Mars Has Been Making Perchlorates Out of Dust for 50 Years. Nobody Knew How.

In 1976, the Viking landers touched down on Mars and found reactive chemistry in the soil — a chlorine signal that puzzled scientists for decades. Nobody could explain how it got there. Fifty years later, the answer is finally in: dust storms charge the air until it discharges, and those discharges make the chemicals Viking detected. The evidence is audio.

A team led by researchers at Washington University in St. Louis has published the most detailed account yet of how electrostatic discharges in Martian dust storms drive the planet's chemistry. The work, published in Earth and Planetary Science Letters, combines Perseverance's in-situ acoustic recordings with lab simulations that reproduce the conditions inside a dust storm at roughly one percent of Earth's atmospheric pressure. Mars is electrically alive, and it has been for three billion years.

The numbers come from Perseverance's SuperCam microphone. Since sol 215 in 2021, the instrument has logged 55 distinct electrical events, 16 of which occurred when dust devils passed directly over the rover. The remaining 35 came with the convective fronts of regional dust storms. SuperCam was not designed to be a lightning detector. It was designed to hear rocks being hit by a laser. But a microphone pointed at the sky on Mars turns out to be a useful electrostatic sensor, because the discharges produce a measurable acoustic signature.

The acoustic observations were first published in Nature in November 2025, led by Baptiste Chide, a planetary scientist at the University of Toulouse and a member of the Perseverance science team. That paper established the event frequency and character. The WUSTL Earth and Planetary Science Letters paper is the mechanism paper: it explains why the discharges matter, not just that they happen.

"The isotopic ratios can only be affected by the major process in a system," Alian Wang, lead author and research professor at WUSTL's McDonnell Center for the Space Sciences, said in the university's announcement. "The substantial heavy isotope depletion of three mobile elements is a smoking gun that nails down the importance of dust-induced electrochemistry in shaping the contemporary Mars surface-atmosphere system."

The evidence is in the isotopes. The WUSTL team ran electrostatic discharge products through two custom simulation chambers, PEACh and SCHILGAR, and found that discharges preferentially exclude heavy isotopes of chlorine, oxygen, and carbon. The result is coherent depletion of 37Cl, 18O, and 13C in the perchlorates, volatile chlorine, and carbonates produced. Per the WUSTL lab, the depletion is substantial and consistent across three independent isotopic systems.

This is not a subtle signal. It matches what Curiosity measured in Gale Crater, where the delta-37Cl value came in at negative 51 per mil. It also matches measurements by the ESA Trace Gas Orbiter, which found a similar trend of heavy isotopic depletions in hydrogen chloride in the Martian atmosphere. Four independent missions have now measured the same isotopic signature. The WUSTL lab can reproduce it in a chamber. The mechanism is electrostatic discharge.

The physics is straightforward. On Earth, the atmosphere is thick enough that charge dissipates before it builds to discharge levels in most dust events. On Mars, atmospheric pressure is less than one percent of Earth's, so the threshold for discharge is about one percent of what it would take on Earth. Dust devils and storm fronts reach it routinely. The energetic electrons produced in an electrostatic discharge event are, at Mars conditions, ten million times more powerful per photon than the ultraviolet photons that drive surface chemistry on Earth. ESD does not just happen on Mars. It dominates.

The geological consequences play out on a human timescale. The WUSTL team calculates that dust-discharge phase changes in surface chemistry would become observable after hundreds of years on Mars. That is short by geological standards. The planet's Amazonian period, the last three billion years of its history, has been long enough for dust electrochemistry to reshape the near-surface chemistry many times over. Viking's reactive chlorine chemistry puzzled scientists for decades, and was later found to include perchlorate — but the mechanism was never established. The new data does not contradict the water story but it adds another: perchlorates on Mars may not be a water story at all. They may be electrical.

Kun Wang, associate professor at WUSTL and a co-author of the Earth and Planetary Science Letters paper, put it plainly: this is the first experimental study to show how electrostatic discharges can affect isotopes in a Martian environment. The experiments did not produce the full range of extreme light signatures seen at Gale Crater, but they demonstrate the process works in the right direction and at the right scale to matter geologically.

Alian Wang's group has been building toward this for four years, with support from NASA's Solar System Working Program. The team includes members from six universities across the United States, China, and the United Kingdom. They built the chambers from scratch because no commercial system could replicate the pressure and gas composition conditions at Mars.

The broader implication is not just Mars. The same dust electrification physics operates wherever there is a thin atmosphere and a dusty surface. Titan has it. Triton probably has it. The moon has it, though the absence of an atmosphere constrains the discharge regime. Earth does not generate the kind of regional dust storm electrification that Mars does, but it does generate localized discharges in haboobs and volcanic plumes. The chemistry is universal. Mars is the only place we have four independent measurements of its output.

Perchlorates matter beyond the science. They are toxic to humans, a fact that complicates any Mars surface mission architecture. If perchlorate distribution on Mars is driven by electrical discharge chemistry rather than water chemistry, the deposition patterns will be different from what a water-driven model predicts. That has operational implications for where future missions should avoid landing and where surface processing infrastructure needs to account for toxic salt accumulation.

The WUSTL Earth and Planetary Science Letters paper is Wang et al., "Isotope effects (Cl, O, C) of heterogeneous electrochemistry induced by Martian dust activities," doi.org/10.1016/j.epsl.2025.119784. The Perseverance acoustic observations are in Chide et al., Nature, November 26, 2025.