We live inside a bubble surrounded by a shooting gallery, but we’ve only realized for a few years how well-designed that bubble is.
The Van Allen radiation belts were detected in 1958 by the Explorer rockets, America’s first satellites to orbit the Earth. Since then, physicists have known they shield the Earth from high-energy particles from the sun, but many questions have remained about the belts’ structure and physical mechanisms — especially, how electrons from the solar wind are accelerated to ultra-relativistic speeds in the belts.
Twin NASA spacecraft, the Van Allen probes, in orbit since 2012, have been revealing new secrets of the Van Allen belts produced by the Earth’s magnetic field. News from MIT describes how "killer electrons" are accelerated to 1,000 kilometers per second inside the belts in mere seconds. Because the probes are one hour apart in their orbit, they were able to watch the process unfold.
On Oct. 8, 2013, an explosion on the sun’s surface sent a supersonic blast wave of solar wind out into space. This shockwave tore past Mercury and Venus, blitzing by the moon before streaming toward Earth. The shockwave struck a massive blow to the Earth’s magnetic field, setting off a magnetized sound pulse around the planet. [Emphasis added.]
Some of the data is analyzed at MIT’s Haystack Observatory, where John Foster acts as associate director. He was surprised at how quickly the particles were accelerated by a factor of 10 in just 60 seconds.
Foster and his colleagues analyzed the probes’ data, and laid out the following sequence of events: As the solar shockwave made impact, according to Foster, it struck "a sledgehammer blow" to the protective barrier of the Earth’s magnetic field. But instead of breaking through this barrier, the shockwave effectively bounced away, generating a wave in the opposite direction, in the form of a magnetosonic pulse — a powerful, magnetized sound wave that propagated to the far side of the Earth within a matter of minutes.
This was "the first time the effects of a solar shockwave on Earth’s radiation belts have been observed in detail from beginning to end." It was a "relatively small shock," the article goes on to say. Solar shocks can be much bigger, posing a threat to astronauts and satellites outside the protected zone. The "killer electrons" produced are so energetic, they can zip right through a spacecraft.
What would happen if these high-energy particles reached the Earth? Well, if they can fry a satellite, imagine what they would do to a living cell or DNA molecule. Multiply the shocks over centuries, millennia or longer, and it’s very unlikely any life could survive on the planet, except perhaps in the deepest oceans, or far underground.
Scientists are just beginning to understand how the magnetic field and Van Allen belts work together. A very interesting phenomenon was announced in Nature last fall: a surprisingly thin layer in the Van Allen belts creates "an almost impenetrable barrier through which the most energetic Van Allen belt electrons cannot migrate." A news item from the University of Colorado calls it a "Star Trek-like invisible shield" thousands of miles above the Earth.
As the first significant discovery of the space age, the Van Allen radiation belts were detected in 1958 by Professor James Van Allen and his team at the University of Iowa and were found to be comprised of an inner and outer belt extending up to 25,000 miles above Earth’s surface. In 2013, [Daniel] Baker — who received his doctorate under Van Allen — led a team that used the twin Van Allen Probes launched by NASA in 2012 to discover a third, transient "storage ring" between the inner and outer Van Allen radiation belts that seems to come and go with the intensity of space weather.
The latest mystery revolves around an "extremely sharp" boundary at the inner edge of the outer belt at roughly 7,200 miles in altitude that appears to block the ultrafast electrons from breeching the shield and moving deeper towards Earth’s atmosphere.
"It’s almost like theses electrons are running into a glass wall in space," said Baker, the study’s lead author. "Somewhat like the shields created by force fields on Star Trek that were used to repel alien weapons, we are seeing an invisible shield blocking these electrons. It’s an extremely puzzling phenomenon."
Mercury, Venus, the moon, and Mars lack a powerful magnetic field that can generate these belts or the structure they create like the impenetrable shield. It’s no wonder that those bodies are all lifeless. How Earth’s "extremely sharp" protective boundary is generated is somewhat of a mystery:
"Nature abhors strong gradients and generally finds ways to smooth them out, so we would expect some of the relativistic electrons to move inward and some outward," said Baker. "It’s not obvious how the slow, gradual processes that should be involved in motion of these particles can conspire to create such a sharp, persistent boundary at this location in space."
Here we see a significant factor that must be considered when evaluating the habitability of an exoplanet. It’s not enough that the planet reside in the "circumstellar habitable zone" where liquid water can exist. Life could not survive the barrage of radiation most stars emit constantly. (The sun, interestingly, is a relatively quiet star within its class, which comprises a minority of all stars.)
Think of how many unrelated "coincidences" have "conspired" to create this effect. A magnetic field generated from deep within the Earth creates a multi-level structure that produces a very sharp invisible shield high above the planet. This shield — responding automatically — is strong enough to capture the highest-energy particles from a star that happens to be among the quietest in its class. But the shield does not block useful radiation from reaching the atmosphere to maintain a temperate climate and energy for plants. These factors, moreover, converge on the only rocky planet that "just happens" to reside in the habitable zone, and, in fact, is inhabited by a profusion of diverse living creatures — including intelligent observers who can study the shield and appreciate it.
The "Star Trek" shield is just one of many factors that make a planet habitable. Some of these are discussed in Illustra’s film The Privileged Planetand the book with that title co-authored by Guillermo Gonzalez and Jay Richards. Additional amazing "coincidences" of astrophysics are shared in Discovery Institute’s recent masterpiece video, Privileged Species, featuring Michael Denton.
"Privileged" is the right word. We carry on our daily lives, even laying out on the beach in the sunshine, oblivious to all that makes that possible.
Image source: NASA via MIT News.