Exploring Auroras on Mars

Panels a and b show adjacent imaging swaths on August 28, 2021, collected 15 minutes apart. Panel c shows the combined image, averaging the two swaths. Panels c–f are typical examples of a crustal field aurora, panels g–i are examples of a patchy non-crustal field aurora, and panels j–l are examples of a sinuous aurora, with panels j and k taken 16 minutes apart. Credit: Lillis et al., 2022.

Auroras are caused by high-speed charged particles entering planetary atmospheres. Protons and electrons are emitted by the Sun in the solar wind, and some of these ions collide with atoms and molecules in the upper atmospheres of planets in the solar system, including Mars. The energy from these collisions is absorbed by the atoms and molecules in the atmosphere and then released as ultraviolet and visible light. These emissions of light are called auroras. One type of aurora is the discrete aurora, which is caused by electron impact on the night side and is strongly localized spatially. While auroras have been observed on Mars for nearly 20 years, no instrument before the Emirates Mars Ultraviolet Spectrometer (EMUS) onboard the United Arab Emirates’ Hope spacecraft has been able to image auroras across the full disk of Mars.

Robert Lillis from the University of California, Berkeley, and colleagues examined images and spectra of Mars in the ultraviolet using EMUS in search of discrete auroras. They found discrete auroras in 77% of their images, with 7% of them being brighter than ten rayleighs. (One rayleigh is ten billion photons per square meter per second). For comparison, the brightest auroras on Earth can be brighter than one million rayleighs. Martian discrete auroras typically occur over regions with weak or vertical magnetic fields, which do not deflect solar electrons away from the martian atmosphere. Lone oxygen atoms are responsible for much of the ultraviolet light in martian auroras, and carbon atoms and carbon monoxide also contribute significantly. Lillis and colleagues identify three common types of discrete aurora: crustal field (confined to regions with weak or primarily vertical magnetic fields), patchy (typically dim and located far from strong magnetic fields), and sinuous (thin auroras thousands of kilometers long). This study of martian auroras contributes to our understanding of the minuscule magnetic field of Mars and its effects on martian magnetospheric dynamics and chemistry. READ MORE