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Ample evidence of water on Mars but less gravity and a thinner atmosphere makes it not suitable for an Earth-like planet – study

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New findings have implications for the search for life on other planets besides Mars.

Mars was once more water-rich than Earth, according to remote sensing studies and analyses of Martian meteorites dating all the way back to the 1980s. NASA’s Viking orbiter spacecraft — and, more recently, NASA’s Curiosity and Perseverance rovers on the ground — have brought back dramatic images of Martian landscapes dotted with river valleys and flood channels.

Despite this evidence, there is no remaining liquid water on the surface. Countless hypotheses were advanced, including a weakening of Mars’ magnetic field, which could have resulted in the loss of a dense atmosphere.

However, a study published in the Proceedings of the National Academy of Sciences the week of Sept. 20 suggests a more fundamental reason why today’s Mars looks so drastically different from Earth’s “blue marble.”

Mars’ fate was predetermined from the start, according to Kun Wang, an assistant professor of earth and planetary sciences in Arts & Sciences and the study’s senior author.

There is almost certainly a limit to the size of rocky planets that can retain enough water to support habitability and plate tectonics, mass exceeding that of Mars, according to the author.

Wang and colleagues used stable isotopes of the element potassium (K) to determine the presence, distribution, and abundance of volatile elements on various planetary bodies for the new study.

Although potassium is a relatively volatile element, the researchers chose to use it as a tracer for more volatile elements and compounds, such as water. This is a relatively new method that differs from previous attempts to determine the amount of volatiles on Mars using potassium-to-thorium (Th) ratios gathered via remote sensing and chemical analysis. Previously, members of the research group examined the moon’s formation using a potassium tracer method.

Wang and his colleagues calculated the potassium isotope compositions of twenty previously confirmed Martian meteorites preferred to be representative of the red planet’s bulk silicate composition.

The researchers determined that while Mars lost more potassium and other volatiles than Earth during its formation, it retained more of these volatiles than the moon and asteroid 4-Vesta — two bodies that are significantly smaller and drier than Earth and Mars.

The researchers discovered a clear relationship between body mass and potassium isotopic composition.

“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,” said Katharina Lodders, research professor of earth and planetary sciences, a co-author of the study.

“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.”

“Martian meteorites are the only samples available to us to study the chemical makeup of the bulk Mars,” Wang said.

“Those Martian meteorites have ages varying from several hundred millions to 4 billion years and recorded Mars’ volatile evolution history. Through measuring the isotopes of moderately volatile elements, such as potassium, we can infer the degree of volatile depletion of bulk planets and make comparisons between different solar system bodies.”

“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,” Wang said.

“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.”

Being too close to the sun (or, in the case of exoplanets, their star) can have an effect on the amount of volatiles a planetary body can retain. This distance from the star is frequently incorporated into indices of “habitable zones” surrounding stars.

“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 now believes that, for planets within habitable zones, planetary size should be emphasized and considered routinely when determining whether an exoplanet is capable of supporting 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.”

The findings could help search for life on other planets, the researchers say.

Image Credit: Getty

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