SAN FRANCISCO, CALIFORNIA—A stream of hot protons from the sun is penetrating deep into the thin atmosphere of Mars, researchers reported here today at a meeting of the American Geophysical Union. The stream, known as the solar wind, is typically deflected by the ionosphere, a layer of ions and electrons forming a shield around Mars. But the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission—a NASA spacecraft orbiting Mars—has found that some protons re-emerge within the ionosphere below altitudes of 200 kilometers. The effect could be used to monitor the strength of the solar wind even at altitudes where mission scientists had not expected to have any handle on it. MAVEN, which arrived in Mars’s orbit in September, needs to catalog the ways energy is deposited in the upper atmosphere in order to achieve one of its main mission goals: explaining how Mars lost much of its atmosphere. Billions of years ago, when Mars was warmer and wetter, the planet is presumed to have had a much thicker atmosphere—one that has been eroded steadily by the solar wind, and also during more catastrophic solar storm events, into the dry, cold landscape seen today (pictured).