by Paul Murdin
In addition to its rivers and springs, Mars also had massive floods that lasted for a matter of weeks, producing lakes and even seas. Mars Global Surveyor saw crusty, dried-up lake beds, and stepped cliffs on the interior walls of craters: platforms cut by waves, showing that the craters had once been filled with water. Rovers, like Curiosity, have established the chemical composition of some minerals on Mars that form only in standing water. A startling discovery by Viking Orbiter was of crater-crowned ‘islands’ standing above a dry plain at Ares Vallis in the Chryse Planitia region. The lozenge shapes of these islands and the fact that their steep cliffs are between 400 and 600 metres high suggests the islands were carved from impact craters by a flood of catastrophic proportions. Mars Pathfinder also saw rounded rocks and boulders of many different compositions that must have been deposited in their present location by floodwater.
Some of these floods were truly awesome in scale. In one case where a natural ice dam had collapsed, something like 100,000 cubic kilometres of water was released in only a few days. In comparison, a typical flood on Earth is only a few cubic kilometres, and the largest known flood in the geological record of our planet released somewhere between 100 and 1,000 cubic kilometres of water.
Melas Chasma is a basin 1,200 metres above the floor of Valles Marineris. It once was filled by a lake and now contains deposits left when the lake dried. Swirling rusty streaks indicate where hardened sediments have been exposed by waves lapping at the edge of the lake.
Where is this martian water now? Some of it is frozen in the polar ice caps (plate XI), but much of it could be hidden under the surface of the planet. At its cold, high-latitude landing site, the Mars Phoenix Lander exposed small quantities of water-ice, which lay millimetres under the surface, by scratching at the ground with a scoop. It is possible that more water-ice is present in a permafrost layer many metres deep. A radar experiment on Mars Express has even suggested that there is a large reservoir of liquid water underground at the martian south pole.
Occasionally, the ice melts. Some recent craters on the surface of Mars, like the one known as Yuty, are surrounded by outward-flowing lobes, which look like the petals of a flower – features not found on the Moon or Mercury. These craters are called ‘splosh’ craters, and seem to have been formed by projectiles that crashed into mud. This is further evidence that the subsurface soil of Mars may contain ice, which was melted into mud by the meteor’s impact, flowed outwards and then resolidified.
The presence of water opens the possibility that life might have developed on Mars at some point in the past. In limited areas of the planet conditions exist that would favour its survival even now. In 2004 astronomers using ground-based telescopes in Hawaii and Chile and the European Space Agency’s Mars Express satellite discovered methane in Mars atmosphere, which is released from several vents during the summer, when the ice melts.
On Earth, methane is puffed into the atmosphere by volcanoes and deep ocean vents – but it is also produced by bacteria and animals. On Mars, the Curiosity rover has measured a strong seasonal cycle in the methane concentration. Perhaps this is a consequence of seasonality in the amount of carbon dioxide in the atmosphere: it freezes out onto the large southern polar cap, making the overall atmosphere thinner, which increases the relative concentration of methane, which doesn’t freeze. Seasonal variations in dust-storms and the levels of UV light could also affect the abundance of methane. In the back of everyone’s mind is the thought that biological activity could well be seasonal. It is too early to say which process is responsible for the methane on Mars.
Mars may not be the only place in the universe where life might once have existed, nor perhaps even the most likely place for astronomers to search for traces of life. There is water elsewhere in the Solar System – lots of it. Jupiter’s satellite Europa, which is about the size of Earth’s Moon, has been investigated by both the Voyager and Galileo spacecrafts. Europa has a completely spherical, nearly smooth surface with a grooved pattern that looks like crazy paving. Its surface is predominantly water-ice, covered with icy plains. The grooves are cracks in the ice, where floes the size of cities have broken off from the main sheet. The floes are mobile because they float on water. Frozen ‘puddles’ of ice smooth over older cracks and warmer material bubbles up from below the surface. Evaporative salts tint the white ice a reddish-brown in some areas. The ice layer of Europa could be a kilometre thick, or more. The pressure of the heavy layer of floating ice combines with the radioactive and tidal heating of Europa’s interior to liquidize the water beneath the ice. Europa has more water than the total amount found in the oceans on Earth.
Jupiter has two further satellites that may have underground oceans. Ganymede and Callisto have a composition that is a mixture of rock and ice. There is no direct sighting of liquid water but there are several lines of argument that point to both Ganymede and Callisto having oceans of salty water under their rocky surfaces. The ocean within Ganymede is perhaps 1,000 kilometres deep, and like Europa’s ocean holds as much as or more water than there is on Earth. Callisto’s ocean is only a few hundred kilometres deep.
All these oceans may have life swimming there. Salty oceans warmed under an icy surface: if there is indeed life anywhere else in the Solar System, it could well be here on the moons of Jupiter. Perhaps life in the Universe is rarer on planets than on moons – perhaps it is we who are atypical.
Volcanoes on Io
A chance discovery
Had the fierce ashes of some fiery peak
Been hurled so high they ranged about the globe?
Alfred, Lord Tennyson, ‘St. Telemachus’, 1892 (‘suggested by the memory of the eruption of Krakatoa’)
‘It was a moment that every astronomer, every planetary scientist lives for’, recalled Linda Morabito of the instant when she guessed what was causing a mysterious crescent-shaped blob to appear on photographs of one of Jupiter’s moons. ‘I had the sense that I was seeing something that no one else had seen before.’ That evening at dinner, Morabito told her father what had happened. ‘He looked at me and said: “Do you realize you may have discovered the first volcanic activity outside the Earth?” ’
Jupiter’s innermost satellite was discovered in 1610 by Galileo Galilei and named ‘Io’ by Simon Marius, after one of the Roman god’s mythological lovers. Until the advent of large telescopes in the last years of the nineteenth century, Io remained a featureless point, its status as a world or moon 3,600 kilometres in diameter (over a quarter the diameter of Earth) a matter of theoretical inference. In the 1890s, using the Lick Observatory telescopes at the University of California, Edward E. Barnard saw changes in Io’s brightness that suggested the colour and reflectivity of its surface varied from area to area (equator and pole). In the twentieth century, R. B. Minton was able to show that Io had reddish-brown poles and a yellow-white equator – its surface colours were very different from the other three ice-covered Galilean satellites. The prevailing theory was that Io’s surface was covered with sodium and sulphur salts.
When the Pioneer space probes passed Jupiter in 1973–74, the mass of Io was calculated from the amount by which it deflected the spacecraft off course. It was clear that Io was denser than Jupiter’s other satellites, with less ice and more rock. Pioneer 11 took a fuzzy picture as it flew over Io’s pole, showing Io’s yellow colour and some mysterious dark patches. The next spacecraft to view Io were the Voyager 1 and 2 probes, which flew past Jupiter in 1979. On 5 March 1979 Voyager 1 was only 20,000 kilometres above Io’s surface and transmitted magnificent close-up images back to Earth. The surface was brightly coloured with reds, oranges and yellows. There were no meteor craters, showing that the moon’s surface was young, and that some form of geological process had erased any craters that had formed earlier. However, Io had pits that resembled calderas, as well as mountains taller than any on Earth and what seemed to be lava flows. It looked like a volcanic landscape. At the press conference immediately after the encounter, the
scientists made much of a multicoloured heart-shaped feature that they thought was indeed a volcano. Proof of this would be discovered a few days later: not by an experienced planetary geologist, but by a young navigation engineer processing routine data.
Linda Morabito had begun working at the Jet Propulsion Laboratories when she was a student at the University of Southern California, and by 1977 had been appointed a navigation engineer for Voyager 1. During the encounter with Jupiter, Morabito was working fourteen hours a day in the Voyager navigation area at the control centre where ‘data was falling down on us like rainfall and the images were coming in at all hours of the day and night’. Morabito’s immediate task during the encounter was to identify background stars in the images from the spacecraft and use them to determine its position so that its trajectory could be corrected in real time. Afterwards the images would be analysed to reconstruct the trajectory as accurately as possible. On the morning of 9 March, after the encounter with Io, Morabito set about this routine analysis. She began processing several images taken by the Voyager 1 spacecraft as it was looking back ‘over its shoulder’ for one last view of the Jovian system. One image, taken from a distance of 4.5 million kilometres, had been put up on the monitors for everyone to see. Morabito ‘stretched’ the image – increased its contrast to look for a particular dim star – and noticed something that no one had been able to see on the raw picture: an ‘anomaly’ to the left of Io, just off the rim of the moon. The anomaly was crescent-shaped and extremely large relative to the overall size of Io.
Careful scientist that she was, Morabito first considered whether the anomaly might have been caused by a piece of debris or a blemish on the camera. When these had been eliminated, only one possible explanation remained – the anomaly had something to do with Io. Io itself was overexposed on the image and it took some work to discover the exact area of Io’s surface where the mysterious object had appeared. The area proved to be the large, heart-shaped feature, now called Pele, which had been shown at the press conference. The anomaly was a plume from a volcano, an ash cloud rising more than 260 kilometres above the satellite’s surface; the heart-shaped feature was the volcano itself, with its bright orange and green slopes, yellow ejecta and black lava flows, surrounded by a ring of red material ejected over the years and extending up to 600 km from the volcano. Nine such plumes were discovered later in footage from the same fly-by, and a second plume was actually visible in Morabito’s discovery image, above a dark patch called Loki, where the volcanic cloud was high above the night side of Io, catching the rays of the rising Sun. The Loki volcano has been continuously active since it was first observed in 1979.
Current estimates suggest that there are at least 400 volcanoes among the hundred or so mountains on Io. The low gravity of Io allows volcanic plumes to reach as high as 500 kilometres above the surface. Although the Galileo spacecraft was prevented from passing too close to Io for fear that the moon’s hostile plasma environment would damage it, Galileo nevertheless saw ten plumes in 1998.
Why – uniquely among Jupiter’s moons – does Io have volcanoes, and why does it have so many? According to calculations made in 1979 by Stan Peale, Patrick Cassen and R. T. Reynolds the interior rocks of Io are compressed and expanded by the gravity of Jupiter and its other moons. The resulting friction and pressure heat the rocks into liquid magma, triggering volcanic eruptions. The eruptions create pits and mountains on Io’s surface, coating it with sulphurous deposits and covering meteor craters with lava flows hundreds of kilometres long. The scale of these flows is enormous: they contain hundreds of times the volumes of lava produced by volcanoes on Earth. Gases released in the eruptions give Io its thin atmosphere, and leak along Jupiter’s magnetic field lines, producing striking aurorae. The aurorae are mainly confined to a kinked oval on the cloud tops. Bright spots (‘auroral footprints’) mark where the magnetic field leads directly from Jupiter’s satellites to the top of Jupiter’s atmosphere.
Saturn and the Gas-Giant Planets
Lords of the rings
I have been battering away at Saturn, returning to the charge every now and then. I have effected several breaches in the solid ring, and now I am splash into the fluid one, amid a clash of symbols truly astounding. When I reappear it will be in the dusky ring, which is something like the state of the air supposing the siege of Sebastopol conducted from a forest of guns 100 miles one way, and 30,000 miles the other, and the shot never to stop, but go spinning away round a circle, radius 170,000 miles.
James Clerk Maxwell, letter to Lewis Campbell in 1857
The four outermost planets in the Solar System – Jupiter, Saturn, Uranus and Neptune – are strikingly different from the terrestrial planets that are closer to the Sun. They are exceptionally large and made mostly of gas and ice (plate XIV) rather than rock and dust. When Galileo first saw Saturn through his telescope, he was astounded to find that the planet had ‘handles’. The ‘handles’ were, of course, Saturn’s rings. Recent discoveries reveal that all the gas-giant planets have these peculiar systems of rings. But where did the rings come from?
Jupiter, Saturn, Uranus and Neptune have always been much larger than the rest of the planets in the Solar System. They originally formed from parts of the solar nebula that were too distant from the Sun for ices to be vaporized by its heat. The gas giants consequently retained their icy material; this is why they are much larger than the inner terrestrial planets, whose ices were ‘cooked’ away by the newly formed Sun, leaving only small quantities of rock and dust to form into planets. As a result, the gas giants are composed principally of hydrogen and helium. They are in this way like stars, but considerably smaller, so they are not as hot and dense, and do not generate energy through nuclear reactions – you could say they failed to become stars. The interior regions of the two larger gas giants, Jupiter and Saturn, are composed of ‘metallic hydrogen’, a form of hydrogen created under high pressure that has some of the properties of metallic crystals. Metallic hydrogen was discovered through theoretical analysis and has never been created in the laboratory because the pressures needed are not consistently achievable on Earth (although some scientists are trying hard): Jupiter and Saturn are, it seems, the only places in the Solar System where metallic hydrogen exists.
Each of the gas giants, most visibly Jupiter, acted as a centre for the formation of a miniature planetary system of small planet-like moons. In addition to its satellites, each also has a system of planetary rings, discs of particles that follow nearly circular orbits near its equatorial plane. Saturn’s distinctive rings are well known and can be seen with a good pair of binoculars, but the rings of the rest of the gas-giant planets were only discovered in the last decades of the twentieth century.
When Galileo turned his telescope on Saturn, his equipment was not powerful enough to reveal the true nature of the rings. In 1610 he described what he saw as ansae (Latin for ‘handles’), which he interpreted as large moons. ‘I have observed [Saturn] to be triple-bodied. To my very great amazement I saw that Saturn is not a single star, but three together, which almost touch each other.’ Two years later the ‘moons’ had disappeared. ‘I do not know what to say about so surprising a case, so unexpected and so novel’, he exclaimed. Saturn’s rings were edge-on to the Earth at the time, and could not easily be seen. In 1616 he saw a complex shape, because his telescope had improved and the aspect of the rings had again changed. ‘The two companions are no longer two small perfectly round globes…but are much larger and no longer round…there are two half ellipses with two little dark triangles in the middle, each contiguous to the middle globe of Saturn, which is always perfectly round.’ Galileo was unable to offer a solution to the puzzle.
In 1655 Dutch astronomer Christiaan Huygens took an interest in Saturn and discovered its largest moon, Titan, which lined up with Saturn and its handles. Huygens and his brother produced a more powerful telescope, and in February 1656 Huygens saw that the ‘handles’ were in fact a ring around the planet.
To establish precedence for his discovery, while buying himself time for further study of the ring, Huygens inserted a cryptic remark in his book about Titan, De Saturni luna observatio nova (‘New Observation of a Moon of Saturn’), that he had found an explanation for the ansae, inviting anyone else in the know to step forward. He then described his discovery using an anagram (into which he had put minimal creative effort): aaaaaaa ccccc d eeeee g h iiiiiii llll mm nnnnnnnnn oooo pp q rr s ttttt uuuuu, which he later revealed meant: Annulo cingitur, tenui, plano, nusquam cohaerente, ad eclipticam inclinato (‘it is surrounded by a thin flat ring, nowhere touching, and inclined to the ecliptic’).
In 1787 the French mathematician Pierre-Simon Laplace suggested that Saturn’s ring was a set of thin solid ringlets, somewhat like bangles looped around a forearm, because a single solid ring could not orbit around the planet. However, in 1849, the French scientist Édouard Roche calculated that if any solid satellite was in orbit too close to its planet, it would break up under the tidal forces that the planet exerts. The ‘Roche limit’, within which a solid satellite cannot survive, is 2.44 times the radius of the planet, a little farther than the external radius of Saturn’s rings. This made it unlikely that Saturn’s ring could be a solid structure. In 1857 the Scottish physicist James Clerk Maxwell showed that ‘the only system of rings which can exist is one composed of an indefinite number of unconnected particles, revolving round the planet with different velocities according to their respective distances.’ This was confirmed in 1895 when Allegheny Observatory director James Keeler found that the inner part of the ring was orbiting Saturn faster than the outer part.
