Lightning discharges: Sparks in Martian dust reveal hidden electricity

• Sometimes you get a small electric shock from touching your car door handle on a dry summer’s day.

The source of these shocks is a spark discharge between your body and the car, produced by static electric charge from two different materials rubbing together. This triboelectric charging, observed since ancient Greece when rubbed materials were attracted to amber, is still familiar in classrooms: rubbing plastic sticks with cat fur or a balloon on your hair.

Now we know that, if you were returning to a parked car on Mars, you could experience a similar shock. A new study has, for the first time, directly demonstrated electrical discharges on the red planet.

The same triboelectric process operates in volcanic eruptions on Earth, where ash particles accumulating charge can initiate large lightning discharges. Lightning is even more common in thunderstorms, where interactions between soft hail and ice crystals cause charge separation.

On Earth, dust storms and dust devils — short-lived whirlwinds formed from rising warm air — are known to electrify through dust collisions. Yet sufficient electrification to trigger spark or lightning discharges usually isn’t reached, owing to Earth’s ~1 bar surface pressure. On Mars, where surface pressure is only 1-10% of Earth’s, spark discharges become likely at lesser levels of electrification.

For decades, it has been thought that dust devils on Mars might produce spark discharges. Laboratory experiments shaking sand in low-pressure carbon dioxide atmospheres have repeatedly recorded highly charged dust and discharges. Still, no direct observations existed. Clues have appeared — dust stuck to the wheels of a NASA rover, for instance — but nothing definitive.

The new, serendipitous observations published in Nature show that electrical discharges do occur in the Martian atmosphere.

These results arose from a small loop in the wire connecting a microphone on the Perseverance rover to its electronics. This wire, and the microphone system attached to it, became an unexpectedly effective accidental lightning detector. The SuperCam microphone was designed to record Mars’s acoustic environment, yet it also picked up small electrical transients.

Investigating these events, researchers found that some transients were followed by sounds. The authors showed that the transients were caused by spark discharges: electromagnetic signals picked up by the coil, followed by acoustic signals from the microphone. This is akin to seeing a flash of lightning and later hearing thunder.

By analysing the time difference between electrical and acoustic signals, the authors found the discharges occurred just a few metres from the lander. They also occurred more often during dust storms or when dust devils swept over the rover.

Generally, two independent sources of corroborating evidence are needed to confirm a new phenomenon. For weak events on Mars, detection from a distance is impossible, so in-atmosphere detection is required. Although the same system recorded both signals, they were conveyed in very different ways — much like a radio crackling from a lightning strike before broadcasting thunder.

That electrical discharges occur on Mars has various implications. Atmospheric electricity can drive chemical reactions linked to complex molecules, perhaps related to life’s origins. There are also practical concerns for future missions: dust can penetrate mechanical systems, and sparks could threaten electronics.

Fortunately, during your next road trip through desert dust devils on Earth, you can just drive through — though it might remind you that on Mars, you might see sparks in the dust.

The Conversation