Habitability of Mars limited by small size, isotope study suggests

Water is essential for life on Earth and on other planets, and scientists have found plenty of evidence for water early in Mars’ history. But Mars does not have liquid water on its surface today. New research from Washington University in St. Louis suggests a basic reason: Mars may simply be too small to hold large amounts of water.

Remote sensing studies and analyzes of Martian meteorites dating back to the 1980s postulate that Mars was once rich in water, relative to Earth. NASA’s Viking spacecraft and, more recently, the ground-based Curiosity and Perseverance rovers returned spectacular images of Martian landscapes marked by river valleys and flood channels.

Despite this evidence, no liquid water remains on the surface. The researchers offered many possible explanations, including a weakening of Mars’ magnetic field that could have resulted in the loss of a thick atmosphere.

But a study published the week of September 20 in the Proceedings of the National Academy of Sciences suggests a more basic reason why today’s Mars is so drastically different from Earth’s “blue marble”.

“The fate of Mars was decided from the start,” said Kun Wang, assistant professor of Earth and planetary sciences in Arts & Sciences at the University of Washington, lead author of the study. “There is probably a threshold on the size requirements of rocky planets to hold enough water to allow habitability and plate tectonics, with a mass exceeding that of Mars.”

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

Potassium is a moderately volatile element, but scientists decided to use it as a kind of tracer of more volatile elements and compounds, like water. This is a relatively new method that departs from previous attempts to use potassium-to-thorium (Th) ratios collected by remote sensing and chemical analysis to determine how much volatiles Mars once had. In previous research, members of the research group used a potassium tracer 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 bulk silicate composition of the Red Planet.

Using this approach, 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.

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

“The reason for the much lower abundances of volatile elements and their compounds in differentiated planets than in primitive undifferentiated meteorites is a long-standing question,” said Katharina Lodders, research professor of Earth and Planetary Sciences. at the University of Washington, co-author of the study. “The discovery of the correlation of K isotopic compositions with planetary gravity is a new discovery with important quantitative implications for when and how differentiated planets received and lost their birds.”

“Martian meteorites are the only samples we have available to study the chemical makeup of the mass of Mars,” Wang said. “These Martian meteorites have ages ranging from several hundred million to 4 billion years and have recorded the volatile evolutionary history of Mars. By measuring the isotopes of moderately volatile elements, such as potassium, we can deduce the degree of volatile depletion of bulk planets and make comparisons between different bodies in the solar system.

“There is no question that there was liquid water on the surface of Mars, but it is difficult to quantify the total amount of water on Mars through remote sensing and rover studies alone,” said Wang. “There are many models for the water content of Mars. In some of them, Mars at the beginning was even wetter than Earth. We don’t think it was.”

Zhen Tian, ​​a graduate student of Wang’s lab and a McDonnell International Academy scholar, is the first author of the article. Postdoctoral research associate Piers Koefoed is a co-author, as is Hannah Bloom, who graduated from the University of Washington in 2020. Wang and Lodders are faculty members at the university’s McDonnell Center for the Space Sciences.

The findings have implications for the search for life on other planets besides Mars, the researchers noted.

Being too close to the sun (or, for exoplanets, being too close to their star) can affect the amount of volatiles a planetary body can hold. This measure of distance to stars is often taken into account in indices of “habitable zones” around stars.

“This study points out 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 Livability. from 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 thinks that, for planets that are in habitable zones, the size of the planet should probably be emphasized more and routinely taken into account when considering whether an exoplanet could support life.

“The size of an exoplanet is one of the easiest parameters to determine,” Wang said. “Based on size and mass, we now know whether an exoplanet is a candidate for life, as a first-order determinant for volatile retention is size. ”