Was Venus Once Habitable? A Case for Further Exploration of Earth’s Twin

Owing to its infernal environment, Earth’s twin has been largely ignored in recent years, but further exploration might reveal if Venus was once habitable.

On March 5, 1982, the Soviet Union’s Venera 4 landed on the surface of Venus after parachuting through its dense atmosphere. It recorded a surface temperature of 465°C and air pressures 94 times higher than those on Earth at sea level. After just under an hour of running soil analysis tests, the lander succumbed to the immense pressures and temperatures. No mission has ever landed on our nearest planetary neighbour since.

Although there have been several successful missions sent to explore Venus from orbit in the past 35 years, interest in our sister planet seems to have almost evaporated in recent years. With Mars getting all the attention, Venus has been all but forgotten, despite the fact it actually has much more in common with Earth than the Red Planet likely ever did. Indeed, the Venusian surface might be the proverbial hell, but it probably wasn’t always this way.

Searching for Earth 2.0

Wikimedia Commons

Home to one of the most hostile environments in the solar system, Venus might sound like a pretty poor match for Earth 2.0, but there’s more to the planet than meets the eye. Firstly, its similar size, composition, mass and gravity are not replicated anywhere else in the solar system. We’re also having a hard time detecting planets of similar size around other stars, since they’re very difficult to detect. Even if when we do find them, we can’t possibly explore them as effectively as we can explore a planet right here on our doorstep.

There’s little doubt that Venus started life in much the same way Earth did. 4.5 billion years ago, they were probably almost identical, forming in the same way at the same time. The only key difference and, indeed, a driving factor in Earth’s evolution, was that Venus never had a Moon. Unlike Earth, the planet wasn’t subject to an enormous impact event that practically ripped the planet in two. Although this has major evolutionary implications, Venus was likely once a much less hostile place than it is today.

Present-day Venus might not exactly stack up as an Earth 2.0 candidate, but recent research suggests that it may have been the first habitable planet in our solar system. The simulations also demonstrate that Venus may have retained a potentially habitable climate until as recently as 715 million years ago, which was about the time when the first animal life started appearing on Earth. The simulations even take into account Venus’s bizarre retrograde rotation rate of 243 Earth days.

What these results clearly demonstrate is that we can and should explore Venus to learn more about the evolution of Earth-like planets and even life itself as well as, perhaps, the catastrophic destruction of habitable environments.

What Happened to Venus?

NASA

Venus might have once been Earth’s twin, but something happened during its troubled history that caused it to take an entirely different direction in its evolution. Today, the planet is home to a completely sterile environment; an endless rocky desert baking under crushing pressures and searing temperatures. Even in upper cloud levels, where temperatures and pressures are much more Earth-like, would-be visitors would still have to content with abrasive sulphuric acid rain.

Venus underwent global resurfacing sometime between 300 and 600 million years ago, possibly due to a mantle overturn event. This cataclysmic event completely transformed the surface, burying almost any evidence of its past far beneath a layer of volcanic basalt that now covers 90% of the planet. In fact, Venus has more volcanoes than any other planet in the solar system yet, most mysteriously, almost all of them are long extinct, and there’s been no direct evidence of recent eruptions on the surface.

The main problem with Venus is that is doesn’t have a magnetic field to protect it from the solar wind. Here on Earth, our magnetic field performs the critical function of protecting our atmosphere and oceans from getting swept away by the ‘electric’ wind of the Sun. Without it, the components that make up our oceans and atmosphere would be broken down and swept away into space. Only heavier materials, such as CO2 and sulphur, which are produced in abundance by volcanos, would remain. This appears to be precisely what happened to Venus.

The loss of Venus’s magnetic field can be attributed to its extremely long rotational period. A day on Venus lasts almost 243 Earth days, making it longer than the Venusian year. Stranger still, the planet rotates backwards, so the sun rises in the west rather than the east. With this very slow rotation, Venus is incapable of maintaining a dynamo effect in its core, which is exactly what creates a magnetic field. Today, the strength of Venus’s magnetic field is 0.000015 of Earth’s.

It’s unlikely that Venus always rotated in such a way, but no one is quite sure of what caused its current situation. Friction with the extremely dense atmosphere and tidal interactions with the sun have both been put forth as possible explanations.

Delving into the Inferno

Wikimedia Commons

Because of its incredibly hostile environment, Venus is notoriously difficult to explore from the surface, and orbital missions can’t tell a great deal about the planet’s past habitability. Additionally, if Venus was ever habitable or was even home to some form of life, any fossil evidence would likely be buried deep beneath the surface or even destroyed entirely. There are, for example, some highland areas that are much older than the lava plains that cover about two thirds of the surface.

Venus’s atmosphere gives few insights to the planet’s past habitability, so it would probably be necessary to send a lander there. This lander would need to be equipped with tools to drill into the surface and analyse the mineral composition of the oldest regions of the planet. As a result, we’d garner invaluable insights into the evolution of the planet’s atmosphere, past surface characteristics and, perhaps, even indications of past life.

According to research conducted by NASA, Venus may have had water oceans and moderate surface temperatures for 2 billion years, which is more than enough time for microbial life to evolve. By contrast, microbial life has existed on Earth for at least 3.7 billion years, although multicellular life did not appear until much later. As such, we can’t reasonably expect to find a Venusian dinosaur fossil but, if life could thrive on Earth before there was even oxygen in the atmosphere, then it seems likely it could have arisen on an ocean-covered Venus as well.

Unfortunately, the world’s space agencies continue to largely ignore Venus, which is understandable given their limited budgets and abundance of other promising opportunities. Last month, however, JPL’s James Cutts laid out a proposal as part of the Planetary Science Vision 2050 workshop that outlined a sample-return mission that would give us an unprecedented opportunity to study Venus’s past. However, if accepted, it’s not likely that such a mission would launch until the 2040s.

Although NASA hasn’t sent a mission there in almost 30 years, Venus hasn’t been completely forgotten about. Scientists from both the US and Russia have recently been discussing the latter’s proposed Venera-D mission, which may launch as early as 2025. Should the mission go ahead, it will herald a new age of exploration of our neighbour, ultimately culminating in a lander that will be able to explore the surface up close.

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It’s hard to imagine a more hellish world than Venus, and perhaps even harder still to imagine it as a habitable planet where life may have even taken a foothold before it did on Earth. How do you imagine our sister planet’s distant past? Share your thoughts and ideas in the comments below!

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