This time we’re moving nearer to home, to the planet closest to our own, Venus, the second rock from the Sun.
Venus is often called Earth’s twin, because it’s similar in size to the Earth (80% of the Earth’s mass) and orbits in a similar location (70% of the Earth’s distance from the Sun). You’ve probably seen Venus in the night’s sky and thought it was a bright star, as its so close you don’t need a telescope to see it.
Because of this similarity, from the 18th Century right up to the early 1960’s, scientists and writers thought Venus would be a good candidate for harbouring alien life. Venus does orbit closer to the Sun than the Earth, but at a distance that liquid water should still be able to cover much of its surface, thus Venus was judged to be in the habitable zone around our Sun.
Many predictions were made of rich jungles and warm oceans covering much of Venus’ surface, a bit like an entire planet of Costa Rica, which doesn’t actually sound too bad. These predictions were best explored in the science fiction of the time, including Perelandra (1943) by C. S. Lewis, in which Venus is depicted as a paradise planet covered in warm oceans and vast floating rafts of vegetation, and Edgar Rice Burroughs Venus Series, in which Venus has rich jungles conveniently full of scantily-clad voluptuous human-looking women, and monsters to slay, fun times. (He also created Tarzan, which explains quite a bit).
Humanity’s first steps towards exploring Venus suggested that this wasn’t going to be the case though. 1962 NASA’s Mariner 2 probe successfully orbited Venus and scanned the planet from space using the infrared and microwave spectrums. Whilst the probe found that the cloud tops were of a balmy temperature, it found the surface of Venus was a sweltering 425°C. Which is about as hot as it gets in Greece in the summer.
The Russians fired a number of probes into Venus’s atmosphere in the late 1960s to further explore our “twin”. The probes didn’t last long. Upon entering Venus’s atmosphere these early spacecraft bravely managed to chirp some data back to Earth, but only for a few tens of minutes before they were obliterated. From these probes scientists learnt that Venus is an even harsher environment than Mariner 2 first revealed, with a surface temperature closer to 480°C, clouds of sulphuric acid, and an immense atmospheric pressure up to 92 times stronger than that of Earth’s (similar to the pressure at a 1km depth in Earth’s oceans). If you walked over the surface of Venus you’d be simultaneously melted and crushed, unless you weren’t engulfed by a lava flow first.
Life under these conditions is almost unimaginable. I say almost, as in the last few decades biologists have found extremophile life in some very harsh environments on Earth. Extremophiles are microorganisms that thrive in tough conditions, like living in ice, or acid, or boiling water (see here for more on extremophiles). However, we’ve not yet found any microorganisms on Earth that could survive on Venus.
Other predictions have also been made about life being able to survive in Venus’s atmosphere; my favourite is from a short story by Stephen Baxter (yes, I’m forever talking about this guy, deal with it), called The Invasion of Venus, in which alien life is discovered living in the cloud layers at an altitude at which liquid water could be found. The aliens are much more technologically advanced than humans, and totally ignore us. It’s a great idea, but not very likely though.
So why is Venus so inhospitable if its our twin planet?
Well, planetary theory suggests that early in its life, Venus may have been a warm planet with abundant liquid water on its surface. But, as Venus was slightly closer to the Sun than the Earth, a greater proportion of its surface water evaporated to form cloud vapour in Venus’s atmosphere. Water vapour is a greenhouse gas, it traps some of the heat emitted by a planet in its atmosphere and further warms the planet, the so-called Greenhouse Effect, a bit like a planet wearing a woolly jumper.
The water vapour in Venus’s atmosphere will have further warmed its surface, causing more water to be evaporated, and causing more heating, in turn causing more evaporation, and so on. This is called a positive feedback cycle, as the initial increase in temperature causes further positive rises in temperature.
This positive feedback effect had another important consequence too. Surface rocks contain a certain amount of carbon, and the hotter the surface, the more carbon is released from such rocks into the atmosphere, primarily as carbon dioxide (CO2), as the carbon reacts with atmospheric oxygen. I’m sure you’ll be aware that CO2 is also a greenhouse gas, and the hotter Venus became, the more CO2 was released into its atmosphere, further contributing to the positive feedback effect on Venus’s surface temperature.
Over time, UV light from the Sun split the water molecules in Venus’s atmosphere into oxygen and hydrogen atoms, the light hydrogen atoms escaped into space, whilst the oxygen atoms remained to react with carbon to form CO2, resulting in an atmosphere with almost no water and abundant carbon dioxide.
To make matters worse, Earth also has a number of processes that remove CO2 from its atmosphere, oceans absorb CO2, photosynthetic life uses CO2 to store energy, and even the process of plate tectonics removes carbon from the surface of Earth and buries it deep in its mantle. As Venus lost its oceans its capacity to remove CO2 from the atmosphere was reduced, it likely had no photosynthetic life, and we believe it has no plate tectonics, partly due to its loss of surface water.
So Venus began slightly warmer than the Earth, but the increased evaporation of water and much greater volume of CO2 in it’s atmosphere led to a runaway Greenhouse Effect that caused it to become the high-pressure cooker planet it is today. Whereas Earth’s atmosphere contains about 0.04% CO2, Venus’s contains up to 97% CO2. The greenhouse gasses in the Earth’s atmosphere make our planet around 30°C warmer than it would be without them, but on Venus they make it up to 500°C warmer. Venus stands closer to the proverbial fire and wears a much thicker woolly jumper than Earth does. For anyone who disputes the Greenhouse Effect, I give you Venus.
So Venus likely doesn’t house alien life, and probably didn’t have liquid water for long enough for life to emerge in its past. But it has taught planetary scientists a good lesson, that the concept of a habitable zone is much more complex and hard to predict than first thought.
Scientists have learnt that the habitable zone isn’t just the band of space around a star that surface liquid water can reside in, the so-called Goldilocks Zone, not too hot, not too cold – but that it depends upon the features of the planetary body too. More distant planetary bodies have also reinforced this lesson, as moons such as Europa and Enceladus are way outside of what is considered to be our Sun’s Goldilocks Zone, but due to some interesting features, these moons probably have sub-surface oceans that may hold life.
Next up, we take a look at the next planet along from Venus, Earth, and how NASA searched for life on our emerald and topaz planet.