Water is essential for life on Earth and other planets, and scientists have found ample evidence of water in Mars" early history. But Mars has no liquid water on its surface today. New research from Washington University in St. Louis suggests a fundamental reason: Mars may be just too small to hold onto large amounts of water.<br><br><br>Remote sensing studies and analyses of Martian meteorites dating back to the 1980s posit that Mars was once water-rich, compared with Earth. NASA"s Viking orbiter spacecraft—and, more recently, the Curiosity and Perseverance rovers on the ground—returned dramatic images of Martian landscapes marked by river valleys and flood channels.<br><br>Despite this evidence, no liquid water remains on the surface. Researchers proposed many possible explanations, including a weakening of Mars" magnetic field that could have resulted in the loss of a thick atmosphere.<br><br>But a study published the week of Sept. 20 in the Proceedings of the National Academy of Sciences suggests a more fundamental reason why today"s Mars looks so drastically different from the "blue marble" of Earth.<br><br>"Mars" fate was decided from the beginning," said Kun Wang, assistant professor of earth and planetary sciences in Arts & Sciences at Washington University, 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."<br><br>For the new study, Wang and his collaborators used stable isotopes of the element potassium (K) to estimate the presence, distribution and abundance of volatile elements on different planetary bodies.
