A new model could solve a Martian mystery, explaining why observations, including those from NASA’s Mars Atmosphere and Volatile Evolution (MAVEN) orbiter, seem to show that electrons lose energy as they traverse the outermost region influenced by the planet, called the magnetosheath.
The electrons in this region originate mostly from the solar wind, charged particles that stream from the Sun into the solar system. Without its own internal magnetic field to shield it, only Mars’s neutral atmosphere stands in the path of these particles, deflecting them around the planet like a rock deflects rushing water in a river.
This deflection forms a shock wave, or bow shock, in front of the planet, which gives any electrons that cross it a jolt of energy as they enter the magnetosheath and continue downstream toward the planet’s atmosphere. But curiously, spacecraft observations have shown that electrons have less energy deeper inside the magnetosheath, as if their energy were dissipating.
Scientists previously invoked complex physics to show how this dissipation might occur as the incoming electrons reached Mars’s atmosphere and collided with atmospheric atoms or molecules. However, one problem with that theory is that the Martian atmosphere is too thin for these collisions to happen frequently in the magnetosheath.
Now Schwartz et al. propose that this apparent “erosion” of energy is an illusion. They suggest that incoming electrons do not lose energy through collisions but that higher-energy electrons can escape from the magnetosheath as they rush past the planet, exiting the bow shock on its flanks, where it is weaker.
Meanwhile, electrons crossing into the bow shock along the flanks get less of an energy boost than electrons that hit the bow shock head-on, which explains the spacecraft readings that appeared to suggest that electrons deep in the magnetosheath have lost energy. The new model shows instead that the bow shock acts like a sieve, allowing highly energized electrons to escape and replacing them with less energized electrons.
The team’s model, which captured all the major physical processes at play in a comparison with MAVEN data, could transform and simplify scientific understanding of the outer layer of the Martian environment. (Geophysical Research Letters, https://doi.org/10.1029/2019GL085037, 2019)
—Mark Zastrow, Freelance Writer