Secrets of the Universe

by Paul Murdin

  As telescopes improved in the seventeenth and eighteenth centuries, astronomers saw that Saturn’s ring was in fact a series of rings. In 1675 Cassini saw two main rings, which were later named ‘A’ and ‘B’, separated by a gap now called the Cassini Division. In 1850 the Harvard Observatory father-and-son team William and George Bond discovered a third ring inside A and B, called the ‘crepe’ or C ring. Keeler discovered in 1888 a 325-kilometre gap in the A ring, which he named the Encke Gap after the German astronomer Johann Franz Encke, who had studied the rings. The gap is actually the orbital path of a small moon, Pan, which sweeps aside or accretes the particles in the rings as it orbits Saturn. The particles are snow-like ice fragments. The accreted particles fall slowly on the equator of Pan and have built up an equatorial wall of snow. Pan has the shape of a flying saucer, or more domestically, a piece of ravioli.

  The Pioneer 11, Voyager and Cassini spacecraft discovered further, thinner rings around Saturn, and a marvellously detailed structure within the rings themselves. The total mass of Saturn’s rings is approximately equivalent to the mass of its satellite Mimas, which fits with the theory that the rings are pulverized remains of a former moon. They consist of fragments 1 centimetre to 5 metres in size (plate XX), made primarily of water-ice, or of micron-sized particles. The beautiful structure of the rings is the result of the complex pulls of Saturn’s larger satellites which, like Pan, act as ‘shepherds’, ushering particles out of some zones to leave gaps, much as a shepherd’s dog might chivvy sheep to leave one area of a field for another.

  The planet Uranus has more than ten rings. The first of these rings was discovered when Uranus passed across a star by chance in 1977. The telescopes of the Gerard P. Kuiper Airborne Observatory, a modified C-141A towing aircraft flying at stratospheric altitudes, were preparing to observe the way that the star’s light faded as it passed behind Uranus’s atmosphere. But before the planet reached its predicted position in front of the star, the starlight unexpectedly dimmed several times. It had been temporarily blocked by Uranus’s planetary rings. After the star exited from behind the planet, it was blocked again by the other side of the rings, with the depth of the dimmings repeating in reverse order. The Voyager 2 spacecraft directly imaged Uranus’s rings in 1986.

  After the discovery of Uranus’s rings, it seemed likely that Neptune might also have rings. Using the same technique, astronomers carefully observed stars for signs of premature dimming as they were occulted by Neptune, and in the 1980s found the evidence they were looking for. To confirm this discovery, NASA engineers reprogrammed the orbit of Voyager 2 as it approached Neptune in 1989, both to learn more about the ring system and also to avoid the risk that dust from the rings would endanger the spacecraft. The Voyager images showed that the planet has four, perhaps five, dusty narrow rings, which have since been named Adams, Le Verrier Lassell, Arago and Galle for the astronomers involved in the discovery of Neptune. The Adams ring is incomplete and has three main arcs named Liberté, Égalité and Fraternité.

  Jupiter’s rings were discovered by the Voyager spacecraft in 1979, and further studied by the Galileo mission. They are very thin and faint but have been imaged by the Hubble Space Telescope.

  Rings of particles and debris do not orbit a planet forever, but fade away – relatively quickly, compared to the age of the Solar System. Saturn’s rings may have been caused by the break-up of a 200-kilometre satellite less than 500 million years ago; other planets’ rings formed from fragments of small icy satellites or captured comets, and some are periodically replenished by sprays of dust from meteoroid impacts on rocky satellites. Our own planet almost certainly had a ring system at least once in its history, after the Moon was formed. The rings would have presented a beautiful spectacle in the sky, whether they were seen only by the uncomprehending eyes of dinosaurs, or whether no creatures had yet evolved that were able to see them at all.

  The close scrutiny given by astronomers to Saturn’s rings revealed its satellites. Christiaan Huygens discovered the first, Titan, in 1655. It is the second-largest moon in the Solar System, just 100 kilometres smaller in diameter than Jupiter’s Ganymede. From Earth Titan is inscrutable, covered by an impenetrable hazy atmosphere. It is the only moon in the Solar System with a thick atmosphere. It is primarily composed of nitrogen. The haze arises because the atmosphere of Titan also contains methane. Sunlight acting on methane produces particles that are akin to smoke: the haze is ‘smog’.

  Titan has been examined from close quarters by an ESA probe named Huygens in tribute to Titan’s discoverer. Carried to the moon by NASA’s Cassini spacecraft, Huygens parachuted through the atmosphere and landed on Titan’s surface in 2005. As it descended, it pictured its landing area, a flat plain adjacent to hills cut by river valleys. It touched down in the damp estuary of one of the valleys, squelching onto a lake bed littered with boulders. It looked like a mundane landscape, but was literally and metaphorically out of this world. Although Titan’s landscape shows similarities to any landscape sculpted by water, the liquid concerned is not water but liquid methane. Methane rain falls on the hills, running into methane rivers that flow down to methane lakes. The lake where Huygens landed was almost dry because of seasonal effects on Titan, and is one lake among many: the lake scenery over Titan was mapped soon afterwards by Cassini as it orbited in the Saturnian system. A radar system on Cassini probed below the haze to view the flat lake surfaces.

  The atmosphere of Titan is like the Earth’s atmosphere in its early years, pre-biotic, before life evolved here and photosynthesis produced oxygen, which chemically combined with methane and similar molecules to produce the atmosphere in which we live today. Titan offers us a glimpse of how the Earth used to be.

  I The Origin of the Milky Way by Jacopo Tintoretto (c. 1575). Jupiter holds his son Hercules, born to the mortal Alcmene, to be nursed by the goddess Juno. Some milk spurts from her breast, forming the Milky Way.

  II William Crabtree Watching the Transit of Venus AD 1639 by Ford Madox Brown (1903). The awestruck draper views the transit of Venus projected into the attic of his shop through darkened shutters and curtained windows, while his wife struggles to keep the children in order.

  III The Galilean satellites of Jupiter. Shown to scale in images from the Galileo probe, the four largest moons of Jupiter are, in order of increasing distance from Jupiter (left to right): Io, showing coloured ash drifts from its volcanoes; Europa, showing a crazed pattern of stained ice floes; and Ganymede and Callisto, their patterns of meteor craters pockmarked in the frozen rock.

  IV The volcano Maat Mons on Venus. The Magellan spacecraft mapped the lava flows that extend for hundreds of kilometres from the base of the volcano across the fractured plains shown in the foreground.

  V Mars. Frosty water-ice clouds at the poles and orange dust storms (lower right) cover the dark markings of Mars that lie among the sand and rock deserts.

  VI Earthrise as seen from the Moon. Apollo astronauts photographed earthrise from their lunar orbiter, encapsulating the growing realization that our Earth is a planet with limited resources.

  VII Asteroid Bennu. The OSIRIS-REx spacecraft imaged Bennu from a range of 24 km (15 miles) to reveal its boulder-littered surface. The asteroid, which is only 500 metres in diameter, is made of material not much changed from the original composition of the solar system.

  VIII Described as the picture of the century, the first close-up photograph of the lunar crater Copernicus was made by Lunar Orbiter 2 in 1966 as it scanned the Moon’s surface for possible Apollo landing sites.

  IX Comet 67P/Churyumov-Gerasimenko. The Rosetta probe pictured this two-lobed comet as material was being loosened from it to make its tail.

  X When Buzz Aldrin roamed around the Apollo 11 landing module, Eagle, in 1969, his footprint demonstrated the engineering qualities of the lunar surface, but also marked the moment and the place at which humans first stepped onto the Moon.

  XI An avalanche has fallen from the steep cliff at the
edge of the northern ice cap of Mars. The cliff, 700 metres high, is made of layers of water ice mixed with red dust, its upper surface covered by a blanket of frozen carbon dioxide.

  XII Comet McNaught. Ripples in the comet’s magnificent tail were caused by periodic releases of dust along the curving orbit and the pressure of solar radiation on the dust particles.

  XIII Omega Centauri. This, the largest globular star cluster in our galaxy, is such a bright conglomeration of millions of stars that it is visible to the naked eye even though it is more than 15,000 light years away.

  XIV Jupiter’s Great Red Spot. The 400-year-old storm is an anti-cyclone that rotates counter-clockwise and creates a turbulent zone in the band of clouds in which it lies. Smaller storms come and go nearby, as shown by the Juno spacecraft.

  XV Caloris Basin on Mercury is one of the biggest impact craters in the solar system. It triggered volcanoes at its lower rim (visible as orange spots).

  THE DYNAMIC UNIVERSE

  Helium

  The cosmic element

  On the subject of stars, all investigations which are not ultimately reducible to simple visual observations are necessarily denied to us. While we can conceive of the possibility of determining their shapes, their sizes, and their motions, we shall never be able by any means to study their chemical composition or their mineralogical structure.

  Auguste Comte, Cours de la Philosophie Positive, 1835

  The discovery of helium in 1868 was a transformative moment for chemists and astronomers, entirely disproving Comte’s notion that the stars were inherently unknowable. The revelation came as a faint yellow line of light, observed during an eclipse of the Sun. It was emitted by a major ingredient in the makeup of stars, which was later found also to be an important building block of substances on Earth. Copernicus and Galileo were right: the Earth and the heavens were made of the same basic materials.

  In the Middle Ages, astrology – the arrangement of the planets in the zodiac – was part of the study of alchemy, an early form of chemistry whose original purpose was to turn base elements into precious metals. The relationships that alchemists perceived between planets and chemicals were reflected in the old names for certain metals. Quicksilver is still called mercury, but other cosmic names have fallen into disuse: copper was once known as Venus, iron as Mars, tin as Jupiter, lead as Saturn, gold as the Sun, and silver as the Moon. As recently as a century ago, household pipes were still stamped with the astrological symbol for the planet Saturn to indicate that they were made of lead.

  Eventually alchemy evolved into the modern science of chemistry. The French chemist Antoine Lavoisier, in a series of experiments conducted between 1782 and 1789, discovered by carefully weighing his chemical compounds before and after they participated in reactions that some chemicals were never broken down into lighter ones. He called these chemicals ‘elements’, publishing in 1789 a list of thirty-three elements (although not a list that modern chemists would entirely agree with). Through the work of nineteenth-century Russian chemist Dmitri Mendeleev, this list evolved into a precursor of the modern Periodic Table, with the known elements grouped into columns and rows according to their observed chemical properties (which we now know are dictated by their atomic structures).

  The empirical arrangement of the elements in the early Periodic Table left a number of empty holes, sparking a search for the missing elements, many of which were subsequently named for celestial objects, reflecting the traditional association of chemicals with planets. Uranium was named for the planet Uranus in the eighteenth century; a few years later, palladium and cerium were named after the recently discovered asteroids Pallas and Ceres. Neptunium and plutonium were named for the planets Neptune and Pluto, tellurium from the Greek word for the Earth and selenium for the Moon. Some of the celestial names that were given to the new elements are no longer used, including aldebarium and cassiopeium, names derived from the star Aldebaran and the constellation Cassiopeia, for the elements now called ytterbium and lutetium, respectively. Denebium (from the star Deneb) was a name given to a rare earth element whose existence was later disproved.

  Such associations between newly discovered elements and the cosmos were only products of a poetic and fanciful naming convention. The first clues that the terrestrial elements were actually to be found elsewhere in the cosmos came from the spectrum of sunlight.

  In 1802 William Wollaston discovered that the spectrum of sunlight had seven gaps, which he regarded as boundaries between the natural colours of the spectrum. But in 1814 the German optician Joseph von Fraunhofer invented a spectroscope with superior resolution and discovered not seven but hundreds of gaps in the solar spectrum. The gaps are now known as the ‘Fraunhofer lines’. Fraunhofer accurately measured their wavelengths (as an aid to making accurate optical instruments) and labelled the more prominent gaps with letters. The German chemists Robert Bunsen and Gustav Kirchhoff discovered that many of the Fraunhofer lines represented light that had the same wavelength as the light that materials emitted when they were heated and vaporized. For example, the Fraunhofer D-lines in the solar spectrum were identical with the yellow sodium emission from salt. This suggested that sodium was a component of the material heated in the atmosphere of the Sun.

  By the end of the 1880s, spectral emissions from fifty of the then known elements had been discovered in the solar spectrum. This proved that the Sun was made of similar elements to the Earth. Applying spectroscopy to the brighter stars, Henry Draper and William Huggins showed that stars also had dark lines in their spectra. The spectroscopists Father Angelo Secchi, H. C. Vogel and E. C. Pickering developed schemes for classifying the spectra of stars and listed the Fraunhofer and other spectral lines that were found in each. The same elements that had been found on the Earth were present not only in the Sun, but also in the stars.

  In 1868–69 there was another dramatic development. Astronomers Norman Lockyer and Jules Janssen observed the solar chromosphere (the Sun’s denser, lower atmosphere) during the total solar eclipse of 1868 and discovered a strong spectral emission at a wavelength near to the sodium D-lines. To make it practical to measure the wavelength of this light, Lockyer and Janssen developed a technique for viewing the spectrum of the chromosphere in the absence of a solar eclipse. Measured at leisure in the solar observatory, the wavelength of the light emitted by the Sun’s lower atmosphere was proved to be different from the sodium D-lines. In fact, the light did not correspond to the emissions from any of the then known elements.

  Janssen and Lockyer realized that they had discovered a previously unknown element. Lockyer named it ‘helium’, after the Greek word for the Sun, helios. This element was isolated on Earth in 1895 by the Scottish chemist William Ramsay as he studied radioactive minerals that give off helium as they decay. Helium is thus unique among the elements in the periodic table, having been discovered in a cosmic object before it was identified on Earth. Most terrestrial helium is made by the radioactive decay of heavier elements on Earth – the same process Ramsay observed in his experiments – but most cosmic helium originated during the Big Bang, or is generated inside stars.

  In the wake of the discovery of helium came many similar claims for new elements, most of which turned out to be cases of mistaken identity. As conditions on the Sun are so different from conditions in a laboratory, its spectrum is easy to misread. New spectral lines were discovered in the chromospheric spectrum during the total solar eclipse of 1869, but the new element, ‘coronium’, invented to account for them, was shown in 1941 by the astronomer Walter Grottrian to be nothing more than iron heated to high temperatures and low densities. Similarly, some spectral lines in nebulae were once attributed to the element ‘nebulium’, but turned out to have been produced by oxygen and other common elements.

  In both the discovery of helium and the rectification of spurious claims for other new elements, scientists used new astronomical discoveries to explain terrestrial phenomena, and vice versa. This was an extension of Copern
icus’s realization that the Earth was a planet like others: the material that the Earth is made of is the same as the material of the rest of the Universe, and the scientific laws that apply here are the same as those that apply everywhere.

  Gravitation

  Determinism and chaos

  Truth is ever to be found in the simplicity, and not in the multiplicity and confusion of things.

  Isaac Newton, Fragments from a Treatise on Revelation, 1680s

  When the theory of gravitation emerged in the seventeenth century it seemed that mathematics had infinite power to see the future. But Newton could only predict the behaviour of one small planet orbiting a lone, absolutely spherical star. The real Universe has many planets, stars and irregular shapes, and by the twentieth century chaos reigned supreme.