The habitability of Mars may be severely limited by the planet’s small size.

Water is one of the most important ingredients for life as we know it. And one day Mars must have had it too. Numerous river-carved gullies and even entire deltas have been found on the surface of the red planet. And robbers have found evidence in craters that they once harbored lakes. It all stands in stark contrast to Mars today. In 2021, no liquid water can be found on the surface of the red planet, and Mars therefore no longer seems as suitable for life as it used to be.

Too small

But why is there no liquid water on Mars anymore? It is often attributed to a weakening of the planet’s magnetic field, which heralded the demise of Mars’ thick atmosphere and eventually caused nearly all of Mars’ water to disappear into space. For example, the red planet would gradually have been transformed from a warm and particularly wet planet to a cold and dry planet.

Doomed

You could conclude that Mars – unlike Earth, which is still very comfortable – just had bad luck. But in a new study, scientists are coming up with an alternative theory. Because of the planet’s small size, Mars was doomed to lose water from the start.

Gravity

It is actually very simple, researcher Kun Wang explains to Scientias.nl. “Gravity – which in turn is determined by a planet’s mass – determines how many volatile elements the planet can hold during its formation.” If the planet is small in size and mass, then gravity is also weaker and volatile elements (such as hydrogen) can more easily escape that gravity and slip into space. “This intuitive idea is not new, but it has never been demonstrated with real data,” Wang said. But that has now changed. “Our research is one of the first studies to show a perfect correlation between the size of a planet and the rate at which volatile substances disappear into space.”

Potassium

For their research, Wang and colleagues focused on potassium: a volatile element, but not as volatile as hydrogen. “The more volatile an element is, the more of that element disappears into space,” Wang explains. “And that makes the element more difficult to measure, because less of it remains. Potassium is a moderately volatile element; it is not so volatile that it is completely lost and is therefore easier to measure.”

Meteorites

Wang and colleagues found 20 Martian meteorites that were between 200 million and four billion years old. “We measured the chemical composition of these meteorites,” Wang said. “We specifically focused on the ratio between a heavy potassium isotope (potassium-41) and a light potassium isotope (potassium-39).” The measurements indicate that during its formation Mars lost more potassium – and other volatile elements – than Earth (which is much larger). But at the same time, the red planet was able to hold much more of these volatile elements than the moon and asteroid 4-Vesta; two celestial bodies that are smaller and drier than Earth and Mars.

Mars

It confirms the previously mentioned suspicion that there is a relationship between the size of celestial bodies and the extent to which they are able to retain volatile elements. But what does that mean for Mars? “There’s no question that billions of years ago liquid water flowed over the surface of Mars,” Wang said. “This is evident from morphological and mineralogical evidence, such as photos of dried-up river valleys and canals. But you can’t determine how much water there was from the start based on that. Some people have painted a very rosy picture of young Mars, arguing that it harbored more water than Earth. And that in turn led many people to wonder: if life originated on Earth, why not on Mars? In our study, we put a limit to the total amount of water and other volatile elements that could collect on Mars, and we argue that these amounts must have been much smaller than on Earth. It also means that the chances of life forming on Mars are smaller. It’s in line with the fact that we haven’t found any signs of life on Mars to date. This new study puts people back on the ground.”

It’s hard to live

Due to its small size, Mars in principle already had less water and volatile elements. And because of that lack of volatile elements, the planet was also doomed to dry out. “The research highlights once again how difficult it is for a planet to be habitable,” Wang said. “The distance from the sun matters, but the size of the planet is also important. Too big would mean it traps too many gases and develops too thick an atmosphere. But if the planet is too small, there are just too few volatile elements.”

The research certainly has implications not only for Mars, but also for the search for life on planets outside our solar system. Researchers regularly discover such exoplanets. And if they are rocky, the question immediately arises whether there might be life. At the moment, to answer that question, the first thing to consider is the distance between the planet and its parent star. Because it dictates whether liquid water can exist on such a planet. A short distance from the parent star means that any liquid water evaporates. And if the distance is too great, any liquid water will actually freeze. A planet must therefore be at a certain distance from the parent star – in the so-called habitable zone – to be able to accommodate liquid water. However, the new research argues in favor of considering the size of the planets in addition to the distance from the parent star. “The study shows that for planets there is a very narrow range of sizes, with the planets harboring just enough – but not too much – water to allow habitable conditions on their surfaces,” said study researcher Klaus Mezger. “These results will help astronomers search for habitable exoplanets in other solar systems.” Wang agrees. “The size of an exoplanet is one of the easiest parameters to determine. Based on size and mass, we can now determine whether a planet is a candidate for life, because size is a first important factor for the retention of volatile elements.”