The Continental Shelf

 

Around the coasts of all the land masses, between low-tide level at about the 100-fathom mark is a shallow platform known as the continental shelf, from which the higher parts project as islands. The shelf slopes gently seaward with an angle of less than one degree. It is well developed off Western Europe, where it extends westward for 200 miles from south-west Britain, and off north-eastern North America, while off the Arctic coast of Siberia it is about 750 miles wide. Detailed hydrographic surveys in the Canadian Arctic have outlined a continental shelf 75 to 100 miles in width to the north of Canada, covered with water to 100 fathoms, and abruptly dropping by the continental slope to deep water. Around the continents it is much narrower, or almost completely absent, especially along coasts where fold mountains run parallel and close to the sea, as along the edge of the eastern Pacific.

Detailed soundings have revealed that the valleys of many rivers seem to continue across the continental shelf. This could be accounted for either by rising sea level or by a sinking of the land; in other words, continents really end at the edge of the shelf. This would explain the fact that the continental shelf is at its widest around the shores of lowland areas, where a slight change of level involves a considerable extent of land. Some continental shelves may be due to in part to wave erosion, or to the building up of an offshore terrace by deposition during long periods of geological time; some authorities even claim deposition by the Quaternary ice sheets may have helped to build up the shelves in the Atlantic.

One difficult problem is that some submarine channels cross the shelf and then continue beyond its edge into deeper water. One of these lies off the Hudson in North America, another in the Bay of Biscay. Some authorities ascribe them to faulting, others to erosion by powerful submarine turbidity currents, or to former river erosion, involving therefore enormous changes in sea level. Another explanation is that a river current, continuing into the sea, keeps its channel clear by depositing material at the sides, thus leaving a trough as sediment accumulates. All these explanations are open to criticism, particularly in the case of the deeper troughs. More difficult still to understand are troughs that do not cross the whole shelf, but are found near its oceanward margin; sometimes they form deep gorges cut into the edge of the shelf. A number have been surveyed in detail by echo-sounding off the New England coast. One explanation attributes these gorges to the sapping action of submarine springs bursting out far down the continental slope.

© John Welford

When will the Sun stop working?

The Sun is a massive nuclear fusion reactor that is converting hydrogen to helium and releasing vast quantities of energy as result. This means that eventually it will run out of fuel and cease producing energy. When that happens there will be no possibility of life continuing on any planet in the solar system. So how long have we got?


We’re all doomed!

All good things must come to an end, and that means that not only will every person, animal and plant currently alive have to “meet its maker” (however you might interpret that expression), but even the Sun itself will eventually die and cease to emit light and heat.

However, there is no need to panic! The Sun has been going strong for 4.6 billion years and will continue to do so for a long time yet.

What will happen, over the next few billion years, is that as the Sun’s nuclear fuel is gradually used up it will become both brighter and bigger - twice as bright and 50% larger.

Around five billion years from now all the hydrogen in the Sun will have been converted to helium and the nuclear fusion process will end. The core will shrink and the outer layers expand massively while at the same time becoming much cooler. The Sun will have become a red giant.

The diameter of the Sun will expand to such an extent that it will swallow up the inner planets, including Earth. However, life on this planet will have become impossible long before this point is reached – even if mankind has not committed collective suicide (on behalf of all living things) many millions of years before this happens.

Eventually the Sun’s outer layers will fade away into space, to leave only a small white dwarf star behind. If the Sun had been considerably more massive at the outset it would have run the risk of exploding as a supernova, but it was never anywhere close to being that category of star.

The total life of the Sun, from birth to death as a fading white dwarf, will have been about 11 billion years, of which the existence of humankind on Earth will have been an extremely small fraction.

At least – when it happens – nobody will be able to blame any President or Prime Minister of the “wrong” political persuasion for their lack of action in allowing the Sun to stop working!

© John Welford

Venus: Earth's fierce sister planet

Venus is the closest planet to Earth in terms of distances between orbits (about 41 million kilometres) and the two planets are quite similar in size, surface area, volume and mass. Venus is therefore sometimes dubbed Earth's sister planet.

However, the sisters have very different characteristics. For one thing, Venus rotates far more slowly than Earth, so that a day on Venus lasts longer than a Venusian year! It also rotates the other way round to Earth, so that the sun rises in the west and sets in the east.

However, the most important difference, and the main one that makes it impossible for life to exist there, or for manned missions ever to be contemplated, is that the surface temperature on Venus averages 450 degrees Celsius. Atmospheric pressure is also enormous, being 90 times as great as that on the surface of Earth.

The main reason for the huge surface temperature is the fact that the Venusian atmosphere consists of 96% carbon dioxide, thus causing the thick cloud cover of Venus to act like extremely efficient greenhouse glass. Temperatures on Venus are therefore 100 degrees hotter than on Mercury, which is closer to the sun by about 50 million kilometres.

It has often been said that Earth risks becoming another Venus if the proportion of greenhouse gases in the atmosphere is allowed to keep rising. However, with carbon dioxide currently representing 0.4% of Earth's atmosphere it clearly has a long way to go before reaching the Venusian level!

© John Welford

Vincent Van Gogh injures himself, December 1888

On 24th December 1888 occurred one of history’s best-known acts of self-mutilation, namely the severing of part of his ear by the Dutch artist Vincent Van Gogh.

Van Gogh had settled in the town of Arles in southern France, where, in February of that year, he had hoped to start a community of like-minded artists. However, nobody else was willing to abandon Paris to join him and it was only when Vincent’s brother Theo offered to pay his train fare that one artist, Paul Gauguin, was persuaded to go south. Theo knew just how mentally unstable Vincent was, and he feared for the consequences if Vincent was left on his own.

To start with, the two artists struck up a relationship and they lived and painted together for two months. However, Gauguin then came to realise that the differences between them were greater than the similarities and he sought to bring the arrangement to an end.

Van Gogh was extremely distressed by this prospect and became violent towards Gauguin. Things came to a head on Christmas Eve when he attacked Gauguin with a razor, although he failed to wound him. It was after this that he shut himself away and used the razor to cut off part of his ear. It was apparently his intention to give it to his favourite prostitute as a somewhat bizarre Christmas present.

Paul Gauguin left the following day and the two artists never saw each other again. Gauguin settled in Brittany before going even further away – to Tahiti and the Marquesas Islands in the South Pacific. Vincent Van Gogh did not stay in Arles for much longer before committing himself to a mental asylum and then returning to Paris, where he shot himself in July 1890. It was a sad end for a tormented genius.

© John Welford

Clues to life on Mars from the Atacama Desert

The idea that there might once have been living organisms on Mars is a fascinating one, and even more so is the notion that some form of very primitive life might still be there. However, clues as to where the evidence for past or present life could be found are coming from a very strange direction, namely studies of the Atacama Desert in Chile, South America.

The Atacama Desert is one of the driest places on Earth and thus offers a reasonably close parallel to conditions on Mars – or at least to Mars as it might have been in fairly recent geological time.

The point is that microbes – very simple life forms – can survive for many years in water-free environments and reactivate when water again becomes available. This has been shown to happen in the Atacama Desert, where microbes survive in a dormant state in salt crusts (see picture) that absorb any water that happens to be around. This need not even be liquid water, because salt can take water directly from the air.

The question is therefore whether similar conditions could exist on Mars, given that it is known that there was once abundant liquid water on the planet that could have given rise to salt deposits as it evaporated. Indeed, such deposits have been discovered in Mars’s southern uplands. Modern Mars is not totally devoid of water, although most of it exists as ice at the north polar cap and beneath the surface. There are also very small amounts of water vapour in the thin atmosphere of the planet.

Given the much drier conditions on Mars, it is conceivable that evolutionary processes might have worked differently than on Earth, such that lifeforms could have survived on Mars that would not have done so on Earth. It is therefore going to be very interesting for future rover missions to explore the salt deposits on Mars to see if primitive life has survived. If it can do so in the Atacama, there is a reasonable chance that this is also possible in comparable environments on Mars.

© John Welford

Tintoretto, a great but crafty Venetian artist

The name “Tintoretto” was a nickname, meaning “the little dyer” or “the son of the dyer” which was given to Jacopo Robusti (c.1519-1594) because he was the eldest of the 21 children of a Venetian dyer of silk cloth.

His father took note of Jacopo’s talent for painting at an early age. The boy had a penchant for daubing pigments on the wall of his father’s workshop in ways that were more creative than the sort of behaviour that might normally lead to a clip round the ear in such circumstances, and so, when he was about 14, his father sent him to the artist Titian (then aged 56) to see if he could be trained to make the best use of his skills.

The story goes that Titian sent him home after only ten days in his studio, on the grounds that the work Tintoretto was producing was so much better than that of his master that Titian grew jealous and refused to have him around. This is unlikely to be the real explanation, and it is far more likely that Titian recognised that the young man’s independent style was such that he had little to teach him. Tintoretto was therefore largely self-taught.

Whether or not Tintoretto regarded his treatment by Titian as a rebuff, it is clear that the two men did not get on well. Artists of the day depended on winning commissions from wealthy patrons, and Titian was out to win as many high-paying commissions as he could get. Tintoretto realised that he could get work by offering to do it for less than Titian demanded. This was hardly likely to endear Tintoretto to the older artist, or indeed to other Venetian artists on whom he pulled the same trick.

Another reason for Tintoretto’s success as an artist is that he was extremely reluctant ever to leave the city of Venice, and he was therefore on the spot when commissions came up, whereas Titian travelled all over Europe to work for foreign monarchs, the Church, and other patrons.

Tintoretto was thus able to have a successful and lucrative career as a Venetian portrait painter. Venice was at the height of its commercial and political power during the 16th century and there were many noblemen and wealthy merchants who were anxious to be immortalized in oils.

In 1550 Tintoretto married the daughter of a Venetian nobleman. The couple were very close, even to the extent that Tintoretto hated to be parted from her for more than a day at a time. It is probably also true that she wanted to keep a close eye on him. On the only occasion when Tintoretto is known to have left Venice, at the age of 62, he insisted that his wife accompany him to visit the court of the Gonzaga family at Mantua.

Tintoretto was able to work extremely quickly when the occasion demanded and this ability, together with his quick wits and opportunistic nature, stood him in very good stead when a major commission was announced, in 1564, for the decoration of the interior of the Scuola (Brotherhood) di San Rocco, a building next door to the church of San Rocco in Venice.

Four painters, including Tintoretto, were invited to present designs for the central roundel of the ceiling, which would then be considered by the committee of the Brotherhood. While the other three painters went away to work on their designs, Tintoretto measured the space in question, painted a canvas in double-quick time, and stuck the result in place. When the committee (and the other artists) protested, Tintoretto offered to give the painting to the Brotherhood for no fee. As their policy was never to turn down a gift, they had no choice but to accept it and, having done so, felt honour bound to give Tintoretto the rest of the commission, which he continued to work on for the next 24 years, including major works in the church next door.

Tintoretto’s other major work was on the Doges’ Palace, which had suffered fires in 1574 and 1577 and which was therefore in need of re-decoration. Tintoretto worked alongside Paolo Veronese (who, as his nickname suggests, was an “import” from Verona). Veronese had worked in the palace in 1553 when he first arrived in Venice, and he did not always appreciate having to share the work (and the fees) with the troublesome Tintoretto, especially after he had been robbed of a commission when the latter had offered to paint a work in Veronese’s style, at a lower fee than Veronese would have charged.

Tintoretto’s greatest and final contribution to the palace restoration was a painting on canvas entitled “Paradise” (1592). It is widely believed to be the largest painting ever done on canvas, measuring 75 feet by 30 feet. It includes hundreds of figures, many of which were painted from life. Tintoretto worked on this with his son Domenico, who was nothing like as talented as his father, and the painting’s lack of Tintoretto’s typical skill in depicting light and space might be due to that fact.

Tintoretto’s style owed much to what is termed Mannerism, which contrasted with both High Renaissance style and Baroque. The term is often used derogatively, to indicate a falling off from the classical perfection of the Renaissance towards an emphasis on structure at the expense of naturalism. Thus limbs might be extended to unnatural lengths or figures placed in strange positions if that suited the artist’s conception of how the painting should be structured.

If Tintoretto’s works sometimes appear “stagey” there is a very good reason for that. He often designed a work by making model figures out of wax that could be twisted into the desired shapes and placed on a board, with flying figures suspended from wires. He would therefore produce something that looked like a stage set from a model theatre, complete with lighting effects, and he would then paint what he saw.

Tintoretto’s use of light had a lot to do with living in Venice. It was customary for heavy shutters to be used to shield inhabitants from the bright Italian sun, but the light that entered rooms would then be diffuse and often be bounced up from the water of Venice’s many canals. This meant that the colours that Tintoretto saw and reproduced would glow but be muted, such as dull gold, crimson, mulberry and sea green, and there would only rarely be a direct light source. The overall effect is fluid and dreamlike, but without the physical immediacy that one associates with Titian.

Tintoretto died on 31st May 1594. He was buried alongside his daughter Marietta who had inherited some of her father’s talent as an artist but had died in 1590 aged about 30. Tintoretto’s name lives on as that of one of the greats of Italian post-Renaissance art. 

© John Welford

Where do fast radio bursts come from?

Astronomers are constantly finding new and mysterious things “out there”, and cosmologists then expend considerable energy on working out what they are!

One of the latest phenomena in this category is the “fast radio burst” or FRB, which is a pulse of radio waves that is extremely brief – a matter of fractions of a second. The first detection of an FRB was made in 2007, from examination of signals received in 2001 by the Parkes radio telescope in Australia. The pulse lasted for only 4.6 milliseconds (a millisecond is a thousandth of a second).

Despite much analysis of data received by radio telescopes across the world, only a handful of FRBs have been found to date, and only one (in 2014) has been observed as it happened.

Astronomers would love to be able to track an FRB to its source, and this can best be done by getting a number of radio telescopes in different countries to get a fix on a signal, once one has been detected. In April 2015 they were able to do this after an FRB was picked up that lasted for less than a millisecond. What they also discovered was that the burst produced a “radio afterglow” that continued for six days. This gave the astronomers at Parkes enough time to alert the Subaru telescope in Hawaii to focus on the source, which in turn led to the discovery that the signal came from an elliptical galaxy some 7 billion light years away. The radio burst therefore started its journey several billion years before Planet Earth existed!

Various theories have been proposed to explain the cause of a fast radio burst, and – as might be expected – alien intelligence of some kind is on the list. However, the theory that is currently being given most weight is that FRBs originate from the collision of two neutron stars (a neutron star is the extremely small and dense residue of a collapsed giant star). Such an event would produce a massive amount of energy that would also include gravitational waves, such as are believed to result from the merging of black holes. It is hoped that a gravitational wave source will be confirmed as coinciding with that of a fast radio burst, but this has yet to happen.

However, not all cosmologists accept the idea that FRBs are caused by colliding neutron stars, or that there need be only one explanation. Another candidate is a flare from a magnetar, which is a neutron star with a powerful magnetic field, the decay of which can produce x-rays and gamma rays.

Speculation will doubtless continue until more observations can produce a more definitive answer to this particular problem.

© John Welford