Mars has always been too small to retain its oceans, streams, and lakes.

New research suggests that Mars was destined to desiccate by its small size.

Observations made by robotic explorers such as NASA's Curiosity and Perseverance rovers, scientists know that, in the ancient past, liquid water traveled the surface of Mars: The Red Planet once hosted lakes, rivers and streams, and possibly even a large ocean that covered much of its northern hemisphere.

But this surface water virtually disappeared approximately 3.5 billion years ago, lost in space with a large part of the Martian atmosphere. This dramatic climate shift occurred after the Red Planet lost its global magnetic field, which had protected Mars' air from being stripped away by charged particles streaming from the sun, scientists believe.

But this immediate cause was underpinned by a more fundamental factor: March is just too small to hold on to surface water in the long run.

"The destiny of Mars was decided from the beginning," said co-author of the study, Kun Wang, Adjunct Professor of Earth and Planetary Sciences at the University of Washington, St. Louis. "There is likely a threshold on the size requirements of rocky planets to retain enough water to enable habitability and plate tectonics." That threshold is more extensive than Mars, the scientists believe.

The study team — led by Zhen Tian, a grad student in Wang's lab — examined 20 Mars meteorites, which they selected to represent the Red Planet's bulk composition. The researchers measured the abundance of various potassium isotopes in these extraterrestrial rocks between 200 and 4 billion years old. (Isotopes are versions of an element that contain different numbers of neutrons in its atomic nucleus. )

Tian and her colleagues used potassium, known by the chemical symbol K, as a tracer for more "volatile" elements and compounds — stuff like water, which transitions to the gas phase at relatively low temperatures. As a result, they found that Mars lost far more birds during its formation than the Earth, which is approximately nine times more massive than the Red Planet. But Mars has been more restrained from its birds than the Earth's Moon and the 329-mile-wide asteroid Vesta (530 kilometers), both much smaller and drier than the Red Planet.

"The reason for far lower abundances of volatile elements and their compounds in differentiated planets than in primitive undifferentiated meteorites has been a longstanding question," co-author Katharina Lodders, a research professor of Earth and planetary sciences at Washington University, said in the same statement. ("Differentiated" means a heavenly body in which the interior has separated into different layers, such as the crust, mantle, and nucleus. )

"The finding of the correlation of K isotopic compositions with planet gravity is a novel discovery with important quantitative implications for when and how the differentiated planets received and lost their volatiles," Lodders said.

The latest study, released online today (September. 20) in the journal Proceedings of the National Academies of Sciences, and earlier research suggests that small size is a dual burden on habitability. Bantam planets lose a lot of water during their formation, and their world magnetic fields also extinguish relatively soon, resulting in a thinning of the atmosphere. (On the other hand, the Earth's global magnetic field is still strong, powered by a dynamo deep down on our planet. )

The new work could also have applications beyond our cosmic court, declared the team members.

"This study emphasizes that there is a minimal size range for planets to have just enough but not too much water to develop a habitable surface environment," co-author Klaus Mezger of the Center for Space and Habitability at the University of Bern in Switzerland said in the same statement. "These results will help astronomers find habitable exoplanets on other solar systems."

This «surface environment denial» is essential in any discussion on habitability. Scientists believe that modern Mars still has the potential to support vital underground aquifers, for example. And moons like the Europe of Jupiter and the Enceladus of Saturn shelter enormous oceans carrying life under their surfaces covered with ice.