The water found on the comet is identical to that on Earth.

All the fascinating details about the Churyumov-Gerasimenko comet have come to light thanks to fortunate coincidences. It all began when Soviet astronomer Klim Churyumov unexpectedly spotted it in images of a completely different comet in 1969. Later, when the Rosetta mission was being prepared in the 2000s to study a comet, it was initially aimed at another celestial body, but they missed the launch window: they had to find a different "target," and the Churyumov-Gerasimenko comet turned out to be a convenient option in every regard.

Undoubtedly, scientists have never regretted that the spacecraft went to this comet. Upon approach, it became evident that it has a rare and unusual "hybrid" shape: two parts of different sizes connected by a "neck."

There are other examples of such distinctive appearances found in space, the most notable being the Arrokoth asteroid in the Kuiper Belt (beyond Neptune). These celestial bodies are referred to as contact-binary minor planets: they are thought to be the result of the collision and merging of two originally separate bodies. There is a theory that sometimes an asteroid and its moon merge in this way.

Subsequently, scientific instruments aboard the Rosetta probe detected that various complex organic compounds were erupting from the comet's surface, along with molecular oxygen, which is extremely rare in space.

All of this is released from the comet's interior along with streams of evaporating water that rise above it and spread out in different directions, like rays, as it approaches the Sun. At its closest point to the star in its orbit, the Churyumov-Gerasimenko comet finds itself between Earth and Mars, while at its furthest point, it is beyond Jupiter.

It is thanks to the Rosetta probe that scientists were able to gain insights into cometary water. The findings were unexpected, and some scientists may have been disappointed: it turned out that the water vapor above this celestial body is strikingly different from terrestrial water. The difference lies in the frequency of heavy hydrogen—deuterium—atoms found in water molecules. It is termed "heavy" because a regular hydrogen atom has only one proton and one electron, while deuterium has an additional neutron.

It became clear that heavy water vapor almost completely envelops the Churyumov-Gerasimenko comet, which is not characteristic of Earth's environment: our water contains relatively little deuterium. From this, many concluded that Earth’s oceans are unlikely to have a cosmic origin.

However, a recent team of planetary scientists from the USA, Switzerland, and France revisited the results of these measurements. In an article for the journal Science Advances, the researchers expressed the opinion that the data itself is accurate, but does not reflect the actual state of water in the Churyumov-Gerasimenko comet: the surrounding material contains many dust particles that effectively hold onto molecules of heavy water.

Thus, regular "light" water evaporates instantly, while its more "massive" counterpart remains. According to the researchers, this creates the impression that the comet's water is rich in deuterium. In reality, the ratio of heavy to regular hydrogen in its water is close to that of Earth. This is an important detail for understanding the role of comets in delivering water to Earth billions of years ago.

In this regard, it is particularly unfortunate that one extremely interesting and important operation within the Rosetta mission did not succeed: its lander, Philae, successfully landed on the surface of the Churyumov-Gerasimenko comet in 2014 and attempted to drill into it, but apparently it did not succeed: it remains unclear whether any of the collected samples made it into the prepared cells.