Missing Martian Atmosphere: Clues In Earth’s Cold Plasma?


Illustration courtesy J. Huart, ESA

It’s been an incredible week for space weather, thanks to a nasty sunspot that hurled a cloud of superheated gas and charged particles toward the Earth.

But some cooler space weather news has snuck into the mix: “Invisible” veils of cold plasma were discovered around Earth, and they might tell us something about Mars’s missing atmosphere.

First, a little bit more on the discovery.

Two space scientists poring over data from the European Space Agency’s Cluster II satellites—four spacecraft which zip around Earth in an elliptical orbit—found evidence of positively charged, slow-moving (hence “cold”) plasma particles as far as 60,000 miles (100,000 kilometers) above Earth’s surface.

That’s about a quarter of the way to the moon, and a region where few researchers suspected any cold plasma lurked. But nobody needs to be hard on themselves, said space scientist Mats André of the Swedish Institute of Space Physics (and leader of the new study about the plasma in Geophysical Research Letters). That’s because cold plasma is really, really tough to detect.

The invisibility of cold matter has a lot to do with like charges repelling like charges. Sunlight in space strips away electrons from atoms, ionizing any “naked” matter out there to have a positive charge. That includes spacecraft (like Cluster II satellites) and atmospheric gas at the edge of Earth’s atmosphere (which creates cold plasma).

Like a magnet’s north pole near another magnet’s north pole, the spacecraft and cold plasma repel and never meet. “Hot” plasma, on the other hand, is moving fast enough to ignore any magnetic repulsion with a spacecraft, so it’s detectable.

Without some clever analysis, cold plasma just doesn’t seem to be out there.

But André knew two things. First, that a spacecraft zooming through cold plasma—if it existed—would create a shockwave (i.e. cold plasma piling in front and zipping around to the back). Second, moving charged particles emit electric fields.

André and his colleague paired these facts up to “see” the subtle shockwaves in Cluster II’s data. The results imply cold matter constitutes between 50 and 70 percent of all charged particles in Earth’s magnetic field! That’s quite a jump from zero percent.

The sudden abundance of cold plasma means a few things. Space weather forecasts stand to improve, as cold plasma particles probably interact with incoming hot matter from solar storms—and that dynamic is missing from computer models. Another effect is that the Earth is bleeding off roughly 2.2 pounds (1 kilogram) of atmospheric gas every second.

When you look at planets with thin atmospheres like Mars (which has just 1 percent the atmospheric pressure of Earth), scientists like André begin to wonder what role the “blood loss” of cold plasma plays in killing atmospheres.


Illustration couretsy NASA/JPL-Caltech

Despite having half the solar intensity, the rate of loss may be about the same as it is at Earth (about one Chihuahua’s weight every second).

“There are all kinds of ways to get rid of a planet’s atmosphere—big asteroid impacts, loss of a dynamo, and so on,” André said. “Well, this is certainly one of them when you apply it over billions of years of time. I don’t know how important it is, but this is on my short list.”

While no one knows for certain how Mars lost its atmosphere, we now have a chilly new suspect.

Dave Mosher is a freelance journalist obsessed with space, physics, biology, technology and more. He lives in New York City and you can stalk him on Twitter as @davemosher.

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