A menagerie of Martian auroras

Fundamentally, an aurora is the emission of light caused by energetic particles—electrons, ions, or neutral atoms, for instance—that deposit their energy in a planetary atmosphere. Beyond the basic definition, there are a huge variety of observational auroral signatures and classifications. They depend on the type of particles that triggered the light and how those particles gain their energy.

The particles often emerge from the Sun, as part of either low-energy plasma in the solar wind or of emissions called solar energetic particle (SEP) events that are associated with violent solar eruptions. Charged particles can also be energized near a planet by plasma waves or when magnetic field lines break and reconnect (see the article by Forrest Mozer and Philip Pritchett, Physics Today, June 2010, page 34). Energetic neutral atoms are typically speedy ions that have recently been neutralized, and so they are ultimately powered by the same processes.

The study of Earth’s auroras has taken giant leaps during the space age. Observations from sounding rockets and satellites have revealed that the characteristics of auroras are controlled by the strong global dipole magnetic field generated by the planet’s core. The magnetosphere shields most of the atmosphere from direct plasma interactions by deflecting or trapping charged particles. At a bow shock located roughly 10–20 Earth radii on the sunward side of the planet, the solar wind flows around the magnetic obstacle like water around a stone.

Earth’s magnetic field controls where and how plasma interacts with the atmosphere. Auroral activity is most common near the North and South Poles because field lines often converge there. Although the magnetic field cannot exert a force on energetic neutral atoms, it controls processes that generate them. For example, ions and electrons trapped in the Earth’s magnetosphere can recombine to produce energetic atoms that rain down on the atmosphere near the equator.

Although a strong global dipole magnetic field drives Earth’s auroras, it turns out that many ways exist for energetic particles to get to the upper atmosphere of a planet. Unlike Earth, Mars possesses a hybrid magnetosphere that’s composed of two sources: an induced component generated by electric currents in the ionosphere and a crustal component from patches of magnetized iron-bearing minerals near the surface. The hybrid magnetosphere protects Mars’s atmosphere from space plasma, but the solar wind is deflected less than one Mars radius above the planet’s surface—much closer than at Earth—and the system can become highly disrupted by space-weather disturbances.

Just as the satellite era jump-started research on Earth’s auroral activity, the fleet of spacecraft that have been sent to study Mars and its atmosphere has made possible astronomers’ understanding of the planet’s auroras. To date, all the observations of Martian auroras have come from spectrometers aboard Mars orbiters and landers, with supporting information provided by in situ particle detectors. With the Sun near the maximum-activity point of its 11-year cycle, many of those instruments are currently observing the most intense events they have ever recorded.

Discrete auroras, which are characterized by intense, well-defined arcs or bands, were the first type of aurora to be discovered on Mars, in 2005. They are caused by electrons that follow vertical magnetic field lines and plunge into the atmosphere, much as they do over Earth’s polar regions. In the case of Mars, the planet’s patchy crustal fields cause auroras to form in cusp regions scattered around the planet (see panel a of the figure).

Depending on the time of day—and thus how Mars’s magnetic field is superposed on the Sun’s—the ends of a magnetic field line may attach nearby on the planet (closed), or one end may lead out into space (open). Closed field lines near twilight can shuttle electrons from the dayside ionosphere onto the nightside, whereas open field lines allow higher-energy plasma from distant regions of the magnetosphere to access the atmosphere. Particles may also be accelerated close to the planet, and the breaking and reconnecting of magnetic field lines could be a significant energizing source for the highly dynamic system.

Diffuse auroras, which are typically fainter and extend over a broader area than do discrete auroras, have also been identified on Mars. Triggered by SEP events, diffuse auroras are somewhat rare and quite dramatic. The small Martian magnetosphere does little to deflect the energetic particles, which can penetrate deep into the atmosphere anywhere on the planet and at any time of day. Depending on the severity of the space-weather event, the phenomenon can light up the Martian atmosphere across the entire planet with diffuse auroras for days at a time (see figure panel b).

Although SEP auroras can occur at any time, there has been a distinct uptick in frequency with the recent high solar activity. Studying diffuse auroras is important for future crewed missions to Mars because the extreme solar outbursts that excite the atmosphere also present a radiation hazard to astronauts on the surface (see the article by Erdal Yiğit, Physics Today, July 2024, page 42).

Proton auroras, in which precipitating neutral hydrogen atoms themselves emit light, have also been identified on Mars. The most common type is driven by a process that does not operate on Earth. At Mars, the solar wind approaches the planet closely enough to interact with its expansive envelope of atomic hydrogen, which is produced by the photolysis of water in the atmosphere. Stealing electrons from that native Martian hydrogen, the solar-wind protons transform into energetic hydrogen atoms that cannot be deflected by magnetic fields. They penetrate through the bow shock and into the upper atmosphere, where they produce an aurora across the dayside of the planet.

Another type of solar-wind aurora occurs when space-weather conditions disrupt Mars’s induced magnetosphere. With the magnetic shield disabled, solar-wind protons can flow directly into the atmosphere in patchy patterns (see figure panel c). Exploring other mechanisms for exciting proton auroras at Mars is an area of active research.

Whether it be a discrete crustal-field aurora, an SEP-driven diffuse aurora best viewed from a radiation shelter, or a proton aurora spurred by a puff of solar wind, the menagerie of Martian auroras reminds us that there are still twists to well-studied physical phenomena lurking beyond our planet. We just need to look for them.

Justin Deighan is a planetary scientist at the Laboratory for Atmospheric and Space Physics at the University of Colorado Boulder.