Mars habitability limited by its small size, isotope study suggests

Artist's rendering of Mars with Earth-like water. Credit: Image: NASA Earth Observatory/Joshua Stevens; NOAA National Environmental Satellite, Data, and Information Service; NASA/JPL-Caltech/USGS; Graphic design by Sean Garcia/Washington University
Water is vital for life on Earth, as well as other planets. Scientists have discovered ample evidence of water in Mars’ early history. However, Mars today has no liquid water. Washington University in St. Louis has just released new research that suggests Mars might not be able to retain large quantities of water.

Remote sensing studies of Martian meteorites from the 1980s have shown that Mars was once water rich, as compared to Earth. NASA's Viking orbiter spacecraft and, more recently, Perseverance and Curiosity rovers on ground returned dramatic images of Martian landscapes highlighted by flood channels and river valleys.

This evidence does not support the claim that there is any liquid water on the surface. There are many explanations for this phenomenon, according to researchers. One possibility is that the magnetic field of Mars has been weakened. This could have led to the loss of an atmosphere.

A study published in the Proceedings of the National Academy of Sciences on Sept. 20 suggests that there is a deeper reason Mars today looks so different to Earth's "blue marble".

Kun Wang, Washington University assistant professor of earth and planetary science in Arts & Sciences, was the senior author of the study. "Mars' fate has been decided from the beginning," he said. "It is possible that there is a threshold for the size requirements of rocky worlds in order to preserve enough water to allow habitability and plate-tectonics. This threshold may be higher than Mars'.

Wang and his collaborators used the stable isotopes (K) of the element potassium to estimate the abundance, distribution, and presence of volatile elements on different planet bodies.

Potassium is a volatile element. However, scientists chose to use it as an indicator for other volatile elements and compounds such as water. This method is relatively new and differs from previous attempts to use potassium to-thorium ratios (Th) gathered via remote sensing and chemical analyses to determine how many volatiles Mars once contained. To study the formation and evolution of the moon, the researchers used a potassium tracer technique.

Wang and his team measured the potassium content of 20 Martian meteorites that had been previously confirmed. These were chosen to represent the bulk silicate composition on the red planet.

This approach allowed the researchers to determine that Mars lost more potassium than Earth, but retained more volatiles than the moon or asteroid 4-Vesta. These two smaller bodies are drier than Mars and Earth.

Researchers found a clear correlation between body size, potassium isotopic composition and body size.

Katharina Lodders from Washington University is a coauthor of this study. She said that the reason for lower levels of volatile elements and compounds on differentiated planets has been a long-standing question. The correlation of K isotopic compositions and planet gravity is an important discovery that has important quantitative implications about when and how differentiated planets lost and received their volatiles.

Wang stated that "Martian meteorites" are the only available samples to study the chemical composition of bulk Mars. These Martian meteorites, which have ages of several hundred million to four billion years, recorded Mars' volatile evolutionary history. We can use the isotopes moderately volatile elements like potassium to infer the level of volatile depletion on bulk planets, and compare different solar system bodies.

Wang stated that while it is indisputable there was once liquid water on Mars' surface, the amount of water Mars once had cannot be quantified using remote sensing or rover studies. There are many models that can be used to estimate the Mars' bulk water content. Some models show that early Mars was wetter than the Earth. "We don't think that this was true."

Zhen Tian is a graduate student in Wang’s lab and a McDonnell International Academy Scholar. He is the first author. Piers Koefoed, a postdoctoral researcher, is also a coauthor. Hannah Bloom, who graduated in 2020 from Washington University, is also a co-author. Wang and Lodders were faculty fellows at the McDonnell Center for the Space Sciences.

Researchers noted that the findings could have implications for searching for life on other planets than Mars.

Exoplanets and sunspots can have an effect on the amount of volatiles a planet body can retain. This measurement of distance from star is often used to calculate "habitable zone" indexes around stars.

Klaus Mezger, co-author of this study, said that the study demonstrates that planets can have enough water but not too much to create a habitable environment. He is also a member of the Center for Space and Habitability, University of Bern. These results will help astronomers to search for habitable exoplanets within other solar systems.

Wang believes that for those planets in habitable zones, the planetary size should be more prominent and considered regularly when considering whether an exoplanet might support life.

Wang stated that the size of an exoplanet was one of the most difficult parameters to determine. We now know whether an exoplanet can support life based on its size and mass. This is because size is the first-order determinant for volatile retention.

More information: The Potassium Isotope Composition of Mars reveals a mechanism of planet volatile retention, Proceedings of the National Academy of Sciences (2021). Information for Journal: Proceedings of The National Academy of Sciences Potassium Isotope Composition of Mars Reveals a Mechanism of Planetary Volatile Retention, (2021). DOI: 10.1073/pnas.2101155118