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Alien Planets Are Even Less Habitable Than We Thought

'[T]here is a very limited size range for planets to have just enough but not too much water'.

It turns out, Mars was always fated for a waterless destiny.

New observations from robotic explorers like NASA's Perseverance and Curiosity have revealed much about the ancient past of the Red Planet, where liquid water flowed throughout the planet's surface. It used to have lakes, streams, rivers, and perhaps even a colossal ocean stretching around the horizon of Mars' northern hemisphere. For decades, scientists have thought the weakening of the Martian magnetic field enabled charged particles from the sun to strip away the atmosphere, literally blowing away the bodies of water.

But a deeper, more primary cause for the move from wetness has come to light: Mars was always too small to retain its surface water forever, according to a new study published in the journal Proceedings of the National Academy of Sciences.

And this could have major implications in the ongoing search for habitable worlds beyond our solar system.

Mars lost far more water during formation than Earth

"Mars' fate was decided from the beginning," said Kun Wang, co-author of the study and assistant professor to Earth and planetary sciences at Washington University in St. Louis, in a statement on the university website. "There is likely a threshold on the size requirements of rocky planets to retain enough water to enable habitability and plate tectonics." And this "threshold" is probably planets the size of Mars. The new study went forward under the leadership of a grad student working in Wang's lab named Zhen Tian, and they analyzed 20 meteorites on Mars, selected to form a representative sample of the bulk composition of Mars. They measured the abundance of multiple potassium isotopes in Martian rocks, which were aged anywhere from 200 million years to four billion years old.

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Potassium served as a tracer for more "volatile" elements and common life-supporting compounds, like water, which changes into a gas at low temperatures. The researchers discovered that Mars lost much more of its volatiles throughout its initial formation than the Earth did, which is roughly nine times the mass of the Red Planet. Mars did, however, retain its volatiles better than our moon. "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 a research professor of Earth and planetary sciences named Katharina Lodders, of Washington University, in the earlier statement.

Mars' small stature forms a compound effect with its diminishing magnetosphere

By differentiated, Lodders refers to a body in space whose interior has divided into separate layers like the crust, mantle, and core. "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." And the new study combined with earlier work shows that the small size of planets like Mars only compounds other tertiary causes for reduced-to-no habitability. Such planets give up colossal amounts of water throughout formation, and their magnetic fields turn off fairly early, causing a rapid thinning in of planetary atmosphere.

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"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 the University of Bern, Switzerland's Center for Space and Habitability's Klaus Mezger, who is also a co-author of the study, in the statement. "These results guide astronomers in their search for habitable exoplanets in other solar systems." In other words, this could mean there are fewer habitable alien worlds out there than we thought. But then again, knowing how a planet's smallness can negatively impact the tendency for water to stick around could also help astronomers know which planets to skip, and which to view one more time.

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