Mars and water. Those words can trigger an avalanche of speculation, evidence, hypotheses, and theories. Mars has some water now, but it’s frozen, and most of it’s buried. There’s only a tiny bit of water vapour in the atmosphere. Evidence shows that it was much wetter in the past. In its ancient past, the planet may have had a global ocean. But was it habitable at one time?
A new study says it wasn’t. Mars lost most of its water, and it’s all to do with the planet’s size.
“Examining the presence, distribution, and abundance of volatile elements and compounds, including water, on Mars has been a central theme of space exploration for the past 50 years,” the authors write in their paper. Many missions to Mars, whether orbiter, lander, or rover, include understanding Martian water in their science objectives. “Follow the Water!” was the easy-to-rally-around catchphrase for NASA’s Mars Exploration Program.
Evidence that Mars was once wet goes back decades. The Viking missions sent orbiters and landers to Mars in the late 1970s. The orbiters took images of geological formations on Mars that indicated the presence of large amounts of water in the past. In the same era, scientists studying Martian meteorites found evidence of aqueous weathering products.
More recent missions have gathered ample evidence that Mars once had water. Cameras on modern orbiters like NASA’s Mars Reconnaissance Orbiter and the ESA’s Mars Express Orbiter have studied Mars intensely. The Jezero Crater has received a lot of orbital attention, in anticipation of the Perseverance rover’s mission there. Jezero is an ancient paleolake with a clearly visible river delta. So nobody seriously doubts that Mars was at one time much wetter than it is now.
What happened to all that water is a burning question in science. The widely accepted theory is that Mars lost its magnetic shield, then its thick atmosphere and the water simply escaped into space. The question is: Was it able to retain enough water long enough for life to originate?
In a new paper titled “Potassium isotope composition of Mars reveals a mechanism of planetary volatile retention,” a team of researchers tackled that question. The first author is Zhen Tian from the Department of Earth and Planetary Sciences, at the McDonnell Center for Space Sciences. The paper is published in the Proceedings of the National Academy of Sciences.
Their answer? Mars is just too small.
“Mars’ fate was decided from the beginning,” said Kun Wang, assistant professor of earth and planetary sciences in Arts & Sciences and senior author of the study. “There is likely a threshold on the size requirements of rocky planets to retain enough water to enable habitability and plate tectonics, with mass exceeding that of Mars.”
That’s the short answer.
The long answer involves potassium isotopes and their presence not only on Mars but on other Solar System bodies.
The team of researchers used stable isotopes of potassium to “estimate the presence, distribution and abundance of volatile elements on different planetary bodies,” according to a press release announcing the study. While potassium itself is only moderately volatile, it can be used as a tracer for more volatile compounds, including water. Members of this team have already used the potassium tracer method to study the Moon’s formation.
Wang and the other researchers studied 20 Martian meteorites that together represent the Martian silicate composition. They found that Mars retained more volatiles like water than the Moon did. It also retained more than asteroids like Vesta, which are both smaller than Mars. But they found the opposite when it came to Earth: Mars lost more volatiles, including water. The correlation between potassium composition and body size is well-defined, according to the team.
Mars and Earth formed from the same solar nebula, the material left over after the Sun formed. As such, they started out with similar compositions. But when the team found that Mars meteorites had heavier concentrations of potassium isotopes than Earth, it implied a greater loss of potassium on Mars than on Earth.
They also found that “the bulk silicate values of Earth, the Moon, Mars and Vesta correlate with surface gravity” and also with H2O abundance.
“The K isotopic composition of BSM <bulk silicate Mars> and the strong correlation between 41K and planet mass reveals that the sizes of planetary bodies fundamentally control their ability to retain volatiles. This could further shed light on the habitability of planets and assist with constraining unknown parent body sizes,” the authors write in their paper.
“The reason for far lower abundances of volatile elements and their compounds in differentiated planets than in primitive undifferentiated meteorites has been a longstanding question,” said Katharina Lodders, research professor of earth and planetary sciences, and a co-author of the study. “The finding of the correlation of K isotopic compositions with planet gravity is a novel discovery with important quantitative implications for when and how the differentiated planets received and lost their volatiles.”
The authors say that this has to do with how planets and other bodies accrete over time. The loss of volatiles like water can vary over time as the bodies grow by accretion. Larger bodies simply retain more volatiles than smaller bodies.
And here’s the kicker: “There is likely a threshold on the size requirements of rocky (exo)planets to retain enough H2O to enable habitability and plate tectonics, with mass exceeding that of Mars,” the authors write in their paper.
Without Martain meteorites of widely different ages striking Earth this work would not have been possible. They date back as far as four billion years, and as recently as several hundred million years.
“Martian meteorites are the only samples available to us to study the chemical makeup of the bulk Mars,” Wang said. “Those Martian meteorites have ages varying from several hundred millions to 4 billion years and recorded Mars’ volatile evolution history. Through measuring the isotopes of moderately volatile elements, such as potassium, we can infer the degree of volatile depletion of bulk planets and make comparisons between different solar system bodies.”
So, was Mars once really wet? Probably. Was it wet long enough for life to give it a go on Mars? According to the researchers, not likely.
One thing the study does is add a little more detail to the idea of a habitable zone, and how we think about exoplanets and habitability. In exoplanet studies, we use the term habitable zone to describe the temperature zone around a given star where a planet could reasonably be expected to have liquid water on its surface, given the right atmosphere. This study adds planet size to the whole idea, though it’s not the first work to bring this idea to light.
If a planet in a star’s habitable zone is too small, it’ll simply lose its water, even if it did start out with a wetter surface.
“This study emphasizes 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 Habitability at 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.”
For senior author Wang, the implications of this research are clear. Planetary size should receive more emphasis when it comes to exoplanets and habitability.
“The size of an exoplanet is one of the parameters that is easiest to determine,” Wang said. “Based on size and mass, we now know whether an exoplanet is a candidate for life because a first-order determining factor for volatile retention is size.”