An international team of researchers has achieved a landmark discovery: they have, for the first time, detected an electromagnetic signal in the Martian atmosphere, which specialists refer to as a “whistler.” This phenomenon arises during lightning discharges, as detailed in a publication in the journal Science Advances.
NASA’s MAVEN spacecraft initially obtained the raw data way back on June 21, 2015, but only now were researchers able to confirm that the wave they registered was indeed this “whistler”—a diffused radio echo created when radiation originating from lightning passes through the planet’s ionosphere. This finding carries immense significance, validating that electrical discharges genuinely occur within the Martian atmosphere. It serves as evidence confirming that the physical laws governing radio wave propagation in plasma are universal across both Earth and Mars.
Up until this point, it remained uncertain whether such phenomena could manifest on both planets, given their differing natures. On Earth, lightning most frequently develops within water vapor clouds, yet the thin Martian atmosphere contains very little water vapor. However, it turns out that charge separation also occurs through the friction of sand particles during the intense dust storms so characteristic of the Red Planet.
The “whistler” is a specific type of signal. When lightning strikes, a broad spectrum of electromagnetic waves is emitted, the lowest frequencies of which penetrate the ionosphere and travel along magnetic field lines. Higher frequencies move faster than the lower ones, causing the signals to stretch out over time; when translated into sound, they resemble a descending whistle or the distant call of a whale.
For a considerable time, scientists theorized that such “whistles” were impossible on Mars due to the planet’s lack of a global magnetic field. Nevertheless, Mars possesses localized areas of magnetized crust, residual features left over from an ancient magnetic field. These regions, as scientists hypothesized decades ago, are capable of acting as conduits for such waves.
Experts analyzed 108,418 recordings from the plasma wave instrument aboard the MAVEN probe; the team from Charles University in the Czech Republic managed to locate a single, yet perfectly matching, signal. It was recorded at night, at an altitude of 349 kilometers above a region where residual crustal magnetism is detected.
Nighttime was chosen because, during the day, solar radiation compresses the Martian ionosphere, hindering the travel of plasma waves. The tracked signal persisted for approximately 0.4 seconds, characterized by a drop in frequency over time, and its power was tenfold greater than background noise.
Modeling the magnetic field and plasma density in the scrutinized area revealed an almost perfect correlation with how the signal would have propagated from the surface up to the spacecraft’s orbit. Calculations suggested: the discharge source was comparable in power to a strong terrestrial lightning strike, even though only a attenuated portion of the signal reached the detector. Events like this are infrequent due to a confluence of multiple factors: a powerful flash is necessary, the magnetic field must be almost vertical, it must occur on the night side of the planet, and the spacecraft with the requisite instruments must be in the right place at the right time. There is a high probability that lightning strikes on Mars actually happen more often, but they have gone undetected because this precise combination of conditions has not previously been met.
This discovery is critically important for astrobiology. Laboratory research concerning the origins of life indicates that electrical discharges can act as a trigger for the synthesis of fundamental organic molecules. If such processes are ongoing on Mars, they constitute yet another factor that must be considered when assessing the planet’s potential to have supported life in the past.
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