Why is sea water salty?

A billion years ago, things weren't like this: water was fresh everywhere. Then, with the rain, the salt in the rocks dissolved and salted the seawater.

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The question is trivial, but it leads to many answers that tell us about the changes the planet has undergone over its very long life: why is seawater salty (about 35 grams of salt, mostly sodium chloride, per kilogram of water)? To put it simply: why does the water we drink on vacation taste so much like the water we add salt to pasta at the end of cooking?

It all began four billion years ago, when the first torrential rains deposited water in depressions in the Earth's crust, forming the first seas (the oceans are about 3,8 billion years old), which were certainly not like today's. According to the National Ocean Service, for example, the first seas were only slightly salty, and the concentration increased over time. A billion years later, the phenomenon intensified, and thus we arrive at the answer to the initial question: seawater is salty because rainwater dissolves the salts in rocks and carries them into the sea. Just as we, to continue with the culinary metaphor, toss salt into the pasta water after cooking.

But not all seas are equally salty. First, salt concentration increases as temperatures rise. This is easy to explain scientifically: higher temperatures translate into greater water evaporation and therefore increased salinity. Then there are seas, like the Dead Sea, where salt levels can be as much as 10 times higher than normal, with peaks around 35%, and where the density is so high that swimming isn't necessary to stay afloat, as the denser the water, the more buoyant it is. Even our Mediterranean Sea has a relatively higher salinity than the global average (38 grams of salt per liter of seawater): this is because it is a nearly closed basin, with little exchange with the less salty ocean waters.

As for the origin of water on Earth, there are currently only more or less well-founded theories about how water could have accumulated on Earth's surface over the last 4,6 billion years in sufficient quantities to form the oceans (which, according to most experts, are approximately 3,8 billion years old). In short, water could have two types of origins: interplanetary and extraplanetary. In the first case, water is the result of the planet's progressive cooling, resulting in the condensation of gases and the creation of the first rain; in the second case, it would have been the impact of one or more meteorites containing water. Both hypotheses likely contain a grain of scientific truth.

Finally, a curiosity: although seawater is as salty as the water used to cook pasta, we can't drink it. Our bodies, in fact, maintain blood salinity at around 0,9%. Drinking water four times saltier than that found in the human body would alter its hydro-saline balance and the functioning of its organs. In the Dead Sea, the water's density is so high that you don't even need to swim to float. You can read the newspaper or sip a drink as if you were on a deckchair.

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