Water in the Solar System
For a long time we believed that water was a scarce and precious resource and that its abundance on Earth was an almost unique feature of our world. However, as we have learned about our Solar System, we have discovered that water is widespread everywhere. On the other hand, if we consider its chemical formula, with two hydrogen atoms and one oxygen, we shouldn't be too surprised. Hydrogen, in fact, is the most abundant element in the Universe, it alone makes up around 75% of the atoms in our galaxy, while oxygen is the third most common element in space.
We should not be surprised, therefore, that water is found almost everywhere, nor that it has played a key role in the formation and evolution of our planetary system. The distribution of water can tell us a lot about how planets, moons, comets and all other bodies formed about four and a half billion years ago from the “proto-planetary disk” composed of gas, dust and ice.
After the formation of the "proto-sun", the central area of the solar system was too hot for the existence of water and only the elements most refractory to heat were able to form the internal "proto-planets". At greater distances, however, temperatures were low enough to allow water to condense. The dividing line, known as the “ice line,” was located near Jupiter's current orbit. Even today, this is the approximate distance from the Sun at which comet ice begins to sublimate.
Water ice, which made up a significant portion of the solid material in those regions, allowed the outer planets to grow much faster than their inner cousins. In the core of Saturn, Uranus and Neptune, and probably in that of Jupiter, there are the icy cores around which the gaseous atmospheres of these planets gathered, transforming them into the giants we observe today.
The large quantities of ice were not all assimilated by the giant planets. Each of those worlds was accompanied by dozens of smaller bodies, some larger than our Moon, others just a few kilometers in diameter. Initially, it was believed that these moons were completely frozen worlds, but this idea was gradually replaced by a more suggestive model. Thanks to data from probes sent around Jupiter and Saturn, we have discovered that, although the surface of some moons is made up of kilometers of ice as hard as granite, inside them there are enormous expanses of liquid water.
The first ocean of this type was identified under the icy blanket of Europa, one of Jupiter's moons first observed by Galileo Galilei. Europa is about the size of our Moon and was studied closely by a NASA probe that orbited the Jovian system for eight years, between the late 1990s and early 2000s. The automated vehicle, which named after the Italian scientist, found that Ganymede and Callisto, two other moons of Jupiter, may also have oceans of liquid water beneath the ice layer.
A similar result was highlighted by the Cassini probe, which studied Saturn and its satellites between 2004 and 2017. The NASA and ESA mission highlighted that Enceladus and Titan, two of the main moons of the "Lord of the Rings" ”, show clear indications of the presence of large quantities of salt water beneath their icy surfaces.
Let's go back to the formation of the Solar System. If the interior was too hot for water to be present in liquid or frozen form, it becomes crucial to explain how the Earth obtained the water that covers two-thirds of its surface. Our world may have acquired it in later times, perhaps transported by water-rich comets or asteroids.
Between 2014 and 2018, NASA's Dawn mission closely studied Ceres, the largest body in the asteroid belt between Mars and Jupiter. Analysis of the dwarf planet revealed the presence of large quantities of water ice, which shows an isotopic composition similar to that of Earth's oceans. This data, along with numerous other geochemical studies of ancient meteorites, suggests that icy asteroid impacts were the primary source of water on Earth.
We don't know all the mechanisms that brought the precious liquid to our planet, but we know how important it is for the evolution of life; just reflect on the fact that our body is made up of over 60% water. About 3.7 billion years ago, the first microbes learned to survive in an atmosphere rich in carbon dioxide. Subsequently, they evolved into cyanobacteria, organisms that, thanks to photosynthesis, were capable of using water, sunlight and carbon dioxide to produce their food. The byproduct of photosynthesis is oxygen, which has thus become an essential component of the atmosphere we breathe today.
So, even if the "blue planet", as the astronauts who admire it from orbit call it, has less water than the moons of Jupiter and Saturn, we have to thank the asteroids, meteorites and all the other processes that have led to the water on our planet. We wouldn't be here without them.
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