The Mars Problem

With the help of NASA rovers including Curiosity and Perseverance, scientists were able to discover that the planet Mars could hold liquid water on the surface in the past. We thus had lakes, rivers, but also an ocean which could cover a large part of the northern hemisphere. This situation did not last, however, since 3.5 billion years ago all water was lost in space, as was also the case with the Martian atmosphere. According to scientists, this tragedy took place after the planet lost its global magnetic field. The latter’s role was to protect the atmosphere against charged particles from the Sun.

This version is the one that is known to scientists today, but a new study has just revealed a fundamental factor in the fact that Mars no longer has water on its surface. According to the results of this study, the Red Planet is just too small to be able to hold water in the long term.

According to Kun Wang, assistant professor of Earth and planetary sciences at Washington University in St. Louis and co-author of the study, the fate of Mars was decided from the start. There is probably a lower limit when it comes to size so that rocky planets can hold enough water for habitability and plate tectonics. Apparently, Mars would be below this limit.

Analyzes carried out

The team of scientists led by doctoral student Zhen Tian examined 20 Martian meteorites. These have been selected to have representativeness of the overall composition of Mars. Researchers have measured the abundance of different isotopes of potassium in these rocks which are 200 million to 4 billion years old.

Tian and his colleagues used potassium as a tracer for more volatile elements and compounds like water. They found that Mars lost significantly more volatile elements during its formation than the Earth, which is nine times more massive. However, the Red Planet retained more volatile elements than the Moon or the asteroid Vesta. These two space objects are both smaller and drier than Mars.

The significance of the results

According to Katharina Lodders, professor of Earth and planetary sciences at Washington University and co-author of the article, the reason behind the lower abundance of volatile elements and their compounds in differentiated planets compared to primitive undifferentiated meteorites remains still a mystery. According to the explanations, “differentiated” means that the interior of the cosmic object in question is divided into several layers like the crust, mantle and core.

Lodders explains that the discovery of the correlation between isotopic compositions of potassium and planetary gravity is a new discovery with important quantitative implications for when and how differentiated planets receive and lose their volatile elements.

The new study and an earlier study both suggest that a planet’s small size is an issue when it comes to habitability. Small planets lose a lot of water during their formation and their global magnetic field goes out very early. This causes the atmosphere to thin.

According to the researchers, this new study emphasizes that there is a very limited margin for planets to have just enough water to develop a habitable surface environment. These results may help astronomers in their search for habitable exoplanets in other solar systems.