A planet’s magnetic field usually protects its atmosphere from being blown away by its star. But new research suggests Mars’s weak magnetic field may have helped its atmosphere escape.

Four and a half billion years ago, Mars boasted a thick atmosphere and abundant surface water—conditions that could have hosted life. But today, only wisps of that atmosphere are left, clinging thinly to the planet.

How did so much Martian air slip into space? One major factor is thought to be the loss of the planet’s magnetic dynamo—the engine in its liquid core that powered its global magnetic field, which mysteriously shut down around 4 billion years ago. Left behind was only a feeble remnant of that field, emanating from the planet’s weakly magnetized crust. Conventional wisdom holds that a planetary magnetic field acts as a shield, protecting the atmosphere from being blown into space by the Sun’s solar wind and radiation (which would have been even stronger when the Sun was younger).

Now Sakata et al. report simulations that add a new wrinkle: A weak magnetic field, like at Mars after its dynamo shut down, may actually bleed its atmosphere faster than no magnetic field at all.

Using magnetohydrodynamic models, the authors explored how varying the strength of Mars’s magnetic field affected the loss of atmospheric ions like oxygen and carbon dioxide into space. As expected, a strong magnetic field that could easily withstand the pressure of the solar wind was able to guard its atmospheric ions. With no magnetic field, the Sun stripped those ions away up to 100 times faster.

But the highest rate of atmospheric ion loss was with a weak magnetic field—6 times faster than with no magnetic field at all. The team found the reason was the magnetic field lines, which guide the motion of charged particles, were easily blown back by the solar wind, creating a path for these ions to escape into space above Mars’s nightside. This means that instead of providing a small measure of protection, Mars’s remnant magnetic field could actually have sped the planet’s transformation into the cold, barren world it is today.

Understanding how planetary magnetic fields can protect against stellar activity is also important to the search for life in other solar systems. Red dwarfs, the most common type of star in the universe, blast their planets with strong stellar winds and extreme ultraviolet radiation. Whether those planets have managed to retain their atmospheres like Earth or have lost them to space like Mars could have a major impact on how common life is in the universe. (Journal of Geophysical Research: Space Physics, https://doi.org/10.1029/2019JA026945, 2020)

—Mark Zastrow, Science Writer