Could lightning occur on Mars?

Researchers in the Czech Republic say they may have observed the signature of a “whistler” in a one-second snapshot captured by the MAVEN probe orbiting Mars. The event, observed in the ionosphere of the planet, would be the first lightning-like electric discharge activity ever to be seen there and the finding will be important for understanding atmospheric processes in the Martian atmosphere.

“Whistlers are well known on Earth and are associated with lightning,” explains space physicist František Němec at Charles University, who led this research effort. “Our result implies that this phenomenon also occurs on our planetary neighbour.”

Unlike Earth, Mars does not have a global magnetic field, but only localized fields created by magnetized materials in the planet’s crust. And because its atmosphere is thin, lightning on this planet does not originate in water clouds but instead in dust storms, similar to those observed in volcanic eruptions here on Earth, and in dust devils.

During dust storms, dust grains become electrically charged as they collide with each other and generate an electric field. On Mars, previous studies have predicted that this field can discharge when its value exceeds the breakdown threshold in the low-pressure Martian atmosphere, which is around 15 kV/m.

Dust devils, for their part, can produce ultralow-frequency radiation on Earth thanks to the electrical charges that fluctuate as the dust swirls around. Since both dust devils and dust storms are much stronger on Mars, theory suggests that they could generate wideband radiation that we could detect on Earth. Despite recent measurements by the Allen Telescope Array, the Mars Global Surveyor (MGS) and Mars Atmosphere and Volatile Evolution (MAVEN) missions and the Mars Express spacecraft, conclusive evidence for Martian lightning has yet to be found.

Analysing electromagnetic radiation

Another way to detect these electric discharges, says Němec, is to analyse the electromagnetic radiation that accompanies them. This radiation lies in the extremely low frequency/very low frequency range and, under some conditions, can reach the ionosphere of a planet. The phenomenon was first identified and observed on Earth shortly before the space era and such electromagnetic waves have successfully been used to provide evidence for lightning on Jupiter, Saturn and Neptune since then.

These waves are known as whistlers, he explains, because of their characteristic spectral pattern in the plasma medium of the ionosphere. Here, higher frequency waves propagate faster and arrive at the observation point sooner than lower frequency ones, resulting in a characteristic “whistling” spectral shape.

The observational challenge is that these waves can penetrate the Martian ionosphere only on the nightside of the planet and when the magnetic field is pointing in the vertical direction. This largely restricts the areas over Mars which whistlers can be observed by spacecraft – namely, to the relatively small crustal field regions in the southern hemisphere of the planet.

Němec says he has now identified the electromagnetic wave signature of a whistler on Mars in a snapshot captured by the MAVEN probe on 21 June 2015. “I first identified it at night in a region with a strong and nearly vertical magnetic field, something that is crucial for the wave to be able to propagate to the altitude of where the probe is orbiting without its signal attenuating too much.”

Out of the many wave snapshots analysed (108,418 in total), only this single event contained a whistler signature, he tells Physics World. “This likely reflects the rarity of the phenomenon itself, as well as the specific ionospheric and magnetic field conditions required for the wave to propagate all the way to the spacecraft.”

The MAVEN probe has been orbiting Mars since 2014 and sent back data to Earth until we lost communication with it last year. While no large-scale dust storms were recorded on the planet at the moment at which the probe captured the whistler, Němec and colleagues say the effect might have come from a local dust event.

Different propagation speeds

“Whistlers are formed because, in the ionized plasma of the ionosphere, different signal frequencies propagate at different speeds,” explains Němec. “As a result, although all frequencies are generated simultaneously during a lightning discharge, the higher frequencies – which propagate faster – arrive at the spacecraft first, followed later by the lower frequencies.”

The researchers, who detail their work in Science Advances, calculated these corresponding time delays and say that their observations agree “very well” with theoretical predictions. They also calculated how the waves attenuate by adapting methods used for Earth to the assumed composition of the Martian ionosphere. The results revealed that higher frequencies are more strongly attenuated, which explains why only the lower-frequency portion of the whistler is observed, says Němec.

The existence of strong lightning-like electrical discharges in the Martian atmosphere highlights the need to better understand the relevant atmospheric processes on the Red Planet, with a particular focus on dust storms and dust devils, he adds. The sudden energy release accompanying such discharges also clearly has the potential to locally alter the atmospheric chemistry.

The Charles University researchers together with their colleagues at the Institute of Atmospheric Physics say they are now working on a detailed analysis of how waves attenuate as they propagate through Mars’ ionosphere. “Importantly, we are actively involved in the design and development of the European Space Agency’s M7 mission candidate M-MATISSE,” reveals Němec. “This two-spacecraft mission, scheduled for launch in 2037, would feature advanced instrumentation for, among other things, wave measurements, and would allow for more detailed investigations of the relevant phenomena.

“We are very excited about this opportunity and hope that the mission will ultimately be adopted.”

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