Dark side of Venus
Earth’s ‘evil twin’ continues to surprise us, and shining light on its chaotic dark side has forced astronomers to rethink some of their ideas about the planet
Earth's evil twin has a chaotic side that scientists have only just got to know better
Venus, the second planet out from the Sun, has long had our attention, and this is hardly surprising; it’s by far the brightest astronomical body in the sky after the Sun and the Moon - a world that's hard to miss.
The most brilliant of the five planets known to ancient astronomers, clearly visible to the naked eye at both dawn and dusk, Venus has been admired for millennia. The ancient Greeks associated the world with their goddess of love and beauty, Aphrodite, while the Romans counterpart if the goddess
Venus – confirming it as the only planet in the Solar System named after a woman!
To the ancient Mayans, however, the planet was linked with the feathered serpent Kukulkan, the patron of war, while in early Christians sometimes referred to the bright ‘morning star’ as Lucifer, the ‘light-bringer’. As we’ve since discovered during the last few decades, the latter is certainly more apt: the name given to Satan before he rebelled and fell from heaven is particularly appropriate for the hellish environment on the planet today.
Venus is often referred to as Earth’s sister or twin, given that the planet is almost the same size – and has a similar mass, density, composition and gravity. Some early science-fiction writers even imagined it to be a lush, bountiful paradise, full of wondrous life. It wasn't until 14 December 1962, when NASA’s Mariner 2 spacecraft flew within 34,760 kilometres (21,600 miles) of the planet, that we truly began to get a real sense of just how different Earth’s so-called twin actually is.
For starters, Venus is far warmer – in fact, it’s the hottest world in the Solar System. Small, rocky Mercury may be physically closer to the Sun, orbiting between 47 and 70 million kilometres
(29 to 43 million miles) away, but Venus – with its relatively circular orbit some 107 to 109 million kilometres (67 to 68 million miles) from the Sun
– is covered by a dense atmosphere of clouds that refelects sunlight. It’s this covering which ensures Venus is the brightest of the planets as seen from Earth, as it reflects around 70 per cent of the light striking it.
Those thick clouds also trap whatever energy reaches the surface, in a runaway ‘greenhouse effect’. Temperatures on the dimly illuminated surface can reach an estimated 471 degrees Celsius (880 degrees Fahrenheit), which is more than enough to melt lead. When the Russian lander Venera 13 landed on the Venusian surface on 1 March 1982, transmitting back colour pictures of its rocky surface for the first time, scientists were surprised that the probe functioned for more than two hours – they had assumed it would last only 30 minutes on the harsh surface.
It’s not just the high temperature that any visitor to Venus has to contend with. The ultra-dry surface of the planet is littered with thousands of volcanoes, active volanoes, which have likely contributed to Venus now having an atmosphere primarily made up of suffocating carbon dioxide, ozone and eroding sulphuric acid. Just to add to the fun, much of the atmosphere cloaking the planet is so thick that the ‘air’ pressure at the surface is an oppressive 90-times greater than you’ll experience at sea level on Earth.
Admittedly, it’s likely that Venus wasn’t always like this. In 2016, NASA researchers at the Goddard Institute for Space Studies published their results from computer models which suggested that, during the first 2 billion years of its existence, the planet could well have been habitable – with a shallow ocean of liquid water, cooler surface temperatures and a thinner, Earth-like atmosphere. This contradicted Cornell University theoretical
“The winds and the cloud composition are somehow connected to the properties of Venus’ surface itself”
Jean-Loup Bertaux
physicist Dima Bolmatov's who, in 2014, proposed that if there were early oceans, they were more likely to have consisted of liquid carbon dioxide.
Clearly, neither is the case today, but that hasn’t stopped a few NASA researchers proposing that we could still explore Venus, or at least its upper atmosphere. After all, on average at their closest approach, Venus and Earth are ‘just’ 40-millionkilometres (25-million-miles) distant, which is considerably closer than Mars and the closest that any planet comes to Earth. Chris Jones, of Langley Research Center in Virginia, was part of a team behind the High Altitude Venus Operational Concept (HAVOC), suggesting that the use of helium-filled airships could even lead to permanent settlements, floating where atmospheric conditions are likely to be far more congenial, some 50 kilometres (31 miles) above the Venusian surface.
Certainly, the most recent human-built visitors to Venus have gone nowhere near ground-level. Much of what we now know about the planet comes from decades of observations made from orbit.
For example, the first successful Venusian orbiter, NASA’s Magellan spacecraft, used radar to generate relief maps of 98 per cent of the planet’s surface – showing details of features as small as 100 metres (328 feet) across – before radio contact was lost in October 1994.
Until December 2014, the European Space Agency’s Venus Express spent eight years orbiting Venus, with a variety of instruments to gain a deeper understanding of her. Among its many findings, it confirmed the presence of lightning within the Venusian clouds, and also detected a surprising strong electric field that may well have helped deplete the planet’s upper atmosphere of oxygen, one of the components of water.
Currently, Japan’s Akatsuki spaceship has been in orbit of Venus since 2015. Among its most fascinating discoveries so far was an enormous bow-shaped bright region stretching for 10,000 kilometres (6214 miles) across the upper atmosphere – a region where clouds usually move at about
100 metres (328 feet) per second in relation to the planet’s surface. Makoto Taguchi and his colleagues at the Japan Aerospace Exploration Agency (JAXA) suggested that this phenomenon, which was hotter than the surrounding atmosphere, was similar to the ‘gravity waves’ generated in the lower atmosphere by air passing over mountains on Earth. (These should not be confused with the recentlydetected ‘gravitational waves’, which are ripples in space-time!)
While ESA’s Venus Express mission ended in 2014, the huge amount of data it collected during its eight years in orbit is still being analysed, with some of its findings seemingly supporting Taguchi’s suggestion. A study published in July 2016 suggested that some of the more extreme wind patterns seen in the planet’s 20-kilometre- (12.4mile) thick cloud layer, which sits between 50 and
70 kilometres (31 and 43.5 miles) above the surface, might indeed be at least partially influenced by the topography of the land underneath.
“We used observations from Venus Express spanning a period of six years, from 2006 to 2012, which allowed us to study the planet’s longer-term weather patterns,” explains Jean-Loup Bertaux of Laboratoire Atmosphères, Milieux, Observations Spatiales (LATMOS) near Versailles, France. “Our results showed that all of these aspects – the winds, the water content, and the cloud composition – are somehow connected to the properties of Venus’ surface itself.”
Bertaux and his colleagues used observations in the infrared part of the spectrum to track the absorption of sunlight by the very small amounts of water vapour which exist in the Venusian atmosphere, enabling them to detect how much was present in each location at cloud-top level.
They found that one particular area, near the planet’s equator, contained more water vapour than its surroundings. This was located above a 4,500 metre (14,764 feet) mountain range, called Aphrodite Terra, suggesting that water-rich air was being forced upwards in a way similar to the gravity waves seen on Earth.
As well as helping us understand more about Venus, these findings have consequences for more general theories about climate. Håkan Svedhem, ESA project scientist for Venus Express, says:
“While our current general circulation models do acknowledge a connection between topography and climate, they don’t usually produce persistent weather patterns connected to topographical surface features. This is the first time that this connection has been shown clearly on Venus – it’s a major result.”
Results of the Venus Express are still being released including the results of wind and uppercloud patterns on the night side of Venus. The new study, published this summer in the journal Nature
Astronomy, showed that the atmosphere on the planet’s night side behaves very differently to that on the side of the Venus facing the Sun.
One of the unique features of Earth’s ‘twin’ is that, unlike most other planets in the Solar System, Venus spins on its axis in the opposite direction to that in which it orbits the Sun. Another is that it does so very slowly: it takes 243 Earth days for Venus to spin once on its axis – what’s called the sidereal day – compared with just 225 Earth days to complete one solar orbit. This means that on Venus, a day is technically longer than a year.
Just to confuse matters, however, the planet’s retrograde rotation means than a Venusian solar day – the time between one sunrise and the next – is actually only approximately 117 Earth-days long.
In contrast to these lethargic speeds, however, winds in the upper Venusian atmosphere sweep around the planet in around four Earth days; a process termed ‘super-rotation’. Until this summer, it was assumed to be essentially uniform on both the ‘day’ and ‘night’ sides of Venus, although an inability to successfully model those observed on the day side suggested that “we might be missing
“While the dayside has been extensively explored, there was still much to discover about the night side” Javier Peralta
some pieces of this puzzle”, according to Javier Peralta of the Japan Aerospace Exploration Agency.
“While the atmospheric circulation on the planet’s dayside has been extensively explored, there was still much to discover about the night side,” said Peralta. “We focused on the night side because it had been poorly explored. We can see the upper clouds on the planet’s night side via their thermal emission, but it’s been difficult to observe them properly because the contrast in our infrared images was too low to pick up enough detail.”
By capturing a ‘cube’ of hundreds of images taken simultaneously at different wavelengths – therefore creating much sharper images than previously – the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) on the Venus Express enabled Peralta and his colleagues to observe the clouds on the night side for the first time. Their findings were ruled to be “unexpected and surprising”.
Simply put, super-rotation seems to be more irregular and chaotic on the night side, with the upper clouds forming different shapes and forms than those seen on the dayside: large, wavy, patchy, irregular and filament-like patterns. Also, supporting the earlier Akatsuki discovery, the cloud patterns were dominated by unmoving phenomena, referred to as stationary waves.
“Stationary waves are probably what we’d call gravity waves. In other words, rising waves generated lower in Venus’ atmosphere that appear not to move with the planet’s rotation,” explained planetary scientist Agustín Sánchez-Lavega, of University del País Vasco in Bilbao, Spain. “These waves are concentrated over steep, mountainous areas of Venus, which suggests that the planet’s topography is affecting what happens up above in the clouds.”
So what of the future? NASA and the Russian Academy of Sciences' Space Research Institute (IKI) possible collaboration on the Institute’s planned Venera-D mission, a successor to the series of probes launched to Venus between 1961 and 1983. The new mission – which would include an orbiter, a lander and potentially a solar-powered airship to explore the atmosphere. The proposed launch date is around 2025.
Meanwhile, NASA continues to fund a range of very-early-stage mission ideas as part of its Innovative Advanced Concepts programme.
These range from HAVOC’s helium-filled crewed airships in the more temperate regions of Venus’ atmosphere, to a ‘steampunk’ rover which would pass on using the electronics that fry in the planet’s environment. Their development and success are by no means guaranteed, but it’s clear that our interest in our closest planetary neighbour continues. Just as well, too. If history teaches us anything, the more we learn about Venus, the stranger still the second world from the Sun becomes.