• Abstract and Figures
• Public Full-text

22 23 The UN has declared 2009 the International Year of Astronomy to celebrate the 400th Enceladus is the dark spot inside the bright flare at the centre of Saturn's E ring. Plumes anniversary of Galileo Galilei’s first astronomical observations with a telescope and of ice and water vapour are erupting off the moon to form this ring. the publication of Astronomia Nova by Johannes Kepler. In that connection, NPD pal- (Photo: Nasa/JPL/Space Science Institute) aeontologist Robert W Williams looks at a Saturn moon’s geology. Hot and cold Saturn’s little ice moon Enceladus looks very alien to ter- restrial geologists. Like Earth, however, this world is crunchy on the outside and soft on the inside. A warm inte- rior requires energy, and an unearthly source is pumping up its heat. 24 25 Enceladus is a tectonically active moon of Saturn and consists mainly of water ice. The prominent tectonic divide visible near the top of the image is Labtayt Sulci, a one- kilometre-deep rift. Note the absence of craters over large areas. (Photo: Nasa/JPL/Space Science Institute) Saturn photographed from within its shadow. Details of the ring system become more distinct when the Sun’s illumination is from the back. Encircling the entire system is the E ring. The icy plumes of Enceladus, whose eruptions supply the ice dust of the E ring, betray the moon's position on the ring's left-hand edge. Located 1.3 billion kilometres away, just above the leftmost edge of the brighter main rings, is the pale blue dot of Earth. (Photo: Nasa/JPL/Space Science Institute) ater is abundant in deep space. the embryonic solar nebula. Countless craft of 1997. Still in excellent health, This is partly because hydrogen icy bodies took shape in the solar dust Cassini is transmitting new information is by far the commonest element cloud through the coagulation of ice on Saturn and its moons even as you in the universe. Oxygen and all grains. read this. Wother atoms heavier than hydrogen and Most of these frozen clumps became helium (disregarding traces of lithium comets and dwarf planets. The rest Dynamic and beryllium) are forged in the cores were either ingested by or formed Data from Cassini proves that Enceladus of giant stars. moons of the giant outer planets – is one of the most geologically dynamic The entire periodic table of ele- Jupiter, Saturn, Uranus and Neptune. bodies in the solar system. Internal heat ments is shot into the galaxies by the The ice worlds orbiting these plan- permits geysers on this little moon to explosive deaths of high-mass, short- ets are geological jewels, with a great spew gossamer plumes of ice hundreds lived stars. Gas makes up the cold, dark diversity of types, and some reveal geo- of kilometres above the surface. This molecular clouds we can see with the logical processes which resemble those creates a torus which forms one of unaided eye as a continuous shadowy found on Earth. These include crustal Saturn’s outermost rings, the tenuous intercalation stretching across the night recycling and plate tectonics. E ring. sky along the band of the Milky Way. Before Galileo viewed the Tuscan Cassini has shown us an extraor- sky through a small telescope in 1610, dinary form of plate tectonics on Clump our solar system contained only eight Enceladus, where new crust is extruded Four and a half billion years ago, our known objects: six planets, the Sun and from spreading ridges in only one solar system accreted from a gravity- our own Moon. direction – resembling an Earthly sub- collapsing clump in a dark molecular Galileo spotted four additional duction zone in reverse. cloud in the outer disk of the Milky Way objects – the largest moons of Jupiter, Such internal heat generation galaxy. Innumerable examples of on- now called the Galilean satellites: is surprising given the small size of going star formation are readily observ- Io, Europa, Ganymede and Callisto. Enceladus. Measuring only 500 kilome- able today, so astronomers can follow Saturn’s five largest moons were tres in diameter, it ranks as the sixth the chain of events. also discovered in the 17th century largest of Saturn’s 60 moons. By com- The cloud of star-stuff, the solar by Christiaan Huygens and Giovanni parison, our Moon has seven times the nebula, consisted of gas, ice and rock Domenico Cassini. diameter, 336 times the volume and dust. Ninety-eight per cent of the mass The frequency of new discoveries 1 000 times the mass. of the cloud was hydrogen and helium accelerated during the following cen- In spite of its large size, however, gas. The remaining two per cent con- turies as optical technology improved. the Moon does not generate heat from sisted primarily of water ice and sec- After 1977, practically all discoveries of radioactive decay and is therefore geo- ondly of frozen methane and ammonia. additional moons were made by four logically dead. Nothing happens there Only 0.5 per cent of the cloud was rock robotic probes sent to the outer plan- except for the occasional meteor strike. dust, in the form of microscopic grains ets. The last large lunar impact, now the the size of smoke particles. These included two Voyager mis- Tycho crater, was 100 million years ago. Copious amounts of water were sions in 1977, the Galileo probe to Although its interior may be as produced when hydrogen and oxygen Jupiter in 1994, and the most advanced hot as 0°C, the surface of Enceladus is combined in the frigid outer reaches of of them all, the Cassini Saturn space- one of the coldest in the solar system. 26 27 Temperatures there reach -201°C at the visual field as a 35-centimetre ball held is a ball of water covered by a shell of field of another body will be pulled friction. But the moon has a sibling warmest. This is because the little moon at arm’s length. ice, with a core of rock in the centre. into an ellipsoid shape. If the bodies are which likes to play tug-of-war. Its name is amazingly white – whiter than fresh It would appear 285 times brighter To understand Enceladus’ alien ener- rotating, rotational energy will induce is Dione, who was the mother of love powder snow, in fact. and feel 2 000 times warmer than the gy source, we have to understand the friction and become converted to heat. goddess Aphrodite in Greek myth. No other known object in the solar Sun because it covers 285 times the bizarre way gravity behaves. That is partly why Earth’s rotation is For each orbit ice moon Dione com- system is whiter than Enceladus. Its ice angular surface area, and has seven slowing down. Our planet rotates pletes around Saturn, Enceladus makes surface reflects 99 per cent of the sun- times the surface density of the Sun. Stretching 30 times faster than the Moon orbits, so two. The orbital periods of these moons light striking it. That observation alone In the Earth’s core and mantle, unsta- Gravity stretches things. Tides on Earth its tidal bulge is always ahead of the sat- are in resonance, a stable position tells us that something is occurring on ble heavy atoms decay into progres- are not caused by the Moon attracting ellite. This transfers angular momentum which orbits tend to establish. Enceladus which is consuming energy. sively lighter atoms, releasing energy in the ocean, but by the Moon stretching to the Moon, slowing Earth’s rotation Dione’s gravity is too weak to induce This is because energy is needed to the process. Fortunately, the crust is not the entire Earth. Because the oceans even more and causing the Earth-Moon notable tidal stretch on Enceladus when keep a white surface pristine in a dusty getting warmer because Earth recycles are more flexible than rock, they get gap to increase by five centimetres the two moons pass each other, but environment like our solar system. As on it and radiates excess heat into space stretched more. every year. does induce eccentricity in the latter Earth, vast regions on Enceladus are not at the same rate as the core and mantel In a tug of war, the rope is stretched We humans think of gravity as an moon’s orbit and makes it more ellipti- pitted with impact craters. A process produce it. equally in both directions, regardless of attractive force only. The reason we do cal. must be coating Enceladus with fresh Unlike Earth, Enceladus may lack the which end the winning team is pulling. not notice that we are being stretched Enceladus moves slightly closer to ice and renewing its crust. So where is necessary unstable atoms in its core to The latter is simply applying a greater when the Moon is overhead is because and slightly farther from Saturn on the energy coming from on this small, generate heat. Density measurements force on the rope in the opposite direc- we are so small. If we were as big as each orbit, which means it experiences frigid globe? from Cassini fly-bys indicate that it may tion to the losing team. From the rope’s Enceladus and located in Saturn’s much a varying intensity of tidal stretch. In be a differentiated body with a large point of view, however, the extra force stronger gravitational field, we would other words, it flexes like an accordion. Renewal silicate core surrounded by a liquid exerted by the winning team would be feel the stretch and we would feel the This orbital eccentricity is maintained Hot interiors and crustal renewal are water-ice mantle.

Load more

  3

Copy Link