Our Sun constantly releases a stream of high-speed charged particles, known as the solar wind. Unlike Earth, Mars does not have a strong global magnetic field to shield its atmosphere from this flow. As a result, the solar wind can interact directly with the upper atmosphere of Mars and gradually strip atmospheric particles into space.
A new Boston University-led study published in Science Advances found that this removal process can occur in a way that is similar to wind blowing across the surface of water. On Earth, wind can generate rolling waves and vortices on the water surface. At Mars, the solar wind can similarly "stir" the edge of the planet's upper atmosphere and generate large boundary waves, known as Kelvin–Helmholtz waves.
First author Chi Zhang, a research scientist at BU's Center for Space Physics, a collaboration between BU's College of Arts & Sciences and College of Engineering, and a team of researchers used observations from both the MAVEN and Tianwen-1 missions in the study. Tianwen-1 served as a solar wind monitor, while MAVEN observed atmospheric ions escaping near Mars, allowing the researchers to directly relate real-time upstream solar wind conditions to atmospheric ion escape at Mars.
Large clouds of plasma in Mars' upper atmosphere facilitate "bulk escape" of atmospheric ions. Zhang explained that, although several mechanisms had previously been proposed to account for the formation of these clouds, their origin remained unclear because direct observational evidence was still lacking. A major challenge in connecting the solar wind to atmospheric escape at Mars was that a single spacecraft could not simultaneously measure both the undisturbed solar wind upstream and the escaping atmospheric ions near Mars. In an earlier study published in Nature Communications, Zhang and colleagues demonstrated that simultaneous observations from MAVEN and Tianwen-1 could directly link variations in the upstream solar wind to the Martian space environment. Building on those dual-spacecraft observations, the new Science Advances study identified Kelvin–Helmholtz waves as a key mechanism driving atmospheric ion escape.
Zhang and colleagues provided clear evidence that these plasma clouds are generated by the Kelvin–Helmholtz waves. They further showed that this process does not occur evenly around the planet. "Instead, it is mainly observed on one side of the planet, depending on the direction of the solar wind electric field," said Zhang. These results establish a direct link between the Kelvin–Helmholtz waves and enhanced atmospheric ion escape from Mars.
"Future research will focus on identifying the conditions that favor the formation and growth of Kelvin–Helmholtz waves and determining how much they contribute to atmospheric escape from Mars," Zhang noted. Further studies will require more spacecraft observations and advanced numerical simulations. With MAVEN transitioning to the closeout stage of its mission, "its rich scientific legacy will be complemented by NASA's ESCAPADE mission, which has already launched and will provide an important new opportunity to investigate solar-wind-driven atmospheric loss at Mars," said Zhang.
"We want to know when these waves are most likely to form, how they evolve, and how strongly they can drive atmospheric escape," said Chuanfei Dong, a BU Center for Space Physics faculty member and a College of Arts & Sciences assistant professor of astronomy. "This process could also occur on other planets that lack a strong magnetic field, including some exoplanets."
"Mars is thought to have once been potentially habitable, with a thicker atmosphere and surface liquid water. Understanding how it became the cold, dry planet we see today is important for understanding how planetary environments evolve over time," said Zhang.
