Billions of years ago, shortly after its formation, Mars was not very different from Earth. It was smaller and somewhat cooler because it was farther from the Sun, but its surface was dominated by continents and oceans . Extensive seas, lakes and even rivers capable of excavating channels on the Martian soil were added to the current mountains, valleys and volcanoes.
Liquid water is currently scarce on the surface of the Red Planet , only occasionally seen falling downhill and mixing with salts and soils. Yet evidence of those ancient seas, rivers, and lakes abounds. The Kasei valley , with its network of channels over 1,600 kilometers long, the Jezero crater, where NASA’s Perseverance rover landed, or the deltas formed over the Eberswalde crater tell us about a wet past on Mars.
Unfortunately, liquid water lasted a relatively short time , just a few hundred million years. The final cause is more or less clear. For water to remain liquid, it not only needs to be at the right temperature, but also at the right pressure. A fairly typical experiment in science demonstrations is to boil water without using a burner or any heat source . This is achieved simply by making a vacuum in the container that contains the water. When the pressure drops low enough, the boiling temperature of water drops below room temperature and the water begins to bubble. It is at that moment that attendees are invited to approach and touch the container, checking that although the water boils, it is at room temperature.
Something similar happened on Mars, but on a planetary scale. A few hundred million years after the planet formed, its magnetic field began to disappear . This gave free rein to the solar wind , which was interacting with the Martian atmosphere , detaching it from the surface of Mars. The same has not happened on Earth because our magnetic field is still active and resisting this solar onslaught. But Mars lost its magnetic field and with it much of its atmosphere. In doing so, the pressure on its surface decreased enough to evaporate its oceans , causing these in turn to become part of that atmosphere that was gradually being lost. Today only a thin layer survives , which exerts a pressure on the planet’s surface about 100 times less than that exerted by our own atmosphere.
A recent study by Japanese researchers sheds some light on the processes that led to the loss of that magnetic field. The Earth’s magnetic field has its origin in the convection currents that take place in the core . These currents move huge amounts of charged particles, which in turn generate the planetary-scale magnetic field. Something similar happened on Mars, although those currents were considerably more ephemeral than on Earth.
A team led by Shunpei Yokoo of the University of Tokyo has modeled the behavior of the Martian core in its early stages. For this they have used a diamond anvil cell , with which they have managed to simulate the incredible pressures of the Martian core. These devices have served on other occasions to even simulate the conditions of the core of Saturn. Inside this cell they introduced liquid iron, mainly enriched with sulfur and hydrogen , but also with traces of other elements, such as carbon and oxygen. Using infrared lasers, they also managed to simulate the temperature of this region.
What they observed is that initially, the mixture of iron, sulfur and hydrogen remains homogeneous, but little by little it differentiates into two different liquids . One consisting of liquid iron enriched mainly by sulfur and another enriched mainly by hydrogen. Being the second less dense, two different layers ended up being formed , a heavier inner one with sulfur and a lighter outer one with hydrogen. During the differentiation of each layer in the Martian core, convection currents comparable to those on Earth must have formed , which would have endowed Mars with a considerable magnetic field. However, when this differentiation ended, the currents ceased (since there was no longer any movement inside) and with them the magnetic field disappeared , the atmosphere succumbed to the solar wind and the Martian oceans evaporated.
This process must have taken several hundred million years , long enough for the Martian water to erode part of its surface, but not enough for it to survive to the present day. A similar differentiation process has probably taken place on Earth, causing us to be able to speak of an inner core and an outer core , but it has not affected our magnetic field too much. If the Earth were to experience a fate similar to that of Mars, it would be billions of years from now, so we have nothing to worry about.
REFERENCIA:
S. Yokoo et al, 2022, Stratification in planetary cores by liquid immiscibility in Fe-S-H. Nat Commun 13, 644, https://doi.org/10.1038/s41467-022-28274-z
