Mars was born with a fatal flaw that doomed it to become a barren lifeless place, according to new research: it was always too small to retain large amounts of water.

Evidence that water was once present on Mars has been mounting for decades and this month NASA's Perseverance rover collected rock samples which have all but confirmed groundwater flowed on the red planet.

But the space agency's scientists weren't sure whether that water was present for tens of thousands of years or for millions of years. Now, more scientists believe they can contribute to the understanding of why that water disappeared.

Mars is ostensibly located within the "habitable zone", a distance from the sun which is neither too hot nor too cold to support liquid surface water and thus life - but it appears to support neither at the moment.

The most popular hypothesis for why Mars is barren today is based on its lack of a magnetosphere.

Unlike the Earth, where molten iron in the core of the planet created a protective magnetic shield around us, Mars' magnetic field is too weak to protect its atmosphere from being stripped away by cosmic forces.

Now a study published in the Proceedings of the National Academy of Science suggests Mars was created with a fatal flaw that meant it was never going to be able to protect that atmosphere and its liquid water.

"Mars' fate was decided from the beginning," explained Kun Wang, an assistant professor at Washington University and senior author of the study.

"There is likely a threshold on the size requirements of rocky planets to retain enough water to enable habitability and plate tectonics, with mass exceeding that of Mars," he explained.

Wang's team analysed potassium isotopes on Mars, something which can be used to trace volatile elements such as water, and compared them to Earth, the moon and an asteroid called 4-Vesta.

They found there was a clear correlation between the size of the cosmic body and the presence of potassium isotopes.

As the only Martian samples available on Earth are those from meteorites, they also often function a little like capsules showing what the planet was like at different stages in its history.

"This study emphasises 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 co-author Klaus Mezger of the Centre for Space and Habitability at the University of Bern. "These results will guide astronomers in their search for habitable exoplanets in other solar systems."

Wang now thinks that, for planets that are within habitable zones, planetary size probably should be more emphasised and routinely considered when thinking about whether an exoplanet could support life.

"The size of an exoplanet is one of the parameters that is easiest to determine," Wang added. "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."