Scientists have detected tiny lightning bolts on Mars for the first time — they were found discharging around NASA’s Perseverance rover and coming from dust-storm fronts and whirling dust devils.
Finding the electrical discharges has solved a major Martian mystery, namely the origin of oxidants such as hydrogen peroxide on the Red Planet, which was discovered on Mars in 2003. These oxidants can react with organic molecules, potentially destroying biosignatures, while other chemical reactions triggered by the lightning can generate new organic molecules.
“This is exciting,” Baptiste Chide of the Institut de Recherche en Astrophysique et Planétologie in Toulouse told Space.com. “It opens a new field of investigation for Mars.”
Chide led a team of Mars rover scientists to find evidence for the electrical discharges hidden in data from the most unexpected of instruments: Perseverance‘s microphone.
Chide’s team discovered 55 electrical events across 29 hours’ worth of microphone recordings, spread out across two Martian years. The recordings each have a distinct audio signature. Initially there is a burst of static, called the overshoot, that lasts less than 40 microseconds. The overshoot is followed by an exponential drop in signal lasting perhaps 8 milliseconds, depending on how far away the microphone is from the discharge. Both the overshoot and subsequent drop are not real acoustic noises: They are the result of interference in the microphone’s electronics from the magnetic field generated by the discharge. The next part of the audio recordings is a real sound. This manifests as a second loud peak in the signal followed by smaller peaks, and these are caused by a modest shockwave produced by the flash of the lightning.
These electrical discharges are not forked lightning bolts lancing down from the sky like we have on Earth, because Mars does not have thunderstorms because it lacks atmospheric water. Instead, for the microphone to hear the electrical discharges, the discharges have to be much closer to the rover.
On Earth, lightning is caused mostly by friction between icy particles in the clouds. On Mars, it is friction between dust particles that prompts the discharges. We see something similar on Earth in volcanic plumes.
However, the conditions on Earth and Mars are very different, evident in their respective “breakdown threshold.” This describes the point when clouds of particles that have become electrically charged are able to discharge.
“The breakdown threshold is higher on Earth than on Mars, and is primarily to do with pressure and also the composition of the atmosphere,” Daniel Mitchard of Cardiff University told Space.com. Mitchard is a physicist who studies lightning, though he is not a member of the rover team and did not participate in this study.
Earth’s predominantly nitrogen–oxygen atmosphere and Mars’ mostly carbon-dioxide atmosphere are electrically insulating, meaning a lot of charge has to build up to overcome the insulating effect and discharge. Because the surface pressure on Earth is one atmosphere, it means that there is a lot of insulating atmosphere that lightning has to pass through, and so the breakdown threshold is quite high, three megavolts per square meter. On Mars, where the surface pressure is just 0.006 atmospheres, there is less insulating atmosphere for an electrical discharge to overcome, so the breakdown threshold is much less, around 15 kilovolts per square meter.
“So this means that we would generally expect lightning on Mars to be weaker than on Earth,” said Mitchard, who likens Mars’ electrical discharges to the static shock that you might receive rubbing a balloon or walking on an insulated flooring.
Of the 55 discharge events detected by Perseverance’s microphone, 54 of them occurred during the top 30% of strongest winds recorded during the 29 hours of recordings. This strongly connects the discharges to localized winds that are able to loft dust into the air, as is commonly found at a dust-storm front. Sixteen of the events also coincided with dust devils passing very close to the rover — the most distant electrical discharge measured is estimated to have been just 6.2 feet (1.9 meters) from Perseverance. Some of the discharges were caused by dust grains in the air, while a handful were actually the rover becoming electrically charged to several kilovolts following collisions with airborne dust particles and then discharging into the ground.
However, the rover and its instruments are well protected from electrical mishap. Nevertheless, Chide and the rover team speculate that the Soviet Mars 3 mission, which landed on Mars in the middle of a dust storm in 1971 and was only active for 20 seconds before going kaput, could have been damaged by electrical discharges.
To ensure that future missions are fully protected, the microphone readings can guide the future design of Mars missions. “Now that we have quantitative data on the energy [of the discharges], we will be able to adjust the specification we put on the design of electronic boards and potentially have new constraints on the space suits needed for astronauts,” said Chide.
So far, only the microphone has picked up evidence for the discharges. Could Perseverance’s cameras potentially capture the flashes of these lightning bolts?
“Imaging the discharges would be hard,” said Chide. This would be partly because many of them take place in the day when dust devils are most active, and those that would otherwise be bright enough might be obscured by dust. The flashes would also be very brief, lasting just microseconds, and most would be only millimeters in length – the largest bolts are those discharges from the rover itself, which extend several tens of centimeters to reach the Red Planet’s surface. To capture short, fast electrical discharges requires a high-speed, high-resolution camera that we don’t currently have on Mars.
“Hopefully, more advanced cameras will eventually make their way there,” said Mitchard. This is now more likely if planetary scientists wish to study the lightning in more detail in the future.
Even then, it would not be a simple matter. “We wouldn’t really know where to point the camera,” said Chide. “We’d have to be very lucky!”
Of more immediate interest is lightning’s connection to oxidants such as hydrogen peroxide. Because such oxidants can react with and chemically alter organic compounds, the presence of lightning is of interest to astrobiologists seeking biosignatures on the Red Planet. In theory, areas with high concentrations of oxidants should experience more dust devil and storm activity and therefore more electrical discharges. For example, dust devil activity in Gusev crater, where the Spirit Mars Exploration Rover landed in 2004, is twenty times higher than in Jezero crater where Perseverance is, while there is barely any dust devil activity on Elysium Planitia. Does this match the distribution of oxidants on Mars, and could scientists improve their chances of finding biosignatures by sending life-seeking missions to areas of Mars that do not experience as many dust devils and dust storms?
“This is a good question,” said Chide. “The quantification of the amount of oxidants produced by this new phenomenon will be the next step, requiring lab experiments and models.”
Whereas lightning has already been discovered in the clouds of the gas giants Jupiter and Saturn, this is the first time that electrical discharges have been discovered on a rocky planet other than Earth. It raises the possibility that similar phenomena could take place on Venus via dust or Saturn’s moon Titan via icy grains.
Meanwhile, the Martian discharges could assist dust storms, since the electrical static reduces the threshold velocity needed for winds to lift dust particles off the surface in the first place, creating a positive feedback loop of dust being helped off the surface, becoming further electrified, which helps more dust to become airborne, and so on. As such, the electrification of the dust could play an important role in Mars’ global dust cycle and hence what passes for its climate.
With thousands of smaller, regionalized dust storms every Martian year, it means that there are thousands of kilometers of electrified dust-storm front that could be crackling with tiny lightning bolts. The shocking story of the electrified Mars may not be over yet.
The research was published on Nov. 26 in the journal Nature.