No one would know anything exists. Nothing matters without consciousness. I would therefore argue the most important body in the Solar System is the one which sustains conscious life which can bestow the quality of 'importance' - that is EARTH.
6) WHICH OBJECT IS THE MOST VULNERABLE BODY IN THE SOLAR SYSTEM?
In the beginning our Solar System resembled a cosmic pool table, in which planet-sized objects were flying about in all directions, sometimes resulting in the most enormous of collisions. It was chaotic. Thankfully things have since calmed down as the planets and moons settled into (more or less) uniform orbits. Stability of this system ensures that although minor objects such as small asteroids and comets may sometimes come to grief on a planet's surface, most of the larger heavenly bodies have a reasonably secure future for aeons to come, until indeed the Sun itself exhausts it's supply of nuclear fuel.
However, though planets seemingly endlessly orbit the sun, and moons endlessly orbit their respective planet, some moons orbit rather too close. These include one of the most unique and impressive, which I have already described; Triton has a decaying revolution and so, in about 100 million years, it will stray too close to mighty Neptune and it will break up.
Mars has two strange moons which are irregular in shape, and ultra small - just a few kilometres in length. They may in fact be captured asteroids. Orbiting at only 6000km from the Martian surface, one of these objects - Phobos - is the closest moon to its parent planet in the Solar System (at least that we know of), and its fate is sealed. The orbit is reducing by nearly 2cm per year, and in 10 to 50 million years it is destined to break up or crash into Mars (with a thud).
Vulnerability of these moons is measured in terms of self destruction. But there is another way in which vulnerability can be gauged - the value and immediacy of loss. If Phobos crashes into Mars in 50 million years, a lump of rock will be lost - we're talking one lump of rock in 50 million years time. But on one planet a huge amount more than rock could be lost within mere decades. Thousands of living species. The most vulnerable planet in terms of the scale of what could be lost within the lifetime of people alive today - is EARTH.
EARTH - BEAUTIFUL, BIOLOGICALLY ACTIVE, UNIQUE, FASCINATING, IMPORTANT - VULNERABLE
Earth - measured against the vastness of just the local Solar System, let alone the unimaginable hugeness of the Galaxy - is the tiniest of pin-head sized jewels. Yet it is the most beautiful and biologically active of heavenly bodies in our star system, probably the most unique and fascinating of heavenly bodies in our Galaxy, and from the perspective of our conscious thought, the most important body in the Universe.
It is also the most vulnerable. Four and a half billion years after its creation, we have been here for barely two million as 'humans', and in our current guise as Homo sapiens, just a couple of hundred thousand. Civilisation is a few thousand years old, and industrial and technological development, a mere few hundred. Our time on Earth has been a tiny, insignificant fraction of the planet's history, yet we are already destroying the life we share the planet with, and our ability to control this destruction is reducing year by year. We possess the potential to remove so much of this planet's beauty, it's biology, its unique fascination, and its importance in the blink of a cosmic eye. The one thing we should remove as a matter of urgency, is its vulnerability.
IF YOU WISH TO LEARN MORE ABOUT THE SOLAR SYSTEM ... BRIAN COX'S TELEVISION SERIES
Hardcover book of the TV series
DVD. Brian Cox's follow-up series to Wonders of the 'Solar System'
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LINK TO MY OTHER PAGES
- Greensleeves Hubs
In addition to astronomy articles, I also write film reviews and travel guides, as well as pages on photography, linguistics and other subjects, including short pieces of creative writing.