The Venus we know today is not the Venus from over 700 million years ago. In fact, the planet was ENTIRELY different. Before Venus became the literal HELL (like my title suggests), it is widely believed that it had a stable climate for billions of years!
Now you might be wondering.. what caused Venus to turn into the physical manifestation of Hell? Well, that's a good question. It is believed that a mysterious resurfacing event COMPLETELY altered the planet's landscape. Based on popular belief amongst scientists who have looked into Venus's past, it seemed that the planet had liquid water on its surface, plate tectonics, and a stable atmosphere with Earth-like temperatures until this mysterious event took place.
In fact, these habitable conditions could have lasted for up to 3 BILLION years based on simulations conducted by scientists.
So, how did we find out that Venus was basically Earth 2.0 back in the early (and I'm talking EARLY) days? Well, in 1978, NASA's Pioneer spacecraft found some evidence on Venus's surface that indicate it that it once had shallow oceans on its surface. That may not sound impressive, but given the current condition of the hellscape, I mean planet, that's insane to me! Since this discovery, several subsequent missions have studied the planet's surface and atmosphere, which have led to the popular beliefs that were mentioned earlier.
Since it is widely believed that the planet had liquid water on its surface with a much more stable atmosphere than it currently has, then it must have supported life as well. (Sound familiar)? Of course, this is all before its atmosphere quite literally became toxic to any life. Now, we are currently unsure of what the exact conditions are for a "habitable" planet. We know through using the Earth as an example, that the planet would need to be at a distance from its star where it can withstand the solar radiation without having its atmosphere blown away. However, the star would also need to provide energy for that planet. We see that on Earth with pretty much all life using the Sun's energy, whether it's for food through photosynthesis or just by us walking outside and feeling the beautiful rays of the Sun and getting Vitamin D in the process. The Earth is in what is called the "habitable zone," where liquid water is able to form on a planet's surface, thus making that planet more capable of supporting life. Venus used to be in the habitable zone in the early days of the Solar System due to the Sun being much dimmer and smaller, so it would be believable due to its mass and distance from the early Sun, that it would resemble something closer to our own home.
The bigger question here is.... what exactly the event was that essentially "killed" the planet.
This event was something large that took place on the planet and filled its atmosphere with a huge amount of gas, which could not be reabsorbed by the rocks on the surface. We have similar events take place on Earth, but nothing quite to this scale. What happened to Venus completely changed the planet, and we may never be able to fully understand this event. The best approximation that scientists have as to what happened to Venus is that a carbonate-silicate cycle (a process in which carbon dioxide is absorbed into rocks) was disrupted after intense volcanic activity took place on the planet's surface.
To answer the initial question of what this "event" was that killed Venus, let's talk about the runaway greenhouse effect. On Earth, we have a nice magnetic field which protects us from the harsh solar rays and flares that the Sun outputs.
Unfortunately, other planets are not so lucky. When the Sun got bigger and brighter, the habitable zone moved outward, to where Earth is. Venus was approaching the inner edge of it, and things took a turn for the worst for our "sister" planet. Since the Sun got bigger, naturally the temperatures started to rise on Venus. This caused its oceans to evaporate and deposit a large amount of water vapor into the atmosphere. Water vapor is known to trap heat very well, which further increased the planet's surface temperatures. The cycle continued with oceans evaporating and water vapor entering the atmosphere, quite literally "cooking" the planet. Venus had turned into a runaway greenhouse, with all of the water trapped in its atmosphere trapping as much heat as possible, and ending the possibility of any life continuing to thrive on the planet. It had become Hell. Without its oceans, all plate tectonics came to a halt, which locked the planet in place. Believe it or not, but plate tectonics play a large role in regulating the amount of carbon dioxide in the atmosphere. As mentioned previously, carbon gets absorbed into rocks in the carbonate-silicate cycle. However, with no plate tectonics around, the carbon would be unable to bind to the elements in the dirt and rocks, which eventually get buried far beneath the surface. Once they are beneath the surface, plate tectonics would cause plates to rub against each other and sink below one another (subduction).
Without plate tectonics, the carbon that was trapped in the rocks and dirt slowly outgassed or was dumped out in massive volcanic events (see the connection to what I discusses earlier)? Basically once the oceans evaporated, the carbon began to take over the atmosphere. With time, the water vapor in the atmosphere got hit with enough sunlight to break it apart, sending the hydrogen into space. Once that happened, the carbon rose up and took its place. As the atmosphere grew thicker, this just continued worsening the conditions on the surface, turning the planet into something that we're more familiar with when we here the name "Venus." Another thing is that the atmosphere might have even had enough drag to literally slow down the rotation of the entire planet itself, which would have given it its present day rate of rotation. Once this was over with, (which probably took about 100 million years or so), the potential for any future life to arise on Venus was over.
The tragic story of our sister planet brings up an important question about our own home planet. The same fate may hold true for us as well... The Sun is not done aging, and it will continue to grow larger as it uses up its fuel, starting to form heavier elements. It will grow brighter and bigger, which means Earth will not be in the habitable zone anymore. It will move outward once again. At some point in the next few hundred million years, the Earth may approach the inner edge of the habitable zone, clinging on to what's left, but that will not stop the Sun. Much like Venus, our oceans will also evaporate, temperatures will rapidly shoot upward, plate tectonics will shut down, carbon dioxide will spill into the atmosphere. Just like that, our Solar System will have gained another Hell.
By then we don't know what will become of human life, if we will even still be around by then, or if we will have found a new planet or moon to call home. Obviously, it won't affect those of us that are alive today and the many generations to come, but looking into the past, present, and futures of not only our own planet, but those that share the same star as us, is a crucial subject to study. Perhaps we should take care of our planet more, it could be our only home for all we know... and we should want to make that last for as long as possible. Venus really was our sister planet, and now she's gone...
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