Anomalous transient enhancement of planetary ion escape at Mars

Atmospheric ion escape plays a crucial role in the evolution of planetary climate and habitability. While Mars has been the focus of extensive in-situ spacecraft observations, our understanding of ion escape at Mars has been constrained by single-point spacecraft measurements, which fail to distinguish spatial and temporal variability. Observations from NASA's Mars Atmosphere and Volatile EvolutioN (MAVEN) mission and China's Tianwen-1 mission provide complementary observations the Martian space environment and a unique opportunity to study the variability of ion escape. Here, we report that ion escape at Mars exhibits unexpected spatial-temporal variability under steady and weak external solar wind conditions. In the hemisphere where the solar wind electric field is directed toward the planet, a condition that usually hinders ion escape into space, we instead observe the transient appearance of escaping planetary ions with high energies and strong escape fluxes. This finding underscores that planetary ion escape can be unsteady and dynamic, even under stable external conditions.