Mars is too small to hold water.
The story of Mars may contain basic facts about planetary habitability.
Artist's depiction of Mars and Earth-like surface water. The Red Planet was a wet world in ancient times. (Photo credit: NASA Earth Observatory/Joshua Stevens; NOAA National Environmental Satellite, Data and Information Service; NASA/JPL Caltech/USGS; Sean Garcia/University of Washington Graphic Design)
Mars is doomed to lose water because of its small size, a new study suggests.
Based on observations by robotic explorers like NASA's Curiosity and Perseverance rovers, scientists know that in ancient times, liquid water flowed across the surface of Mars: The Red Planet once hosted lakes, rivers and streams, and perhaps even a vast ocean that covered much of its northern hemisphere 39bet-xsmb-xổ số tây ninh-xổ số binh phước-xổ số binh dương-xổ số đồng nai.
But about 3.5 billion years ago, all but disappeared, along with most of the Martian atmosphere. Scientists believe this drastic climate change occurred after the Red Planet lost its global magnetic field, which protects the Martian air from being carried away by charged particles from the sun.
But according to the new study, there's a more fundamental driver behind this immediate cause: Mars is too small to hold surface water for long.
"The fate of Mars was decided from the very beginning," study co-author Kun Wang, an assistant professor of Earth and planetary sciences at Washington University in St. Louis, said in a statement. "There may be a threshold for the size requirements of rocky planets to retain enough water for habitability and plate tectonics." Scientists believe this threshold is larger than Mars.
The team, led by Zhen Tian, a graduate student in Wang Kun's lab, probed 20 Martian meteorites, which they chose to represent the overall composition of the Red planet. The researchers measured the abundance of various potassium isotopes in these extraterrestrial rocks, which range in age from 200 million to 4 billion years. (Isotopes are elements that contain different numbers of neutrons in their nuclei.)
Tian and her colleagues used potassium, the chemical symbol K, as a tracer for more "volatile" elements and compounds - things like water that transition to the gas phase at relatively low temperatures. They found that Mars lost much more volatiles during its formation than Earth, which has about nine times the mass of the Red Planet. But Mars holds its volatiles better than the moon and the 329-mile-wide (530-kilometer) asteroid Vesta, both smaller and drier than the Red Planet.
"The reason why the abundance of volatile elements and their compounds is so much lower in differentiated planets than in primitive undifferentiated meteorites has been a long-standing question," co-author Katharina Lodders, a research professor of Earth and planetary sciences at the University of Washington, said in the same statement. (A "divergence" is a celestial body whose interior has been divided into different layers, such as the crust, mantle, and core.)
"The finding that K isotopic composition correlates with planetary gravity is novel and has important quantitative implications for when and how diverging planets receive and lose volatiles," Lodes said.
The new study, published online today (Sept. 20) in the journal Proceedings of the National Academy of Sciences, follows previous research showing that small size is a double whammy for habitability. Bantam planets lose a lot of water as they form, and their global-scale magnetic fields shut down relatively early, causing their atmospheres to thin. (In contrast, Earth's global magnetic field is still strong, powered by deep within our planet.)
The new work could also have applications beyond our own universe, team members said.
"This study highlights that planets have a very limited range of sizes, with enough but not too much water to develop habitable surface environments," co-author Klaus Metzger of the Center for Space and Habitability at the University of Bern, Switzerland, said in the same statement. "These results will guide astronomers in the search for habitable exoplanets in other solar systems."
The "surface environment" disclaimer is important in any discussion of habitability. For example, scientists believe modern Mars still supports underground aquifers that could support life. Moons like Jupiter's Europa and Saturn's Enceladus have huge, potentially life-supporting oceans beneath their ice-covered surfaces.
The "surface environment" disclaimer is important in any discussion of habitability. For example, scientists believe modern Mars still supports underground aquifers that could support life. Moons like Jupiter's Europa and Saturn's Enceladus have huge, potentially life-supporting oceans beneath their ice-covered surfaces.