Science.- Size matters to limit the habitability of Mars

21-09-2021 Artistic interpretation of a Mars with surface water similar to that of the Earth. RESEARCH AND TECHNOLOGY POLICY NASA EARTH OBSERVATORY / JOSHUA STEVENS


The small size of Mars is a fundamental reason why the red planet could not retain large amounts of water like it did in its early history, a study reveals.

Remote sensing studies and analyzes of Martian meteorites dating back to the 1980s suggest that Mars was once rich in water, compared to Earth. NASA’s ‘Viking’ spacecraft – and, more recently, the ‘Curiosity’ and ‘Perseverance’ rovers on the ground – returned spectacular images of Martian landscapes punctuated by river valleys and flood channels.

Despite these tests, there is no liquid water left on the surface. The researchers came up with many possible explanations, including a weakening of Mars’ magnetic field that could have caused the loss of a thick atmosphere.

But the new study, published in the journal Proceedings of the National Academy of Sciences, suggests a more fundamental reason why today’s Mars looks so dramatically different from Earth’s “blue marble.”

“The fate of Mars was decided early on,” Kun Wang, associate professor of Earth and planetary sciences in Arts and Sciences at the University of Washington, lead author of the study, said in a statement. “There is likely a threshold at the size requirements of rocky planets to retain enough water to allow habitability and plate tectonics, with a mass greater than that of Mars. “

For the new study, Wang and his colleagues used stable isotopes of the element potassium (K) to estimate the presence, distribution, and abundance of volatile elements in different planetary bodies.

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Potassium is a moderately volatile element, but the scientists decided to use it as a kind of tracker for more volatile elements and compounds, such as water.

It is a relatively new method that departs from previous attempts to use relationships between potassium and thorium (Th) collected by remote sensing and chemical analysis to determine the amount of volatiles that Mars had. In previous research, members of the research group used a potassium tracing method to study the formation of the Moon.

Wang and his team measured the potassium isotope compositions of 20 previously confirmed Martian meteorites, selected to be representative of the red planet’s silicate composition.

Using this method, the researchers determined 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 bodies much smaller and drier than Earth and Mars. .

The researchers found a well-defined correlation between body size and the isotopic composition of potassium.

“Why the abundance of volatile elements and their compounds is so much lower in differentiated planets than in primitive undifferentiated meteorites is a long-standing question,” says Katharina Lodders, research professor of Earth and Planetary Sciences. from the University of Washington, co-author of the study. “The finding of the correlation of the isotopic compositions of K with the gravity of the planet is a novel discovery with important quantitative implications on when and how the differentiated planets received and lost their volatiles.”

“Martian meteorites are the only samples we have to study the chemical composition of the bulk of Mars – Wang adds -. Those Martian meteorites have ages that vary between several hundred million and 4,000 million years and record history of the volatile evolution of Mars. By measuring the isotopes of moderately volatile elements, such as potassium, we can infer the degree of volatility depletion on the planets and make comparisons between different bodies in the solar system. “

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Wang admits that “it is indisputable that there was liquid water on the surface of Mars, but it is difficult to quantify the total amount of water that Mars had just from remote sensing and rover studies. There are many models of the water content on Mars. In some of them, early Mars was even more humid than Earth. We don’t think it was like that, “he says.

The findings have implications for the search for life on planets other than Mars, the researchers note. Being too close to the Sun (or, in the case of exoplanets, being too close to its star) can affect the amount of volatiles a planetary body can retain. This measure of the distance to the star is usually taken into account in the indices of “habitable zones” around the stars.

“This study shows that there is a very limited size range for planets to have enough, but not excessive, water to develop a habitable surface environment – says Klaus Mezger, from the University’s Center for Space and Habitability. from Bern, Switzerland, co-author of the study. “These results will guide astronomers in their search for habitable exoplanets in other solar systems.”

Wang now thinks that for planets within habitable zones, more emphasis should probably be placed on planetary size and routinely taken into account when considering whether an exoplanet might support life.

“The size of an exoplanet is one of the easiest parameters to determine,” explains Wang. “Based on size and mass, we now know if an exoplanet is a candidate for life, because a first-order determining factor for life. volatile retention is size. ”

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