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Mars was doomed to desiccation by its small size, a new study suggests.
Thanks to observations from robotic explorers such as NASA’s Curiosity and Perseverance rovers, scientists know that in the past, liquid water flowed across the Martian surface: the Red Planet was once home to lakes, rivers and streams. , and maybe even a huge ocean that covered much of its northern hemisphere.
But this surface water had all but disappeared around 3.5 billion years ago, lost in space along with much of the Martian atmosphere. This dramatic climate change came after the Red Planet lost its global magnetic field, which had shielded the air of Mars from charged particles from the sun, scientists believe.
Related: The search for water on Mars (photos)
But that immediate cause was underlying a more fundamental driver, according to the new study: Mars is simply too small to hold surface water for the long term.
“The fate of Mars was decided from the start,” study co-author Kun Wang, assistant professor of earth and planetary sciences at Washington University in St. Louis, said in a statement. . “There is probably a threshold on the size requirements of rocky planets to hold enough water to allow habitability and plate tectonics.” This threshold is greater than Mars, scientists believe.
The study team – led by Zhen Tian, a graduate student from Wang’s lab – examined 20 Martian meteorites, which they selected to be representative of the overall makeup of the Red Planet. Researchers measured the abundance of various isotopes of potassium in these extraterrestrial rocks, which ranged in age from 200 million years to four billion years. (Isotopes are versions of an element that contain different numbers of neutrons in their atomic nuclei.)
Tian and his colleagues have used potassium, known by the chemical symbol K, as a tracer of more “volatile” elements and compounds, such as water, which changes to the gas phase at relatively low temperatures. They found that Mars lost significantly more volatiles during its formation than Earth, which is about nine times the size of the Red Planet. But Mars has preserved its volatiles better than Earth’s moon and the 329-mile-wide (530-kilometer) asteroid Vesta, both much smaller and drier than the Red Planet.
“The reason for the much lower abundances of volatile elements and their compounds in differentiated planets than in primitive undifferentiated meteorites is a long-standing question,” said co-author Katharina Lodders, research professor of science in Earth. Earth and Planets at the University of Washington, said in the same statement. (“Differentiated” refers to a cosmic body whose interior has separated into different layers, such as the crust, mantle, and core.)
“The discovery of the correlation of K isotopic compositions with planetary gravity is a new discovery with important quantitative implications for when and how differentiated planets received and lost their birds,” Lodders said.
The new study, which was published online today (September 20) in the journal Proceedings of the National Academies of Sciences, and previous work together suggests that small size is a double whammy for livability. Bantam planets lose a lot of water during their formation, and their global magnetic fields also shut down relatively early, causing the atmosphere to thin. (In contrast, Earth’s global magnetic field is still strong, powered by a dynamo deep within our planet.)
The new work could also have applications beyond our own cosmic backyard, team members said.
“This study points out 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,” co-author Klaus Mezger, of the Center for Space and ‘habitability of the University of Bern in Switzerland, said in the same statement. “These results will guide astronomers in their search for habitable exoplanets in other solar systems.”
This “surface environment” disclaimer is important in any discussion of livability. Scientists believe modern Mars still supports potentially vital underground aquifers, for example. And moons like Jupiter’s Europe and Saturn’s Enceladus are home to huge oceans, perhaps essential to life, beneath their ice-covered surfaces.
Mike Wall is the author of “The low“(Grand Central Publishing, 2018; illustrated by Karl Tate), a book on the search for alien life. Follow him on Twitter @michaeldwall. Follow us on twitter @Spacedotcom or Facebook.
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