Читать книгу The Planets - Professor Cox Brian - Страница 10

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.

‘We found three-and-a-half-billion-year-old lake sediments that contained a diversity of organic materials. It tells us there’s actually organic matter present. What we found in those rocks, is what we expected of natural organic matter. It’s what you would expect to find on Earth.’

Jennifer Eigenbrode, astrobiologist

In contrast, the younger terrain of Hesperia Planum displays much less evidence of regular erosion by water, but bears the scars of occasional catastrophic floods that cut deep valleys over very short periods of time and may have formed temporary large lakes or seas.

Winter frosts at the North Pole of Mars are disappearing, revealing the surface features of the ice cap.

Alluvial fans are gently sloping wedges of sediment deposited by flowing water. Some of the best-preserved examples on Mars are in Saheki crater. On Earth, they are found in deserts, for example, in Death Valley, California.

Geological faults have disrupted layered deposits, creating a striking landscape in the northern Meridiani Planum region.

The Grand Canyon, in Arizona, began to form about 1,200 million years ago in the late Proterozoic period. Mars was already long frozen by this point.

The Amazonis Planitia region shows little sign of flowing water, fewer impact craters and less evidence of active volcanism, suggesting it was formed more recently when Mars was significantly less geologically active.

The persistence of surface features over many billions of years in the Noachian, Hesperian and Amazonian regions has led to the historical epochs of Mars being named after the distinctive terrains that still bear the characteristic marks of the climate and geological activity that formed and sculpted them.

The Noachian Period was the earliest and wettest, and coincided with the origin of life on Earth around 4 billion years ago, when conditions on both worlds appear to have been very similar. The Martian atmosphere may have been denser than Earth’s, and dominated by carbon dioxide, but significant questions remain about how such an atmosphere could have warmed Mars sufficiently to deliver the warm, wet climate and how that atmosphere was lost. The MAVEN spacecraft currently in orbit around Mars aims to answer this question, as we’ll discuss later in this chapter. The Noachian Period ended as Mars became increasingly cold and arid around 3.5 billion years ago, just as life was gaining a foothold on Earth.

The Hesperian Period, the time of catastrophic floods, ran from the end of the Noachian to around 3 billion years ago, when Mars entered its current frozen, arid phase, punctuated by occasional volcanic activity and the large-scale movement of ice, but with very little evidence of flowing water. The long 3-billion-year freeze from the end of the Hesperian to the present day is known as the Amazonian.

This is a summary of what we know about Mars; the whys pose a significant challenge to planetary scientists. Given a warm, wet and seemingly stable world early in its history, what triggered the loss of atmosphere and descent into modern-day aridity? What happened to the water on Mars? Was it lost to space or does it persist today as surface ice or in subsurface rocks or reservoirs? If so, how much water is still accessible? Could we exploit the ancient reservoirs of Mars to support a human colony? And perhaps most significantly of all, did life arise on the planet during the Noachian Period, coincident with the origin of life on Earth, and could that life still be present on Mars today?

The current fleet of spacecraft in orbit around Mars and roving across its surface has been designed to answer these questions.

The history of Mars from formation to the present, including major geological events, is shown in comparison to Earth’s timeline. Time is measured in Ga (giga annum): billions of years before the present. The Phanerozoic and Amazonian eons extend to and include the present day on the two planets.

An artist’s illustration of NASA’s Mars Reconnaissance Orbiter (MRO) passing above Nilosyrtis Mensae, a portion of the planet.

THE MARTIAN FLEET

Mars today is a planet buzzing with activity. Communications to Earth and the Martian Internet are managed by the Mars Reconnaissance Orbiter (MRO), an orbiting bridge between worlds. MRO carries the HiRISE instrument, a camera with resolution high enough to see basketball-sized features on the Martian surface. The Mars Color Imager (MARCI) camera monitors Martian weather, and the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) identifies mineral deposits, particularly those formed in the presence of surface water.

Orbiting with the MRO is the Mars Atmosphere and Volatile Evolution Mission (MAVEN). This camera-less spacecraft operates between 150 kilometres and 6,000 kilometres above the Martian surface, measuring the composition of the atmosphere at different altitudes and observing how the tenuous gases are stripped from the planet by the solar wind.

Mars Odyssey is the veteran of the orbiting fleet, having arrived in 2001 and still being operational in a polar orbit, searching primarily for water ice on the surface. Mars Express is a European Space Agency mission that is delivering high-resolution photographs, mineralogy data, radar investigation of the near sub-surface and atmospheric measurements, including the search for methane, a gas that on Earth is associated with biological activity. India’s Mangalyaan space probe is primarily a technology demonstrator, but it carries a secondary scientific package capable of investigating atmospheric composition.

‘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.’

President Lyndon B. Johnson

An Opportunity eye view of the surface of Mars, taken from the rover’s front hazard-avoidance camera (Hazcam).

The newest arrival at Mars is the joint European Space Agency/Russian ExoMars Trace Gas Orbiter, which will observe seasonal changes in the Martian atmosphere and search for subsurface water deposits. The spacecraft will form the communications bridge for ESA’s ExoMars rover, due to land in 2021.

The two most recent explorers of Mars are the Opportunity and Curiosity rovers. The Opportunity rover landed on the Meridiani Planum close to the Martian equator on 25 January 2004, with a planned lifetime of 90 Earth days. In a spectacular testament to the Jet Propulsion Laboratory’s engineering excellence, Opportunity remained operational until a planet-wide dust storm covered its solar panels in June 2018, after over 14 years and a journey of 45 kilometres on the surface of Mars, exploring the Endurance, Victoria and Endeavour craters. On 13 February 2019, Opportunity was finally declared ‘dead’.

Подняться наверх

Читать книгу The Planets - Professor Cox Brian - Страница 10

EARTH+MARS

EARTH
+
MARS