Mars may be too small to hold on to large amounts of water, a new study suggests.
While water is essential for life on Earth and other planets, and scientists have found evidence of water in Mars’ early history, the red planet has no liquid water on its surface today.
Researchers have suggested there could be a number of explanations of why there is no longer any water on Mars, including a weakening of its magnetic field that could have resulted in the thinning of its atmosphere.
But the new study offers a more fundamental reason why today’s Mars looks so drastically different to Earth.
Kun Wang is assistant professor of earth and planetary sciences at Washington University and was a senior author of the study.
“Mars’s fate was decided from the beginning,” he said.
“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.”
For the new study, the researchers used stable isotopes of the element potassium to estimate the presence, distribution and abundance of volatile elements on different planetary bodies.
Researchers measured the compositions of 20 previously confirmed Martian meteorites, selected to be representative of the bulk silicate composition of the red planet.
They found that Mars lost more potassium and other volatiles than Earth during its formation, but retained more of these volatiles than the Moon and asteroid 4-Vesta – two much smaller and drier bodies than Earth and Mars.
According to the study, there was a well-defined correlation between body size and potassium isotopic composition.
Katharina Lodders, research professor of earth and planetary sciences at Washington University, and a co-author of the study, said: “The finding of the correlation of K [potassium] 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.”
Prof Wang said: “Martian meteorites are the only samples available to us to study the chemical make-up of the bulk [composition of] Mars.
“Those Martian meteorites have ages varying from several hundred millions to four billion years and recorded Mars’s volatile evolution history.
“It’s indisputable that there used to be liquid water on the surface of Mars, but how much water in total Mars once had is hard to quantify through remote sensing and rover studies alone.
“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.”
The researchers say their findings have implications for the search for life on other planets.
Being too close to the Sun (or, for exoplanets, being too close to their star) can affect the amount of volatiles that a planetary body can retain.
This distance-from-star measurement is often factored into indexes of habitable zones around stars.
Klaus Mezger is professor of geochemistry at the Centre for Space and Habitability at the University of Bern, Switzerland and a co-author of the study.
“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,” he said.
“These results will guide astronomers in their search for habitable exoplanets in other solar systems.”
Prof Wang now thinks that for planets that are within habitable zones, more emphasis should be placed on planetary size and this should routinely be 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,” he 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.”
