Last year, scientists made an intriguing discovery: Earth’s electromagnetic beating pulse between the ground and the lower ionosphere — known as the Schumann resonance — could be observed not just from the planet’s surface, but could also be detected from space using NASA’s satellite-based Vector Electric Field Instrument (VEFI).
Now a new paper in The Astrophysical Journal describes how it might be possible to use the Schumann resonance to study other planets in the solar system. (You can see a nifty NASA animation of Schumann resonances here.)
Nikola Tesla was the first to document what we now call Schumann resonances in his Colorado laboratory in 1899. Based on various experiments, he calculated that the natural resonant frequency of the Earth was around 8 Hertz. He was definitely in the ball park; that range was confirmed in the 1950s by Winfried Otto Schumann, for whom the phenomenon is named.
To get Schumann resonances, two conditions are necessary. First, you need an enclosed spherical cavity around a planet or similarly sized body, with conducting lower and upper layers separated by an insulating layer. This forms a planet-sized waveguide.
Second, you need lightning, or a similar powerful source of electromagnetic energy capable of creating the big discharges that then build up and resonate in the cavity to create that telltale pulse.
Both conditions exist on Earth, where the surface and the conductive ionosphere create the needed cavity. Lightning flashes 50 times per second around the Earth, and those discharges create the requisite reverberations.
It’s possible Schumann resonances may also exist on five other bodies in our solar system: Venus, Mars, Jupiter, Saturn, and Saturn’s moon, Titan. But those resonances might not be the same frequency as those on Earth. The frequency of Schumann resonances is determined in part by the size of a planetary body, as well as the kinds of atoms and molecules in the atmosphere, which can alter the electrical conductivity.
Therein lies the potential usefulness of this new technique. It makes it possible to determine the chemical composition of a planet’s atmosphere from 600 miles above the surface, particularly the so-called “volatiles” — water, methane and ammonia. Other techniques exist to make those same determinations, but exploiting the Schumann resonance as well provides additional information about the global density of these chemicals around the entire planet.
When combined with other instrumentation, it should be possible to gain a far more accurate inventory of various chemical abundances in a given planet’s atmosphere. And that would give astronomers valuable cues about the chemical abundances that originally existed in the parent nebula from whence our solar system formed.
Specifically, it might shed light on an as-yet-unanswered question: why the outer planets in our solar system have a much higher rate of volatiles in their atmospheres than the inner planets.
The current working theory about the formation of the solar system holds that after the sun formed, the extra dust and gas left over flattened into a spinning protoplanetary disk. Over time, all the rocky particles within that disk kept colliding and sticking together to form larger pieces of rock — thereby gravitationally attracting even more bits of rock and dust.
These bodies continued to get bigger to form “planetisimals,” eventually becoming the rocky inner planets. Further away from the sun, excess gases froze into giant spheres, developing into the gas giant planets.
Astronomers aren’t sure why this separation happened. One hypothesis is that the solar wind used to be much stronger than it is today, thereby blowing lighter elements like hydrogen and helium away from the inner regions. But the solar wind dropped off as those elements hit the outer orbits, and gravity did the rest, forming giant planets with solid rock and icy cores surrounded by gas.
But then came the discovery in 1995 of a distant hot gas giant the size of Jupiter, 51 Pegasi b, orbiting close to its sun, suggesting that after forming far from the star, the planet then migrated inward into a closer orbit. So a new theory is needed to explain this suspected migration, and better information about chemical abundances in planetary atmospheres could help.
Images: (top) Lightning at the shuttle launchpad at NASA’s Kennedy Space Center in Florida. Credit: NASA/Bill Ingalls. (center) NASA Goddard Space Flight Center. Public domain. (bottom) The solar system. Source: How Stuff Works.