Kitabı oku: «The Planets», sayfa 4

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It wasn’t to last. Slowly the young Sun grew brighter, its increased energy output causing temperatures to gradually rise, which in turn began to lift more and more water vapour into the air, thickening the atmosphere and sealing the planet’s fate. Although the oceans of Venus may have persisted for billions of years, as the surface warmed and the atmosphere thickened, the destiny of this planet was already set, driven by an unstoppable process we have recently become very familiar with here on Earth.

The greenhouse effect is a process that has the power both to protect and to destroy a planet, but despite this power it actually boils down to some pretty simple physics. It’s all about how sunlight – solar radiation – interacts with the constituent parts of an atmosphere. In the case of the Earth, as solar radiation hits our atmosphere some of it is reflected straight back out into space, some is absorbed by the atmosphere and clouds, but most of the sunlight (about 48 per cent) passes straight through the atmosphere and is absorbed by the Earth’s surface, where it is heated up. The reason so much solar radiation makes it to the surface is because the gases in our atmosphere, like water vapour and carbon dioxide, are transparent to light in the visible spectrum. When you think about it, that’s pretty obvious because there’s a source of visible light in the sky, the Sun, and we can all see it! But it’s a different story when that sunlight heats the surface of the Earth and re-radiates back out not as visible light but as the longer-wave infrared light – thermal radiation.

‘Venus hasn’t stopped heating up, and we believe that as the Sun continues to age, billions of years into the future, it’s going to continue getting hotter. Eventually that means that Earth will go the way of Venus.’

David Grinspoon, astrobiologist

We can’t see this light, but as it radiates back out from the Earth’s surface, carbon dioxide and water vapour absorb the infrared, trapping that energy, and so the planet maintains a higher temperature that is intimately linked to the constituent parts of the atmosphere. The higher its level of gases like water vapour, carbon dioxide, methane and ozone, the greater the greenhouse effect and the bigger the uplift in temperature. Despite the very real threat that this now poses to the future of our planet, the greenhouse effect on its own is not necessarily a bad thing – the Earth would be at an average temperature of around minus 18 degrees Celsius without it – but as we are currently witnessing here on Earth, shift the balance of those gases and things can change very quickly.

At some point in Venus’s past, the levels of water vapour lifted into the atmosphere by the warming sun pushed the greenhouse effect to become more intense. With less and less of the Sun’s energy escaping, the ambient temperatures began to rise exponentially until the day came when the last raindrops fell onto the surface of the planet, the heat evaporating the rains long before they could reach the ground. Venus had reached a tipping point: with the increasing temperatures feeding more and more water vapour into the atmosphere, a runaway greenhouse effect took hold, driving away the oceans. This led to the surface of the planet getting so hot that carbon trapped in rocks was released into the atmosphere, mixing with oxygen to form increasing amounts of another greenhouse gas – carbon dioxide. With no water left on the surface and no other means to remove it, carbon dioxide built up in the atmosphere, setting the planet on a course that would result in the scorched body that we see today.

In 1977, in the days before computer-generated imaging, NASA commissioned artist Rick Guidice to paint illustrations of the surface of Venus, based on images received from Pioneer probes.

And so Venus’s moment in the Sun came to an end. Earthlings take note: when it comes to the greenhouse effect, there is a precariously thin line between keeping a planet warm and frying it.

THE END OF EARTH?

Of the four rocky worlds, only one has managed to navigate through the instability and constant change of our Solar System over the last 4 billion years and maintain the characteristics needed to support life. Mercury lost its fight early as it was flung inwards towards the Sun, Venus flourished at first, before slowly coming to the boil, and Mars, the runt of the litter, became a frozen wasteland long ago. Only Earth, uniquely amongst the planets, has persisted with an adequate stability over the last 4 billion years to allow liquid water to remain on its surface and an atmosphere just thick enough to keep its climate calm – not too hot and not too cold. Events have rocked us and extremes of temperature have waxed and waned, but never outside of the parameters needed to harbour life. In a chaotic solar system, filled with planetary might-have-beens, Earth is a shining example of stability, and the evidence for this is to be found in every nook and cranny of the planet.

Today Earth is dominated by life; the land and seas are teeming with millions upon millions of species, with thousands of new life forms discovered each year. Somehow, even when disaster threatened, the Earth has remained a living world; while endless species have come and gone, life has always persisted. It’s woven into the fabric of the planet – an integral part of every continent and every ocean. Life plays a crucial role in maintaining the balance of the atmosphere that keeps our planet temperate, but we know for certain it cannot last.

In a chaotic solar system, filled with planetary might-have-beens, Earth is a shining example of stability.

The Kamchatka Peninsula in Eastern Siberia is one of the most inhospitable places on Earth. A volcanic wasteland, peppered with thousands of hot springs, it’s here that we find some of the toughest living things. Extremophiles survive here that are able to withstand temperatures and pH levels higher than any other land-based life forms we have ever discovered. Kamchatka is part of the Pacific ring of fire, and despite its remoteness, biologists have long been enticed here to explore its toxic, bubbling cauldrons for signs of life. Complex life, animals and plants struggle to survive in temperatures above 50 degrees Celsius, so searching for life here is all about searching for single-celled life forms, bacteria and archaea – ancient microorganisms – that are somehow able to endure in this hostile environment. Life forms like Acidilobus aceticus, an archaea that can be found in a hot spring where the water is so acidic it reaches a pH of 2, and where temperatures rise to 92 degrees Celsius. In other parts of the hydrothermal field, bacteria like Desulfurella acetivorans have been discovered, which happily live in pools that are touching 60 degrees Celsius, but it’s these that are the real hotheads. In one of the biggest and hottest pools investigated by scientists, a large number of microbes have been found living in temperatures approaching 97 degrees – making it one of, if not the hottest environment ever studied for signs of life on land.

But to find the greatest hotheads on Planet Earth you need to look not on land but deep beneath the sea. In the furthest depths of the Atlantic, around the black smoker hydrothermal vents blurting out of the ocean floor, we’ve found strains of archaea that can survive temperatures of 122 degrees Celsius, and perhaps even higher.

These rare life forms live at the very edges of biology. Unique adaptations to their cellular chemistry enable the proteins and nucleic acids that create the structure of the microorganism to function, while the membranes that are protecting the cells utilise different fatty acids and lipids to keep the cell stable at the higher temperatures.

Russia’s Kamchatka Peninsula is one of Earth’s most inhospitable areas; the volcanic landscape gives us an insight as to how planet Earth might appear when it becomes too hot for life.

Arcturus, one of the brightest stars in the Northern Hemisphere, which in its early history would have had similar characteristics to Earth.

Perhaps there are even tougher life forms that we are yet to discover, but the thermophilic microorganisms that we have so far identified and investigated in places like Kamchatka all point to the fact that life has its limits. Evolution by natural selection can only adapt so much, and even though it’s impossible to imagine what life on Earth will look like in a few hundred million or even a few billion years’ time, we know that biology is constrained by thermodynamics, and so we can say with some certainty that there will come a time when the Earth is too hot for any living things to exist. Natural selection will eventually run out of options as the laws of physics outplay it, and all life will come to an end.

The blue white-dwarf star Sirius B (pictured to the right of Sirius A) has burned out to a core the size of Earth, giving us an insight into the future of our planet.

When this will happen no one can be certain, but as the Sun ages and grows hotter, temperatures on Earth will rapidly rise. Today the average surface temperature on the planet is 14.9 degrees Celsius, but with just a 10 per cent rise in the Sun’s luminosity, the average temperature will rise to 47 degrees Celsius and climbing. The increased temperatures will raise great storms across the planet. The rains will remove carbon dioxide from the atmosphere and it will be locked away as newly formed sedimentary rock. Trees and plants will struggle as they are robbed of the gas that sustains them, until eventually photosynthesis will cease. The lungs of our planet will fail and the precious oxygen that green plants and algae produce will dwindle. With the primary food source gone, the food chain will collapse and the age of complex life on Earth will draw to a close.

The diamond-shaped constellation, Boötes, has been known to scientists for centuries, described by Ptolemy in the second century.

Astrobiologist David Grinspoon on Venus as a window on Earth’s future

‘Left to its own devices, Earth will go the way of Venus. Now, this is nothing to lose sleep over right now because we’re talking at least a billion years, probably more like a couple of billion years in the future. We have more immediate concerns, but as we do compare the planetology and look at the exoplanets around other stars and consider the variety of planets in the universe and consider not just the past, but the future of our climate in our solar system, it is something to think about, that the current state of Venus is probably some kind of a window into the distant future of Earth under the warming sun.’

Heat-loving extremophiles may flourish for millions of years more, but eventually nuclear physics will have its way and as average temperatures race above 100 degrees Celsius, the last pockets of life will be extinguished from the Earth.

We can say with confidence this is going to happen because we can plot the future of our Sun far more precisely than the future of the Earth. Our understanding of nuclear physics allows us to predict what happens inside the cores of stars and thus we can see the past, present and future of stars like ours written across the night sky.

The heavens are filled with shining examples of stars that give us a glimpse into the future of our Sun. Arcturus, for example, in the constellation Boötes, is one of the brightest stars in the Northern Hemisphere. It’s around the mass of the Sun, perhaps a little bit heavier, and so in the distant past would have had remarkably similar characteristics to our own star. Today, though, Arcturus is 6 to 8 billion years old, potentially 3 billion years older than the Sun, and as it is no longer a main-sequence star, it is now in the red giant phase. Its fuel exhausted, it has swollen up to 25 times its original diameter and is around 170 times as luminous, despite the fact that as its core slowly burns out it is cooling.

To see even further into the future, we need to look towards the brightest star in the northern sky – Sirius. The dog star, as it is commonly known, is twice the mass of the Sun and still fully in the main sequence. But obscured by the glare of Sirius A is a faint companion, Sirius B. This is a star that has already burnt through its fuel, swollen into a red giant and the outer layers have drifted off into space, leaving the fading core of the star about the size of the Earth, known as a white dwarf.

These stars are just two examples amongst many that point us towards the ultimate fate of our Sun, a fate that we believe will play out over the next 5 billion years or so.

Just like Arcturus, as the Sun exhausts its hydrogen fuel, its outer edge will inflate and it will enter a red giant phase. Expanding millions of kilometres out into space, it will engulf Mercury first. Venus’s fate will be sealed next as the Sun expands further. Some models predict that Earth may just escape the fiery end of its neighbours – heated to 1,000 degrees Celsius but hanging on beyond the edge of the dying star as its orbit extends out due to the lessening mass of the Sun. Dead but not destroyed, Earth and Mars will orbit as burned-out relics of their former selves. The era of the four rocky inner planets will be over, the billions of lives lived on the surface of one of them nothing but a distant memory, but within our Solar System lies another family of rocky worlds whose moment in the Sun may be to come.

A NEW HOPE

Far beyond the asteroid belt, millions of miles away from the sun-drenched planets of the inner Solar System, the gas giants of Jupiter and Saturn are home to another family of rocky worlds. Jupiter alone has 79 known moons orbiting it, a menagerie of satellites of multiple shapes and sizes. We’ve been peering at these moons since Galileo Galilei spotted four of them (Io, Europa, Ganymede and Callisto, known as the Galilean moons) over 400 years ago, with his telescope, transforming our understanding of our place in the Solar System.

Today we have explored the Galilean moons not just from afar but close up and found them to be dynamic worlds. Io is fiercely volcanic and Europa, the ice moon, shows tantalising evidence on its surface pointing to a sub-surface ocean sitting below its icy crust. Ganymede and Callisto make up the final two Galilean moons, and just like Europa they are rocky worlds with an abundance of water ice on their surfaces and perhaps their own oceans lurking beneath. These three rocky, frozen worlds are all sitting in the cold outreaches of our Solar System, touched by the distant Sun but barely warmed, lying dormant until perhaps one day the ageing Sun will reach out and turn these bodies into ocean worlds for the very first time.

Created by images taken by the Galileo spacecraft in the late 1990s, this colour view shows Saturn’s icy moon Enceladus – perhaps our closest candidate for sustaining life as we know it.

Titan, a frozen moon shrouded in its own atmosphere, as seen from Saturn.

‘The world is my country, science is my religion.’

Christiaan Huygens

The next planet out, Saturn, also has its ever-growing family of moons. Amongst its collection of over 60 confirmed satellites are Titan, the only known moon with a dense atmosphere and liquid lakes on its surface (though they are primarily methane, not water), and Enceladus, a frozen ice moon just like Europa with a liquid ocean deep beneath its ice. We will come to Enceladus in detail in Chapter 4, but for now it’s intriguing to note that this icy moon may be our best current candidate as a second life-sustaining world in our Solar System. Until we go back and explore further we can’t be certain what lies below its surface, but the possibilities that the Cassini probe has so tantalisingly hinted at make it one of the most exciting places for us to visit within the next generation of interplanetary expeditions.

All these ice worlds, sitting dormant in the frozen reaches of the Solar System, offer the promise of a very different future, one in which the rocky worlds of the inner Solar System have been reduced to cinders, and a new generation of worlds waits to awaken. Ice worlds will become water worlds, warmed by the expanding Sun, until our dying star ultimately collapses into a white dwarf.

From left to right, the moons of Jupiter – Ganymede, Callisto and Io – are dynamic worlds; the former two lie dormant, waiting to be awakened by the warmth of the Sun.

No 2

EARTH
+
MARS

THE TWO SISTERS

PROFESSOR BRIAN COX

WAR OF THE WORLDS

Mars is a mirror for our dreams and nightmares. To the naked eye, the planet exhibits a reddish hue, blood red in the imagination; God of War, Star of Judgement. Through a small telescope, it is the most Earth-like of planets, with cinnabar deserts and white polar ice caps. A world we could imagine visiting, perhaps even settling in. Nineteenth-century astronomers convinced themselves they saw plains and mountain ranges and canals delivering meltwater from high latitudes to arid equatorial cities. Some thought the Martians a peaceful civilisation, far in advance of our own. Others saw threat. ‘Across the gulf of space, minds that are to our minds as ours are to those of the beasts that perish, intellects vast and cool and unsympathetic, regarded this earth with envious eyes,’ wrote H.G. Wells in his classic science-fiction novel The War of the Worlds, in 1897.

The nature of Mars remained a mystery until well into the twentieth century because the planet is small and far away and therefore difficult to view with ground-based telescopes. Even the Hubble Space Telescope, high above the distorting effects of Earth’s atmosphere, produces images which would not at first sight have prevented Wells from publishing. With a little imagination, the ice caps, high clouds and dark regions circling the deserts could be mistaken for evidence of a water cycle feeding the seasonal advance and retreat of vegetation.

The topography of Mars, as captured by NASA’s Hubble Space Telescope. The white ice clouds and orange dust storms characterise the planet’s hostile weather systems.

Photographs from the first flyby of Mars by NASA’s Mariner 4 spacecraft on 15 July 1965 abruptly laid to rest the romantic notion of Mars as Earth’s habitable twin or potential foe. These images revealed an arid surface reminiscent not of our blue planet but of our desiccated Moon. Overnight, we discovered for certain that Earth is the only planet in the Solar System capable of supporting complex life, and contemporary accounts of the impact of the Mariner 4 flyby suggest that this was a powerful realisation. In November 1965, the Bulletin of the Atomic Scientists carried an article entitled ‘The Message From Mariner 4’ – and the message was bleak. ‘The shock of Mariner’s photographic and radiometric reports is caused not only by their denial of the terrestrial image of Mars, but by the revelation that there is no second chance, at least not in the solar system.’ President Lyndon B. Johnson was reported as commenting, ‘It may be – it may just be that life as we know it, with its humanity, is more unique than many have thought.’ The hesitation in the first few words is revealing. Here is Mars as a symbol of our cosmic isolation. It is as though deep, or perhaps not so deep, in the subconscious, the 1960s’ power brokers all the way up to the President suddenly understood that the Earth is far more fragile and precious than a dispassionate analysis of their Cold War brinkmanship might suggest. Or perhaps the perspective delivered by exploration is always shocking. Apollo 8’s Earthrise, the photograph that delivered such a positive end to a troubled 1968 by setting the blue Earth against the grey Moon, was three years away, but red Mars provided a foretaste.

‘The flight of Mariner 4 will long stand as one of the really great advances in man’s unending quest to extend the horizons of human knowledge.’

Lyndon B. Johnson

The Mariner 4 spacecraft began its historic journey to Mars on 28 November 1964. It sent back its first pictures to NASA’s Jet Propulsion Laboratory on 15 July 1965.

One site on Mars seen three ways. First imaged in 1965 by Mariner 4.

‘… if there were intelligent life on Mars … a photographic system considerably more sophisticated than Mariner 4 would be required to detect it.’

Carl Sagan

In 2017 by the HiRISE camera on the Mars Reconnaissance Orbiter (MRO).

Image was hand-coloured by NASA employees based on data transmitted back by Mariner 4.

President Lyndon B. Johnson sharing in man’s ‘quest to extend the horizons of human knowledge’ as Dr William H. Pickering (left), director of the Jet Propulsion Laboratory, shows him the first Mariner 4 photos.

Lyndon B. Johnson – extracts from Remarks Upon Viewing New Mariner 4 Pictures From Mars, 29 July 1965

‘Dr Webb, Dr Pickering, Dr Leighton, Members of Congress, distinguished guests:

Unaccustomed as I am to welcoming men from Mars, I am very happy to see you gentlemen here this morning. As a member of the generation that Orson Welles scared out of its wits, I must confess that I am a little bit relieved that your photographs didn’t show more signs of life out there …

The flight of Mariner 4 will long stand as one of the really great advances in man’s unending quest to extend the horizons of human knowledge. In the history books of tomorrow, unlike the headlines of today, the project’s name may be lost but the names of the men of vision, men of imagination and faith who made this enterprise such a historic success are going to be honored in the world for many generations to come.

This advance for mankind is awe-inspiring. It is all the more so when we realize that such capabilities have come into being within a short span of a very few years …

It may be – it may just be that life as we know it with its humanity is more unique than many have thought, and we must remember this.’

In response, Carl Sagan co-authored a paper suggesting, somewhat playfully, that all was not lost. Mariner 4 took only 22 photographs with a resolution of over a kilometre in a strip crossing the region in which the astronomer Percival Lowell had sketched canals from his observatory in Flagstaff, Arizona, at the turn of the twentieth century. From the warmth of the Arizona Desert, Lowell wrote that Mars was chilly, but no more so than the South of England, which certainly supported a civilisation of sorts. Using several thousand photographs of a similar resolution taken by meteorological satellites in Earth’s orbit, Sagan and his co-authors found only a single feature that unambiguously indicated the presence of a civilisation – Interstate Highway 40 in Tennessee. They concluded that Mariner 4 would not have detected human civilisation had it flown by Earth. ‘We do not expect intelligent life on Mars, but if there were intelligent life on Mars, comparable to that on Earth, a photographic system considerably more sophisticated than Mariner 4 would be required to detect it.’

The non-existence of an extant Martian civilisation was confirmed by the Mariner 9 mission in November 1971 – the first spacecraft to orbit another planet. Mariner 9 achieved a photographic resolution of 100m per pixel, and no sign of intelligent life, past or present, was detected. The twin Viking landers in 1976 failed to detect even microbial life, although the combined results of the suite of microbiology experiments carried by the spacecraft are not considered to be unequivocal because Martian soil chemistry is, to coin a phrase from the official NASA report, enigmatic, and could conceivably have masked any biological activity.

In hindsight, the fact that Mars is not teeming with life today is not so surprising. Mars orbits 50 million miles further from the Sun than Earth and receives less than half the solar energy. It is a small world with a tenuous atmosphere that provides little insulation or greenhouse warming. NASA’s Curiosity rover in the Gale crater has measured midday temperatures above 20 degrees Celsius, but in the early hours of the morning it has experienced minus 120. As Alfred Russel Wallace wrote in 1907, any attempt to transport water across the Martian surface today would be ‘the work of mad men rather than of intelligent beings’. There are no canals, no cities and no envious eyes. The planet is a frozen hyper-arid desert too far from the Sun to support complex life.

Yet it hasn’t always been this way. Observations from our fleet of orbiting spacecraft and landers have revealed a complex and varied past. Once upon a time the red planet was glistening blue. Streams ran down hillsides and rivers wound their way through valleys, carved by a water cycle from land to sky and down again from mountains and highlands to the sea. This presents a great challenge for planetary scientists. Put simply, nobody would have been surprised if Mars had always been an inert rock because it is a small planet far from its star. But the geological evidence is unequivocal; the surface tells a different story.

Mars, then, remains an enigma. As a wandering red star, it stirred the imagination of the ancients. As a telescopic image, too small and shifting for visual or intellectual clarity, it became our twin. When spacecraft flew by, it shocked us into considering our cosmic isolation. The red planet was relegated in our collective consciousness to the status of just one more rock glistening in the night. Then we landed, and discovered a world that was once habitable, and could be again.

READING THE MAPS OF MARS

A map of Mars can be read like a history book. Unlike Earth, where constant weathering, tectonic activity and volcanism have erased the deep geological past, Mars has been relatively quiescent for most of its life. The scars of collisions from the first turbulent billion years after the formation of the Solar System can still be seen from orbit; ancient cataclysms documented below a thin film of dust.

NASA’s Mars Global Surveyor spacecraft spent four and a half years mapping Mars in the late 1990s and provided detailed maps such as the one shown on the previous page, with colours corresponding to differences in altitude. Just as on Earth, there is significant variation, but the geological features on our smaller sister world are much bigger and bolder.

The highest elevations on Mars are found on the Tharsis Rise, a great volcanic plateau and home to the largest volcano in the Solar System, Olympus Mons. At over twice the height of Everest, Olympus Mons towers 25 kilometres above the lowlands of Amazonis Planitia to the west, and its base would fit inside France, just about. Cutting a deep scar across Tharsis to the south-east of Olympus Mons is Valles Marineris, named after the Mariner 9 spacecraft that discovered it, a canyon that dwarfs anything on Earth; the Grand Canyon would fit into one of its side channels.

The lowest points on Mars are found in the Hellas impact basin, the largest clearly visible impact crater in the Solar System. From the highest points on the crater rim to the floor, Hellas is over 9 kilometres deep; it could contain Mount Everest. The atmospheric pressure at the floor is twice that at the rim; high enough for liquid water to exist on the surface in a narrow range of temperatures.

These are extreme altitude differences for a small world; over 30 kilometres from the summit of Olympus Mons to the floor of Hellas. On much-larger Earth, for comparison, there is only 20 kilometres difference between the summit of Everest and the Challenger Deep in the depths of the Mariana Trench.

The most striking and ancient elevation difference on Mars is that between the Northern and Southern Hemispheres of the planet, known as the global dichotomy; Mars is an asymmetric world. The Northern Hemisphere is on average 5.5 kilometres lower in altitude than the Southern. There is no consensus as to how the dichotomy formed, other than that it was early in the planet’s history and before the large impacts which created the Utopia and Chryse Basins around 4 billion years ago. At some later time, the Northern lowlands were resurfaced by volcanic activity in a similar fashion to the smooth lunar seas, which accounts for their lack of cratering relative to the much more ancient terrain to the south.

The oldest terrain on Mars is found in the Noachis Terra region of the Southern Highlands. It is characterised by heavy cratering reminiscent of the far side of the Moon. Even small craters in the Noachian Highlands are heavily eroded, which suggests the regular, if not persistent, presence of liquid water. There are dry river valleys and deltas and evidence of water pooling in the craters and overflowing their walls, forming interconnected networks of lakes. This is how we know Mars was once a warmer and wetter world, at least occasionally; the evidence is written across the Land of Noah.

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