ST. LOUIS – In recent decades, scientists have theorized and found evidence that the planet Mars had, at one time, liquid water on its surface. Images from NASA spacecraft and the Curiosity and Perseverance rovers reveal a Martian landscape dotted with river valleys and flood channels that have long since dried out.
But why is Earth’s neighbor devoid of water at the surface? One such theory posits Mars’ magnetic field was weakened over a long time, resulting in the loss of a thick atmosphere and, thus, liquid water.
A study published this week in the “Proceedings of the National Academy of Sciences” suggests a different reason: Mars could be too small to retain large amounts of water at the surface.
Kun Wang, an assistant professor of earth and planetary sciences at Washington University and lead author of the study, says while it’s “indisputable” water once flowed on the surface of Mars, just how much liquid water is unknown.
“There are many models out there for the bulk water content of Mars. In some of them, early Mars was even wetter than the Earth. We don’t believe that was the case,” Wang said.
Wang and collaborators used stable potassium isotopes “to estimate the presence, distribution and abundance of volatile elements on different planetary bodies.” They measured potassium isotope compositions of 20 confirmed Martian meteorites and determined Mars “lost more potassium and other volatiles than Earth during its formation” some 4.6 billion years ago. Mars kept more of its volatiles than Earth’s moon and the asteroid 4-Vesta, which are smaller and drier than Earth and Mars.
Wang believes there is a correlation between a planetary object’s size and its potassium isotopic composition.
The study may have implications in the search for life on other planets, researchers say.
The proximity of a planet or object to its star (in our case, the sun) can affect the amount of volatiles it can retain.
“This study emphasizes that there is a very limited size range for planets to have just 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, a co-author of the study. “These results will guide astronomers in their search for habitable exoplanets in other solar systems.”
Wang believes when looking at exoplanets in a star’s “habitable zone,” planetary size should be a bigger factor when considering or discussing if that exoplanet could support life.
“The size of an exoplanet is one of the parameters that is easiest to determine,” Wang said. “Based on size and mass, we now know whether an exoplanet is a candidate for life, because a first-order determining factor for volatile retention is size.”