Venus is often seen similar to Earth, due to its closeness in mass, size and chemical composition to our planet. And now, Venus shares another common trait with Earth: it may have once contained oceans.

However, Venus' oceans weren't like those we know on Earth. Scientists now believe that the oceans on Venus were mostly carbon dioxide.

Of course, the similarities between Earth and Venus are only in their shared basic traits. Venus is actually a planet with an infernal atmosphere where sulfuric acid hangs in clouds over an immensely hot desert. And unlike Earth, it's completely incapable of sustaining life.

The oceans once contained by today's hot and dry Venus were nothing like ours, made of carbon dioxide and probably covered most of the planet, running about 80 feet deep.

But where did those oceans come from? On Earth, that kind of water would come from precipitation, but Venus is too hot for water to cool down and become precipitation. So this is why scientists believe that Venus' oceans were carbon dioxide instead, especially considering how common that chemical compound is on the planet's surface.

So how do scientists know this? They considered something called carbon dioxide's supercritical state. This is when extreme heat and pressure affect carbon dioxide, making it take on a form that is, basically, both liquid and gas at the same time.

The scientific team behind this research used computer simulations to study this supercritical state of carbon dioxide, something we don't currently know much about. Those simulations unveiled the mystery of carbon dioxide in that state and showed how it could shift between having liquid properties to having gas properties.

Considering that Venus' atmospheric pressure is already more than 90 times that of Earth's, scientists believe that its early atmosphere was far greater and lasted up to 200 million years. This extreme pressure could create the supercritical version of carbon dioxide that existed on the planet's surface. This means that the oceans of Venus probably resembled something like a soapy bath: gas bubbles covered by liquid.

Not only that, the supercritical carbon dioxide oceans that once flowed over Venus' surface probably affected the surface of the planet, creating the features we see today.

"This in turn makes it plausible that geological features on Venus like rift valleys, riverlike beds, and plains are the fingerprints of near-surface activity of liquidlike supercritical carbon dioxide," says Dima Bolmatov, a theoretical physicist at Cornell University in Ithaca, New York.

[Photo Credit: NASA/JPL]