by Paul Murdin
The French mathematician Joseph Fourier first noticed the greenhouse effect in the Earth’s atmosphere in 1827. The Earth receives light from the Sun, of which about 70% is absorbed. This sunlight warms the land, atmosphere and oceans, which radiate energy back towards space as infrared light. But most of the infrared emitted from the surface does not escape into space but is absorbed in the atmosphere by greenhouse gases and clouds. It heats the lower air, just as the glass roof of a greenhouse or the windows of a car allow sunlight in but trap the infrared light radiated by everything inside, causing the interior of the greenhouse or the car to become hot.
Throughout the nineteenth and twentieth centuries, scientists gradually came to understand precisely how the greenhouse effect works on the Earth. In 1861 the Irish physicist John Tyndall discovered that water vapour and carbon dioxide are the most important greenhouse gases in the Earth’s atmosphere. Astronomer Samuel Langley, director of Allegheny Observatory in Pittsburgh, Pennsylvania, used spectrum mapping in 1884 to show exactly how carbon dioxide and water vapour were opaque to infrared radiation. As Fourier and Tyndall had suspected, these two atmospheric gases allow incoming sunlight to pass through the atmosphere, but block much of the outgoing infrared radiation.
As they learned more about the properties of greenhouse gases, scientists began to wonder whether an increase in the amount of them in the atmosphere might raise the temperature of the Earth. In a series of publications from 1896 to 1908, the Swedish chemist Svante Arrhenius speculated that changes to the level of carbon dioxide in the Earth’s atmosphere could alter its surface temperature through the greenhouse effect. Not only did geologically produced changes cause the Ice Ages, Arrhenius thought, but the burning of fossil fuels (such as coal) would produce anthropogenic global warming. Between 1928 and 1938, British meteorologists George Simpson and Guy Stewart Callendar finally succeeded in calculating the full extent of the greenhouse effect on Earth. By 1961 physicists Gilbert N. Plass and Lewis D. Kaplan were making increasingly realistic climate models for the Earth and the phrase ‘greenhouse effect’ began to be used in connection with global warming. In the mid-1960s the first conferences on global warming were held and the first official reports on global warming appeared.
Scientists were also applying the same ideas to other planets. Between 1908 and 1922, astronomers Frank Very, Charles G. Abbott and Edward A. Milne made various studies of the greenhouse effect in the atmospheres of Venus and Mars, on the basis of existing knowledge regarding their compositions. Astronomer Rupert Wildt even speculated in 1937–40 that the surface temperature of Venus would be exceptionally high ‘on account of the greenhouse effect of the carbon dioxide’, estimating that large quantities of this gas could raise the surface temperature of the planet by up to 50 ºC above what would otherwise be expected. Compared to the Earth, the greenhouse effect on Venus is extreme, so much so that Carl Sagan suggested it had become unbalanced and run away with itself.
Venus’s atmosphere was originally carbon dioxide, but the greenhouse effect had warmed the planet so much that the high temperature changed the composition of its surface, generating more greenhouse gases in the atmosphere and raising the temperature still further. Accurate calculations were difficult in Sagan’s day because too little was known about Venus in the 1960s. This gap in knowledge was a major impetus for the exploration of Venus by spacecraft.
The US Mariner fly-bys of Venus began in 1962 with Mariner 2. This mission confirmed the high temperature of Venus’s surface and the high density of its atmosphere (ninety-three times denser than the Earth’s). In 1967 the Soviet Venera spacecraft series, initially designed by pioneering Soviet space engineer Sergei Korolyov, began their scientific explorations of Venus. The Venera missions were the first to venture onto the planet’s surface, followed by US Pioneer probes. The spacecraft glided and parachuted through the atmosphere, measuring its properties, but they landed too fast to survive the impact. Later Venera missions between 1967 and 1982 successfully landed softly on the surface, as did two Russian Vega landers in 1985. These missions discovered that Venus’s surface was composed of black, scaly volcanic rock under a yellow sky. The surface has a temperature of 740 K (467 °C), compared to the Earth’s 287 K (14 °C). Its hot, dense atmosphere is indeed primarily made up of carbon dioxide. Clouds of sulphuric acid droplets are thought to be responsible not only for the yellow sky but also for the intensity of the present greenhouse effect on Venus. Other minor components include nitrogen and water vapour with hydrogen chloride and hydrogen fluoride. Like sulphuric acid, these last two substances are very powerful acids and the landers withstood their corrosive hot rain for only between a few minutes and two hours.
Owing to these severe technical difficulties, no landers have been sent to Venus since 1985. However, it has become possible to maintain orbiters around Venus for longer durations. The Magellan spacecraft lasted four years from 1990 to 1994 and mapped the surface of Venus with radar that penetrated through the clouds. It proved to be a surface of volcanoes and lava flows (plate IV). The space exploration of Venus continued with the European Space Agency’s Venus Express, which arrived at Venus in 2005 and identified some regions where volcanic activity was still happening.
Since 1827, when Fourier first identified the greenhouse effect on Earth, carbon dioxide in the Earth’s atmosphere has increased by about a third due to man-made emissions, principally from industrial processes; an increase in the amount of methane generated by modern farming practices augments the change. In the absence of man-made emissions, the moderate greenhouse effect on the Earth naturally raises the planet’s overall temperature by 33 °C, keeping the climate livable. A doubling of the amount of carbon dioxide by anthropogenic means has the potential to increase the temperature by 3 to 5 °C, causing serious global climate change. However, the runaway greenhouse effect on Venus raises its temperature by 500 °C, completely changing the planet’s atmosphere and its climate. Venus is a horrific vision of what catastrophic climate change could look like on Earth.
Mars
The drying, dying planet
But who shall dwell in these worlds if they be inhabited?…Are we or they Lords of the World?
Johannes Kepler, Harmony of the World, translated by Robert Burton, 1618
Why did Mars – the planet in the Solar System most similar to the Earth – fail to sustain life? Early science-fiction writers portrayed Mars as a dying planet inhabited by desperate, warlike aliens. We now know that Mars suffered a global climatic catastrophe early in its development.
Mars is the red planet, the fourth from the Sun. It is smaller, colder and drier than the Earth and has a much thinner atmosphere. Galileo saw the disc of Mars with his telescope, but could not see its surface markings, which were discovered in 1659 by the Dutch astronomer Christiaan Huygens, who determined that Mars had a rotation period of about twenty-four hours – its days were almost the same length as the Earth’s. In 1666 the Italian-French astronomer Gian Cassini discovered Mars’s polar caps, which were assumed to be ice caps like the Earth’s; 350 years later, space probes confirmed that the polar caps are deposits of ice and dry ice, 2 to 3 kilometres thick (plate V).
In 1840 the German banker and amateur astronomer Wilhelm Beer and his colleague Johann H. von Madler made the first maps of Mars, showing dark areas that seemed variable in colour and intensity. Initially these dark areas were thought to be seas, but the French astronomer Emmanuel Liais suggested in 1860 that they could be large patches of vegetation, showing seasonal variations in colour. When Giovanni Schiaparelli mapped Mars in 1877, he labelled the dark patches as ‘continents’, ‘islands’ and ‘bays’, linked by numerous long, straight canali (‘channels’).
The canali led to speculation that Mars was inhabited by intelligent life. The American astronomer Percival Lowell’s many maps of Mars (dating between 1894 and 1911) made Schiaparelli’s ‘canals’ straighter and thinner, adding unwarranted credibility to the idea that they were artificial irrigati
on canals carrying water between darker ‘cultivated’ areas. Although other astronomers scorned Schiaparelli and Lowell’s overly detailed maps as works of fantasy, their vision of Mars took hold in science fiction and popular culture, where the planet was depicted as an old world, inhabited by warlike aliens looking to colonize the Earth because, despite their efforts at irrigation, their own world was dying. But Turkish-born French astronomer Eugenios Antoniadi proposed that the canals were only psychological interpretations of faint, blotchy structures seen through the terrestrial atmosphere. In 1903 the Greenwich astronomer Edward Maunder used schoolboys as test subjects to demonstrate that a defective telescope causes an area with many point-like features (such as a group of craters) to appear as a network of lines.
Mars has two small, potato-shaped satellites. The satellites were discovered in August 1877 by the American astronomer Asaph Hall at the US Naval Observatory during a particularly favourable time when the Earth was unusually close to Mars. Hall’s first glimpse of a satellite occurred just before fog from the River Potomac rolled in and shut his observing window. During the ensuing cloudy weather he slept at the observatory so as to take advantage of any brief, clear interval. He found the satellite again a week later, and was so full of his discovery that, in his excitement, he couldn’t keep it to himself:
Until this time, I had said nothing to anyone at the Observatory of my search for a satellite of Mars, but on leaving the observatory after these observations of the 16th, at about three o’clock in the morning, I told my assistant, George Anderson, to whom I had shown the object, that I thought I had discovered a satellite of Mars. I told him also to keep quiet as I did not wish anything said until the matter was beyond doubt. He said nothing, but the thing was too good to keep and I let it out myself. On 17 August between one and two o’clock, while I was reducing my observations, Professor Newcomb came into my room to eat his lunch and I showed him my measures of the faint object near Mars which proved that it was moving with the planet.
Hall discovered the second moon later that night:
For several days the inner moon was a puzzle. It would appear on different sides of the planet on the same night, and at first I thought there were two or three inner moons, since it seemed very improbable to me at that time that a satellite should revolve around its primary in less time [7 hrs 39 min] than that in which the planet rotates [24 hrs 36 min]. To settle this point, I watched this moon throughout the nights of 20 and 21 August, and saw, in fact, that there was but one inner moon.
The moons were christened after the horses that drew Mars’ war-chariot, as related in the Iliad: Phobos (the inner moon) and Deimos (the outer) – Fear and Dread. Phobos is spiralling down towards Mars and will impact in 50 million years.
Mars is repeatedly bombarded by meteors. The discovery that Mars is heavily cratered came in 1964 in a fly-by mission by the US space probe Mariner 4. Mars has no tectonic plates, so its surface is not regularly churned over in the same way as the Earth’s, and weather erosion of the landscape is minimal because of the thin, dry atmosphere. Much of the crust of Mars is therefore very old; some terrains formed over 3.8 billion years ago and preserve traces of every subsequent meteoric bombardment. There are a few newer terrains on Mars, chiefly ash-strewn volcanoes and lava flows. Mariner 9 discovered recent volcanic activity on Mars in 1971–72. The largest volcano is Olympus Mons, which at 24 kilometres high is three times higher than Mount Everest. Other dramatic landscapes were shaped by water, including vast, now-dry flood plains and glacial features. Mariner 9 sent back pictures of the eponymous Valles Marineris, a huge canyon system 600 kilometres wide and 7 kilometres deep, which extends 4,000 kilometres east–west along the martian equator.
NASA’s Viking missions of 1976 were the first to land on Mars and to image its desert-like surface at close range. The landers looked for and failed to discover organic material in the soil – there did not seem to be life on the desert surface, although it is still possible that evidence may be found elsewhere on the planet. Mars Pathfinder and the Mars Rovers landed in 1997 and 2004, respectively, and Mars Global Surveyor (2001) and Mars Express (2003) mapped the surface at high resolution for several years, confirming that there were massive floods and glaciers on areas of Mars in the past and discovering that water and ice still produce changes on Mars.
The martian atmosphere is made of carbon dioxide, nitrogen and argon. Because it is so thin, it is easy to see clouds, sand storms and seasonal exchanges of material between the polar caps. During the northern winter and southern summer, great dust-storms sometimes cover virtually the whole planet. More frequently, smaller tornados twist across the desert surface as ‘dust-devils’. The orbit of Mars changes with time, causing the seasons and climatic cycles to vary dramatically. The martian equivalents of Milankovič cycles are therefore more extreme than those on Earth.
Mars has no magnetic field, but in 1999 Mars Global Surveyor discovered residual magnetism in old surface rocks in the southern hemisphere, laid down as they drifted over the convective iron core of the planet. At some time about 4 billion years ago the liquid core cooled enough to solidify, and the martian dynamo died away. Since Mars does not have a protective magnetosphere like the Earth’s, its atmosphere is exposed directly to the solar wind and has gradually been stripped away. This means that ultraviolet light and solar particle radiation reach the surface at levels that would be deadly to surface-dwelling life. The weak atmosphere means that the martian air pressure is only 1% of the Earth’s atmospheric pressure, too low for liquid water to remain liquid for long. Without a thick atmosphere to insulate it, the planet does not retain much of the heat it receives from the Sun, causing severe frosts at night with temperatures plunging as low as -140 ºC in the polar regions.
These discoveries suggest that Mars was once wet and warm. Its climate changed catastrophically when the planet lost its magnetic field and became the dry and sterile place that it is today. Yet it is still possible that life developed in the wet and warm era and survives in niche environments even now.
Water on Mars and Jupiter’s Satellites
Evidence for extraterrestrial life?
A moment of happiness, you and I sitting on the
verandah, apparently two, but one in soul, you and I.
We feel the flowing water of life here, you and I,
with the garden’s beauty and the birds singing.
The stars will be watching us, and we will show
them what it is to be a thin crescent moon.
You and I unselfed, will be together, indifferent
to idle speculation, you and I.
The parrots of heaven will be cracking sugar as we
laugh together, you and I. In one form upon this earth,
and in another form in a timeless sweet land.
Mewlana Jalal ad-Din Rumi, ‘You and I’, fourteenth century CE
Water is the key ingredient for life on Earth. Wherever we find water in the Solar System, it is possible that we may also find evidence of life. ‘Splosh’ craters, traces of catastrophic floods, glaciers and channels carved by underground rivers suggest that life could at one time – or might even now – exist on Mars. But the most promising places for extraterrestrial life may be on Jupiter’s moons, like Europa, which contains more water than the Earth.
When Mariner 9 reached Mars in November 1971 it was the first spacecraft to enter into orbit around another planet. A martian dust-storm was in progress at the time and all that could be seen in Mariner’s first transmissions were the south pole and the tops of four high volcanoes. Controllers waited two anxious months before the atmosphere cleared and Mariner began photographing the surface of Mars. By the time it shut down in October 1972 the spacecraft had sent almost seven thousand images back to Earth.
Mariner’s success paved the way for the two Viking missions that were launched in 1975. Upon reaching Mars, each spacecraft separated into a lander and an orbiter. The Viking landers showed close-up images of a sa
ndy, wind-blown desert, strewn with angular rocks that had fallen as debris from meteor craters. The Viking missions were followed by Mars Pathfinder in 1997, and in 2004 by the Spirit and Opportunity Rovers, which were mobile and able to range outside their immediate landing areas. Spirit was active until it got stuck in a sand dune in 2010, Opportunity until it was immobilized by a sandstorm in 2018. NASA’s Mars Science Laboratory mission used a remarkable crane to lower the Curiosity rover onto the surface of Mars in 2012. Larger and faster than either Spirit or Opportunity, Curiosity has the capability to analyse the composition of rocks at a distance of 7 metres. Another NASA lander, InSight, successfully touched down on Mars in 2018 in order to measure seismic activity on Mars as a means to investigate its interior structure.
Mars Phoenix Lander touched down in 2007 at the most northerly latitude of Mars yet explored. The Viking Orbiters launched in 1975 had mapped the surface in detail from a distance, a task later shared by Mars Global Surveyor (1997–2006), Mars Odyssey (launched 2001), Mars Express (launched 2003), and the Mars Reconnaissance Orbiter (launched 2006). These spacecraft have discovered that the surface of Mars has wide-ranging networks of valleys. Unlike watercourses on Earth, some of these dry riverbeds, which are the remains of an extinct drainage system, have no small streams and tributaries, only large rivers that emerge full-size at their source. There are also numerous glacial features on Mars. It seems likely that these riverbeds were not made by the runoff of rain, but were carved by groundwater flow: rivers flowing at first underground, then emerging from beneath icy glaciers that channelled the water. Other valleys were created when the permafrost above ground was melted by geothermal springs, causing the roof structures of subterranean rivers to collapse.
