Water is necessary for life on Earth and other planets, and scientists have discovered plenty of evidence of water on Mars throughout its early past. However, there is no liquid water on Mars now. A basic explanation, according to new research from Washington University in St. Louis, is that Mars is simply too small to contain substantial volumes of water.
Mars was formerly water-rich, according to remote sensing investigations and analysis of Martian meteorites dating back to the 1980s. The Viking orbiter spacecraft, as well as the Curiosity and Perseverance rovers on the ground, have returned stunning photographs of Martian landscapes dotted with river valleys and flood channels.
Despite this, no liquid water can be seen on the surface. Many theories have been presented, including a weakening of Mars’ magnetic field, which could have caused the loss of a thick atmosphere.
However, a research published in the Proceedings of the National Academy of Sciences the week of Sept. 20 offers a more basic explanation why today’s Mars looks so different from Earth’s “blue marble.”
“Mars’ fate was predetermined from the start,” said Kun Wang, senior author of the study and assistant professor of earth and planetary sciences at Washington University. “The size requirements of rocky planets to retain enough water to support habitability and plate tectonics, with mass exceeding that of Mars, are expected to reach a limit.”
Wang and his colleagues employed stable isotopes of potassium (K) to assess the presence, distribution, and quantity of volatile elements on several planetary bodies for the new study.
Despite the fact that potassium is a fairly volatile element, scientists decided to employ it as a tracer for more volatile elements and molecules, such as water. This is a relatively novel method that differs from past attempts to quantify the amount of volatiles Mars originally had using potassium-to-thorium (Th) ratios acquired by remote sensing and chemical analysis. Members of the research team previously studied the creation of the moon using a potassium tracer approach.
Wang and his colleagues analyzed the potassium isotope compositions of 20 previously confirmed Martian meteorites that were chosen to represent the red planet’s bulk silicate composition.
Using this method, the researchers discovered that during the formation of Mars, it lost more potassium and other volatiles than Earth, but retained more than the moon and asteroid 4-Vesta, two much smaller and drier worlds than Earth and Mars.
The scientists discovered a clear link between body size and potassium isotope composition.
“The reason for much lower abundances of volatile elements and their compounds in differentiated planets than in primitive undifferentiated meteorites has been a longstanding question,” said Katharina Lodders, a coauthor of the study and a research professor of earth and planetary sciences at Washington University. “The discovery of a link between K isotope compositions and planet gravity is a groundbreaking finding with significant quantitative implications for when and how differentiated planets received and lost volatiles.”
“The only samples accessible to us to analyze the chemical makeup of the bulk of Mars are Martian meteorites,” Wang stated. “The ages of those Martian meteorites range from several hundred million to four billion years, and they represented Mars’ turbulent evolution history.” We can assess the degree of volatile depletion of bulk planets and draw comparisons between other solar system worlds by detecting isotopes of moderately volatile elements like potassium.
“It’s undeniable that there was once liquid water on Mars’ surface,” Wang said, “but how much water there was in total is difficult to calculate by remote sensing and rover investigations alone.” “There are numerous models for the bulk water content of Mars available. Early Mars was even wetter than Earth in some of them. That was not the case, in our opinion.”
The paper’s first author is Zhen Tian, a graduate student in Wang’s lab and a McDonnell International Academy Scholar. Piers Koefoed, a postdoctoral research associate, and Hannah Bloom, a Washington University graduate, are both co-authors. Wang and Lodders are McDonnell Center for Space Sciences faculty fellows at the institution.
The discoveries have ramifications for the search for life on worlds other than Mars, according to the researchers.
A planetary body’s ability to retain volatiles is influenced by its proximity to the sun (or, in the case of exoplanets, their star). This measurement of distance from the star is frequently used in indexes of “habitable zones” around stars.
“This study highlights that there is a very narrow size range for planets to have just enough but not too much water to establish a livable surface environment,” said Klaus Mezger, a co-author of the study from the University of Bern in Switzerland. “These findings will help astronomers find habitable exoplanets in other solar systems.”
Wang now believes that, for planets in habitable zones, planetary size should be stressed and evaluated more frequently when determining whether or not an exoplanet might host life.
“One of the criteria that is easy to establish is the size of an exoplanet,” Wang said. “We now know whether an exoplanet is a contender for life based on its size and mass, because size is a first-order determining factor for volatile retention.”