An article published in the journal “Nature Astronomy” describes the discovery of proton aurorae on the planet Mars. Scientists from NASA’s MAVEN mission were studying the red planet’s atmosphere using the space probe’s IUVS instrument and noticed a strange glow in its upper layers. The investigation led to the discovery of this rare type of aurora caused by energetic protons carried by the solar wind, a phenomenon that can occur on a large scale on Mars and perhaps also on Venus and Titan while it’s limited on Earth.
Aurorae are a well known phenomenon far beyond the Earth and for example NASA’s Juno space probe detected a lot of useful data to study Jupiter’s aurorae. They’re generated by energetic particles that strike a planet’s atmosphere bombarding its gases with the consequence of making them glow. Generally the electrons are the charged particles that generate aurorae but in rare cases protons can have the same effect.
NASA’s MAVEN space probe is equipped with instruments suitable for examining the phenomena occurring in Mars atmosphere and therefore aurorae as well. The mission scientists noted that sometimes the Imaging UltraViolet Spectrograph (IUVS) instrument detected ultraviolet emissions from the hydrogen present in the atmosphere’s upper layers and that brightness lasted for a few hours.
Subsequently, another MAVEN space probe instrument that aims to study the interactions between the solar wind and the Martian atmosphere, the Solar Wind Ion Analyzer (SWIA), detected solar wind protons that were charged. This phenomenon also initially raised perplexity in the scientists because protons are generally diverted in a boundary area of the atmosphere of Mars called in jargon bow shock.
The key to this phenomenon is that the protons carried by solar wind reach Mars atmosphere and capture electrons from the edge of the large hydrogen cloud that surrounds the red planet. The bow shock is a magnetic obstacle that divert charged particles such as protons but the ones that capture electrons become neutral hydrogen atoms so they pass through it. At that point, they start colliding with the atmosphere’s molecules emitting photons at ultraviolet wavelengths that form proton aurorae.
The image (NASA / MAVEN / Goddard Space Flight Center / Dan Gallagher) illustrates that situation, with protons coming from the Sun, capturing electrons and becoming hydrogen atoms that pass through the bow shock to form a proton aurora. Another effect is that the hydrogen present in the Martian cloud increases along with the energy deposited by the protons that enter it.
Such a phenomenon is much rarer and more limited on Earth because its powerful magnetic field manages to divert many more solar wind particles. It could be more common on Venus, where the magnetic phenomena are generated by the interaction between the solar wind and the atmosphere, and on Titan, one of Saturn’s moons, also without its own magnetic field and with an atmosphere where hydrogen is abundant. Basically, it’s a type of aurora so far little known only because it’s very limited on Earth.