The Planets

‘I can find no reason … for denying that she may be considered the abode of creatures as far advanced in the scale of creation as any which exist upon the Earth.’

Richard Proctor, English astronomer, 1870

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A hazy and cloud-shrouded Venus, photographed by Pioneer Venus Orbiter.

Nobel Prize-winning chemist Svante Arrhenius was one of the most renowned scientists to fuel the mythology of what lurked behind Venus’s cover. Like many of the scientists of his era, Arrhenius let his curiosity wander into many different realms, including astronomy, and he hypothesised at length about the Venusian environment. Assuming the clouds of Venus were composed of water, he wrote in his book The Destinies of the Stars that ‘a very great part of the surface of Venus is no doubt covered with swamps’, creating an environment not unlike the tropical rainforests found here on Earth.

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Artwork showing the successful landing of the Venera 9 spacecraft, which in its 53 minutes on the surface of Venus returned the first ever images of the planet.

© Science History Images / Alamy Stock Photo

Svante Arrhenius, Nobel Prize-winning chemist

‘Everything on Venus is dripping wet … A very great part of the surface … is no doubt covered with swamps corresponding to those on the Earth in which the coal deposits were formed … The constantly uniform climatic conditions which exist everywhere result in an entire absence of adaptation to changing exterior conditions. Only low forms of life are therefore represented, mostly no doubt belonging to the vegetable kingdom; and the organisms are nearly of the same kind all over the planet.’

Expanding on this picture, he suggested that the complete cloud cover of the planet created a uniformity totally unlike the extremes of weather that define different parts of the Earth. In Arrhenius’s imagination this stable environment, with a consistently uniform climate all over the planet, meant that any life on Venus lived without the evolutionary pressures of changing environments that drive natural selection here on Earth, leaving Venus in an evolutionary limbo akin to the Carboniferous Period. Describing a world full of prehistoric swamps and dank forests, Arrhenius created the perfect canvas for science-fiction writers of the time to conjure up a menagerie of curious life forms lurking beneath the clouds.

Today Arrhenius is far less known for his fertile imaginings on the wildlife of Venus than he is for his work on the climate of Earth. In 1896 he was the first scientist to use basic principles of chemistry to demonstrate the impact that the atmosphere can have, in particular levels of carbon dioxide, on the surface temperature, a process that was called the Arrhenius effect but is now known as the greenhouse effect. An effect that would not only have profound consequences for our understanding of our impact on our own planet, but would also be vital in explaining the true nature of Venus beneath the clouds.

By the 1920s, as ground-based technology improved, we stopped painting the surface of Venus with our imaginations and started filling in the gaps with facts. The first spectroscopic analysis of the planet’s atmosphere suggested that it wasn’t water or oxygen that filled the clouds of Venus, so some thought this hinted at an arid, desert land beneath. Others speculated that formaldehyde filled the air, leading to the belief that Venus was not only a dead planet but a pickled one, too. But come the 1950s the true nature of Venus began to be revealed, as more accurate Earth-based observation suggested the presence of overwhelming levels of one defining gas in the Venusian atmosphere. This was not a planet shrouded in clouds of water and oxygen, nor pickled in formaldehyde, this was a planet engulfed in a blanket of carbon dioxide, and as Arrhenius had demonstrated on Earth, this almost certainly meant that whatever lay beneath the clouds, the heat would be beyond the limits of even the most resilient life forms on Earth. As the first spacecraft were being built to explore our sister world, it was becoming increasingly clear that visiting Venus would be far from easy and she would be far from welcoming.

In the early 1960s, the Soviet Union began a series of missions under the programme name Venera, which attempted to explore the atmosphere and surface of Venus directly for the first time. The initial launches of the Venera programme failed before they had even left Earth’s orbit, but within a couple of years the programme began to slowly see some success.

Venera 1 was successfully launched on 12 February 1961. Designed as a flyby mission, it is thought to have passed within 100,000 kilometres of Venus, but a total telemetry failure on the craft meant that no data was returned to Earth. As far as we know, Venera 1 is still in an orbit around the Sun to this day.

Venera 3 attempted to go a step further and was designed to enter the Venusian atmosphere to take the first direct measurements. However, on crossing the atmospheric boundary the probe’s systems failed and no data was returned as it plummeted towards the ground. All that was left for Venera 3 was the historic position as the first human-built object to crash into another planet’s surface.

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In March 1982, Venera 13 returned these photographs of the surface of Venus, with part of the spacecraft visible in the foreground.

Despite multiple failures, the Soviets didn’t give up and in October 1967 Venera 4 entered the atmosphere of Venus and sent back data supporting the Earth-based observations, revealing for the first time that the blanket of cloud surrounding Venus was made up of primarily carbon dioxide (90 to 95 per cent), 3 per cent nitrogen and just trace amounts of oxygen and water vapour. Venera 4 confirmed beyond all doubt that this was no second Earth: as it descended through the thick clouds, the temperature rose to 262 degrees Celsius, the atmospheric pressure increased to 22 standard atmospheres (2,200kpa) – and this was still 26 kilometres above the surface. As Venera 4 parachuted its way down to the surface it provided data back to Earth while confirming its own imminent death. This was a spacecraft that was not designed to survive the intense pressures and temperatures it was measuring, let alone the lack of the water landing it was designed for. The craft failed during the descent and was lost long before it reached the surface.

Gradually, through the following missions, the Soviet scientists began to overcome each and every challenge Venus put in front of them. Venera 7 was built to survive the most violent of landings, and even though its parachute failed, it made it to the surface intact in 1970 and was able to use its damaged antennae to transmit limited temperature data for 23 minutes before it expired.

Venera 9 not only made it to the surface and operated for 53 minutes in October 1975 but was also the first craft to successfully deploy its camera on the ground and transmit an image back to Earth. In the first-ever picture taken from the surface of another planet, the black and white fractured image revealed a rocky, desolate landscape with measurements confirming it to be a blistering 485 degrees Celsius, with an atmospheric pressure of 90 atm (standard atmosphere) crushing down.

By the time Venera 13 launched, on 30 October 1981, the ambition of the missions and the confidence in delivering data from the surface had been radically transformed. Venera 13 functioned for 127 minutes in recorded temperatures of 457 degrees Celsius and a pressure of 89 Earth atmospheres. The probe’s cameras deployed, taking the first colour image from the surface of Venus, spring-loaded arms measured the compressibility of the soil, while a mechanical drill arm took a sample of the Venusian surface that was analysed by an onboard spectrometer. If that wasn’t enough, onboard microphones were deployed to record the vicious winds that were assumed to be whipping the surface of Venus, the first-ever recording of the sound of another planet.

As the Venera missions came to a close in 1983, not even the smallest doubt remained of Venus’s hostility. Far from the benign water world we had once imagined, the reality was that this was not a sister we recognised – in our search of the heavens for a place like home we’d found a toxic, fiery hellscape.

Venus is an enigmatic world – almost Earth-like in size, position and potential, and yet as far from paradise as it’s possible to imagine. If Mercury’s story is one of catastrophic orbital change and Earth’s of balance and stability, the story of Venus is a tragedy; a tale of subtle, yet relentless decline. So why did it all go wrong for Venus? Why did a world born with such similarities to the Earth take such a different path? To answer that, we need to look beyond the tortured planet we see today and go back to a time when Venus was a young thriving planet.

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Soviet scientists worked hard on the Venera missions, tweaking the spacecraft at every new incarnation, to get more time to explore Venus’s hostile landscape. This is a model of the Venera 9 spacecraft.

Through the following missions, the Soviet scientists began to overcome each and every challenge Venus put in front of them.

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A diagram of Venera 1, the first mission.

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The Venera 1 display in the space (Kosmos) pavilion at the All-Russia Exhibition Centre, in Moscow, Russia.

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This radar image taken by Venera 15 and 16, offers a fascinating insight into the terrain of Venus, revealing the Maxwell Montes mountain range in the centre and the 100-km-wide Cleopatra crater.

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Sediment and rocks visible on the landscape, imaged by Venera 9.

THE BIRTH OF VENUS

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Rising centrally in this computer-generated image is the volcano Maat Mons, surrounded by cascading lava. This three-dimensional image was created using data relayed by Venera 13 and 14.

‘Today Venus is incredibly hostile … so hot, so dry, but what did it start out like, was it ever more Earth-like? We don’t know for sure, so we want to make future spacecraft missions to nail down that early history.’

David Grinspoon, astrobiologist

Four billion years ago, Venus was a familiar world. A world created from the same dust as the Earth, born just about the same size and settled into an orbit that seemed just far enough away from the glare of the Sun to allow a precious process to begin to take hold. In almost every conceivable way, Venus’s early life mirrored that of our own world. As its newly formed crust settled and cooled from the violent heat of its birth, an atmosphere began to grow around the young planet, fed by gases bubbling up from the molten rock below its surface, as well as captured from the clouds of gas and dust it swept through on its orbit around the Sun. Clinging to the young Venus, this thin layer of gas would have certainly contained nitrogen, oxygen and carbon dioxide, but most intriguing of all, we are certain it would have also contained large amounts of water vapour.

High in the Venusian atmosphere this water vapour eventually cooled enough to change state from vapour to liquid. And with that transformation, a process began, that perhaps for the first time on any of the planets would have seen the conditions become just right for droplets of liquid water to take shape and begin falling from the Venusian sky. These were the first rains of the Solar System, showering down onto the dry plains of Venus. Gradually these rains would have not just fallen but flooded the surface, rivers would have flowed and shallow oceans taken hold of large swathes of the planet’s surface. Venus, perhaps before even the Earth, became a water world, a planet with skies full of clouds and a surface full of oceans, feeding the cycle of water around this young planet.

How can we be certain this blue version of Venus existed? Unlike Mars, where we can see the evidence of its watery past etched onto its surface, we have no such direct evidence of the presence of liquid water on the surface of Venus. The only physical evidence we have suggests that the planet’s watery past comes from measurements taken by NASA’s Pioneer Venus spacecraft back in 1978. One of its most surprising discoveries revealed an unexpected amount of deuterium (heavy water) in the atmosphere compared with hydrogen. This D/H ratio is far smaller on Venus than it is on Earth, and that’s interesting because when the two planets formed the ratio would have almost certainly been the same. Because hydrogen is far more easily lost from an atmosphere than deuterium, this smaller ratio suggests that Venus has lost a lot more water than the Earth over its lifetime – the signature of a long-lost primordial ocean. As cosmochemist Larry Nittler explains:

‘Scientists believe that Venus once had a lot of water in its oceans, but lost it over time, and perhaps in oceans as recently as a billion years ago. The reason we can tell this is from the isotopic composition of hydrogen measured in its atmosphere by spacecraft. Now, hydrogen has two flavours of isotopes, whereas most hydrogen atoms are just a single proton in the nucleus. Some, a small fraction, are what we call deuterium, that have a proton and a neutron, so they weigh twice as much as the regular hydrogen. What happens when you have evaporation of water from a planet, or the atmosphere, is that the water molecules that contain hydrogen are much lighter than the water molecules that contain deuterium, so they evaporate more easily, and can be lost more easily. So, over time, as you evaporate water, deuterium-bearing molecules stay behind relatively to the regular ones, and you build up a deuterium to hydrogen ratio. And by back-calculating from the measured ratio today, we can figure out how much water has been lost over the billions of years of evolution, and [on Venus] it’s quite a lot.’

© United States Naval Observatory

The transit of Venus – as the planet crosses the face of the Sun – was captured in photographic plates as early as 1882.

None of this is solid proof, but it does begin to point us in one direction, and with no further exploration of the surface we have had to rely on an accumulation of indirect evidence to begin to paint a more detailed picture of Venus’s watery past.

As with almost all of our understanding of the planets, the evidence that built this picture has been accumulated through decades of exploration. Starting with the Venera missions’ first touchdown on the planet to the Pioneer Venus orbiter, and to the more recent Magellan mission, which not only relayed extraordinary radar soundings of the surface of Venus but provided the first full topographical map of the planet collated over a period of four years in orbit.

Combining all of the data that has been accumulated over decades of exploration has allowed us to peer deep into the planet’s past, using the same tools that enable us to model the future of climate change here on Earth to create climate models of Venus in the past, present and future. The results of this analysis, conducted most recently by a team from NASA’s Goddard Institute for Space Studies (GISS), all point to the same conclusion – in the distant past Venus was a planet covered in shallow primordial oceans.