The Sun continuously sends a fast stream of charged particles through space, a phenomenon known as the solar wind. Earth is largely protected from this flow by its global magnetic field, but Mars has no comparable shield. This leaves the planet’s upper atmosphere exposed, allowing the solar wind to knock atmospheric particles away and carry them into space.
A new study led by Boston University and published in Science Advances suggests that this atmospheric loss can resemble wind moving across water. On Earth, wind passing over a body of water can create rolling waves and swirling vortices. At Mars, the solar wind appears to produce a similar effect by “stirring” the outer boundary of the upper atmosphere.
This interaction generates enormous boundary waves called 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, worked with colleagues to analyze data from the MAVEN and Tianwen-1 missions. Tianwen-1 measured the incoming solar wind before it reached Mars, while MAVEN tracked atmospheric ions escaping near the planet.
By combining the two sets of observations, the researchers could directly compare changing solar wind conditions with the movement of Martian atmospheric particles into space.
Two Spacecraft Reveal Atmospheric Escape
Large clouds of plasma in the upper atmosphere can produce what researchers call the “bulk escape” of atmospheric ions. Scientists had previously proposed several possible explanations for how these clouds formed, but their origin remained uncertain because direct evidence was limited.
One major obstacle was that a single spacecraft could not observe the undisturbed solar wind upstream of Mars while also measuring atmospheric ions escaping closer to the planet.
In an earlier study published in Nature Communications, Zhang and his colleagues showed that simultaneous measurements from MAVEN and Tianwen-1 could connect changes in the incoming solar wind with conditions around Mars. The new Science Advances study builds on that work and identifies Kelvin-Helmholtz waves as an important mechanism behind atmospheric ion loss.
The researchers found clear evidence that these waves generate the large plasma clouds associated with atmospheric escape. They also discovered that the process is not distributed evenly around Mars.
“Instead, it is mainly observed on one side of the planet, depending on the direction of the solar wind electric field,” said Zhang.
The findings provide a direct connection between Kelvin-Helmholtz waves and increased atmospheric ion escape from Mars.
Measuring the Waves That Drain 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.
Answering those questions will require additional spacecraft measurements and advanced computer simulations. MAVEN is now moving toward the closeout stage of its mission, but researchers expect future missions to continue its work.
“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.”
A Clue to Mars’ Lost Habitability
Understanding atmospheric escape could help explain the dramatic transformation Mars experienced over its history.
“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.