Water and Mars. These words can set off a torrent of speculations, evidence, hypotheses and theories. Mars does have some water, but it is frozen and most of it has been buried. Only a small amount of water vapour is found in the atmosphere. It was much drier in the past, evidence shows. It may have had a global sea in its ancient history. But was it ever habitable?
New research shows that it was not. Mars lost the majority of its water due to its size.
The authors note that space exploration has focused on the distribution, abundance, and distribution of volatile elements and compounds on Mars for 50 years. Understanding Martian water is an important part of many missions to Mars. Follow the Water! was NASA's catchphrase for the Mars Exploration Program.
There is decades of evidence that Mars was once wet. In the late 1970s, Viking missions sent landers and orbiters to Mars. Images were taken by the orbiters of geological formations found on Mars, which indicated that there was a lot of water in the past. Scientists studying Martian meteorites also found evidence of aqueous weathering products.
This image taken from Viking 1 shows Ravi Valles. It clearly appears like it was created by flowing water. Image Credit: NASA image selected by Jim Secosky. http://history.nasa.gov/SP-441/ch4.htm, Public Domain, https://commons.wikimedia.org/w/index.php?curid=8646399
Recent missions have provided ample evidence to show that Mars had once been inhabited by water. Modern orbiters, such as NASA's Mars Reconnaissance Orbiter or the ESA's Mars Express Orbiter, have been closely studying Mars. In anticipation of the Perseverance Rovers mission, the Jezero Crater has been the focus of much orbital attention. Jezero is an old paleolake that has a visible river delta. Nobody can deny that Mars was once wetter than it currently is.
Orbital view of Jezero crater showing its fossil river delta. The different colours are minerals that have been chemically modified by water. Credit: NASA/JPL/JHUAPL/MSSS/BROWN UNIVERSITY
Science is still trying to figure out what happened to all of that water. It is widely believed that Mars lost its magnetic shield and then lost its thick atmosphere, and water simply escaped into space. It is now a question of whether it was able to keep enough water for long enough to support life.
A team of researchers addressed this question in a paper entitled Potassium isotope content of Mars reveals a mechanism for planetary volatile retention. Zhen Tian, from the Department of Earth and Planetary Sciences at the McDonnell Center for Space Sciences is the first author. The paper was published in Proceedings of the National Academy of Sciences.
Their solution? Mars is too small.
Kun Wang, senior author and assistant professor of earth and planetary science in Arts & Sciences, stated that Mars' fate was determined from the beginning. It is possible that there is a limit on the size of rocky planets in order to preserve enough water for habitability and plate-tectonics.
This is the short answer.
The long answer is potassium isotopes, and their presence on Mars as well as other Solar System bodies.
According to a press release, the team used stable potassium isotopes to determine the abundance, distribution, and presence of volatile elements on different planet bodies. Although potassium is not volatile by itself, it can be traced for other volatile compounds such as water. The potassium tracer method has been used by members of the team to study the Moons formation.
Wang and other researchers examined 20 Martian meteorites, which together make up the Martian silicate structure. They discovered that Mars retained more volatiles than the Moon, including water. It retained more volatiles than smaller asteroids, such as Vesta. They found that Mars lost more volatiles including water, but the reverse was true for Earth. According to the team, there is a clear correlation between body size and potassium composition.
The study's image shows the potassium to thorium ratios against the corresponding K concentrations in martian meteorites, Earth's surface (GRS), Earth's mid-ocean basalts and Earth's ocean island basalts. It also includes bulk silicate Earth. This figure suggests a volatile-rich, early Mars. For a more thorough explanation, see the study. Image Credit: Wang et al 2021.
The same solar nebula material that formed after the Sun created Mars and Earth, which is the leftover material from its formation, formed Earth and Mars. They started with similar compositions. The team discovered that Mars meteorites contained higher concentrations of potassium than Earth's, which meant that there was a greater loss on Mars of potassium than on Earth.
The bulk silicate values of Earth and Mars, as well as Vesta, were also found to correlate with surface gravity, and H2O abundance.
The K isotopic composition in BSM The strong correlation between 41K mass and planet size reveals that planet bodies' sizes fundamentally affect their ability to retain volatiles. The authors wrote in their paper that this could shed more light on planet habitability and help to constrain unknown parent bodies sizes.
The study's image shows the surface gravity and potassium abundance for Vesta and Mars. It is clear that potassium has a direct correlation with the mass of the body. Image Credit: Wang et al 2021.
Katharina Lodders is a research professor in earth and planetary sciences and co-author of this study. She has long wondered why there are so many more volatile elements and their compounds on differentiated planets than they do in primitive undifferentiated meteorites. This new discovery of the correlation between K isotopic compositions and planet gravity has important quantitative implications. It will help us understand when and how volatile elements were received by differentiated planets.
According to the authors, this is due to how planets and other bodies accumulate over time. As bodies grow, the loss of volatiles such as water can change over time. Smaller bodies retain more volatiles than larger ones.
The best part? There's a limit to the size of exoplanets that can retain enough H2 O to allow habitability and plate-tectonics.
This work would have been impossible without the Martain meteorites from different ages that struck Earth. They can be traced back to as far as four billion years ago, or as recent as several hundred millions years.
Wang stated that the only sample we have of Martian meteorites is the one that can be used to study the chemical composition of bulk Mars. These Martian meteorites had ages that ranged from several hundred million to four billion years. They also recorded Mars' volatile evolution 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.
The study's figure A shows how volatiles can be lost or retained by bodies. Figure A illustrates how a planet could experience volatile depletion as its size increases. This can be due to a variety of mechanisms, including impacts. Figure B shows how a planet must reach critical size in order to retain volatiles. Image Credit: Wang et al 2021.
Was Mars really once wet? Probably. Did it stay wet enough to allow life to try it on Mars? Researchers say it is unlikely.
The study adds some detail to the concept of a habitable area and changes how we think about exoplanets. The term habitable zone is used in exoplanet research to describe the temperature range around a star that a planet can reasonably expect to have liquid water. Although this study increases the planet size of the idea, it is not the first to do so.
A planet that is too small to be in the stars habitable zone will lose its water.
Klaus Mezger, co-author of this study, stated that planets have a limited range of sizes in which they can get enough water but not too much to create a habitable environment. These findings will help astronomers to search for other habitable exoplanets within the solar system.
Senior author Wang sees the clear implications of this research. Exoplanet size and habitability should be given more attention.
Wang stated that the easiest parameter to measure is the size of an exoplanet. 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.
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