Mars appears to be limited by its small size , according to new research by planetary scientists at Washington University in St. Louis, whose work explores the limits of the habitability of rocky planets.
Size Matters
It seems that some planets and moons may not be large enough to retain molecules that could be crucial for the formation of life, and Mars is among them, as our neighbor, the red planet, is 53% the diameter of Earth. (little more than half).
The researchers looked at the presence of different types of stable potassium isotopes present in Martian meteorites to estimate the presence, distribution, and abundance of volatile elements in different planetary bodies. Potassium is a “volatile”, the group of elements and compounds that planetary bodies can lose (water is another of them).
They measured the potassium isotope compositions of 20 previously confirmed Martian meteorites, selected to be representative of the bulk silicate composition of the Red Planet. They are of ages ranging from several hundred million to 4 billion years and have recorded the history of the volatile evolution of Mars.
Using this approach, they determined that Mars lost more potassium and other volatiles than Earth during its formation, but retained more than the Moon or the asteroid Vesta, two bodies much smaller and drier than Earth and Mars. A well-defined association between planetary size and potassium isotopic composition was clear.
“The fate of Mars was decided from the beginning,” says Kun Wang of Washington University in St. Louis and co-author of the work published in the journal PNAS. “It is likely that there is a threshold in the size requirements of rocky planets to retain enough water to allow habitability and plate tectonics, with a mass greater than that of Mars.”
The importance of retaining volatiles
We know that life on Earth requires liquid water, so conditions that allow for the presence of liquid water are one of the key elements on the planet’s habitability assessment list. We know that Mars used to have water on the surface; we’ve seen evidence of it in multiple Martian meteorites. Today, however, Mars is dusty, dry, and desert, and any water on its surface is frozen.
And precisely this relationship between the gravity of a body and the amount of volatiles has implications for how wet Mars may have been in the past.
“This study emphasizes that there is a very limited size range for planets to have enough but not too much water to develop a habitable surface environment,” said Klaus Mezger of the Center for Space and Habitability at the University of Bern. Switzerland and co-author of the study. “These results will guide astronomers in their search for habitable exoplanets in other solar systems.”
Referencia: Zhen Tian et al. 2021. Potassium isotope composition of Mars reveals a mechanism of planetary volatile retention. PNAS Proceedings of the National Academy of Sciences. 118 (39): e2101155118; doi: 10.1073/pnas.2101155118
