Schumann resonances are electromagnetic waves that resonate in the cavity that is naturally formed between the conductive planetary surface and lower ionosphere, containing the atmosphere.
Their wavelength is of the order of the planet’s perimeter, typically several thousand km, and therefore they are of Extremely low frequency (few Hz). They are an indicator of electromagnetic activity in the planetary atmosphere that is contained within the cavity. They were first predicted in 1952 by Schumann and measured on Earth in 1962 by Balser and Wagner. The main source of excitation on Earth is lightning, and the SR parameters are linked to global weather effects, such as the global temperature of the planet of El Nino o La Nina events.
The Huygens descent module onboard the Cassini mission to Saturn carried an electromagnetic sensor that measured them in the atmosphere of Titan, the largest moon of Saturn. This corresponds to the first in-situ measurement of this kind of resonances in a celestial body other than Earth. The Schumann Resonance characteristics give information about the ionosphere – atmosphere – surface of the planet, and in the case of Titan it revealed the existence of a buried Ocean containing (highly conductive) liquid water below the surface (Béghin et al. 2012).
In Mars, so far the Schumann Resonance has not been measured in its atmosphere. We know that some key ingredients are present, such as an ionosphere and an atmosphere to contain them. It is unclear if the Martian atmosphere has any electrical activity that can trigger the resonances.
Dust devils have been proposed as a possible source of atmospheric electrification, as it occurs in Earth’s deserts, but it is unclear if they are strong and frequent enough as to excite or maintain the resonances on Mars. There is a remote detection by Ruf et al. (2009) using the Deep Space Network Antennas that is consistent with an indirect observation of Schumann Resonances. However, direct evidence of their existence has not been provided so far.
The first mission that carried instrumentation to characterize the Martian atmospheric electricity and potentially detect SR was the descent module Schiapparelli onboard ExoMars 2016, but a problem during the landing sequence impeded a soft landing and the instrumentation was damaged. The next opportunity to characterize the electric activity on Mars and the eventual existence of Schumann Resonances is ExoMars 2020. The surface platform will carry a larger version of the antenna that was included in ExoMars 2016.
Simulations To Characterize Mars Atmosphere
We performed simulations based on our current understanding of the Martian atmosphere and ionosphere to know what to expect and where to look for the Schumann Resonances on Mars. There are two main characteristics that differentiate this cavity from the one on Earth. First, Mars atmosphere is known to host very large dust storms in large portions of the planet. These storms severely affect the conductivity of the atmosphere because of the charged dust grains. These events are not always present, introducing a large variability of the resonant cavity.
In addition, the lack of a strong magnetic field of Mars makes the ionosphere – atmosphere system of Mars very vulnerable to solar wind. Therefore, the characteristics of the illuminated (dayside) and the night side atmosphere are very different, creating a strong asymmetry of the cavity. We modeled the cavity using numerical simulations to account for these two factors. We find that the frequency of the first resonant mode can vary between 9 – 14 Hz, as opposed to Earth where it is quite stable at 7.8 Hz. The first mode should be the one with more energy and therefore the easiest to detect.
Nighttime conditions of the Martian electromagnetic cavity are better for propagating the waves, and it should be easier to detect Schumann Resonances in the Martian night, according to our findings. In addition, new resonant modes are possible in the night side of the cavity owing to wave reflection at the terminator, i.e., the ionospheric boundary that separates the illuminated (day) from the dark (night) regions. Finally, our simulations indicate that these resonances would be fainter than on Earth, with very low-quality factors, or in other words, the energy of the resonances degenerates into broader tones.
This study, Schumann resonances at Mars: Effects of the day-night asymmetry and the dust-loaded ionosphere was recently published in the journal Geophysical Research Letters.*NOTE: New DOI's are registered weekly Friday and may not function until then.