1. Isotopes: definition, measurement and function

Water covers three quarters of our planet's surface. It's everywhere, and in many forms: rain, rivers, seas, oceans, lakes, groundwater, steam, clouds, ice, not forgetting all the water contained in soil and vegetation. All these elements contribute to what is known as the "water cycle". In fact, since it appeared on Earth some 4 billion years ago, the quantity of water present on the planet, estimated at over 1 billion km ³ in total, has not changed. It's still the same volume of water that never ceases to transform, passing through the various stages of vapor, liquid water and ice, perpetuating the permanent water cycle.

With a simple chemical formula (H ₂ O), it is a complex body resulting from the combination of 3 oxygen isotopes (...