The Hunt for Martian Lightning

Mars is one of the few remaining planets in the Solar System where lightning has not yet been detected. Lightning has been observed in the atmospheres of Jupiter, Saturn, and Venus. There is also evidence on the planet suggesting the presence of lightning, such as signs on the ground of potential lightning strikes or the formation of particles in the soil examined by Viking landers that could be explained by lightning discharge. Still, there haven’t been any direct observations or confirmations yet.

The most direct evidence so far came through observations conducted in 2006. That search found indications of lightning in radio waves that corresponded to dust storms occurring on the planet at the same time. Most recently, in 2010, another search in radio was conducted by a team at UC Berkeley in an effort to confirm the observations from 2006 by using the Allen Telescope Array. Both searches used a combination of special techniques to try and capture a glimpse of Martian lightning.

Resonance

In order to figure out at which frequencies we can observe the lightning, a clever property of lightning on Earth was used. In 1952, Winfried Schumann predicted that the cavity between the surface of the Earth and the ionosphere, which is conductive, can serve as a resonant cavity for electromagnetic waves. Just as a string held at both ends (like in a violin) can only naturally vibrate at certain frequencies, the electromagnetic waves inside the cavity formed by the Earth’s surface and the ionosphere can only naturally support certain frequencies, the resonant frequencies. It turns out that lightning causes global resonances to occur in the cavity, observable at these resonant frequencies that were predicted by Schumann.

Similarly, it is expected that if we want to observe lightning on Mars, we can detect these resonant frequencies. Calculating the Schumann resonances for the cavity between the Martian surface and ionosphere gives the specific frequencies at which we can expect to detect Martian lightning.

Kurtosis

In order to distinguish between the radio emission from lightning and another natural (thermal) source, a statistic called the spectral kurtosis is used. The spectral kurtosis helps measure how different a signal is from a Gaussian signal. Most astronomical signals that are studied are those resulting from sources that can be approximated as Gaussian distributions (often also called a normal distribution or a bell curve). An example of this is thermal radiation.

Typically in astronomy, being able to identify how far a signal is from a Gaussian one is very helpful in eliminating noise and interference originating from the Earth. While objects in the sky are more likely to emit Gaussian distributed signals, those originating from human ground based sources, like radio stations and electronics, are more likely to emit peaked signals. Using the spectral kurtosis, the peaked signals can be more easily eliminated, leaving behind the astronomical signals.

In the case of detecting Martian lightning, the opposite is wanted. A lightning strike on Mars would be seen as a more sudden and peaked signal rather than a Gaussian distributed signal. The spectral kurtosis can help identify the lightning strikes and separate them from the thermal emissions coming from Mars’s natural temperature.

The most recent experiment conducted by the team at UC Berkeley was unfortunately unsuccessful at making a detection of Martian lightning during the about 30 hours of observations spread out over a few months. There were some peaks detected, but all of these were ultimately found to have come from man-made sources (which does help show the usefulness of the kurtosis at removing interference).

Although it is still not confirmed, astronomers are making progress in developing tools and techniques so that we can catch a glimpse if Mars does indeed support lightning. And even if there is never a positive sighting, it is still extremely amazing to see how it is possible for lightning strikes on another planet to be detected in radio waves right here on Earth.

Sources

  • Anderson, M., Siemion, A., et al. 2011, arXiv:1111.0685v1
  • Ruf, C., Renno, N., et al. 2009, Geophysical Research Letters, 36, L13202
  • Karkare, K. “Looking for Lightning on Mars." astrobites: 8 November 2011.